Potassium Arsenates

Potassium Orthoarsenate, K₃AsO₄, is formed when excess of potassium hydroxide reacts with aqueous arsenic acid or from a mixture of potassium carbonate and potassium monohydrogen arsenate. The salt crystallises in needles. The heat of formation is 396,200 calories.

By subjecting potassium arsenate in a slowly moving stream of hydrogen to a silent electric discharge, reduction to arsenic occurs. Suitable conditions for this result are to apply 80 volts to the primary circuit and 15,000 volts to the reaction vessel, using a transformer instead of an induction coil; the hydrogen should pass at the rate of 2 litres per hour.

The arsenate considerably accelerates the fermentation of hexose diphosphate, and inhibits the combination of sucrose with phosphoric acid (phosphorylation).

Potassium Monohydrogen Orthoarsenate, K₂HAsO₄, is formed when potassium carbonate is added to aqueous arsenic acid until effervescence no longer occurs; on evaporating the solution to dryness a white residue is obtained. It is soluble in water and may be crystallised in the form of triclinie prisms of the monohydrate K₂HAsO₄.H₂O. These lose their water of crystallisation at 110° to 120° C. The heat of formation is 339,800 calories. When ignited, fusion occurs and, on cooling, a white glass said to be potassium pyroarsenate, K₄As₂O₇, remains.

Potassium Dihydrogen Orthoarsenate, KH₂AsO₄, is prepared by fusing together equal quantities of arsenious oxide and potassium nitrate, dissolving the product in water and allowing to crystallise; or by adding sufficient aqueous arsenic acid to potassium carbonate to form a solution acid to litmus, when it may be crystallised from the solution. It forms tetragonal crystals with axial ratio a:c = 1:0.9380, and isomorphous with the corresponding phosphates of potassium and ammonium. The density is 2.851 at ordinary temperatures and 2.8675 at 9.2° C., and the specific heat is 0.175. The heat of formation is 234,000 calories. When heated, the salt loses 1 molecule of H₂O at 240° to 290° C., and at a dull red heat fusion occurs and potassium metarsenate is formed. The specific heat of the latter is 0.1563 between 17° and 99° C. When dissolved in water the dihydrogen orthoarsenate is re-formed.

The solubility of potassium dihydrogen orthoarsenate in water is 18.87 g. per 100 g. H₂O at 6° C. and increases with temperature. The solution is acid to litmus. The salt is insoluble in alcohol. The dielectric constant is 31.