Sodium bromide, NaBr

The bromide is prepared by neutralizing sodium hydroxide or carbonate with hydrobromic acid; or by the action of bromine on sodium hydroxide, the bromate simultaneously formed being subsequently reduced by heating with charcoal:

6NaOH+3Br₂=5NaBr+NaBrO₃+3H₂O;

2NaBrO₃+3C =2NaBr+3CO₂.

Sodium bromide is a white crystalline substance belonging to the rhombic system. The values recorded for its melting-point exhibit a lack of concordance similar to that cited for the chloride, the limits of temperature given being 733° to 765° C. Victor Meyer, Riddle, and Lamb give 757.7° C.; Guareschi, 760° C.; MacCrae, 761.1° C.; and Ruff and Plato, 765° C. Ramsay and Eumorfopoulos give 733° C., and Huttner and Tammann 749° C., but their results are probably unreliable. For the density of the anhydrous salt Clarke gives the mean value 3.014, and Brunner has investigated the density of fused sodium bromide at temperatures up to 1000° C. The salt forms two hydrates, a dihydrate of density 2.176 at 20° C., and a pentahydrate.

Solubility-curve of sodium bromide.

Sodium bromide dissolves readily in water. Meyerhoffer has plotted the solubility-curve. The transition temperature of the dihydrate into the anhydrous salt (D) is 50.674° C., and that of the pentahydrate into the dihydrate (C) is -24° C.

At its melting-point the salt loses only a trace of bromine, but heating with excess of iodine induces a rapid elimination of bromine.

For the heat of formation of sodium bromide from its elements Thomsen gives 85.77 Cal. The heat of hydration of the anhydrous salt to the dihydrate is 4.52 Cal.

References are appended to investigations of the solubility of sodium bromide in organic solvents, and to others dealing with such properties of its aqueous solutions as specific heat, density, refractive index, vapour-pressure, molecular depression of the freezing-point and elevation of the boiling-point, electrical constants, and the influence of sulphur dioxide on the solubility of the bromide.

The compressibility is given by Richards and Jones as 5.1×10^-6.