The Determination of Some Less Common Anions

Developments in the determination o some less common anions and in the separation of anions of similar species are described. The anions which are considered are the Group Va hexalluorides. vanadates, tungstates. the per-acids. oxyanions of phosphorus and some organic acids. A new method for the determination of chloride, based on GLC. and capable of very high sensitivity is mentioned. This method could probably be applied to the simultaneous determination of chloride, bromide and iodide at very low concentrations. In general, anions have provided more difficulties in their determination than cations, but in recent times many of the older problems have either disappeared or appear likely to be solved soon, partly because of the development of new techniques and partly because of the discovery of new reagents and reactions1. Despite these advances, however, some new problems have arisen during the last decade. New anions have been prepared and have become of commercial importance. Moreover, anions having very similar chemical properties, which were once grouped together for convenience, now require to be differentiated because of their industrial importance, e.g. the oxyanions of phosphorus. Several investigators have examined these problems and notable progress has been made; the present account describes some of these advances. HEXAFLUOPHOSPHATE, -ARSENATE AND -ANTIMONATE These anions are very similar in chemical behaviour, especially hexafluophosphate and fluoarsenate; fluoantimonate is much more readily hydrolysed. Their applications are wide and have been described in several reviews26. Lange7 and his co-workers, who first synthesized hexafluophosphate in 1928, noted that the reactions were very similar to those of perchlorates and the first determinations were made by precipitating and weighing the nitron salt. The less stable difluoride also precipitated, but this interference could be eliminated by first hydroiysing. No other reagents were to be used for more than a quarter of a century. Dess and Parry8 determined hexafluoarsenate gravimetrically after precipitation with tetraphenylarsonium chloride (TPA); a similar method for hexafluophosphate was described some years later9 by Affsprung and Archer. A comparison was made with the nitron method and it was