Potassium hexafluoroarsenate

Because of the special stabiHty of the hexafluoroarsenate ion, there are a number of appHcations of hexafluoroarsenates. For example, onium hexafluoroarsenates (33) have been described as photoinitiators in the hardening of epoxy resins (qv). Lithium hexafluoroarsenate [29935-35-1] has been used as an electrolyte in lithium batteries (qv). Hexafluoroarsenates, especially alkaH and alkaline-earth metal salts or substituted ammonium salts, have been reported (34) to be effective as herbicides (qv). Potassium hexafluoroarsenate [17029-22-0] has been reported (35) to be particularly effective against prickly pear. However, environmental and regulatory concerns have severely limited these appHcations. 

Arsenates are oxidizing agents and are reduced by concentrated hydrochloric acid or sulfur dioxide. Treatment of a solution of orthoarsenate with silver nitrate in neutral solution results in the formation of a chocolate-brown precipitate of silver orthoarsenate, Ag AsO, which may be used as a test to distinguish arsenates from phosphates. With hydrofluoric acid, orthoarsenate solutions yield hexafluoroarsenates, eg, potassium hexafluoroarsenate [17029-22-0] (KAsFg)2 H2O. Arsenates of calcium or lead are used as insecticides sodium arsenate is used in printing inks and as a mordant. 

In the same way a mixture of arsenious oxide and potassium fluoride, yields potassium hexafluoroarsenate, a product which can be explained only by postulating the acid BrFgAsFg as an intermediate. 

Initially, 3.97 g of potassium hexafluoroarsenate (17.4 mmol) in 36 mL of distilled water is added slowly to 140 mL of an aqueous solution that contains 6.42 g of tetrabutylammonium iodide [Alfa Inorganics] (17.4 mmol) at 60°. A pale yellow precipitate of tetrabutylammonium hexafluoroarsenate forms immediately. The mixture is stirred for 10 min and then allowed to cool to room temperature. The precipitate is removed by filtration and is washed with five 10-mL portions of distilled water. The crude product is recrystallized twice fix)m hot methanol. The yield is 5.77 g of white needles of [CH3(CH2)3]4N[AsFJ (13.4 mmol, 80%) (mp 245-248°). 

Hexafluoroarsenic acid [17068-85-8] can be prepared by the reaction of arsenic acid with hydrofluoric acid or calcium fluorosulfate (29) and with alkaH or alkaline-earth metal fluorides or fluorosulfonates (18). The hexafluoroarsenates can be prepared directly from arsenates and hydrofluoric acid, or by neutrali2ation of HAsF. The reaction of 48% HF with potassium dihydrogen arsenate(V), KH2ASO4, gives potassium hydroxypentafluoroarsenate(V)... 

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