Barium Carbonate, BaCO₃

Barium Carbonate, BaCO₃, occurs naturally as the mineral witherite, in orthorhombic crystals isomorphous with aragonite and strontianite. A mineral consisting of mixed crystals of barium and calcium carbonates, barytocalcite, is also found.

Barium carbonate may be readily obtained by precipitation from a solution of a barium salt by an alkali carbonate, or by the action of carbon dioxide on barium hydroxide solution. It can be formed by the union of carbon dioxide with anhydrous barium oxide if the temperature is raised to about 550° C. It may also be obtained in crystalline form by fusion of barium chloride with sodium carbonate and extraction with water, or by fusion of the precipitated carbonate with sodium chloride, or by heating with ammonium nitrate or carbonate solution in a sealed tube at 150°-180° C.

Commercially it is prepared from the sulphate by reduction to the sulphide and treatment of a hot solution of the latter with carbon dioxide.

The density of witherite is 4.3 – 4.5, and of the precipitated carbonate 4.22 – 4.37. The specific heat is 0.14483.

Barium carbonate is more difficult to decompose than either calcium or strontium carbonate. The dissociation pressure only reaches one atmosphere at 1350° C., when, according to Finkelstein, a basic carbonate, BaO.BaCO₃, is formed. This basic carbonate melts below 950° C. It can be formed by prolonged heating at 1120° C., and it dissolves barium oxide and, at higher temperatures, barium carbonate. By extrapolation the temperature of decomposition of the basic carbonate is found to be 1454° C.

Le Chatelier gave the temperature of fusion in an open vessel as 795° C., but this low value was no doubt due to the formation of barium oxide. It only partially fuses when heated at 1380° C. in a current of carbon dioxide. The melting-point under a pressure of carbon dioxide appears to be about 1740° C., when determined by extrapolation from the fusion curve of mixtures of calcium and barium carbonates. There are two transition points, one at 811° C., from witherite or γ-carbonate to carbonate, a hexagonal form, and the other at 982° C., from β- to α-barium carbonate, which is regular in crystalline form.

If carbon dioxide be passed through a methyl alcoholic solution of barium oxide, barium carbonate separates in a gelatinous form and behaves as a reversible colloid, giving a clear solution which is practically a non-conductor of electricity so long as it is not completely freed from methyl alcohol.

By the action of sodium on barium carbonate, carbon is liberated and the oxide formed. Carbon also separates when a solid mixture of chloride and carbonate is electrolysed at 550°-600° C.

The solubility of barium carbonate in water is very small, but greater than that of the other alkaline earth carbonates. The results obtained by different investigators are rather variable, as the solubility depends on the pressure of carbon dioxide, and there are probably present in solution hydroxyl ions due to the hydrolysis of the salt, and hydrogen carbonate ions owing to the formation of bicarbonate. By electrical conductivity determinations the solubility was found to be 24 mgm., or 1.22×10^-4 grm.-mol. per litre at 18° C. Weissenberger determined the solubility in the presence of a small quantity of caustic soda to suppress the hydrolysis, and observed a minimum solubility at a hydroxyl-ion concentration of 1.25×10^-5 grm.-ion per litre. With a higher concentration the hydroxide is formed, and therefore the solubility is apparently increased. The true values of the solubility were considered to be as follows: -

Temperature °C	14	18	23	27	32	38
Gram-molecules BaCO3 per litre×104	4.32	4.57	4.89	5.22	5.69	6.27

The solubility is increased by the presence of ammonium chloride, probably due to the formation of a double chloride in solution, and also by potassium and sodium chlorides and sulphates, magnesium and zinc sulphates, and by calcium nitrate and chloride, all of which probably bring about partial decomposition of the carbonate.

McCoy and Smith, working under pressures of 0.2 - 30 atmospheres of carbon dioxide, found as the value of the solubility product [Ba^••]×[CO₃''], 8.1×10^-9 at 25° C. The maximum solubility was reached at 22 atmospheres and was 7.3 grm. of barium bicarbonate per litre at 25° C. Above this pressure, therefore, it should be possible to separate the solid bicarbonate. This compound may apparently be precipitated from ice-cold solutions of barium chloride by ammonium bicarbonate, preferably with the addition of gelatine to stabilise it, its composition being approximately represented by the formula BaCO₃.1.5H₂CO₃. A compound of composition 2BaO.3CO₂ was described by Boussingault as formed by the action of sodium sesqui- carbonate on barium chloride.

A double salt with potassium carbonate, BaCO₃.K₂CO₃, is formed by precipitation from saturated solutions of barium chloride and potassium carbonate at room temperature. The same compound, as well as a similar sodium salt, may also be formed by fusion.