Methodical instruction
for students of pharmaceutical faculty
LESSON 13 (practical – 6 hours)
THEME: Gravimetric analysis. Determination of Fe content (Precipitation gravimetry).
AIM: to Learn theoretical bases of gravimetric analysis, to learn to carry out calculations of sample weight for gravimetric definitions, and also to determinate dampness (loss on drying) of medical products and the dry rest in tinctures.
PROFESSIONAL MOTIVATION OF STUDENTS
In the previous section we used four examples to illustrate the different ways that mass can serve as an analytical signal. These examples also illustrate the four gravimetric methods of analysis. When the signal is the mass of a precipitate, we call the method precipitation gravimetry. The indirect determination of PO33– by precipitating Hg2Cl2 is a representative example, as is the direct determination of Cl– by precipitating AgCl.
In electrogravimetry the analyte is deposited as a solid film on one electrode in an electrochemical cell. The oxidation of Pb2+, and its deposition as PbO2 on a Pt anode is one example of electrogravimetry. Reduction also may be used in electrogravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a direct analysis for Cu2+.
When thermal or chemical energy is used to remove a volatile species, we call the method volatilization gravimetry. In determining the moisture content of food, thermal energy vaporizes the H2O. The amount of carbon in an organic compound may be determined by using the chemical energy of combustion to convert C to CO2.
Finally, in particulate gravimetry the analyte is determined following its removal from the sample matrix by filtration or extraction. The determination of suspended solids is one example of particulate gravimetry.
Except for particulate gravimetry, which is the most trivial form of gravimetry, it is entirely possible that you will never use gravimetry after you are finished with this course. Why, then, is familiarity with gravimetry still important? The answer is that gravimetry is one of only a small number of techniques whose measurements require only base SI units, such as mass and moles, and defined constants, such as Avogadro’s number and the mass of 12C. The result of an analysis must ultimately be traceable to methods, such as gravimetry, that can be related to fundamental physical properties. Most analysts never use gravimetry to validate their methods. Verifying a method by analyzing a standard reference material, however, is common. Estimating the composition of these materials often involves a gravimetric analysis.
Therefore it is necessary to know theoretical bases of gravimetric analysis end know how to make calculations of sample weight for gravimetric definitions.
THE SELF-PREPARATION PROGRAM
1.Theory of the gravimetric analysis and classification of its methods.
2.Advantages and defects of the gravimetric analysis
3.Precipitation gravimetry: techniques of performance, the requirement to precipitations.
4.The requirements to precipitate’s form
5.Requirements to the weighed form
6.Requirements to precipitants.
7.Conditions of precipitation of crystal precipitates
8.Conditions of precipitation of amorphous precipitate
9.Types of impurities.
10. Choice of a rinsings liquid.
11. Particulate gravimetry: theory, calculations, application examples in the pharmaceutical analysis.
12. Volatilization gravimetry: theory, calculations, and application examples in the pharmaceutical analysis.
13. Use of the gravimetry in the pharmaceutical analysis.
TESTS AND REAL-LIFE SITUATIONS FOR SELF-ASSESSMENT
1. What conditions are necessary for formation of crystal precipitate?
A. Slow precipitation from the hot diluted solutions.
B. Fast precipitation from the hot diluted solutions.
C. Slow precipitation from the cold diluted solutions.
D. Fast precipitation from the hot concentrated solutions.
E. Slow precipitation from the cold concentrated solutions.
2. Which method is method of gravimetric analysis?:
A. A particulate method.
B. A neutralisation method.
C. The Faience-Hodakov method.
D. A polarimetric method.
E. Mhor method.
3. For determination a content of water in drugs and medicinal substances use:
A. A volatilization gravimetric method .
B. A particulate gravimetry method.
C. Mhor method.
D. A precipitation gravimetric method.
E. A permanganatometric method.
4. To advantages and lacks of the gravimetric analysis it is necessary to note:
A. High accuracy and duration of the analysis.
B. High sensitivity and duration of the analysis.
C. High selectivity and duration of the analysis.
D. Low sensitivity and low accuracy.
E. Low sensitivity and high accuracy.
5. At first requirement which concern to the weighed (gravimetric) form is:
A. Exact conformity of structure to the chemical formula.
B. The content of a defined element should be whenever possible big.
C. The content of a defined element should be whenever possible smaller.
D. Insignificant hygroscopicity of the weight form.
E. High chemical stable of the weight form.
6. Which a precipitant choice at first?
A. Are flying substances.
B. Are group reagents.
C. Do not form supersaturated solutions.
D. Have small molarity weight.
E. Have big molarity weight.
1. An ore containing magnetite, Fe3O4, was analyzed by dissolving a 1.5419-g sample in concentrated HCl, giving a mixture of Fe2+ and Fe3+. After adding HNO3 to oxidize any Fe2+ to Fe3+, the resulting solution was diluted with water and the Fe3+ precipitated as Fe(OH)3 by adding NH3. After filtering and rinsing, the residue was ignited, giving 0.8525 g of pure Fe2O3. Calculate the %w/w Fe3O4 in the sample.
2. A 0.6113-g sample of Dow metal, containing aluminum, magnesium, and other metals, was dissolved and treated to prevent interferences by the other metals. The aluminum and magnesium were precipitated with 8-hydroxyquinoline. After filtering and drying, the mixture of Al(C9H6NO)3 and Mg(C9H6NO)2 was found to weigh 7.8154 g. The mixture of dried precipitates was then ignited, converting the precipitate to a mixture of Al2O3 and MgO. The weight of this mixed solid was found to be 1.0022 g. Calculate the %w/w Al and %w/w Mg in the alloy.
3. A sample of slag from a blast furnace is analyzed for SiO2 by decomposing a 0.5003-g sample with HCl, leaving a residue with a mass of 0.1414 g. After treating with HF and H2SO4 and evaporating the volatile SiF4, a residue with a mass of 0.0183 g remains. Determine the %w/w SiO2 in the sample.
4. A 26.23-mg sample of MgC2O4•H2O and inert materials is heated to constant weight at 1200 °C, leaving a residue weighing 20.98 mg. A sample of pure MgC2O4•H2O, when treated in the same fashion, undergoes a 69.08% change in its mass. Determine the %w/w MgC2O4•H2O in the sample.
5. A 200.0-mL sample of water was filtered through a preweighed glass fiber filter. After drying to constant weight at 105 °C, the filter was found to have increased in mass by 48.2 mg. Determine the total suspended solids for the sample in parts per million.
6. A 38.63-mg sample of potassium ozonide, KO3, was heated to 70 °C for 1 h, undergoing a weight loss of 7.10 mg. Write a balanced chemical reaction describing this decomposition reaction. A 29.6-mg sample of impure KO3 experiences a 4.86-mg weight loss when treated under similar condition. What is the %w/w KO3 in the sample?
7. A series of samples consisting of any possible combination of KCl, NaCl, and NH4Cl is to be analyzed by adding AgNO3 to precipitate AgCl. What is the minimum volume of 5% w/v
AgNO3 necessary to completely precipitate the chloride in any 0.5-g sample?
8. The water content of an 875.4-mg sample of cheese was determined with a moisture analyzer. What is the %w/w H2O in the cheese if the final mass was found to be 545.8 mg?
9. The iron content of an organometallic compound was determined by treating a 0.4873-g sample with HNO3 and heating to volatilize the organic material. After ignition, the residue of Fe2O3 was found to weigh 0.2091 g. What is the %w/w Fe in this compound? The carbon and hydrogen in a second sample of the compound were determined by a combustion analysis. When a 0.5123-g sample was carried through the analysis, 1.2119 g of CO2 and 0.2482 g of H2O were collected. What are the %w/w C and %w/w H in this compound? What is the compound’s empirical formula?
ANSWERS TON THE SELF-ASSESSMENT
Tests: 1. A., 2. A., 3. A., 4. A., 5. A., 6. A.
Tasks: 1., 2., 3., 4., 5., 6., 7., 8., 9..
REFERENCES:
1. The lecture’s material.
2. David Harvey. Modern Analytical Chemistry // www.mhhe.com
METHOD OF IMPLEMENTATION OF PRACTICAL WORK
Dry Residue of Extracts
In a flat-bottomed dish about 50 mm in diameter and about 30 mm in height, introduce rapidly 2.00 g or 2.0 ml of the extract to be examined. Evaporate to dryness on a water-bath and dry in an oven at 100-105 °C for 3 h. Allow to cool in a desiccator over diphosphorus pentoxide R or anhydrous silica gel R and weigh. Calculate the result as a mass percentage or in grams per litre.
Work 1. Definition of the dry residue in tinctures.
In weighed weighing bottle or flat-bottomed dish (preliminary finished to constant weight) introduce 2,0 ml of the tincture to be examined. Evaporate to dryness on a water-bath and dry in an oven at 100-105 °C for 3 h. Allow to cool in a desiccator over diphosphorus pentoxide R or anhydrous silica gel R and weigh. Calculate the result as a mass percentage or in grams per litre.
Results of definition write down under the form:
mdish =
mdish +tinct =
mtinct =
mdish + residue =
mresidue =
The Dry Residue of tincture count under the formula:
W = 
Ww/v=
Loss on Drying of Extracts
In a flat-bottomed dish about 50 mm in diameter and about 30 mm in height, weigh rapidly 0.50 g of the extract to be examined, finely powdered. Dry in an oven at 100-105 °C for 3 h. Allow to cool in a desiccator over diphosphorus pentoxide R or anhydrous silica gel R and weigh. Calculate the result as a mass percentage.
Work 2. Definition loss on drying of drugs.
In weighed weighing bottle or flat-bottomed dish (preliminary finished to constant weight) introduce 1,0 ml of the substance to be examined (lactose, glucose, analginum). Dry in an oven at 100-105 °C for 3 h. Allow to cool in a desiccator over diphosphorus pentoxide R or anhydrous silica gel R and weigh. Calculate the result as a mass percentage.
Results of definition write down under the form:
mdish =
mdish + substance =
m substance =
mdish + drying substance =
mloss =
The loss on drying count under the formula:
W =
Definition of ash
Definition of the general ashes.
Use Method I unless otherwise directed in the monograph.
Method I
For vegetable drugs
Incinerate 2 to 3 g of the ground drug in a tared platinum or silica dish at a temperature not exceeding 450° until free from carbon, cool and weigh. If a carbon-free ash cannot be obtained in this way, exhaust the charred mass with hot water , collect the residue on an ashless filter paper, incinerate the residue and filter paper, add the filtrate, evaporate to dryness and ignite at a temperature not exceeding 450°. Calculate the percentage of ash with reference to the air-dried drug.
For other substances
Carry out the above method using 1 g, unless otherwise stated. Calculate the percentage of ash.
Method II
Heat a silica or platinum crucible to redness for 30 min, allow to cool in a desiccator and weigh. Unless otherwise prescribed, evenly distribute 1.00 g of the substance or the powdered vegetable drug to be examined in the crucible. Dry at 100 °C to 105 °C for 1 h and ignite to constant mass in a muffle furnace at 600 °C ± 25 °C, allowing the crucible to cool in a desiccator after each ignition. Flames should not be produced at any time during the procedure. If after prolonged ignition the ash still contains black particles, take up with hot water, filter through an ashless filter paper and ignite the residue and the filter paper. Combine the filtrate with the ash, carefully evaporate to dryness and ignite to constant mass.
Definition of acid-insoluble ash.
Use Method I unless otherwise directed in the monograph.
Method I
Boil the ash for 5 minutes with 25 ml of 2M hydrochloric acid, collect the insoluble matter in a sintered-glass crucible or on an ashless filter paper, wash with hot water and ignite. Calculate the percentage of acid-insoluble ash with reference to the air-dried drug.
Method II
Ash insoluble in hydrochloric acid is the residue obtained after extracting the sulphated or total ash with hydrochloric acid, calculated with reference to 100 g of drug.
To the crucible containing the residue from the determination of sulphated or total ash, add 15 ml of water R and 10 ml of hydrochloric acid R, cover with a watch-glass, boil the mixture gently for 10 min and allow to cool. Filter through an ashless filter, wash the residue with hot water R until the filtrate is neutral, dry, ignite to dull redness, allow to cool in a desiccator and weigh. Reheat until the difference between 2 consecutive weighings is not more than 1 mg.
Definition of sulphatic ash.
Use Method I unless otherwise directed.
Method I
Heat a platinum dish to redness for 10 minutes, allow to cool in a desiccator and weigh. Unless otherwise specified in the monograph, place 1 g of the substance being examined in the dish, moisten with sulphuric acid, ignite gently, again moisten with sulphuric acid and ignite at about 800°. Cool, weigh again, ignite for 15 minutes and repeat this procedure until two successive weighings do not differ by more than 0.5 mg.
Method II
Ignite a suitable crucible (for example, silica, platinum, porcelain or quartz) at 600 ± 50 °C for 30 min, allow to cool in a desiccator over silica gel or other suitable desiccant and weigh. Place the prescribed amount of the substance to be examined in the crucible and weigh. Moisten the substance to be examined with a small amount of sulphuric acid R (usually 1 ml) and heat gently at as low a temperature as practicable until the sample is thoroughly charred. After cooling, moisten the residue with a small amount of sulphuric acid R (usually 1 ml), heat gently until white fumes are no longer evolved and ignite at 600 ± 50 °C until the residue is completely incinerated. Ensure that flames are not produced at any time during the procedure. Allow the crucible to cool in a desiccator over silica gel or other suitable desiccant, weigh it again and calculate the percentage of residue.
If the amount of the residue so obtained exceeds the prescribed limit, repeat the moistening with sulphuric acid R and ignition, as previously, for 30 min periods until 2 consecutive weighings do not differ by more than 0.5 mg or until the percentage of residue complies with the prescribed limit.
Work № 3. Definition of Iron
Iron can be determined gravimetrically by precipitating as Fe(OH)3 and igniting to Fe2O3. The sample to be analyzed is weighed and transferred to a 400-mL beaker where it is dissolved in 50 mL of H2O and 10 mL of 6 M HCl. Any Fe2+ that is present is oxidized to Fe3+ with 1–2 mL of concentrated HNO3.
Fe2+ + NO3– + 2H+ = Fe3+ + NO2 + H2O
After boiling to remove the oxides of nitrogen, the solution is diluted to 200 mL, brought to boiling, and Fe(OH)3 is precipitated by slowly adding 1:1 NH3 until the odor of NH3 is detected.
Fe3 + + 3NH3 + 3H2O FFe (OH) 3¯ + 3NH4 +.
The solution is boiled for an additional minute, and the precipitate is allowed to settle to the bottom of the beaker. The precipitate is then filtered and washed with several portions of hot 1% w/v NH4NO3 until no Cl– is found in the wash water. Finally, the precipitate is ignited to constant weight at 500–550 °C, and weighed as Fe2O3.
2Fe (OH) 3 Fe2O3 + 3H2O.
Answer on the next question:
1. If the ignition is not carried out under oxidizing conditions (plenty of O2 present), the final product will contain some Fe3O4. What effect would this have on the reported %w/w Fe?
2. The precipitate is washed with a dilute solution of NH4NO3. Why is NH4NO3 added to the wash water?
3. Why does the procedure call for adding NH3 until the odor of ammonia is detected?
4. Describe how you might test the filtrate for Cl–.
5. Calculate gravimetric factor for this determination of iron.
BASIC LEVEL OF KNOWLEDGE AND SKIILS:
A STUDENT MUST KNOW:
1. The theoretical bases of gravimetric analysis, using it in the pharmaceutical analysis.
2. The theoretical bases of precipitation gravimetry.
3. The theoretical bases of particulate gravimetry.
4. The theoretical bases of volatilization gravimetry.
A STUDENT MUST BE ABLE:
1. To define the ash (the general, sulphatic, acid-insoluble) in drugs.
2. To use a precipitation gravimetry for quantitative definition of substances.
3. To carry out calculations a mass sample to be examined, quantity of precipitants, volume of a washing liquid, the content of a defined component.
4. To carry out calculations a mass sample to be examined by volatilization gravimetry.
Prepared by Associate Prof. Vronska L.V., Mykhalkiv M.M.
