PH equilibria in anhydrous liquid hydrogen fluoride: The acidity of BF 3, PF 5, NbF 5, TaF 5 and AsF 5

Abstract

Fluoride ion concentration in liquid anhydrous hydrogen fluoride, interrelated to hydrogen ion concentration, can be determined by means of a hydrogen electrode. The ionic product of HF ([H +][F −] = 10 −19·3) is much smaller than had been assumed hitherto. The evaluation of stability constants for the formation of the fluoro complexes of several fluoride ion acceptors is possible by pH titration. PF 5 (PF 5 + F −⇌ PF 6 −; log K PF 6 − = 3·85) is a very weak fluoride ion acceptor in AHF. BF 3 (log K BF 4 = 7·26), NbF 5 (log K NbF 6 = 6·88) and TaF 5 (log K TaF 6 = 8·88) are fluoride ion acceptors of moderate strength, and AsF 6 (log K AsF 6 = 16·9) is a quite strong fluoride ion acceptor. The particle As 2F 11 − appears as intermediate during the formation of AsF 6 −. In solutions of SbF 5 the hydrogen electrode does not work properly. Fluorosulfonic- and trifluoromethylsulfonic-acids are nonelectrolytes in AHF.