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Potassium Arsenates

Potassium Orthoarsenate, K3AsO4

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

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

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.

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