Potassium arsenite, K₃AsO₃

The Potassium arsenite, K₃AsO₃, is very unstable, but from its solution containing excess of arsenious acid potassium hydrogen diarsenite, KHAs₂O₄ or K₂H₂As₄O₄, can be isolated by covering it with a layer of alcohol.

At 25° C. the ternary system potassium oxide - arsenious oxide - water indicates the existence of two arsenites of potassium soluble in water without decomposition. They have the formulae K₂As₄O₇ and K₆As₄O₉,12H₂O.