ATOMIC-ABSORPTION AND EMISSION ANALYSIS. EXTRACTION AND EXTRACTION-PHOTOMETRIC ANALYSIS
FLAME EMISSION ANALYSIS: THEORETICAL BASES, PRINCIPAL SCHEMA, QUALITATIVE AND QUANTITATIVE ANALYSIS, USAGE
Flame emission analysis is based on measurement of radiation by excited metal in flame.
Me* = Me + hn
§ This analysis is the part of emission spectroscopy, in which as excitation source is used flame of different types.
In flame emission spectrometry, the sample solution is nebulized (converted into a fine aerosol) and introduced into the flame where it is desolvated, vaporized, and atomized, all in rapid succession. Subsequently, atoms and molecules are raised to excited states via thermal collisions with the constituents of the partially burned flame gases. Upon their return to a lower or ground electronic state, the excited atoms and molecules emit radiation characteristic of the sample components. The emitted radiation passes through a monochromator that isolates the specific wavelength for the desired analysis. A photodetector measures the radiant power of the selected radiation, which is then amplified and sent to a readout device, meter, recorder, or microcomputer system.
Combustion .flames provide a means of converting analytes in solution to atoms in the vapor phase freed of their chemical surroundings. These free atoms are then transformed into excited electronic states by one of two methods: absorption of additional thermal energy from the flame or absorption of radiant energy from an external source of radiation.
In the first method, known as flame emission spectroscopy (
Schema of processes in the flame
Salt solution ® aerosol liquid-gas ® aerosol solid matter-gas ® vapour of salt ® salt dissociation ® Me
Characteristic of some types of flame
§ Lighting gas – air 1700 – 1840 °С
§ Propyl hydride – air 1925 °С
§ Acetylene – air 2125 – 2397 °С
§ Hydrogen – air 2000 – 2045 °С
§ Lighting gas– oxygen 2370 °С
§ Acetylene – oxygen 3100 – 3137 °С
§ Oxalonitrile – oxygen 4380 °С
Schema of transformation of metal atoms in the flame
The intensity, I, of an emission line is proportional to the number of atoms (concentration), populating the excited state
Reason for rejection from linearity:
§ viscosity and interfacial tension of solution
§ self-absorption
§ ionization
§ Formation slightly dissociated, nonvolatile compounds in flame
The main part of flame photometer:
§ System of gas delivering and air (compressor)
§ flame
§ optical filter or monochromator
§ System of photo cell and amplifier
Instruments:
§ Flame photometer
§ Flame spectrophotometer
INSTRUMENTATION FOR FLAME SPECTROMETRIC METHODS
The basic components of flame spectrometric instruments are discussed in this section. These components provide the following functions required in each method: deliver the analyte to the flame, induce the spectral transitions (absorption or emission) necessary for the determination of the analyte, isolate the spectral lines required for the analysis, detect the increase or decrease in intensity of radiation at the isolated lines(s), and record these intensity data.
Pretreatment of Sample
Flame
Sample Delivery
The device that introduces the sample into the flame or plasma plays a major role in determining the accuracy of the analysis. The most popular sampling method is nebulization of a liquid sample to provide a steady flow of aerosol into a flame. An introduction system for liquid samples consists of three components: a nebulizer that breaks up the liquid into small’ droplets, an aerosol. modifier that removes large droplets from the stream, allowing only droplets smaller than a certain size to pass, and the flame or atomizer that converts the anaIyte into free atoms.
Nebulization
Pneumatic nebulization is the technique used in most atomic spectroscopy determinations. The sample solution is introduced through an orifice into a high- velocity gas jet, usually the oxidant. The sample stream may intersect the gas stream in either a parallel or perpendicular manner. Liquid is drawn through the sample capillary by the pressure differential generated by the high-velocity gas stream passing over the sample orifice. The liquid stream begins to oscillate, producing filaments. Finally, these filaments collapse to form a cloud of droplets in the aerosol modifier or spray chamber.
In the spray chamber the larger droplets are removed from the sample stream by mixer paddles or broken up into smaller droplets by impact beads or wall surfaces. The final aerosol, now a fine mist, is combined with the oxidizer/fuel mixture and carried into the burner.
A typical distribution range of droplet diameters is shown. Droplets larger than about 20 μm are trapped in the spray chamber and flow to waste. The distribution of drop sizes is a function of the solvent as well as the components of the sampling system. In AAS only a small percentage (usually 2% or 3%) of the nebulized analyte solution reaches the burner.
Atomization
The atomization step must convert the analyte within the aerosol into free analyte atoms in the ground state for
Flame Atomizers. The sequence of events involved in converting a metallic element, M, from a dissolved salt, MX, in the sample solution to free M atoms in the flame is depicted. After the aerosol droplets containing 1v.IX enter the flame, the solvent is evaporated, leaving small particles of dry, solid 1v.IX. Next, solid 1v.IX is converted to NIX vapor. Finally, a portion of the MX molecules are dissociated to give neutral free atoms. The efficiency with which the flame produces neutral analyte atoms is of equal importance in all the flame techniques.
If the events proceed vertically from the top down in Figure 9.4, the efficiency of free- atom production is high. Processes that branch horizontally interfere with the production of free analyte atoms. These processes include: excitation and emission of radiation by MX(g) molecules, reaction of M(g) atoms with flame components at high temperatures to produce molecules and ions that also absorb and emit radiation, and formation of M+x ions, which, in addition to reducing the efficiency of free-atom production,. complicate the analysis by adding lines to the spectrum. The flame remains the most generally useful atomizer for atomic spectroscopy despite the developments in electrothermal atomization. A satisfactory flame source must provide the temperature and fuel/oxidant ratio required for a given analysis. The maximum operating temperature of the flame is determined by the identities of the fuel and oxidant, whereas the exact flame temperature is fixed by the fuel/oxidant ratio. In addition, the spectrum of the flame itself should not interfere with the emission or absorption lines of the analytes. Components of the flame gases limit the usable range to wavelengths longer than 210 nm.
Ionization Interference
At elevated flame and furnace temperatures, atoms with low ionization potentials become ionized. Any ionization reduces the population of both the ground state and the excited state of neutral free atoms, thus lowering the sensitivity of the determination. This problem is readily overcome by adding an excess (ca. 100- fold) of a more easily ionized element such as K, Cs, or Sr to suppress ionization in both sample and calibration solutions. The more easily ionized atoms produce a large concentration of electrons in the vapor. These electrons, by mass action, suppress the ionization of analyte atoms. Thus, suppressant should be added to samples that contain variable amounts of alkali metals analyzed by acetylene/air flames to stabilize free-electron concentrations. The addition of suppressants is even more important in analyses that require the hotter acetylene/nitrous oxide flames.
Technique of analysis
§ Preparation of sample to analysis (dissolution)
§ Introduction of sample solution into the flame
§ Selection of analytical spectral line of atoms of investigated element
§ Measurement of intensity spectral line
§ Calculation of investigated element (substance) concentration in the sample
Quantitative flame emission analysis
It is based on measurement of dependence intensity from concentration metal atoms in solution
Methods of quantitative analysis:
§ A method of calibration chart
§ Method of additives
§ Comparison method (sometimes method of limiting solutions)
§ or as method of limiting solutions:
Advantages of flame emission analysis
§ High sensitivity of flame photometry: for Sodium – 0.001 µg/mL, for Potassium and another alkali metals – 0.01 µg/mL, for another metals – 0.1 µg/mL
§ High accuracy (error of method – 1-3 %)
§ High reproducibility of results
§ It is necessary to define many metals
APPLICATIONS
Most applications of
ATOMIC ABSORPTION SPECTROSCOPY: THEORETICAL BASES, PRINCIPAL SCHEMA, QUALITATIVE AND QUANTITATIVE ANALYSIS, USAGE
Atomic absorption, along with atomic emission, was first used by Guystav Kirchhoff and Robert Bunsen in 1859 and 1860, as a means for the qualitative identification of atoms. Although atomic emission continued to develop as an analytical technique, progress in atomic absorption languished for almost a century. Modern atomic absorption spectroscopy was introduced in 1955 as a result of the independent work of A. Walsh and C. T. J. Alkemade.18 Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident.
This method is based on absorption by free metal atoms resonance emission at it travels through atomic vapour
Schema of process in the flame at atomic absorption determination
§ Absorption resonance emission by free metal atoms
Me + hn = Me*
n – frequency (on Bohr):
Instrumentation
Atomic absorption spectrophotometers (Figure) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers. There are, however, several important differences that are considered in this section.
Figure Photo of a typical atomic absorption spectrophotometer. Courtesy of Varian, Inc.
Main parts of atomic absorption spectrophotometer:
§ monochromatic light source (hollow cathode lamp);
Me + Ar+(e-) ® Me* ® Me + hnreson.
The source for atomic absorption is a hollow cathode lamp consisting of a cathode and anode enclosed within a glass tube filled with a low pressure of Ne or Ar (Figure).
Schematic diagram of a hollow cathode lamp showing mechanism by which atomic emission is obtained
When a potential is applied across the electrodes, the filler gas is ionized. The positively charged ions collide with the negatively charged cathode, dislodging, or “sputtering,” atoms from the cathode’s surface. Some of the sputtered atoms are in the excited state and emit radiation characteristic of the metal from which the cathode was manufactured. By fashioning the cathode from the metallic analyte, a hollow cathode lamp provides emission lines that correspond to the analyte’s absorption spectrum.
The sensitivity of an atomic absorption line is often described by its characteristic concentration, which is the concentration of analyte giving an absorbance of 0.00436 (corresponding to a percent transmittance of 99%).
Usually the wavelength providing the best sensitivity is used, although a less sensitive wavelength may be more appropriate for a high concentration of analyte.
A less sensitive wavelength also may be appropriate when significant interferences occur at the most sensitive wavelength. For example, atomizing a sample produces atoms of not only the analyte, but also of other components present in the sample’s matrix. The presence of other atoms in the flame does not result in an interference unless the absorbance lines for the analyte and the potential interferant are within approximately 0.01 nm. When this is a problem, an interference may be avoided by selecting another wavelength at which the analyte, but not the interferant, absorbs.
The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer’s law. Normally the monochromator’s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation.
§ atomizer (graphite electrochemical atomizer or flame);
Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used: flame atomization and electrothermal atomization. A few elements are atomized using other methods.
Flame Atomizers
In flame atomization the sample is first converted into a fine mist consisting of small droplets of solution. This is accomplished using a nebulizer assembly similar to that shown in the inset to Figure.
Flame atomization assembly equipped with spray chamber and slot burner. The inset shows the nebulizer assembly.
The sample is aspirated into a spray chamber by passing a high-pressure stream consisting of one or more combustion gases, past the end of a capillary tube immersed in the sample. The impact of the sample with the glass impact bead produces an aerosol mist. The aerosol mist mixes with the combustion gases in the spray chamber before passing to the burner where the flame’s thermal energy desolvates the aerosol mist to a dry aerosol of small, solid particles. Subsequently, thermal energy volatilizes the particles, producing a vapor consisting of molecular species, ionic species, and free atoms.
Thermal energy in flame atomization is provided by the combustion of a fuel–oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9.
Of these, the air–acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio; however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure. This burner provides a long path length for monitoring absorbance and a stable flame.
The burner is mounted on an adjustable stage that allows the entire burner assembly to move horizontally and vertically. Horizontal adjustment is necessary to ensure that the flame is aligned with the instrument’s optical path. Vertical adjustments are needed to adjust the height within the flame from which absorbance is monitored. This is important because two competing processes affect the concentration of free atoms in the flame. An increased residence time in the flame results in a greater atomization efficiency; thus, the production of free atoms increases with height. On the other hand, longer residence times may lead to the formation of metal oxides that absorb at a wavelength different from that of the atom. For easily oxidized metals, such as Cr, the concentration of free atoms is greatest just above the burner head. For metals, such as Ag, which are difficult to oxidize, the concentration of free atoms increases steadily with height (Figure).
Other atoms show concentration profiles that maximize at a characteristic height.
The most common means for introducing samples into a flame atomizer is continuous aspiration, in which the sample is continuously passed through the burner while monitoring the absorbance. Continuous aspiration is sample intensive, typically requiring 2–5 mL of sample. Flame microsampling provides a means for introducing a discrete sample of fixed volume and is useful when the volume of sample is limited or when the sample’s matrix is incompatible with the flame atomizer. For example, the continuous aspiration of a sample containing a high concentration of dissolved solids, such as sea water, may result in the build-up of solid deposits on the burner head. These deposits partially obstruct the flame, lowering the absorbance. Flame microsampling is accomplished using a micropipet to place 50–250 mL of sample in a Teflon funnel connected to the nebulizer, or by dipping the nebulizer tubing into the sample for a short time.
Dip sampling is usually accomplished with an automatic sampler. The signal for flame microsampling is a transitory peak whose height or area is proportional to the amount of analyte that is injected.
The principal advantage of flame atomization is the reproducibility with which the sample is introduced into the spectrophotometer. A significant disadvantage to flame atomizers is that the efficiency of atomization may be quite poor. This may occur for two reasons. First, the majority of the aerosol mist produced during nebulization consists of droplets that are too large to be carried to the flame by the combustion gases. Consequently, as much as 95% of the sample never reaches the flame.
A second reason for poor atomization efficiency is that the large volume of combustion gases significantly dilutes the sample. Together, these contributions to the efficiency of atomization reduce sensitivity since the analyte’s concentration in the flame may be only 2.5 *10–6 of that in solution.
Electrothermal Atomizers
A significant improvement in sensitivity is achieved by using resistive heating in place of a flame. A typical electrothermal atomizer, also known as a graphite furnace, consists of a cylindrical graphite tube approximately 1–3 cm in length, and 3–8 mm in diameter (Figure).
Diagram of an electrothermal analyzer
The graphite tube is housed in an assembly that seals the ends of the tube with optically transparent windows.
The assembly also allows for the passage of a continuous stream of inert gas, protecting the graphite tube from oxidation, and removing the gaseous products produced during atomization. A power supply is used to pass a current through the graphite tube, resulting in resistive heating.
Samples between 5 and 50 mL are injected into the graphite tube through a small-diameter hole located at the top of the tube. Atomization is achieved in three stages. In the first stage the sample is dried using a current that raises the temperature of the graphite tube to about
Electrothermal atomization provides a significant improvement in sensitivity by trapping the gaseous analyte in the small volume of the graphite tube. The analyte’s concentration in the resulting vapor phase may be as much as 1000 times greater than that produced by flame atomization. The improvement in sensitivity, and the resulting improvement in detection limits, is offset by a significant decrease in precision. Atomization efficiency is strongly influenced by the sample’s contact with the graphite tube, which is difficult to control reproducibly.
Miscellaneous Atomization Methods
A few elements may be atomized by a chemical reaction that produces a volatile product. Elements such as As, Se, Sb, Bi, Ge, Sn, Te, and Pb form volatile hydrides when reacted with NaBH4 in acid. An inert gas carries the volatile hydrides to either a flame or to a heated quartz observation tube situated in the optical path. Mercury is determined by the cold-vapor method in which it is reduced to elemental mercury with SnCl2. The volatile Hg is carried by an inert gas to an unheated observation tube situated in the instrument’s optical path.
§ monochromatizater;
§ photo multiplier.
Preparing the Sample
Flame and electrothermal atomization require that the sample be in a liquid or solution form. Samples in solid form are prepared for analysis by dissolving in an appropriate solvent. When the sample is not soluble, it may be digested, either on a hot plate or by microwave, using HNO3, H2SO4, or HClO4. Alternatively, the analyte may be extracted via a Soxhlet extraction. Liquid samples may be analyzed directly or may be diluted or extracted if the matrix is incompatible with the method of atomization. Serum samples, for instance, may be difficult to aspirate when using flame atomization and may produce unacceptably high background absorbances when using electrothermal atomization. A liquid–liquid extraction using an organic solvent containing a chelating agent is frequently used to concentrate analytes. Dilute solutions of Cd2+, Co2+, Cu2+, Fe3+, Pb2+, Ni2+, and Zn2+, for example, can be concentrated by extracting with a solution of ammonium pyrrolidine dithiocarbamate in methyl isobutyl ketone.
Standardizing the Method
Because Beer’s law also applies to atomic absorption, we might expect atomic absorption calibration curves to be linear. In practice, however, most atomic absorption calibration curves are nonlinear, or linear for only a limited range of concentrations. Nonlinearity in atomic absorption is a consequence of instrumental limitations, including stray radiation from the hollow cathode lamp and a nonconstant molar absorptivity due to the narrow width of the absorption line. Accurate quantitative work, therefore, often requires a suitable means for computing the calibration curve from a set of standards. Nonlinear calibration curves may be fit using quadratic and cubic equations, although neither works well over a broad range of concentrations.
When possible, a quantitative analysis is best conducted using external standards.
Unfortunately, matrix interferences are a frequent problem, particularly when using electrothermal atomization. For this reason the method of standard additions is often used. One limitation to this method of standardization, however, is the requirement that there be a linear relationship between absorbance and concentration.
Quantitative analysis
Bouguer-Lambert-Beer law:
§ Reduction of intensity of resonant light in atomic absorption spectroscopy is proportional atom number into atomic vapour (concentration of this metal in solution)
Reasons of absorbance nonlinearity and metal ions concentration in solution
§ physical:
– Instability of work of different equipment part
– Not monochromatic emission lines
– Metal atomic ionization
§ chemical:
– anionic effect
– cationic effect
Methods of quantitative analysis in atomic absorption:
§ method of calibration chart
§ Method of additives
Advantages of atomic absorption analysis:
§ Very high selectivity
§ Wide spectrum of investigated ions (nearly 70)
§ Low determined minimum (10-5 – 10-6 %)
§ High reproducibility (error ~5% to 10 %)
Lacks of ААА in comparison with flame emission analysis:
§ Low sensitivity
§ Complexity of equipment
§ Presence of nonselective absorption (because dispersion of light and molecular absorption)
Taking into account nonselective absorption:
§ automatic
§ usage of polarized light
|
Atomic absorption using either flame or electrothermal atomization is widely used for the analysis of trace metals in a variety of sample matrices. Using the atomic absorption analysis for zinc as an example, procedures have been developed for its determination in samples as diverse as water and wastewater, air, blood, urine, muscle tissue, hair, milk, breakfast cereals, shampoos, alloys, industrial plating baths, gasoline, oil, sediments, and rocks.
Developing a quantitative atomic absorption method requires several considerations, including choosing a method of atomization, selecting the wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. Each of these topics is considered in this section.
MOLECULAR PHOTOLUMINESCENCE SPECTROSCOPY: THEORETICAL BASES, PRINCIPAL SCHEMA, QUALITATIVE AND QUANTITATIVE ANALYSIS, USAGE
Luminescence is excess of temperature radiation of substance in the event that this excessive emission owns final duratioearly 10-10 с and more.
Types of luminescence at nature of initiation energy:
§ Triboluminescence is an optical phenomenon in which light is generated when asymmetrical crystalline bonds in a material are broken when that material is scratched, crushed, or rubbed. The phenomenon is not fully understood, but appears to be caused by the separation and reunification of electrical charges.
§ Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons is generated by an electron gun (e.g. cathode ray tube) and then impacts on a luminescent material such as a phosphor, causing the material to emit visible light.
§ Chemiluminescence (sometimes “chemoluminescence“) is the emission of light with limited emission of heat (luminescence), as the result of a chemical reaction. Given reactants A and B, with an excited intermediate ◊,
[A] + [B] → [◊] → [Products] + light
§ For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:
luminol + H2O2 → 3-APA[◊] → 3-APA + light
where:
§ 3-APA is 3-aminophthalate
§ 3-APA[◊] is the excited state fluorescing
as it decays to a lower energy level.
A chemoluminescent reaction carried out in an erlenmeyer flask producing a large amount of light.
§ Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field. This is distinct from light emission resulting from heat (incandescence) from the action of chemicals (chemiluminescence), the action of sound (sonoluminescence), or other mechanical action (mechanoluminescence).
§ The amount of light given off is proportional to the dose of radiation received. In thermoluminescence dating, this can be used to date buried objects that have been heated in the past (eg pottery) since the dose received from radioactive elements in the soil, cosmic rays etc is proportional to age. This phenomenon has been used for Thermoluminescent dosimeters, to measure the radiation dose received by a chip of suitable material that is carried around by a person or placed with an object.
Photoluminescence is divided into two categories: fluorescence and phosphorescence.
Absorption of an ultraviolet or visible photon promotes a valence electron from its ground state to an excited state with conservation of the electron’s spin. For example, a pair of electrons occupying the same electronic ground state have opposite spins (Figure a) and are said to be in a singlet spin state.
Difference between singlet and triplet states
Absorbing a photon promotes one of the electrons to a singlet excited state (Figure b). Emission of a photon from a singlet excited state to a singlet ground state, or between any two energy levels with the same spin, is called fluorescence. The probability of a fluorescent transition is very high, and the average lifetime of the electron in the excited state is only 10–5–10–8 s. Fluorescence, therefore, decays rapidly after the excitation source is removed. In some cases an electron in a singlet excited state is transformed to a triplet excited state (Figure c) in which its spin is no longer paired with that of the ground state. Emission between a triplet excited state and a singlet ground state, or between any two energy levels that differ in their respective spin states, is called phosphorescence. Because the average lifetime for phosphorescence ranges from 10–4 to 104 s, phosphorescence may continue for some time after removing the excitation source.
The use of molecular fluorescence for qualitative analysis and semiquantitative analysis can be traced to the early to mid-1800s, with more accurate quantitative methods appearing in the 1920s. Instrumentation for fluorescence spectroscopy using filters and monochromators for wavelength selection appeared in, respectively, the 1930s and 1950s. Although the discovery of phosphorescence preceded that of fluorescence by almost 200 years, qualitative and quantitative applications of molecular phosphorescence did not receive much attention until after the development of fluorescence instrumentation.
Molecular Fluorescence and Phosphorescence Spectra
To appreciate the origin of molecular fluorescence and phosphorescence, we must consider what happens to a molecule following the absorption of a photon. Let’s assume that the molecule initially occupies the lowest vibrational energy level of its electronic ground state. The ground state, which is shown in Figure, is a singlet state labeled S0.
Energy level diagram for a molecule showing pathways for deactivation of an excited
state: vr is vibrational relaxation; ic is internal conversion; ec is external conversion, and iscis intersystem crossing. The lowest vibrational energy level for each electronic state is indicated by the thicker line.
Absorption of a photon of correct energy excites the molecule to one of several vibrational energy levels in the first excited electronic state, S1, or the second electronic excited state, S2, both of which are singlet states. Relaxation to the ground state from these excited states occurs by a number of mechanisms that are either radiationless, in that no photons are emitted, or involve the emission of a photon. These relaxation mechanisms are shown in Figure.
The most likely pathway by which a molecule relaxes back to its ground state is that which gives the shortest lifetime for the excited state.
Radiationless Deactivation
One form of radiationless deactivation is vibrational relaxation, in which a molecule in an excited vibrational energy level loses energy as it moves to a lower vibrational energy level in the same electronic state.
Vibrational relaxation is very rapid, with the molecule’s average lifetime in an excited vibrational energy level being 10–12 s or less. As a consequence, molecules that are excited to different vibrational energy levels of the same excited electronic state quickly return to the lowest vibrational energy level of this excited state.
Another form of radiationless relaxation is internal conversion, in which a molecule in the ground vibrational level of an excited electronic state passes directly into a high vibrational energy level of a lower energy electronic state of the same spin state. By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. A related form of radiationless relaxation is external conversion in which excess energy is transferred to the solvent or another component in the sample matrix.
A final form of radiationless relaxation is an intersystem crossing in which a molecule in the ground vibrational energy level of an excited electronic state passes into a high vibrational energy level of a lower energy electronic energy state with a different spin state. For example, an intersystem crossing is shown in Figure between a singlet excited state, S1, and a triplet excited state, T1.
Fluorescence
Fluorescence occurs when a molecule in the lowest vibrational energy level of an excited electronic state returns to a lower energy electronic state by emitting a photon. Since molecules return to their ground state by the fastest mechanism, fluorescence is only observed if it is a more efficient means of relaxation than the combination of internal conversion and vibrational relaxation. A quantitative expression of the efficiency of fluorescence is the fluorescent quantum yield, Ff, which is the fraction of excited molecules returning to the ground state by fluorescence.
Quantum yields range from 1, when every molecule in an excited state undergoes fluorescence, to 0 when fluorescence does not occur.
The intensity of fluorescence, If, is proportional to the amount of the radiation from the excitation source that is absorbed and the quantum yield for fluorescence Fluorescence is generally observed with molecules where the lowest energy absorption is a p → p* transition, although some n → p* transitions show weak fluorescence.
Most unsubstituted, nonheterocyclic aromatic compounds show favorable fluorescence quantum yields, although substitution to the aromatic ring can have a significant effect on Ff. For example, the presence of an electron-withdrawing group, such as —NO2, decreases Ff, whereas adding an electron-donating group, such as —OH, increases Ff. Fluorescence also increases for aromatic ring systems and for aromatic molecules with rigid planar structures.
A molecule’s fluorescence quantum yield is also influenced by external variables such as temperature and solvent. Increasing temperature generally decreases Ff because more frequent collisions between the molecule and the solvent increases external conversion. Decreasing the solvent’s viscosity decreases Ff for similar reasons.
For an analyte with acidic or basic functional groups, a change in pH may change the analyte’s structure and, therefore, its fluorescent properties. Changes in both the wavelength and intensity of fluorescence may be affected.
As shown in Figure abov, fluorescence may return the molecule to any of several vibrational energy levels in the ground electronic state. Fluorescence, therefore, occurs over a range of wavelengths. Because the change in energy for fluorescent emission is generally less than that for absorption, a molecule’s fluorescence spectrum is shifted to higher wavelengths than its absorption spectrum.
Phosphorescence A molecule in the lowest vibrational energy level of an excited triplet electronic state normally relaxes to the ground state by an intersystem crossing to a singlet state or by external conversion. Phosphorescence is observed when relaxation occurs by the emission of a photon. As shown in Figure, phosphorescence occurs over a range of wavelengths, all of which are at a lower energy than the molecule’s absorption band. The intensity of phosphorescence, Ip, is given by an equation similar to equation for fluorescence
Ip = 2.303kFpP0ebC
where Fp is the quantum yield for phosphorescence.
Phosphorescence is most favorable for molecules that have n→p* transitions, which have a higher probability for an intersystem crossing than do p → p* transitions.
For example, phosphorescence is observed with aromatic molecules containing carbonyl groups or heteroatoms. Aromatic compounds containing halide atoms also have a higher efficiency for phosphorescence. In general, an increase in phosphorescence corresponds to a decrease in fluorescence.
Since the average lifetime for phosphorescence is very long, ranging from 10–4 to 104 s, the quantum yield for phosphorescence is usually quite small. An improvement in Fp is realized by decreasing the efficiency of external conversion. This may be accomplished in several ways, including lowering the temperature, using a more viscous solvent, depositing the sample on a solid substrate, or trapping the molecule in solution.
Excitation Versus Emission Spectra
Photoluminescence spectra are recorded by measuring the intensity of emitted radiation as a function of either the excitation wavelength or the emission wavelength. An excitation spectrum is obtained by monitoring emission at a fixed wavelength while varying the excitation wavelength.
Figure shows the excitation spectrum for the hypothetical system described by the energy level diagram in Figure above. When corrected for variations in source intensity and detector response, a sample’s excitation spectrum is nearly identical to its absorbance spectrum. The excitation spectrum provides a convenient means for selecting the best excitation wavelength for a quantitative or qualitative analysis.
Stokes – Lommel’ law: always emission spectrum and its maximum accents in part larger wavelength in comparison with absorbance spectrum and its maximum.
Example of molecular excitation and emission spectra
In an emission spectrum a fixed wavelength is used to excite the molecules, and the intensity of emitted radiation is monitored as a function of wavelength. Although a molecule has only a single excitation spectrum, it has two emission spectra, one for fluorescence and one for phosphorescence.
§ Rule of mirror symmetry: absorbance spectrum and fluorescence (construct in frequency diagram) nearly symmetry rather straight line, which pass through point of their crossing.
§ Distance between maximums of absorption spectrum and luminescence spectrum is named Stokes displacement.
§ Luminescing substances are characterised by size Stokes displacement. Than more its value, substances a luminescent method especially reliably define.
Instrumentation
The basic design of instrumentation for monitoring molecular fluorescence and molecular phosphorescence is similar to that found for other spectroscopies. The most significant differences are discussed in the following sections.
Molecular Fluorescence A typical instrumental block diagram for molecular fluorescence is shown in Figure. In contrast to instruments for absorption spectroscopy, the optical paths for the source and detector are usually positioned at an angle of 90°.
Block diagram for molecular fluorescence spectrometer
Two basic instrumental designs are used for measuring molecular fluorescence.
In a fluorometer the excitation and emission wavelengths are selected with absorption or interference filters. The excitation source for a fluorometer is usually a lowpressure mercury vapor lamp that provides intense emission lines distributed throughout the ultraviolet and visible region (254, 312, 365, 405, 436, 546, 577, 691, and 773 nm). When a monochromator is used to select the excitation and emission wavelengths, the instrument is called a spectrofluorometer. With a monochromator, the excitation source is usually a high-pressure Xe arc lamp, which has a continuum emission spectrum. Either instrumental design is appropriate for quantitative work, although only a spectrofluorometer can be used to record an excitation or emission spectrum.
The sample cells for molecular fluorescence are similar to those for optical molecular absorption. An analyte that is fluorescent can be monitored directly. For analytes that are not fluorescent, a suitable fluorescent probe molecule can be incorporated into the tip of the fiber-optic probe. The analyte’s reaction with the probe molecule leads to an increase or decrease in fluorescence.
Quantitative Applications Using Molecular Luminescence
Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte’s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section.
Fluorescent analysis is divided on:
§ direct (immediate measure)
§ indirect (fluorescence with indicator and show on ending of process of substance definition, especially at acid-base titration – acridine, luminol, salicylic acid, anthranilic acid).
Quantitative analysis in fluorometry
§ Method of direct fluorometric analysis (on Vavilov law):
Ф= К × С
§ for small concentration 10–7 – 10–4 mol/L
Methods of quantitative analysis:
§ A method of calibration chart
§ Comparison method
Method of additives
Advantages of method is high sensitivity (~10-5 %).
Error of direct fluorometry is 5-7 %.
TURBIDIMETRY AND NEPHELOMETRY: ESSENCE AND USAGE
Turbidimetry and nephelometry are two related techniques in which an incident source of radiation is elastically scattered by a suspension of colloidal particles. In turbidimetry, the detector is placed in line with the radiation source, and thedecrease in the radiation’s transmitted power is measured.
In nephelometry, scattered radiation is measured at an angle of 90° to the radiation source. The similarity of the measurement of turbidimetry to absorbance, and of nephelometry to fluorescence, is evident in the block instrumental designs shown in Figure.
Block diagrams for (a) a turbidometer and (b) a nephelometer
Choosing between turbidimetry and nephelometry is determined by two principal factors. The most important consideration is the intensity of the transmitted or scattered radiation relative to the intensity of radiation from the source. When the solution contains a small concentration of scattering particles, the intensity of the transmitted radiation, IT, will be very similar to the intensity of the radiation source, I0. As we learned earlier in the section on molecular absorption, determining a small difference between two intense signals is subject to a substantial uncertainty. Thus, nephelometry is a more appropriate choice for samples containing few scattering particles. On the other hand, turbidimetry is a better choice for samples containing a high concentration of scattering particles.
The second consideration in choosing between turbidimetry and nephelometry is the size of the scattering particles. For nephelometry, the intensity of scattered radiation at 90° will be greatest if the particles are small enough that Rayleigh scattering is in effect. For larger particles, as shown in Figure 10.37, scattering intensity is diminished at 90°. When using an ultraviolet or visible source of radiation, the optimum particle size is 0.1–1 mm. The size of the scattering particles is less important for turbidimetry, in which the signal is the relative decrease in transmitted radiation.
In fact, turbidimetric measurements are still feasible even when the size of the scattering particles results in an increase in reflection and refraction (although a linear relationship between the signal and the concentration of scattering particles may no longer hold).
§ Nephelometry is a method in which the intensity of scattered radiation is measured at an angle of 90° to the source.
§ At light passage through disperse systems dispersion or light absorption by firm parts is observed.
§ Light intensity, which disperse by small particle of cloud, is described by Rayleigh equation:
§ In nephelometry, the relationship between the intensity of scattered radiation, IS, and the concentration (% w/v) of scattering particles is given as
if F, V, r, b is constant
Methods of quantitative analysis in nephelometry:
§ A method of calibration chart
§ Comparison method
Turbidimetry is a method in which the decrease in transmitted radiation due to scattering is measured.
§ Reduction of light intensively (because of dispersion and absorbance) is described by equation:
§ or in identical conditions:
Methods of quantitative analysis in turbidimetry:
§ A method of calibration chart.
For very dilute solution of suspension (less 100 mg/L)
Advantages: high sensitivity (in comparison with quality reactions)
Lacks: small precision of analyses, high error (more than 10 %).
EXTRACTION. SUBSTANCE DISTRIBUTION BETWEEN TWO LIQUIDS
§ Extraction – is the process by which a solute is transferred from one phase to a new phase.
Extraction consists of three consecutive steps: (1) the mixing of the feed (starting mixture) with the extractant, (2) the mechanical separation of the two phases formed, and (3) the removal of the extractant from both phases and its recovery. After the mechanical separation, a solution of the extracted substance in the extractant (extract) and the residue of the starting solution (raffinate) or solid are obtained. The separation of the extracted substance from the extract and the concurrent recovery of the extractant are accomplished by distillation, evaporation, crystallization, or salting-out.
§ Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Аaq « Аor
§ Process of dissolved substance transferring from one phase to another phase, which are immiscible or restrictedly miscible, is named liquid-liquid –partition or partition between two phase of liquids.
Main concepts:
§ Extraction is process of transferring substance of a water phase in organic
§ Extraction reagent is reagent which with investigated substance forms compound which then is extracted
§ Extragent is organic solvent which is used for extraction
§ Latent solvent
§ Extract – is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol or water. Extracts may be sold as tinctures or in powder form
§ Re-extraction is process of transferring substance of organic phase in water
§ Re-extragent
Conditions of a choice of solvent which is used as extragent:
1. Should not mix up with water.
2. Should be selective.
3. Should have the big capacity in relation to extractive.
4. The density of extragent should be difference from water density.
5. Should have the minimum viscosity.
6. Should be inexpensive.
7. Cannot be explosive.
THE MAIN QUANTITATIVE CHARACTERISTICS OF EXTRACTION
Partition law of Nernst – Shilov:
§ The relationship of dissolved substance concentration in both phases at constant temperature is constant and does not depend on concentration of the dissolved substance:
D = CAorg / CAaqu
§ D – the distribution ratio remains constant if there are no processes:
§ dissociation or association
§ polymerization or other transformations of the dissolved substance
§ The ratio of substance activity in one certain form in organic solvent phase to its activity in a water phase is named a distribution constant
§ The distribution ratio and constant are connected with substance solubility
§ Factor or extraction efficiency
R = ν (A) / ν (A)o
where ν (A) – moles of solute in organic phase
ν (A)o – moles of solute initially present in water phase
§ Extraction efficiency for single extraction
§ Extraction efficiency:
for multiple (m) extraction
§ Separation coefficient (factor) of A, B ions equal relationship the distribution ratio this ions
Enrichment factor (S)
Extraction constant for process:
Mn++n(HL)O=(MLn)o+
Extraction constant is function from:
§ formation constant of complex, which is extracted ()
§ Acid dissociation constant of extraction reagent if it forms extraction compound ()
§ Distribution ratios of extraction reagent (¯)
§ Distribution ratios of extraction complex ()
§ рН of medium (optimum)
Types of extraction systems
1. Halogenides with covalent linkage: HgCl2, HgJ2, SbJ3, AsBr3, GeCl4, element iodine etc.
2. Intracomplex salts: dithizonate, dithiocarbamates, oxyquinolines, oxyns, β-diketonate, and also di-(2-ethylhexyl)-phosphates actinoids, rare-earth and some other elements, etc.
3. Complex metal acids: HFeCl4, HІnBr4, HSbCl6, etc.
4. coordinatively not solvated (a) and coordinatively solvated (b) salts:
a) Salts tetraphenyl arsonium, tetraphenyl phosphonium etc.
b) The compounds which is formed at extraction uranyl nitrate and nitrate of thorium by tributyl phosphate from nitrate solutions.
5. heteropoly compounds of phosphorus, arsenic, silicon, vanadium, molybdenum, tungsten etc.
!!!! Most widely is used in extraction process intracomplex salts, complex metalo halogenide acids and coordinatively solvated (b) salts
|
|||||
|
|||||
|
|||||
|
|||||
TYPES OF EXTRACTION SYSTEMS
Extraction
Liquid-liquid extractions using a separatory funnel are essentially the only kind of extraction performed in the organic teaching labs. Liquid-liquid means that two liquids are used in the extraction procedure. The liquids must be immiscible: this means that they will form two layers when added together, like oil and water. Some compounds are more soluble in the organic layer (the “oil”) and some compounds are more soluble in the aqueous layer (the “water”).
The photo above illustrates how two liquid layers separate. The red layer is simply red food coloring in water. Water is immiscible with the other liquid, which is methylene chloride. Methylene chloride is heavier (denser) than water, therefore, the clear methylene chloride layer is under the red, aqueous food coloring layer.
In a particular experiment in simple extraction or in chemically active extraction, you will be able to figure out which layer, aqueous or organic, will contain the compound you want to isolate. You will also need to know which layer will be on top in the separatory funnel. This is determined by the density of the two solvents.
How to do an Extraction
Support the separatory funnel in a ring clamp on a ringstand. Ring clamps come in many sizes. If yours is too large, you can add pieces of cut tygon tubing to the ring to cushion the funnel, or wrap the ring with tape until it is thick enough to hold the separatory funnel. Make sure the stopcock of the separatory funnel is closed.
Place a stemmed funnel in the neck of the separatory funnel. Add the liquid to be extracted, then add the extraction solvent. The total volume in the separatory funnel should not be greater than three-quarters of the funnel volume.
Insert the stopper in the neck of the separatory funnel.
Pick up the separatory funnel with the stopper in place and the stopcock closed, and rock it once gently.
Point the stem up and slowly open the stopcock to release excess pressure. Close the stopcock. Repeat this procedure until only a small amount of pressure is released when it is vented.
Shake the funnel vigorously for a few seconds. Release the pressure, then again shake vigorously. About 30 sec total vigorous shaking is usually sufficient to allow solutes to come to equilibrium between the two solvents. Vent frequently to prevent pressure buildup, which can cause the stopcock and perhaps hazardous chemicals from blowing out. Take special care when washing acidic solutions with bicarbonate or carbonate since this produces a large volume of CO2 gas.
Put the funnel back into the ring and let it rest undisturbed until the layers are clearly separated. While waiting, remove the stopper and place a beaker or flask under the sep funnel.
Carefully open the stopcock and allow the lower layer to drain into the flask. Drain just to the point that the upper liquid barely reaches the stopcock.
If the upper layer is to be removed from the funnel, remove it by pouring it out of the top of the funnel. Often you will need to do repeat extractions with fresh solvent. You can leave the upper layer in the separatory funnel if this layer contains the compound of interest. If the compound of interest is in the lower layer, the upper layer must be removed from the separatory funnel and replaced with the drained-off lower layer, to which fresh solvent is then added. When you are finished, remember to store the stopper separately from the funnel.
http://www.youtube.com/watch?v=Wo2AQK-vaW4
Microscale Extraction
For microscale separations, pipet layer separation is convenient and normally very little product loss is incurred. Since the two solvents are already in a reaction tube, instead of transferring the small volumes of solvent to another piece of glassware and ultimately losing product, the solvents can be mixed and separated directly from the reaction tubes. Use a Pasteur pipet to gently mix the layers. This can easily be accomplished by gently drawing the liquid up and down with the pipet. Do not simply swirl the tube. This mixing method will not allow the two layers to mix properly and decreasing the success of the extraction. Once the layers are thoroughly mixed, use the pipet to draw up the bottom layer as shown in Figure below.
The ether and water layers are now separated. Normally, two or more ether extractions would be completed to ensure the complete removal of the organic compound. Both the macroscale and microscale separations are typical examples of how liquid/liquid extraction can be used to separate water soluble inorganic materials from organic products. Finally, the ether or other organic solvent could then be evaporated, leaving the mixture of organic product with traces of starting material and by-products (often called the crude product). This can be purified by recrystallization or sublimation.
THE MAIN ORGANIC REAGENTS WHICH USE IN EXTRACTION METHOD
Choosing a Solvent System
One important aspect when choosing a solvent system for extraction is to pick two immiscible solvents. Some common liquid/liquid extraction solvent pairs are water-dichloromethane, waterether, water-hexane. Notice that each combination includes water. Most extractions involve water because it is highly polar and immiscible with most organic solvents. In addition, the compound you are attempting to extract, must be soluble in the organic solvent, but insoluble in the water layer. An organic compound like benzene is simple to extract from water, because its solubility in water is very low. However, solvents like ethanol and methanol will not separate using liquid/liquid extraction techniques, because they are soluble in both organic solvents and water.
There are also practical concerns when choosing extraction solvents. As entioned previously, the two solvents must be immiscible. Cost, toxicity, flammability should be considered. The volatility of the organic solvent is important. Solvents with low boiling points like ether are often used to make isolating and drying the isolate material easier. If ether is used (bp =
Identifying the Layers
One common mistake when performing an extraction is to mix-up the layers and discard the wrong one. The densities of the solvents will predict which solvent is the top or bottom layer. In general, the density of nonhalogenated organic solvents are less than 1.0 g/mL and halogenated solvents are greater than 1.0 g/mL. One common solvent pair is dichloromethane and water. The density of dichloromethane is 1.325 g/mL and water is 1.000 g/mL. Dichloromethane is more dense that water; therefore, dichloromethane will be the bottom layer and water will be the top layer. Table lists the densities of some extraction organic solvents.
(For a complete list of physical properties of some common organic solvents, please see the table located in the front of your laboratory notebook.) Although the density is the physical property that determines which layer is on top or bottom, a very concentrated solute dissolved in either layer can reverse the order. The best method to avoid making a mistake is a drop test. Add a few drops of water to the layer in question and watch the drop very carefully. If the layer is water, then the drop will mix with the solution. If the solvent is the mistaken organic layer, then the water drop will create a second layer. In general, this method can help determine the identity of the layer.
However, it is still best to keep ALL the layers until the extraction is complete and your product has been isolated.
The main organic reagents which use in extraction method
§ 8-oxyquinoline reacts with more than 50 elements
§ Acetylacetonate forms compound with more than 60 elements
§ Thionyl trifluoride acetone is used for excretion and separation actinoids.
§ dithizon is used for determination of Pd, Au, Hg, Ag, Cu, Bi, Pt, In, Zn, Cd, Co, etc.
!!! It is of great importance in the toxicological analysis.
§ Sodium diethyl dithiocarbamate reacts with several tens of elements
!!! It is of great importance in the toxicological analysis.
Emulsions
An emulsion is a suspension of tiny droplets of one solvent mixed in the other. Emulsions are common in extraction because proper mixing is essential. In Italian salad dressing, an emulsion is desired to keep the water and oil mixed. Additives are added to the dressing in order to keep the two normally immiscible solvents miscible. In a liquid/liquid or solid/liquid extraction however, an emulsion will lead to a poor separation. Gentle shaking and swirling the separatory funnel is the best technique to avoid emulsions. However, if an emulsions occurs, there are several simple methods to destroy it. The first is time. Over time the layers will eventually separate. With a severe emulsion, you may not have time during a three hour lab period to wait.
Another method is to add brine or salt water to the mixture. Since ether is less soluble in a highly ionic solution such as salt water, the ether and water will be forced to separate. This method works well with small emulsions. If you have a more difficult emulsion, separate the layers as much as possible and dry the organic layer with a drying agent. The water will be removed from the organic layer along with the drying agent. Subsequent extractions should proceed without further trouble.
Drying Agents
One significant problem with liquid/liquid extraction is that no solvent is COMPLETELY insoluble in another solvent. In practice, one additional step is usually carried out before evaporating the organic solvent: drying over anhydrous sodium sulfate or other drying agent.
Drying a liquid might seem like a peculiar concept, since we normally think of all liquids as being wet. Drying an organic liquid in the organic lab has a special meaning to chemists. It means to remove all traces of water. Even water and hexane are slightly soluble in each other. After separating the two solvents, residual water will remain in the hexane or ether organic layer. This will remain and stick to the solid product when we remove the more volatile solvent. Therefore, chemists remove the water from the organic layer by adding an insoluble inorganic solid to the solution which will absorb the water, thus “drying” it. Granular anhydrous sodium sulfate is the drying agent most often used although other drying agents are also available. All of the inorganic solids work by reacting with the water to form hydrates, which is their preferred form if water is available.
These compounds will associate or hydrate themselves with water. Table lists some common drying agents along with their speed, capacity, and hydration.
These drying agents do not dissolve in the solvent they are “drying”. They may change somewhat, for example, sodium sulfate will clump together as it reacts with water, but they will remain solids iormal extraction solvents. This makes them easy to remove by decantation (pouring off) of the liquid or by gravity filtration. Usually the organic solvent will go from cloudy to clear in the process of being “dried”. You should be careful to remove all of these solid drying agents before solvent evaporation or you might think they are your product. When you take a melting point and the product doesn’t melt by
It is relatively inexpensive and fast.
It is recommended that the drying agent you choose be in a granular form. After the drying agent has removed the residual water, it is easier to remove large granular particles. Drying a solvent however, is not an exact science. An excess of drying agent should be used to ensure that all the water is removed. If the water remains after the materials are collected, it could interfere with the analysis. Add drying agent until there are no longer clumps of drying agent stuck to the sides or bottom of the flask. The drying agents should be free floating in the beaker, like snow.
Do not be afraid to use too much. There are many other choices for drying agents including molecular sieves and sodium metal. There are benefits and disadvantages to each one. Sodium, for example, is an excellent drying agent, however it violently decomposes in water to create NaOH and H2 gas and may ignite spontaneously. Therefore it should be used with caution and only when removing very small amounts of water. Many times a particular drying agent will work better than others in a certain situation.
Acid/Base Extraction
There are also three special cases of liquid/liquid extraction that are extremely useful for isolating and purifying amines, carboxylic acids and phenols. All three of these functional groups can be interconverted from non-ionic organic-soluble forms to water-soluble ionic forms by changing the pH.
Solid/liquid or liquid/liquid extractions rely on the solubility of the solute to be extracted. In acid/base extraction, the molecule to be extracted is transformed so that we impose a new solubility on the molecule. One specific example is benzoic acid, an organic acid. Benzoic acid is soluble in most organic solvents including dichloromethane and ether. However, this acid can be easily deprotonated with base to give a charged ionic species that is readily soluble in water.
Solid/liquid or liquid/liquid extractions rely on the solubility of the solute to be extracted. In acid/base extraction, the molecule to be extracted is transformed so that we impose a new solubility on the molecule. One specific example is benzoic acid, an organic acid. Benzoic acid is soluble in most organic solvents including dichloromethane and ether. However, this acid can be easily deprotonated with base to give a charged ionic species that is readily soluble in water.
Acid/base extraction is one of the more difficult principles in organic chemistry to understand.
The most straight forward approach to understanding this subject is to create a flow chart (mentally or on paper) to follow which species has been created and where the molecule resides.
If you can imagine the molecule changing and moving to the appropriate layer, you will be able to complete the unknown separation very easily.
Figure is a detailed flow chart of the separation of a strong organic acid, a weak organic acid, an organic base, and a neutral component.
If you can follow the steps involved below, the unknown extraction in this chapter will be much easier to understand.
Whether you use acid/base, solid/liquid or liquid/liquid, extraction is a useful organic tool to separate a mixture of compounds. From the early drugs that were extracted from trees and plants to modern day pharmacology, extraction is still used to separate and purify organic molecules.
The following experiments demonstrate both acid/base extraction on a microscale and solid/liquid extraction on a macroscale.
Classification extraction processes:
§ Periodical extraction – is the process in which separatory funnel (which contain substance which extragent) is shaken
§ Continuous extraction
A continuous extraction apparatus and a continuous extraction process for extracting an extractable component of a liquid or particulate feed material with an immiscible liquid solvent for the extractable component there of, which apparatus and process are both especially useful for extracting glycerine from soap, utilize an extraction column with oscillatably movable contact promoting means therein which move in a horizontal plane and have openings therein through which the immiscible feed and the extracting liquid, in different phases, may pass during oscillations, at which time the contact promoting means make repeated contacts with the different phases to renew contact surfaces of the liquids and to bring them into intimate contact with each other, whereby extraction of extractable material from the feed by the extracting liquid is promoted. A plurality of spaced vertical screens positioned substantially radially from the axis of a cylindrical extraction column constitute preferred contact promoting means which oscillate rotationally and horizontally and thereby help to improve extraction column efficiency while preventing or minimizing axial mixing. Instead of the screens a woven or knitted screening, mesh or fabric of filamentary material, having openings therein, may be used and may be wrapped around or otherwise suitably held to a central vertical shaft which is rotationally oscillatable horizontally in the extraction column.
http://organometallica.tumblr.com/post/19806788830/a-continous-liquid-liquid-extractor-in-action
Schematic diagram of a Soxhlet extractor.
1: Stirrer bar/anti-bumping granules
2: Still pot (extraction pot) – still pot should not be overfilled and the volume of solvent in the still pot should be 3 to 4 times the volume of the soxhlet chamber.
3: Distillation path
4: Soxhlet Thimble
5: Extraction solid (residue solid)
6: Syphon arm inlet
7: Syphon arm outlet
8: Expansion adapter
9: Condenser
10: Cooling water in
11: Cooling water out
Schematic diagram of a Soxhlet extractor
Continuous & Fully Automatic Solvent Extraction Plan
§ Countercurrent extraction
A counter current extractor in which material to be extracted is caused to move in counter current with an extracting liquid by a screw conveyor characterized in that the direction of rotation of the screw conveyor is intermittently reversed. A process for extracting soluble and dispersible materials using such a counter current extractor is also disclosed.
http://www.chem.uoa.gr/Applets/AppletCraig/Appl_Craig2.html
The solids (in a suitable form) are fed by gravity or metered by volume to the lower end of the process-vessel and are transported upwards through the extractor by 2 counter-rotating mirrored helicoidal screws with a controlled slip. The solids pass in the form of 2 overlapping cylinders, rotating at about half the rate of the screws.
The fresh extraction liquid enters at the top-end and flows by inclination-controlled gravity down as a submerged stream in the bottom half of the cross-sectional area through the slowly rotating solid in true, well-controlled countercurrent with close to ideal plug-flow in both phases. The speed of the screws is adjusted to give the neccesary residence time for the solid. The flow of liquid is adjusted independently to give an optimum concentration of the extract.
The extract leaves at the lower end through special self-cleaning filters and a level-control reservoir. The extracted solid is slightly squeezed at the top end and continuously discharged.
Adjustment and automatic control is very simple. Scale-up from pilot plant tests is well proven.
Extraction is diffusion- and equilibrium-controlled. The solid is only submerged in the liquid half of the time, but the rate-limiting diffusion takes place all the time. The counterflow ensures the best possible equilibrium limit. Therefore, a high yield and a high extract concentration can be obtained simultaneously: the overall efficiency equals 3 to 5 ideal conter-current equilibrium/separation-stages and the yield is typically adjusted to an overall optimum of around 90% of total extractables.
The equipment has been successfully used for many beverages, pharmaceuticals and generally all kinds of difficult vegetable materials such as leaves, stems and roots. The solid must be cut, milled, rolled or shredded as finely as possible to maximize extraction speed, however with a limit of typically max. 10% smaller than
Cross-section of a two-stage countercurrent mono-stage centrifugal extractor
Niro Counter Current Extraction for coffee, tea, herbs, and health products
USAGE OF EXTRACTION IN THE DRUG ANALYSIS
Extraction is used for:
1. Separation of elements
2. Concentrating impurities
3. Clearings of the basic component from impurities in the process of synthesis of substances of drugs
4. Definition of the basic component from impurities in the process of synthesis of substances of drugs
5. For identification and quantitative definition of chemical agent or substances-markers in the process of the analysis of phytogenesis drugs
6. Increase of sensitivity and selectivity of reactions
7. Studying of formation constant of complexs
8. Studying of substance condition in a solution (a charge, polymerisation degree)
§ Reception of extracts, tinctures, fermental preparations, antibiotics, preparations from a different biological material.
Usage of extraction as method of concentrating and definition
Absolute concentrating is reached at usage of smaller volume of an organic phase in relation to initial volume of a water solution.
Relative concentrating is an increase in impurity concentration in relation to the main component.
Especially important role extraction is by connection with physical and physical-chemical methods of the analysis – hybrid methods of the analysis which have such advantages:
§ High sensitivity
§ Selectivity
§ Specificity
§ rapid analysis method
Extraction of medicinal herbs includes stages:
§ Drying (sometimes it is not necessary)
§ Crumbling up
§ Sifting
§ Selection of optimum extragent
§ Choice of an optimum technique extraction
§ Extraction (moisten, passage extragent through pores, dissolution of substances in the middle of cell, diffusion of substance molecules through cellular covers, mass-carrying extraction substances from a surface of particles in extragent).
Analytical techniques of reception of an extract for the purpose of medicinal herbs analysis on high quality:
§ Extraction to a full attrition (percolation with
§ Single extraction of raw material shot (by boiling
§ Equilibrium extraction (for 4-5 hour balance between internal extrageny in medicinal herbs and the external extract, analyzes an extract part)
§ !!! In the biochemical and toxicological analysis extraction is used for excretion of substances from animal and herb tissues, as fresh and dried up.
