Potassium bromide, KBr

The Potassium bromide, KBr, is prepared by the action of bromine on warm, concentrated potassium-hydroxide solution, the bromate simultaneously formed being converted into bromide by subsequent heating:

6KOH + 3Br₂ = 5KBr + KBrO₃ + 3H₂O.

It forms white cubes, m.p. 733° C., 740° C., 745.5° C., or 750° C.

The mean density is 2.690, other values found being 2.756 at 20° C., 2.679 at 23.4° C., and 2.73 at 25° C. The specific heat is 0.1132 between 16° and 98° C., or 0.102 between the temperature of liquid air and 15° C. The molecular electric conductivity of potassium bromide between 745.2° and 868.6° C. is given by the formula

μ_t = 90.09 + 0.1906(t - 750).

The heat of formation of potassium bromide from the elements is given as 95.3 Cal. and 95.6 Cal. At 20° C. 100 grams of water dissolve 65 grams of the bromide; and at 25° C. 100 grams of ethyl alcohol dissolve 0.142 gram.

In aqueous solution potassium bromide reacts with bromine, polybromides with the formulae KBr₃ and KBr₅ being formed in solution. In the neighbourhood of its melting-point the salt evolves bromine freely. Potassium bromide finds application in medicine as a soporific, and in photographic development as a "restrainer."