Manufacture of washing soda

The chemical name of washing soda is sodium carbonate decahydrate, and its chemical formula is Na₂CO₃.10H₂O. So washing soda is hydrated salt of sodium carbonate containing 10 molecules of water of crystallization. Sodium carbonate is manufactured by Solvay process. The basic raw materials which are used in the process of manufacture of sodium carbonate in Solvay process are.

1. Sodium chloride (NaCl)
2. Ammonia (NH₃)
3. Limestone (CaCO₃)

The manufacture of sodium carbonate by Solvay process includes many steps which are discussed below:

1. Saturation of Sodium Chloride with Ammonia

First of all a cold and concentrated solution of sodium chloride called brine is saturated with ammonia to get a solution called ammoniacal brine.

2. Carbonation

In the second step the ammoniacal brine prepared in first step is dropped from the top of a tower called carbonating tower and carbon dioxide gas is passed into the tower from near its base. The carbonating tower has partitions which are made up of iron plates with hole in the center and covered with perforated plate. The function of these partitions is to slow down the down coming ammoniacal brine solution so that the upcoming carbon dioxide has enough time to react with it completely.

So as the ammoniacal brine comes down from the top of the carbonating tower, it reacts with carbon dioxide to form precipitates of sodium hydrogencarbonate (also called sodium bicarbonate):

Function of Limestone (Calcium Carbonate)

The carbon dioxide used in the carbonating tower is actually obtained by heating the limestone (calcium carbonate) which decomposes into carbon dioxide and calcium oxide after heating:

The calcium oxide produced in this reaction is then treated with water to obtain slaked lime (calcium hydroxide):

        CaO      +      H₂O          Ca (OH)₂

    Calcium oxide              Water                         Calcium hyroxide

This slaked lime (calcium hydroxide) is heated with ammonium chloride which is obtained as a by-product in the carbonating tower to produce ammonia, which can be used again:

3. Filtration and Heating

The precipitates of sodium bicarbonate (sodium hydrogen carbonate) are separated by the process of filtration and are then dried and heated. On heating, sodium bicarbonate decomposes to form sodium carbonate as given below:

So by the Solvay process we get anhydrous sodium carbonate. But the washing soda contains 10 molecules of water of crystallization so anhydrous sodium carbonate is mixed with water to form washing soda.

    Na₂CO₃   +   10H₂O              NaCO₃.10H₂O

   Sodium            Water                                     Sodium carbonate decahydrate
  carbonate

Test your understanding and answer these questions:

1. What is solvay process? Explain.