Dispersion methods :
In these methods, large particles of a substance are broken into particles of colloidal dimensions in the presence of dispersion medium. Since the sols formed in this manner are unstable, they are stabilised by adding suitable stabilizers. Some of the methods employed for carrying out the dispersion are described as follows:
(i) Mechanical dispersions : Many substances can be reduced to colloidal size in a "colloidal mill" consisting of a series of discs rotating in opposite directions with only a small gap between them at the rate of `10,000` rpm. The dispersion medium together with the substance to be dispersed and a stabilizer is passed through the mill and after sometime, a colloidal solution results. The protective material stabilises the sol and prevents the particles from coagulating. See fig.1.
(ii) Electrical disintegration or Bredig's arc method : This process involves dispersion as well as condensation. Colloidal solutions of metals such as gold, silver, copper, platinum etc., can be prepared by this method. In this method electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourizes some of the metal, which then condenses to form particles of colloidal size. A slight trace of electrolyte stabilises the sols formed. See fig.2.
(iii) Peptization : This is a process of converting a precipitate into colloidal solution by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is generally applied to convert fresh precipitate into colloidal solutions because such precipitates are simply aggregates of colloidal particles held by weak forces.
`text(Cause of peptization)` : During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. The adsorbed ion is generally common with those of precipitate. This causes the development of positive or negative charge on the precipitates, which ultimately breaks into particles of colloidal dimensions. For example, when freshly precipitated ferric hydroxide is shaken with aqueous solution of ferric chloride (peptizing agent) it adsorbs `Fe^(3+)` ions and thereby split into colloidal particles of the type `[Fe(OH)_3)]Fe^(3+)`. Similarly, a precipitate of `AgCl` on shaking with dilute solution of `AgNO_3` adsorbs `Ag^+` ion and get peptised to colloidal particles of the type `[AgCl]Ag^+`. In some cases, peptization can also be achieved by organic solvents. For example, cellulose nitrate is peptised by ethanol. The colloidal solution of cellulose nitrate in ethanol is called 'collodion'.
(iv) Washing Methods : It is common experience in analytical chemistry that a precipitate tends to pass through the filter paper while being washed free from electrolytes. It is probable that the electrolytes have caused the primary colloidal particles to form a precipitate and their removal may result in a return to the colloidal state.
(i) Mechanical dispersions : Many substances can be reduced to colloidal size in a "colloidal mill" consisting of a series of discs rotating in opposite directions with only a small gap between them at the rate of `10,000` rpm. The dispersion medium together with the substance to be dispersed and a stabilizer is passed through the mill and after sometime, a colloidal solution results. The protective material stabilises the sol and prevents the particles from coagulating. See fig.1.
(ii) Electrical disintegration or Bredig's arc method : This process involves dispersion as well as condensation. Colloidal solutions of metals such as gold, silver, copper, platinum etc., can be prepared by this method. In this method electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourizes some of the metal, which then condenses to form particles of colloidal size. A slight trace of electrolyte stabilises the sols formed. See fig.2.
(iii) Peptization : This is a process of converting a precipitate into colloidal solution by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is generally applied to convert fresh precipitate into colloidal solutions because such precipitates are simply aggregates of colloidal particles held by weak forces.
`text(Cause of peptization)` : During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. The adsorbed ion is generally common with those of precipitate. This causes the development of positive or negative charge on the precipitates, which ultimately breaks into particles of colloidal dimensions. For example, when freshly precipitated ferric hydroxide is shaken with aqueous solution of ferric chloride (peptizing agent) it adsorbs `Fe^(3+)` ions and thereby split into colloidal particles of the type `[Fe(OH)_3)]Fe^(3+)`. Similarly, a precipitate of `AgCl` on shaking with dilute solution of `AgNO_3` adsorbs `Ag^+` ion and get peptised to colloidal particles of the type `[AgCl]Ag^+`. In some cases, peptization can also be achieved by organic solvents. For example, cellulose nitrate is peptised by ethanol. The colloidal solution of cellulose nitrate in ethanol is called 'collodion'.
(iv) Washing Methods : It is common experience in analytical chemistry that a precipitate tends to pass through the filter paper while being washed free from electrolytes. It is probable that the electrolytes have caused the primary colloidal particles to form a precipitate and their removal may result in a return to the colloidal state.
