Manufacture of Sodium Carbonate and Bicarbonate
Ernest Solvay (1838 – 1922) was a Belgium chemist. In 1861 he developed an ammonia-soda process for the manufacture of sodium carbonate (soda) and sodium bicarbonate (baking soda) whose ingredients are readily available and inexpensive.
The Solvay process is an intricate industrial process but this article outlines its main parts and focuses on the important chemistry.
Solvay or Ammonia-Soda Process
Solvay’s method is called the ammonia-soda process but it is also named after himself as “The Solvay Process”, a two-stage process first making soda by treating concentrated solutions of common salt with ammonia and carbon dioxide, yielding crude sodium bicarbonate (Figure 1a). In the next stage, the bicarbonate is converted into soda ash by heat and carbon dioxide with water being expelled (Figure 1b).
Steps in the Solvay Process
An explanation of each step of the process is given below with numbers corresponding to step numbers on the conceptual diagrams, Figure 1a-b.
1. A mixture of limestone and coke is fired in a kiln to give carbon dioxide and quicklime
2. The carbon dioxide is made to react with a saturated solution of ammonia in brine. CO2 + H2O --> H+ +HCO3-. Ammonia unites with the H+ ions to form NH4+ ions. The concentration of HCO3- ions becomes sufficient to precipitate sodium bicarbonate, NaHCO3, which is not soluble in water and less soluble in brine. The NH4+ and Cl- ions remain in solution.
3. The crude bicarbonate is filtered off from the ammonium chloride liquor.
4. It is then decomposed by heat to give a residue of soda ash (anhydrous sodium carbonate). 2NaHCO3 --> NaCO3 + H2O + CO2
5. The soda ash is treated in three ways:
a) It is recrystallised from water to give washing soda (sodium carbonate decahydrate)
b) Its solution is treated with carbon dioxide to produce refined sodium bicarbonate as fine white crystals. Na2CO3 + H2O +CO2 --> 2NaHCO3
c) Soda ash is also treated with slaked lime to produce caustic soda
6. By treating the ammonium chloride (from step 3) with slaked lime, ammonia is evolved and is ready for use in the process again.
The chemistry pathways of the Solvay process are summarized in Figure 2.
Uses of Sodium Carbonate and Bicarbonate
Sodium carbonate is used in the manufacture of glass, water glass (sodium silicate solution), and caustic soda. It is a constituent of soap powders, used for washing clothes.
Sodium bicarbonate, or bicarbonate of soda has a wide range of uses. For example, it is used in medicinal powders for neutralizing the acid of gastric juices. This is preferable to the carbonate because it is not so strongly alkaline and there is less danger of it attacking sensitive mucous membranes.
Sodium bicarbonate is also a well-known constituent of self-raising flours and baking powders – hence its name “baking soda” – baking powder contains baking soda together with a feebly acid substance such as potassium hydrogen tartrate (cream of tartar), primary calcium phosphate, or sodium alum. The acidic substance supplies H+ ions in the presence of moisture and these react with HCO3- ions from the baking soda to produce CO2, which causes the dough to rise.
References:
- Fundamental Chemistry. J.A. Saul. Hall's Book Store Pty Ltd, Melbourne, Australia. 1967.
- Solvay Process for the Production of Sodium Carbonate. AUS-e-TUTE.
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