SODIUM HYDROGEN CARBONATE AND SODIUM CARBONATE

Sodium Carbonate (Na₂CO₃)

Popularly known as washing soda or soda ash, sodium carbonate is a commercially important compound. In earlier days, it was obtained from the ash of plants and from natural deposits in India and Egypt.

Manufacture of Sodium Carbonate

1. Raw Materials
2. Sodium chloride – NaCl (brine solution)
3. Ammonia – NH₃
4. Limestone – CaCO₃ (for carbon dioxide supply)

Principal Reactions

Process

The first step in the manufacture of sodium carbonate is to generate carbon dioxide. This can be got by heating limestone. The Plant Used in the Solvay Process

Saturation of Brine With Ammonia

Brine solution is pumped into the ammonia absorber. From the ammonia recovery tower, ammonia mixed with a little carbon dioxide enters the absorber and saturates brine. Impurities of calcium and magnesium present in brine are precipitated as carbonates. These impurities can be removed by pumping the liquid through the filter press and then passed through cooling pipes.

Carbonation

Ammoniacal brine next enters the carbonating tower from the top. This tower is partitioned using horizontal plates that have a central hole and covered with a perforated plate. The ammoniated brine meets the rising stream of carbon dioxide to form crystals of sodium bicarbonate and ammonium chloride, which remains in the solution.

Filtration

The viscous milky liquid from the carbonating tower is filtered using a rotary vacuum filter. The solid sodium bicarbonate left on the filter cloth can be periodically scrapped. The filtrate is pumped to the top of the ammonia recovery tower.

Calcination

Sodium bicarbonate is then heated strongly to form anhydrous sodium carbonate or soda ash.

Recrystallization

To get washing soda (Na2CO3.10H2O) from soda ash, anhydrous sodium carbonate is dissolved in water and recrystallized.

NaO₂CO₃ + 10H₂O -> NaO₂CO₃.10H₂O

(Washing soda)

Economics of the Solvay Process

The calcium oxide or quick lime formed in the lime kiln is changed to slaked lime [Ca(OH)₂] and pumped into the ammonia recovery tower. This then reacts with the filtrate from step 3 (filtration) to regenerate ammonia. Since ammonia is very expensive, this recycling cuts down the expenditure of buying fresh ammonia every time.

Properties of Washing Soda

1. Transparent crystalline solid with ten molecules of water per molecule
2. Soluble in water
3. Washing soda solution is alkaline due to hydrolysis
4. Has detergent or cleansing properties
5. Less corrosive
6. Efflorescent

Efflorescence

Efflorescence is the loss of water of crystallization from a hydrated salt, when exposed to air.

Uses of Sodium Carbonate

1. as washing soda in laundry as a cleansing agent
2. for softening hard water
3. in manufacturing glass, paper, soap and caustic soda
4. as a valuable laboratory reagent
5. in quantitative analysis to standardise acid solutions
6. in qualitative analysis in the detection of acid radicals of insoluble salts

Carbon Cycle

The cyclic changes that carbon undergoes in nature is referred to as the Carbon cycle. Carbon in the form of carbon dioxide gets added to the air and gets removed from the air constantly. This addition and subtraction is so well balanced that the percentage of carbon dioxide remains remarkable steady at 0.03 to 0.04

Addition of carbon dioxide to air

1. By the respiration of living organisms.
2. By the combustion of carbon present in carbon compounds, such as wood, coal, coke, petroleum oil, vegetable oil, alcohol etc.
3. By the decay of organic matter, like the dead bodies of animals and plants.
4. From chemical industry such as, heating of limestone, fermentation of molasses to form alcohol, in beer and wine making processes etc.

Removal of carbon dioxide from air

1. By photosynthesis

Green plants take in carbon dioxide to synthesize carbohydrates. The carbohydrates formed are consumed by animals, digested, absorbed and used in respiration. During respiration, the carbohydrate is oxidised to carbon dioxide, which is released into the air.

2. By dissolution

Carbon dioxide of the air dissolves in water, forming a very dilute solution of carbonic acid. This reaction is an unstable one and depends upon water temperature. If water gets warmer, some of the dissolved carbon dioxide gets released into the air.