Some Important Compounds of Sodium

Four sodium compounds dominate industrial chemistry: sodium carbonate, sodium chloride, sodium hydroxide and sodium hydrogencarbonate. Each is manufactured on a large scale by a distinct industrial process.

Sodium Carbonate (Washing Soda), $Na_2CO_3\cdot 10H_2O$

Industrially, sodium carbonate is made by the Solvay process, which exploits the comparatively low solubility of sodium hydrogencarbonate. $CO_2$ is passed into a concentrated brine solution that has been saturated with ammonia; the ammonia first forms ammonium carbonate, which is then further carbonated to ammonium hydrogencarbonate:

This ammonium hydrogencarbonate then reacts with the sodium chloride present, precipitating sodium hydrogencarbonate — the least soluble species in the mixture:

The precipitated $NaHCO_3$ crystals are then heated (calcined) to give the carbonate:

Ammonia is regenerated (and recycled) by treating the leftover $NH_4Cl$-containing liquor with slaked lime, which also yields calcium chloride as a by-product:

This process cannot, however, be adapted to manufacture potassium carbonate, because potassium hydrogencarbonate is too soluble to precipitate out of a saturated potassium chloride solution the way its sodium analogue does.

Pure sodium carbonate is a white crystalline solid that normally exists as the decahydrate, $Na_2CO_3\cdot 10H_2O$ — this is washing soda — and dissolves freely in water. On heating it progressively loses its water of crystallisation, first to the monohydrate and then, above 373 K, to the fully anhydrous white powder known as soda ash:

Its aqueous solution is alkaline because the carbonate ion is hydrolysed by water:

Sodium carbonate is used for water softening, laundering and general cleaning; in the manufacture of glass, soap, borax and caustic soda; across the paper, paint and textile industries; and as a standard laboratory reagent for both qualitative and quantitative analysis.

Sodium Chloride, $NaCl$

Sea water — roughly 2.7–2.9% salt by mass — is the most abundant natural source of sodium chloride, and in tropical countries such as India it is recovered simply by evaporating sea water in the sun (around 50 lakh tonnes are produced annually in India this way). The crude salt obtained by crystallising brine carries sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities; the calcium and magnesium chlorides are particularly troublesome because they are deliquescent and pull moisture from the air. To purify the salt, the crude material is redissolved in the minimum amount of water, filtered to remove insoluble matter, and the filtrate is saturated with hydrogen chloride gas; pure sodium chloride crystallises out while the more soluble calcium and magnesium chlorides stay in solution.

Sodium chloride melts at 1081 K, and its solubility (36.0 g per 100 g of water at 273 K) barely changes with temperature. Beyond its everyday use as table salt, it is the starting material for the industrial production of $Na_2O_2$, $NaOH$ and $Na_2CO_3$.

Sodium Hydroxide (Caustic Soda), $NaOH$

Sodium hydroxide is manufactured commercially by electrolysing brine in a Castner–Kellner cell, using a mercury cathode and a carbon anode. Sodium metal deposited at the cathode immediately alloys with the mercury to form sodium amalgam, while chlorine gas is liberated at the anode:

Treating the amalgam with water then liberates the hydroxide together with hydrogen gas, regenerating the mercury:

Sodium hydroxide is a white, translucent solid, melting at 591 K, and dissolves readily in water to give a strongly alkaline solution. Its crystals are deliquescent, and a freshly exposed solution will slowly absorb atmospheric $CO_2$ at the surface to form $Na_2CO_3$. …