I?iTRODUCTION
In previous papers in this series, the development of a general method for the determination of indium in zinc-base alloys, involving the use of the linear-sweep cathode polarograph was described' and methods for the determination of this element in "aluminium-free"" and "lead-free"" zinc alloys have been presented. l'hc first of these requires special instrumentation, not yet generally available, while the other two are not applicable to rnazak alloys (aluminium about 4%; lead up to 50 p,p,m.) in which it was desired to determine iridium down to a concentration of O.OOOI~/~ (I p.p.m.1. This paper described the development of a method applicable to mazak alloys which may bc carried out by conventional polarography. In the course of the work already reported, the use of a "collection" procedure for selective removal of indium from cadmium, employing zinc oxide, was described". This proviclecl a rapid separation of indium from all interfering elements usually found in zinc alloys, with the exception of aluminium. This element was precipitated at the pn of the "collection" (4.95) and its gelatinous nature made filtration tedious. Significant amounts of other impurities were also co-precipitated.
This method was thcrcfore confined to those zinc alloys of negligible aluminium content. No other feasible method of separating inclium could, however, be found and it was therefore decided to re-examine the "collection" procedure to ascertain if it could bc modified so that the precipitated aluminium could be used as the collector. It was also necessary to overcome the considerable difficulties caused by co-precipitation of interfering elements. In order to check the efficiency of separations easily and cluickly, radio-chemical techniques were used for the initial work as described in a previous paperr. As before, radio-active indium was used, the isotope being rrQIn of half-life 49 days, EXPERIMENTAL AND DISCUSSION A mazak alloy, of approximately the composition given in Part I of this series', was used in this work. Polarography was carried out, as previously, on three types of conventional polarograph2 at 25" and vwws the mercury pool anode. All potentials are quoted on the European sign convention. On this convention the potential of saturated calomcl electrode (S.C.E.) has been taken as +0.246 V ~~7~11s the normal hydrogen electrode (N.H.E.).