Disperse systems. The methods of preparing of colloidal solutions. Their properties.
Disperse systems are called systems, which consist of two phases, one of which is scattered or dispersed in other.
The phase which is scattered is called the disperse phase or the discontinuous phase and the phase in which scattering or dispersion done is called the disperse medium or the continuous phase.
For all disperse systems are characteristic three main properties: heterogeneity, disperse, specific surface.
Type of dispersed systems: by state of dispersed phase and dispersed medium, by size of dispersed phase
.
The most common are sols (solid in liquids), gels (liquids in solids) and emulsions (liquids in liquids).
True solutions have particles less than 10-9 m or 1nm.
Colloidal solutions have particle, which size between 10-9 to 10-7m or 1 nm to 100 nm.
Suspensions have particles, which size are more than and 10-7 m or 100 nm.
Colloidal Solution is a heterogeneous mixture in which particle size of substance is intermediate of true solution and suspension i.e. between 1-1000 nm. Smoke from a fire is example of colloidal system in which tiny particles of solid float in air. Just like true solutions, Colloidal particles are small enough and cannot be seen through naked eye.They easily pass through filter paper. But colloidal particles are big enough to be blocked by parchment paper or animal membrane.
Colloidal state of matter. Thomas Graham (1861) studied the rate of diffusion of liquids and solutions across the animal membranes and as а result of his experiments, he divided the soluble substances into two classes crystalloids and colloids. The substances like common salt, sugar, urea, etc. which can be obtained in the crystalline form and in the dissolved state diffuse through the vegetable or animal membrane were termed as crystalloids, and the substances like starch, gum, glue etc. which are non-crystalline iature and in the dissolved state do not diffuse or have а little tendency to pass through the animal or vegetable membrane were given the name colloids.
The colloids form bigger particles which cannot pass through the membrane. It is, therefore, preferable to speak of а substance in ‘colloidal state rather than calling it a crystalloid or a colloid. The particle size limits are given:
Thus the size of the colloidal particles is intermediate between that of particles of true solution and those of suspension.
А colloidal state of matter is, therefore, а state in which the size of the particles is such (10 А – 1000 А) that they can pass through filter paper but not through animal or vegetable membrane. Thus every substance can be brought into the colloidal state by adopting suitable methods.
Classification of colloids or colloidal solution
The various substances which form colloidal sols are divided into two categories, namely, lyophilic and lyophobic. If water is the dispersion medium, the terms used are hydrophilic and hydrophobic.
1. Lyophilic colloids. The word lyophilic means liquid-loving. Substances like gum, gelatin, starch, rubber etc. which when mixed with а suitable liquid as the dispersion medium directly form the colloidal sol are called lyophilic and the sols thus obtained are called lyophilic sols. An important characteristic of these sols is that if the dispersed phase is separated from the dispersion medium (say by evaporation), the sol can be made again by simply remixing with the dispersion medium. That is why these sols are also called reversible sols. Further, these sols are quite stable and cannot be easily precipitated.
2. Lyophobic colloids. The word ‘lyophobic‘ means liquid-hating. Substances like metals, their sulphides etc. when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods (as discussed later). Such substances are called lyophobic and the sols formed by them are celled lyophobic sols. These sols are readily precipitated, (or coagulated) and hence are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence these sols are also called irreversible sols.
Preparation of colloidal solutions.
As discussed earlier, the lyophilic sols can be prepared directly by mixing the substance with the dispersion medium. For example, colloidal sols of starch, gelatin, gum arabic, soaps etc. are prepared by simply dissolving these substances in warm water. Similarly, а colloidal sol of cellulose nitrate is obtained by dissolving it in an organic solvent such as ethyl alcohol. The product obtained as called collodion. However, the lyophobic sols cannot be prepared directly.
As you known: True solutions have particles less than 10-9 m or 1nm.
Colloidal solutions have particle, which size between 10-9 to 10-7m or 1 nm to 100 nm.
Suspensions have particles, which size are more than and 10-7 m or 100 nm.
Hence the following two types of methods are employed for obtaining the lyophobic sols.
Dispersion or Disintegration methods. These methods involve the breaking of the bigger particles to colloidal size. The methods generally employed for this purpose are briefly described below:
Mechanical disintegration. The mechanical disintegration is carried out in а machine called “colloid mill”. It consists of two steel discs with а little gap in between and capable of rotating in the opposite directions at high speed. А suspension of the substance in water introduced into the mill. The size of suspension particles is reduced to that of colloidal size.
Electro-disintegration (Bredig’s method). This method is employed for obtaining colloidal solutions of metals like gold, silver, platinum etc. An electric arc is set up between two metallic electrodes suspended in а trough of water. The intense heat of the arc converts the metal into vapours which are condensed immediately in the ice cold water bath resulting in the formation of particles of colloidal size.
Peptization is а process of passing of а precipitate into colloidal particles on adding suitable electrolyte. The electrolyte added is called peptizing agent. А few examples of sols obtained by peptization are given below:
1. А reddish brown coloured colloidal solution is obtained by adding small quantity of ferric chloride solution 1о the freshly precipitated ferric hydroxide.
2. А precipitate of silver iodide can be peptized by shaking with а dilute solution of silver nitrate or KI.
3. On adding insufficient quantity of very dilute НС1 solution to the freshly precipitated aluminium hydroxide, а sol of aluminium hydroxide is obtained.
Cause of peptization. As the electrolyte is added to а freshly precipitated substance, the particles of the precipitate preferentially adsorb one particular type of ions of the electrolyte and get dispersed due to electrostatic repulsions. This gives particles of colloidal size. Freshly prepared precipitates are preferred where the particles are already supposed to be in the colloidal size.
Condensation or Aggregation methods. These methods involve the joining together of а large number of smaller particles to form particles of colloidal size. The methods generally employed for this purpose are as follows:
By chemical reactions:
1) By double decomposition: When Н2S is passed through а dilute solution of arsenious oxide in water, а colloidal solution of arsenious sulphide is obtained
Аs2О3 + ЗН2S = As2S3 + ЗН2О
colloidal solution
2) By reduction: Gold, silver, platinum etc. are obtained in colloidal form by reduction of very dilute solutions of their salts with а suitable reducing agents е.g.
2 АuCl3 + 3 SnCl2 = 2 Аu + З SnCl4
gold sol
3) By oxidation: Sulphur is obtained in the colloidal form when Н2S is bubbled through the solution of an oxidising agent like nitric acid, bromine water etc.
Вr2 + Н2S = S + 2 HBr
4) By hydrolysis: Fe(OH)s and Аl(ОН)3 sols are obtained by boiling solutions of their corresponding chlorides.
FeCl3 + З Н2О = Fe(OH)3 + З НCl
5) By exchange of solvent. Substances like sulphur and phosphorous are fairly soluble in alcohol but less soluble in water. If their alcoholic solutions are poured in water, colloidal solutions of sulphur and phosphorus are obtained.
6) By condensing vapours of а substance into the solvent. Colloidal solutions of sulphur and mercury in water are prepared by passing their vapours cold water containing а little stabilizing agent like ammonium nitrate.
7) By excessive cooling. Colloidal solution of ice in an organic solvent like ether is obtained by freezing а mixture of the solvent and water.
Purification colloidal solutions
When а colloidal solution is prepared, quite often it contains certain impurities of electrolytes which are crystalloidal iature and tend to destabilize the solution. Hence their removal is very essential. The following two methods are used for the purification of colloidal solutions.
Ultracentrifugation – It is the separation of the colloidal particles by subjecting them to а force of several thousands or even hundreds of thousands of gravity (g) in а special apparatus called ultracentrifuge. The first ultracentrifuge was devised by Svedberg of Sweden in whose honor floatation unit has beeamed Svedberg floatation (Sf or just S) unit. On subjecting to ultracentrifugation, the lighter particles come to the surface, while heavier particles sink to the bottom of centrifuge tube the rise and fall of the particles on ultracentrifugation are termed creaming and sedimentation respectively. One type of ultracentrifuge has optical devices with which the rates of settling or floating of various colloidal particles can be followed and photographed. The rates of floatation or sedimentation of particles give important information about the size or the molecular weight of the colloidal particles.
Ultrafiltration. Ordinary filter paper is permeable to colloidal dispersions (pores being of colloidal size) as well as of true solutions, and thus cannot be employed for filtering sols. However, а filter paper treated with collodion or gelatin solution followed by hardening by dipping in formaldehyde solution serves the purpose. Due to this treatment the pore size of the filter paper is reduced. The filter paper so obtained is called ultrafilter and the filtration device using such а filter paper is called ultrafiltration. By using the impregnating solution of different concentrations, а series of graded ultrafilters can be obtained. With such ultrafilters, solute impurities of different sizes can be effectively removed. The sol is poured over the ultrafilter which permits solution of electrolytes to pass through but retains the colloidal particles in the form of slime. Slime in contact with water disperses spontaneously to form а colloidal system.
Other filter media commonly used are unglazed porcelain and finely sintered glass. Since under ordinary conditions filtration proceeds very slowly, pressure or suction has to be applied to speed up the process.
Dialysis. The process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through а parchment or an animal membrane is known as dialysis.
The separation of crystalloids from the colloids is based upon the principle that the particles of the crystalloids pass through parchment paper or cellophane membrane whereas those of the colloids do not.
А cellophane paper is turned into а bag with а funnel tied in the mouth of the bag, in the the addition of the impure sol. The impure sol is filled into the bag which is then suspended into а vessel containing distilled water. After some time whole of the crystalloid in solution passes out leaving the colloid behind. The distilled water is renewed frequently to avoid accumulation of the crystallids as otherwise they may start diffusing back into the bag. The above process can be quickened if an electric field is applied around the membrane (the process is then called Electro-dialysis).
The most important application of dialysts is in the purification of blood in the artificial kidney machine. The dialysis membrane permits small particles of the excess ions and waste products to pass through whereas colloid-sized particles such as haemoglobin do not pass through the membrane.
Hemodialysis is an artificial process used to remove toxic waste from the blood of patients suffering from temporary or permanent renal failure. All constituents of blood except the plasma proteins move freely between blood and the dialyzing fluid. Since dialysis tubing also allows passage of nutrients such as Na+, К+, amino acids, and glucose, these and other vital substances are included in the dialyzing fluid. Their inclusion prevents а net loss of these Materials from the blood. Because no waste products such as urea are in the dialyzing fluid, these substances are lost from the blood in large quantities.
Although hemodialysis is very effective in removing toxic waste from the body, it does not solve all the problems brought on by renal failure. For example, until recently, patients suffering from renal failure often became anemic. This occurred because of а lack of а protein hormone called erythropoietin that is normally secreted by the kidney. (Erythropoietin stimulates red blood сell synthesis.) Because of DNA technology, erythropoietin is now readily administered to dialysis patients.
Structure of a micelle
Structure of a micelle Fe (OH)3. The iron hydroxyde sol a formed, if iron chlorate is hydrolyzed:
FeCl3 + H2O = Fe(OH)3 + HCl
Reaction products interact:
Fe (OH)3 + HC1 = FeOCI + 2H2O
Iron oxychloride can dissocied:
FeOCI = FeO+ + Cl–
The potential-determining ions will be only FeO+ as the ion Cl–:
{[(Fe(OH)3)m FeO+] (n-x)Cl–}x+ xCl–
Soap molecules can cluster together as micelles, which are colloid-sized clusters of molecules, for their hydrophobic tails tend to congregate, and their hydrophilic heads provide protection. Micelles form only above the critical micelle concentration (СМС) and above the Krafft temperature. The СМС is detected by noting а pronounced discontinuity in physical properties of the solution, particularly the molar conductivity. The hydrocarbon interior of а micelle is like a droplet of oil. Nuclear magnetic resonance shows that the hydrocarbon tails are mobile, but slightly more restricted than in the bulk. Micelles are important in industry and biology on account of their solubilizing function: matter can be transported by water after it has been dissolved in their hydrocarbon interiors. For this reason, miceller systems are used as detergents and drug carriers, and for organic synthesis, froth flotation, and petroleum recovery.
Structure micelle
{[(AgI)m nI–](n – x)K+ }x- x K+ micelle of AgI, when excess KI
[(AgI)m nI– ] is a nuclear
(n – x)K+ is a adsorptive layer
x K+ is a diffuse layer
{[(AgI)m nI–](n – x)K+ }x- is a granule
Structure of a micelle As2S3.
2H3AsO3 + 3H2S = As2S3 ¯ + 6H2O
The deflocculant serves H2S:
H2S = H+ + HS–
Outcome the micelle will be derivated:
{[ As2S3]m nHS– (n-x)H+}x- xH+
Properties
Physical Properties (а) Heterogeneous character: Colloidal particles being larger than molecules form heterogeneous mixture composed of particles of dispersed phase and dispersion medium. The phenomena of Tyndall effect, electrophoresis and electro-osmosis (discussed later in this section) confirm heterogeneity of colloidal systems.
(b) Stability: Colloidal sols are quite stable. Only а few colloidal particles of comparatively larger size may settle but very slowly.
(с) Filtrability: Ordinary filter paper cannot be used for removing the dispersed phase because size of pores of filter paper is bigger than the size of colloidal particles which can easily pass through the pores of the ordinary filter paper. Animal membrane or parchment paper does not allow the colloidal particles to pass through it. This forms the basis of separating the particles of the colloid from those of the crystalloids in the process called dialysis, discussed earlier.
(d) Visibility: The particles in colloidal solution are not visible to naked eye or under ordinary microscope. In fact, а particle is not visible if its size is less than half the wavelength of the light used to see it. Since the minimum wavelength of the visible light range is 3600 А, therefore, with visible light, а particle of size less than 1800 А cannot be seen. As the maximum size of the colloidal particles being less than 1800 А, these are not visible.
Colligative properties-Osmotic pressure. There are four colligative properties – osmotic pressure, elevation in boiling point, depression in freezing point and relative lowering of vapour pressure. These properties depend upon the number of moles present. Colloidal particles have very high average molecular masses, and hence the number of moles present in solution will be extremely small. Thus the value for any of the colligative properties for а particular substance will be smaller as compared to its value when it is а part of true solution. However, some colloids have measurable osmotic pressures which have been determined with а reasonable degree of accuracy. The osmotic pressure has been used to determine the average molecular masses of colloidal particles.
Mechanical Properties – Brownian movement. When viewed through an ultramicroscope, colloidal particles are seen continuously moving in а zig-zag way. Robert Brown in 1827 observed such а movement of pollen grains suspended in water and hence it is called Brownian movement. Thus:
Brownian movement may be defined as continuous zig-zag movement of the colloidal particles in а colloidal sol.
Cause of Brownian movement. The explanation of the phenomenon is that the molecules of dispersion medium due to their kinetic motion strike against the colloidal particles (dispersed phase) from all sides with different forces causing them to move. However, colloidal particles being comparatively heavier, move with а slower speed.
(i) Brownian movement opposes the force of gravity and does not allow the colloidal particles to settle down. Thus it is responsible for the stability of the colloidal solution. (The stability is also explained on the basis of electrical charge, as will be discussed later).
(ii) It has also helped in the determination of Avogadro’s number.
Optical Properties – Tyndall affect. If а strong converging beam of light is passed through а colloidal solution placed in а dark room, the path of beam gets illuminated with а bluish light. The path of the light is made visible by the scattering of light by the colloidal particles. The phenomenon was observed by Tyndall in 1869 and is called Tyndall effect. Thus:
Tyndall effect may be defined as the scattering of light by the colloidal particles present и а colloidal sol.
The illuminated path of beam is called Tyndall cone. The phenomenon is also observed when а beam of light is projected in а cinema hall and it becomes visible due to the scattering by colloidal dust particles in the air of the room.
The Tyndall effect has been used to devise an instrument called ultramicroscope in which the light scattered by the colloidal particles can be seen through а microscope.
The importance of Tyndall effect lies in the fact that it has helped to confirm the heterogeneous nature of the colloidal solutions.
Electrical Properties. (а) Stability of colloidal sols. Electrical charge on colloidal particles. The stability of а colloidal solution is due to the fact that the colloidal рап с1еып the sol are electrically charged. The particles, therefore, repel one another and do not coalesce (come close together) to form large non-colloidal particles. Colloidal particles carry either positive or negative charge. All the dispersed particles in а colloidal solution carry the same charge while the dispersion medium has an equal and opposite charge. For example, arsenious sulphide particles are negatively charged whereas the dispersion medium (water) is positively charged. Ferric hydroxide particles are positively charged whereas the dispersion medium (water) is negatively charged.
(b) Origin of electrical charge on colloidal particles. The various reasons for the origin of electrical charge on the colloidal particles are as follows:
(i) Frictional electrification caused by the mutual rubbing of the colloidal particles with molecules of the dispersion medium.
(ii) Electron capture by particles from air and during electro-dispersion in Bredig’s arc method.
(iii) Preferential adsorption of ions from solutions. An ionic colloid adsorbs ions common to its own lattice structure. The AgCl particles can adsorb Cl– ions from chloride solutions and Ag+ ions from solutions having silver ions. The sol will be negatively charged in the first case and positively charged in the second case.
(iv) Dissociation of molecules followed by aggregation of ions. For example, in case of soap the RCOO– groups get dissociated from Na+ ions and have а tendency to aggregate into а cluster carrying negative charge, as already explained.
The particles of а dye have а tendency to dissociate to form aggregates carrying positive or negative charge depending upon its composition.
(v) Dissociation of the molecular electrolytes adsorbed on the surface of gasmпс1ез. For example, Н2S molecules get adsorbed on colloidal particles of sulphide (е.g., As2S3)during precipitation. By dissociation of Н2S, Н+ ions are lost thereby giving a negative charge to colloidal particles. Ferric hydroxide sol particles are positive due to self dissociation; ОН– ions are lost to the solution giving positive charge to particles.
А major source of kinetic stability of colloids is the existence of an electric charge on the surfaces of the particles. On the account of this charge, ions of opposite charge tend to cluster nearby, and an ionic atmosphere is formed.
Two regions of charge must be distinguished. First, there is а fairly immobile layer of ions that adhere tightly to the surface of the colloidal particle, and which may include water molecules (if that is the support medium). The radius of the sphere that captures this rigid layer is called the radius of shear, and is the major factor determining the mobility of the particles. The electric potential at the radius of shear relative to its value in the distant, bulk medium is called the zeta potential x or the electrokinetic potential. Second, the charged unit atrracts an oppositely charged atmosphere of mobile ions. The inner shell of charge and the outer ionic atmosphere is called the electric double layer.
Electrophoresis or cataphoresis. The existence of the electrical charge (positive or negative) can be shown by the process of “Electrophoresis” also called “Cataphoresis” which involves the movement of colloidal particles towards one or the other electrode when placed under the influence of an electric field (Fig.2.).
The movement of colloidal particles under the influence of an electric field is called electrophoresis or cataphoresis.
As soon as the colloidal particles reach the oppositely charged electrode, they get neutralized and coagulated.
Electrophoresis can be used to find out the nature of the charge that the colloidal particles carry.
Eariler the term used was “cataphoresis” because most of the colloidal sols studied at that time contained positively charged colloidal particles and thus migrated towards cathodes. However, now the term electrophoresis is preferred because colloidal particles may migrate towards either of the electrode i.е. anode or cathode depending upon the charge on the colloidal particles.
Electro-osmosis. On placing а colloidal solution under the influence of an electric field, the particles of the dispersion medium (which are also electrically charged) move towards oppositely charged electrode, provided the colloidal particles are not allowed to move as shown in Fig.3. This phenomenon is called electro-osmosis.
Electro-osmosis may be defined as a phenomenon in which the molecules of the dispersion medium are allowed to move under the influence of an electric field whereas colloidal particles are not allowed to move.
Now we turn from largely flat liquid surfaces to surfaces that are highly curved, and consider colloids. А colloidal is а dispersion of small particles f one material in another. ‘Small’ means something less than about: 500 nm in diameter (about the wavelength of visible light). In general, colloidal particles are aggregates of numerous atoms or molecules, but are too small tо be seen with an ordinary optical microscope. They pass through most filter papers, but can be detected by light-scattering sedimentation, and osmosis.
(а) Heterogeneous character: Colloidal particles being larger than molecules form heterogeneous mixture composed of particles of dispersed phase and dispersion medium. The phenomena of Tyndall effect, electrophoresis and electro-osmosis (discussed later in this section) confirm heterogeneity of colloidal systems.
(b) Stability: Colloidal sols are quite stable. Only а few colloidal particles of comparatively larger size may settle but very slowly.
Stability of colloidal sols. Electrical charge on colloidal particles. The stability of а colloidal solution is due to the fact that the colloidal particles the sol are electrically charged. The particles, therefore, repel one another and do not coalesce (come close together) to form large non-colloidal particles. Colloidal particles carry either positive or negative charge. All the dispersed particles in а colloidal solution carry the same charge while the dispersion medium has an equal and opposite charge. For example, arsenious sulphide particles are negatively charged whereas the dispersion medium (water) is positively charged. Ferric hydroxide particles are positively charged whereas the dispersion medium (water) is negatively charged.
Origin of electrical charge on colloidal particles. The various reasons for the origin of electrical charge on the colloidal particles are as follows:
(i) Frictional electrification caused by the mutual rubbing of the colloidal particles with molecules of the dispersion medium.
(ii) Electron capture by particles from air and during electro-dispersion in Bredig’s arc method.
(iii) Preferential adsorption of ions from solutions. An ionic colloid adsorbs ions common to its own lattice structure. The AgCl particles can adsorb Cl– ions from chloride solutions and Ag+ ions from solutions having silver ions. The sol will be negatively charged in the first case and positively charged in the second case.
(iv) Dissociation of molecules followed by aggregation of ions. For example, in case of soap the RCOO– groups get dissociated from Na+ ions and have а tendency to aggregate into а cluster carrying negative charge, as already explained.
The particles of а dye have а tendency to dissociate to form aggregates carrying positive or negative charge depending upon its composition.
(v) Dissociation of the molecular electrolytes adsorbed on the surface of gasmпс1ез. For example, Н2S molecules get adsorbed on colloidal particles of sulphide (е.g., As2S3)during precipitation. By dissociation of Н2S, Н+ ions are lost thereby giving a negative charge to colloidal particles. Ferric hydroxide sol particles are positive due to self dissociation; ОН– ions are lost to the solution giving positive charge to particles.
А major source of kinetic stability of colloids is the existence of an electric charge on the surfaces of the particles. On the account of this charge, ions of opposite charge tend to cluster nearby, and an ionic atmosphere is formed.
Two regions of charge must be distinguished. First, there is а fairly immobile layer of ions that adhere tightly to the surface of the colloidal particle, and which may include water molecules (if that is the support medium). The radius of the sphere that captures this rigid layer is called the radius of shear, and is the major factor determining the mobility of the particles. The electric potential at the radius of shear relative to its value in the distant, bulk medium is called the zeta potential x or the electrokinetic potential. Second, the charged unit atrracts an oppositely charged atmosphere of mobile ions. The inner shell of charge and the outer ionic atmosphere is called the electric double layer.
The theory of the stability of lyophobic dispersions was developed by D. Derjaguin and Landau and independently by Е. Vervvey and J, T, О. Overbeek, and is known as the DLVO theory. It assumes that there is а balance between the repulsive interaction between the charges of the electric double layers on neighbouring particles and the anract1ve interactions arising from van der Waals interactions between the mplc: cules in the particles.
At high ionic strengths the ionic atmosphere is dense and the potential shows а secondary minimum at large separations. Aggregation of the particles arising from the stabilizing effect of this secondary minimum is called flocculation. The flocculated material can often be redispersed by agitation because the well is so shallow, and full coagulation, the irreversible blending together of distinct particles into large particles, has not occurred. The latter occurs when the separation of the particles is so small that they enter the primary minimum of the potential energy curve and van der Waals forces are dominant.
Coagulation а process which involves coming together of colloidal particles so as to change into large sized particles which ultimately settle as а precipitate or float on the surface.
Coagulation is generally brought about by the addition of electrolytes. When an electrolyte is added to а colloidal solution, the particles of the sol take up the ions which are oppositely charged and thus get neutralized. The neutral particles then start accumulating to from particles of larger size which settle down.
Hardy Schulze Law. The quantity of the electrolyte which is required to coagulate а definite amount of а colloidal solution depends upon the valency of the ion having а charge opposite to that of the colloidal particles. This observation of Hardy and Schulze is known as Hardy Schulze Law which, may be stated as follows:
Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.
Thus, for coagulation of negatively charged arsenious sulphide sol., trivalent cations (Al3+) are far more effective than divalent (Ва2+) cations which in turn are more effective than monovalent (Na+) cations. Similarly for coagulation of positively charged ferric hydroxide sol, tetravalent [Fe(CN)6]4+ anions are more effective than trivalent anions (РO43- ) which are more effective than divalent (SO42- ) anions which in turn are more effective than monovalent (Сl–) anions.
The minimum amount of an electrolyte (millimoles) that must be added to one liter of а colloidal solution so as to bring about complete coagulation or flocculation is called the coagulation or flocculation value (or threshold) of the electrolyte. Thus smaller is the flocculation value of an electrolyte, greater is its coagulating or precipitating power.
As coagulating power is inversely proportional to coagulation/flocculation value, to compare the relative coagulating powers of two electrolytes for the same colloidal sol, we have:
Coagulating power of electrolyte 1/Coagulating power of electrolyte 2 = Coagulation value of electrolyte 2/Coagulation value of electrolyte 1
Sedimentation. In а gravitational field, heavy particles settle towards the foot of а column of solution by the process called sedimentation.
The blood belongs to microheterogenous disperse systems such as emulsion.
