Sodium carbonate, Na₂CO₃

The carbonate is present in the ashes of sea-plants, its principal source prior to the French Revolution, when Le Blanc devised a method for its production. It is also found in the form of solid deposits, and in solution in many natural waters. Pozzi-Escot believes the Peru deposits to have originated in the reduction to sodium sulphide, by means of plants and algae, of sodium sulphate dissolved from the soil, the sulphide formed being converted into sodium carbonate or sodium hydrogen carbonate by the action of carbon dioxide from the air, or from the decomposition of vegetable matter.

Sodium carbonate is manufactured from sodium chloride by three processes: Le Blanc's process, Solvay's ammonia-soda process, and the electrolytic process.

Le Blanc's Process

This process involves three stages: the conversion of sodium chloride into sodium sulphate, or " salt-cake process "; the reduction of the sulphate to sulphide by means of carbon, and the conversion of the sulphide into sodium carbonate by the action of calcium carbonate, or " black-ash process "; and the extraction of the sodium carbonate with water, or " lixiviation process ":

NaCl+H₂SO₄=NaHSO₄+HCl;

NaCl+NaHSO₄ =Na₂SO₄+HCl;

Na₂SO₄+2C=Na₂S+2CO₂;

Na₂S+CaCO₃ = Na₂CO₃+CaS.

The salt-cake process takes place in two stages, the reaction represented by the second equation being carried on at a higher temperature in a reverberatory furnace. The hydrochloric acid constitutes a valuable by-product.

The sulphate prepared by the salt-cake process is pulverized, and mixed with an equal weight of chalk and half its weight of coal or coke. The mixture is then fused in a rotatory furnace. At first only carbon dioxide is evolved, but at the end of the operation carbon monoxide is generated, and burns as it escapes into the atmosphere:

2CaCO₃+2C =2CaO+4CO.

The sodium carbonate is lixiviated with water to separate it from the calcium sulphide or " alkali-waste," and from other impurities such as sodium chloride, sulphate, silicate, and aluminate; calcium oxide, sulphite, and thiosulphate; iron oxide; and alumina.

Ammonia-soda Process

This process is said to have been devised by the apothecary Gerolamo Forni in 1836. It was perfected by Solvay, and on the continent of Europe it has largely displaced the older Le Blanc process. A solution of sodium chloride is treated alternately with ammonia and carbon dioxide under pressure, sodium hydrogen carbonate separating out from the concentrated solution of ammonium chloride. On heating, the sodium hydrogen carbonate is converted into sodium carbonate, the evolved carbon dioxide being utilized again in the formation of sodium hydrogen carbonate. The ammonia is recovered from the ammonium-chloride solution by distilling it with lime obtained as a by-product in the generation of the carbon dioxide from limestone. The reactions 1 involved are represented by the equations

2NaCl + 2NH₃+2CO₂+2H₂O = 2NH₄Cl+2NaHCO₃;

2NaHCO₃=Na₂CO₃+CO₂+H₂O;

CaCO₃=CaO + CO₂;

2NH₄Cl + CaO = 2NH₃ + CaCl₂+H₂O.

The process yields a purer initial product than the Le Blanc method, but has the disadvantage of leaving the chlorine of the sodium chloride in the form of calcium chloride, for which the demand is limited.

The equilibrium of the four salts sodium chloride, sodium hydrogen carbonate, ammonium chloride, and ammonium hydrogen carbonate has been studied by Toporescu, and he has constructed a diagram to facilitate calculation of the proportion of each salt which will crystallize on progressive evaporation of a solution of known initial composition. His diagrammatic method has been applied by Le Chatelier to ascertaining the proportion of water or salt which must be added to get the maximum yield of pure sodium hydrogen carbonate under manufacturing conditions.

The use of sodium nitrate as a substitute for sodium chloride in the ammonia-soda process has been studied by Fedotiew and Koltunow.

Electrolytic Process

The production of sodium carbonate by the electrolysis of sodium-chloride solution is gradually supplanting the older methods. The solution of sodium hydroxide formed is converted into carbonate by the action of carbon dioxide, the sodium hydrogen carbonate formed being decomposed by heat.

Sodium carbonate is also manufactured to some extent from other materials, such as cryolite, a double fluoride of sodium and aluminium found in Greenland, and sodium nitrate.

Sodium carbonate Properties

Anhydrous sodium carbonate is a white solid, density 2.476, m.p. 851° to 853° C. Its specific heat is 0.246 between 18° and 48° C. (Kopp), and 0.2728 between 16° and 98° C. (Regnault). Its heat of formation from the elements is given as 272.64, 270.8, or 271.97 Cal.

Solubility-curve of sodium carbonate

Sodium carbonate forms three hydrates. The decahydrate is monoclinic, and has the density 1.455, other values being 1.446 at 17° C. and 1.493 at the temperature of liquid air. At 20° C. 100 grams of water dissolve 21.4 grams, reckoned as Na₂CO₃. The heptahydrate appears to exist in both a rhombic and a metastable rhombohedral form, but only between narrow limits of temperature. Its solubility diminishes with rise of temperature. The solubility of the monohydrate at 50° C. is 47.5 grams Na₂CO₃ in 100 grams of water. The transition-points of the hydrates are given by Wells and McAdam: 10 to 7 (D), 32.0° C.; 7 to 1 (F), 35.37° C.; 10 to 1 (intersection CD and GF), 32.96° C. Some of their solubility-data are given in the table, and the solubility-curve in Fig.

The transition-temperature of the decahydrate to the heptahydrate is given by Richards and Fiske as 32.017° C.

According to Berzelius, and also Schindler, the decahydrate in air at 12.5° C. becomes transformed into a pentahydrate. The existence of other hydrates described is even more doubtful. The boiling solution of sodium carbonate absorbs carbon dioxide from the atmosphere. Dubovitz Jpund that exposure of the solid carbonate to atmospheric carbon dioxide and moisture for thirteen days produced between 15 and 20 per cent, of the primary salt, NaHCO₃, and that with a large excess of carbon dioxide and moisture complete conversion could be attained.