The special method of preparation of washing soda is named as Solvay process. This is a method of series of events and is based on the low solubility of Sodium bicarbonate. Bicarbonates are also called hydrogen carbonate. Anyhow the compound is easily soluble and therefore decomposes into sodium carbonate when heated. But the production of Sodium bicarbonate follows any events and they all together are named as Solvay process. The first event is reaction of ammonia gas with water and CO2 to form ammonium carbonate. This product reacts with water and carbon dioxide again to form ammonium hydrogen carbonate. The reaction of ammonium bicarbonate with sodium chloride yields Sodium bicarbonate. The reactions are expressed in terms of their equation along with their byproducts below;
1. 2NH3 +H2O + CO2 gives (NH4)2CO3
2. (NH4)2CO3 + H2O + CO2 gives 2NH4HCO3
3. NH4HCO3 + NaCl gives NaHCO3 + NH4Cl
The sodium bicarbonate formed in third reaction is having low solubility. So these crystals are heated to give sodium carbonate.
2NaHCO3 gives Na2CO3 + H2O + CO2

The sodium carbonate formed is hydrated suitably to form the washing soda. But this process is not suitable for preparation of potassium carbonate because the potassium bicarbonate forming will be more soluble and cannot be used. So Solvay process is only for Sodium element. 
NaCl+H2O+Co2+NH3.=>NaHCO3+NH4Cl. 2NaHCO3=>Na2CO3+H2O+CO2