Double decomposition .
An Arsenic sulphide Sol is prepared by passing a slowstream of hydrogen sulphide gas through a cold solution of arsenious oxide. This is continued till the yellow colour of sol attains maximum intensity.
As2​O3​+3H2​S→yellowAs2​S3​​+3H2​O
Excess hydrogen sulphide is removed by passing in a stream of hydrogen.

Preparation of Colloidal Solution of Arsenious Sulphide

Double Decomposition Method

The colloidal solution of arsenious sulphide can be prepared by the double decomposition method, in which two soluble salts are mixed together to form an insoluble precipitate.

The steps involved in the preparation of colloidal solution of arsenious sulphide by double decomposition method are:

1. Preparation of the stock solutions:

- A solution of sodium sulphide is prepared by dissolving sodium sulphide in distilled water.
- A solution of arsenious trioxide is prepared by dissolving arsenious trioxide in distilled water.

2. Mixing of the stock solutions:

- The solution of sodium sulphide is added slowly to the solution of arsenious trioxide with constant stirring.
- The mixture is allowed to stand for several hours.

3. Filtration and washing of the precipitate:

- The precipitate of arsenious sulphide is filtered using a filter paper.
- The precipitate is washed several times with distilled water to remove any impurities.

4. Peptization of the precipitate:

- The precipitate is then peptized by adding a small amount of dilute hydrochloric acid or nitric acid to the precipitate.
- Peptization causes the precipitate to break up into smaller particles, which form a colloidal solution.

5. Stabilization of the colloidal solution:

- The colloidal solution is stabilized by adding a small amount of sodium chloride or potassium chloride to the solution.
- The addition of these salts prevents the particles from aggregating or coagulating.

Thus, the colloidal solution of arsenious sulphide is prepared by the double decomposition method.