To Prepare Colloidal Solution of Arsenious Sulphide

To Prepare Colloidal Solution of Arsenious Sulphide , [As₂ S₃]

Chemistry Lab ManualNCERT Solutions Class 12 Chemistry Sample Papers

Theory
Arsenious sulpide, As₂S₃ is a lyophobic colloid. It is obtained by the hydrolysis of arsenious oxide (AS₂0₃) with boiling distilled water, followed by passing H₂S gas through the solution obtained.

In the colloidal solution of arsenious sulphide, each particle is surrounded by HS- ions, produced by the dissociation of H₂S. This sulphide ion layer is further surrounded by the counter ion layer of H+ ions.

Apparatus
Conical flasks (250 ml), beaker (250 ml), round-bottom flask (500 ml), glass-rod, funnel, glass tubing, filter-paper, tripod stand, wire-gauze, iron stand with clamp, burner, etc.

Materials Required
Solid arsenious oxide, H₂S gas and distilled water.

Procedure

1. Clean a conical flask (250 ml) by the use of steaming-out process.
2.  To this cleaned flask, add 0.2 g of As₂0₃ solid and add 100 ml of distilled water.
3. Boil the solution for about 10 minutes (Fig).
   
4. Filter the above hot solution through flutted filter paper and receive the filtrate in another beaker (Fig).
   
5. Pass a slow current of H₂S into As₂0₃ solution as shown in Fig.
   
   The solution develops a yellow colour due to formation of As₂S₃. Continue passing H₂S till the intensity of colour does not change further.
6. Expel excess of H₂S gas from the sol by boiling the sol till the escaping gas does not turn lead acetate paper black.
7. Filter the solution through flutted-filter paper and collect the bright yellow filtrate in a dry conical flask and cork it. Label it as “Arsenious Sulphide Sol”.

Precautions

1. Use cleaned apparatus since As₂S₃ sol is affected by even traces of impurities.
2. Handle arsenious oxide with care since it is highly poisonous. Wash your hands immediately every time after handling this chemical. While doing this experiment do not eat or drink anything.