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Sodium Hydroxide
The hydroxide is manufactured either by (a) the Gossage (lime-soda) process or (b) electrolytically.
In the Gossage process, the reaction is between sodium carbonate solution and a suspension of slaked lime:
2Na+ + CO32- + Ca2+ + 2OH-
2Na+ + 2OH- + Ca2+ + CO32-
The position of equilibrium is determined by the solubility products K1 and K2 of the two compounds, i.e.
K1 = [Ca2+] [OH-]2
And, K2 = [Ca2+] [CO32-]
And since [Ca2+] is the same for both compounds:
It is therefore desirable, in theory, to keep the hydroxide ion concentration as low as possible in order to effect a high conversion to sodium hydroxide. In practice, economic considerations require that the hydroxide ion concentration be maintained at a reasonable level. Excess lime is therefore added as a suspension (milk of lime) to warm sodium carbonate solution; sludge of calcium carbonate and hydroxide is then formed at the bottom of the iron tank in which the action is carried out. The solution is filtered off and evaporated somewhat; sodium carbonate (which is sparingly soluble in sodium hydroxide owing to the common ion effect) crystallizes out, and complete evaporation by heating then yields sodium hydroxide in the fused state, which is cast into sticks or pellets. Some sodium carbonate is not in the sludge in the form of the sparingly soluble double carbonate, Na2CO3.CaCO3.5H2O (gaylussite).
Sodium hydroxide is manufactured electrolytically from aqueous sodium chloride, chlorine being an equally important product; but the electrolysis of sodium sulphate (a by-product from the artificial silk industry) has been carried out on an industrial scale in Germany. To obtain sodium hydroxide, anode and cathode must either be separated by a diaphragm, to prevent mixing of the products (diaphragm cell), or the cathode may be of mercury (mercury cell) giving sodium amalgam.
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In the Gossage process, the reaction is between sodium carbonate solution and a suspension of slaked lime:
2Na+ + CO32- + Ca2+ + 2OH-
2Na+ + 2OH- + Ca2+ + CO32-The position of equilibrium is determined by the solubility products K1 and K2 of the two compounds, i.e.
K1 = [Ca2+] [OH-]2
And, K2 = [Ca2+] [CO32-]
And since [Ca2+] is the same for both compounds:
It is therefore desirable, in theory, to keep the hydroxide ion concentration as low as possible in order to effect a high conversion to sodium hydroxide. In practice, economic considerations require that the hydroxide ion concentration be maintained at a reasonable level. Excess lime is therefore added as a suspension (milk of lime) to warm sodium carbonate solution; sludge of calcium carbonate and hydroxide is then formed at the bottom of the iron tank in which the action is carried out. The solution is filtered off and evaporated somewhat; sodium carbonate (which is sparingly soluble in sodium hydroxide owing to the common ion effect) crystallizes out, and complete evaporation by heating then yields sodium hydroxide in the fused state, which is cast into sticks or pellets. Some sodium carbonate is not in the sludge in the form of the sparingly soluble double carbonate, Na2CO3.CaCO3.5H2O (gaylussite).
Sodium hydroxide is manufactured electrolytically from aqueous sodium chloride, chlorine being an equally important product; but the electrolysis of sodium sulphate (a by-product from the artificial silk industry) has been carried out on an industrial scale in Germany. To obtain sodium hydroxide, anode and cathode must either be separated by a diaphragm, to prevent mixing of the products (diaphragm cell), or the cathode may be of mercury (mercury cell) giving sodium amalgam.
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