Lithium borates

A borate of the formula LiBO₂,8H₂O crystallizes from mixed solutions of lithium hydroxide and boric acid. At 47° C. it melts in its water of crystallization, loses seven molecules of water at 110° C., and the eighth at 160° C. The tenacity with which the final molecule of water is retained points to the possibility of the salt being an orthoborate, LiH₂BO₃,7H₂O. At 14.7° C. its density is 1.397.

The anhydrous metaborate, LiBO₂, is precipitated by the interaction of alcoholic solutions of a lithium salt and boric acid, and can also be obtained by fusing lithium carbonate with boric acid. It forms triclinic leaflets of pearl-like lustre, and melts at 843° C. Boiling with water converts it into the octahydrate.

Le Chatelier's data for the solubility of the anhydrous metaborate in water are given in the following table -

Solubility of Lithium Metaborate.

Temperature, °C	0	10	20	30	40	45
Grams LiBO2 in 100 grams of water	0.7	1.4	2.6	4.9	11	20

For the heat of hydration Le Chatelier found

[LiBO₂]+8H₂O=[LiBO₂,8H₂O]+21.7 Cal.,

and for the heat absorbed by solution of the hydrate

[LiBO₂,8H₂O]+Aq. =LiBO₂,Aq. -14.2 Cal.

A polyborate, Li₂B₈O₁₃, is obtained in crystalline form by heating lithium carbonate with excess of boric acid at 500° to 600° C. for a long time.