Sodium arsenates

Excess of sodium hydroxide converts arsenic acid into trisodium orthoarsenate, Na₃AsO₄, mean density 2.835. From solution it crystallizes as dodecahydrate, density 1.832, m.p. 86.3° C. At ordinary temperature 100 grams of water dissolve 28.6 grams of the salt, yielding a strongly alkaline solution. The heat of formation in aqueous solution from the elements is given by Thomsen as 381.5 Cal., and that of the solid by Mixter as 360.8 Cal.

Other salts described are disodium hydrogen orthoarsenate, Na₂HAsO₄, converted by heat into sodium pyroarsenate, Na₄As₂O₇; and sodium dihydrogen orthoarsenate, NaH₂AsO₄.