Chemical Methods Chemistry Notes
Chemical Methods :
→ Double decomposition method : It is generally used to prepare sulphide sols. When HS gas is passed through cold and dilute solution of the salts of metals then metal sulphide sols are obtained.
→ Hydrolysis method : With the help of this method, the colloidal solutions of oxides and hydroxides of heavy metals are prepared. Example:
Similarly colloidal solutions of Al(OH)3, and CH(OH)3 can also be prepared.
→ Oxidation : By this method, colloidal solutions of non-metals are prepared.
→ Reduction : The colloidal solution of some metals are prepared by reducing the aqueous solution of their salts. Example :
By reducing the solution of AuCl3 with the help of SnCl3, the colloidal solution of Au is obtained. It is known as “Purple of cassius”.
Dispersion Methods for the Preparation of Lyophobic sols :
→ In this method, the particles are bigger in size, then these particles are broken down to get the particles of colloidal size. The following are some important dispersion methods :
→ Mechanical Dispersion : In this method, a mixture of dispersed phase and dispersion medium is taken in colloidal mill which gives the colloidal solution. Tooth powder, paint, varnishes, toothpaste, telecom powders lipstick etc., are prepared with the help of this method.
→ In this method, colloid of substance is formed by making suspension and mixing of its fine powder with suitable solvent. Then, this suspension is taken in specific type of colloidal mill. There are two metallic plates in this mill which are located at some distance to each other and revolve in opposite direction to each other with the help of belt.
→ The suspension of substance is added from upper part which passes through rotating belt where large particles are converted into fine particles by mixing in this mill and converted into colloidal size. The distance of plates can be maintained according to appropriate size of particles.
→ Electro-dispersion-Bredig’s Arc method : This method comprises of both dispersion and condensation. With the help of this method, colloidal solution of metals like gold, silver, platinum is prepared. The metal whose colloidal solution is desired is taken in the form of electrodes. These electrodes are now dipped in dipersion medium.
→ An arc is struck between the metal electrodes under the surface of dispersion medium like water which have some suitable stabilizing agent such as a trace of KOH. The dispersion medium like water is cooled by immersing the container in an ice bath. The very high temperature of the are vaporizes the metal but the vapours are readily condensed because they are in contact with cold water. This condensation generates a colloidal solution of metal.
→ Ultrasonic dispersion : Wood and Loomis first time used ultrasonic sound waves for dispersion. Actually the sound waves of high frequency is called ultrasonic waves. With the help of these ultrasonic sound waves, colloidal solution can be prepared. For examples, sols of Hg, sulphur, sulphides and oxides of metal can be prepared easily by using ultrasonic waves.
→ Peptization: The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte is called peptization. Peptization is a reverse process of coagulation.
→ The electrolyte which is used to convert precipitate into colloidal solution is called peptizing agent. This method is applied generally to convert freshly prepared precipitate into a colloidal sol.
→ Example : In freshly prepared precipitate of Fe(OH)3, small amount of dilute solution of FeCl3 is mixed then precipitate [Fe(OH)3] adsorbs Fe3+ ions on its surface. This causes the development of positive charge on precipitate. Due to repulsion created by similar positive charge, precipitate ultimately breako into smaller particles of the size of a colloid. In this way, positively charged shiny red coloured colloidal solution of [Fe(OH)3] is obtained.
→ Similarly, in precipitate of AgCl, dilute solution of HCl is mixed and shaked then white negatively charged colloidal solution is obtained.
In precipitate of AgCl, if dilute solution of AgNO, is mixed and shaked then positively charged colloidal solution is obtained.
Purification of Colloidal Solutions :
→ The colloidal solutions, which are prepared by above given methods, have excessive amount of soluble impurities and electrolytes. For the stability of colloidal solutions, trace amount of an electrolyte is essential but the presence of large amount of electrolyte may cause coagulation. Therefore, it is necessary to remove excess concentration of these electrolyte.
→ “The process used for decreasing the concentration of impurities to a requisite minimum amount is called purification of colloidal solution.” The following given methods are used for the purification of colloidal solutions.
→ As we know that colloidal particles cannot pass through the parchment paper, cellophane or animal membrane while the particles present in true solution can easily be passed through them. It is due to difference in the size of their particles. “The process of removing excess dissolved impurities of electrolytes from a colloidal solution by means of diffusion through a parchment paper or animal membrane is called dialysis.
→ The apparatus used in this process is known as dialyser.” In this method, a bag of parchment paper or animal membrane or cellophane sheet containing the impure colloidal solution is suspended in a container through which fresh water is flown continuously. The molecules or ions present in colloidal solution as impurities, which are diffused through membrane into the outer water and pure colloidal solution is left behind.
→ Dialysis is a slow process. It can be made faster by carried out this process under the influence of an electric field. Then this process is called electrodialysis. In this process the bag containing impure colloidal solution is suspended between two electrodes while the pure water is taken in a compartment on each side. The cations and anions present in the colloidal solution migrate out to the oppositely charged electrodes.
The main drawback of this method is that it cannot be used for the removal of non-electrolyte impurities like sugar and urea.
→ As we know that collodial particles can be passed generally through normal filter paper because the pore size of such filter paper is larger than the size of colloidal particles. Hence the pores of the filter paper can be made smaller by special method. In this method, the normal filter paper is soaked in a solution of gelatin or a collodion and then in turn hardening them by soaking in formaldehyde and then finally drying it.
→ The usual colloidion is a 4% solution of nitrocellulose in a mixutre of alcohol and ether. Now the pore size is reudeed and colloidal particles cannot pass through them. This special filter paper is known as ultrafilter paper and the process of filtration is known as ultrafiltration Ultrafiltration process is a slow process which can be speed up by using suction pump. The colloidal particles left on the ultra-filter paper and then stirred with fresh dispersion medium (solvent) to get a pure colloidal solution.
In this method, the colloidal sol is taken in a tube which is placed in an ultra centrifuge. On rotating the tube at high speeds, the colloidal particles settle down at the bottom of the tube while the impurities remain in the solution called centrifugate. The settled colloidal particles are removed and then mixed with an appropriate dispersion medium to prepare pure colloidal solution.