rodate acts LIS a strong oxidising agent in acid medium and its reduction proceeds cithcr to iodide, iodine, or unipositivc iodine state, depending upon the acidity conditions'. The quantitative reduction of ioclatc to iodide is difficult to attain. However, the prcscncc of mercuric ions facilitates this reduction to form a stable complex,
[J-Jig LP, at low acidity%. The complex is dccomposcd at high acidity and the liberated iodide is oxidiscd to iodine and then to unipositivc iodine by excess of iodate added. Conscqucntly, in the titration of a rcductant such as As(III) with ioclatc in acid medium and in the presence of mercuric ions two ond-points are possible, the first corresponding to the reduction of ioclate to iodide at low acidity and the second corresponding to the conversion of iodide to the unipositivc iodine stage at a comparatively high acidity, As the re'duction of iodatc proceeds to iodide, a red coiour of iodine is gradually dcvcloped and its intensity increases to maximum half way through the titration and again falls to a ln~nimum value as this iodine is reduced to iodide. this conversion being facilitated by the binding of iodide by J-Q -tz ions. At high acidity, as the mercuric iodide complex is decomposed and the liberated iodide is oxidisccl by excess iodatc, there is again a devclopmcnt of iodine colour, which, as in the former cast, grows in intensity and subsequently falls to a minimum constant value corresponding to the iodine monochloride end-point.
The. present investigation was carried out in order to utilisc these observations for the ~pcctrop~~otometric titrations of various reductants, such as arscnic(iII), hydrazinc, thiosulphatc ancl thiocyanate, by iodate procedures. The detcrminationof arsenic was carried out both in hydrochloric ancl sulphuric acid medium, while the titrations of hydracine, thiosulphatc and thiocyanate were found to bc feasible only in the hydrochloric acid medium. Direct titration of thiocyanate with iodate was inconveniently slow bo Ihc indirect proccclurc was adopted using arsenious oxide for back-titration of'tha cxccss iodate. 'The titrattons wore carrtcd out in a Hilgcr Spckkcr nbborptiomcter unit (F.-f 7Go) with a bltrc filter havmg a maximum transmlsslon m the range of q50-500 rnp. Alquots of the todatc sofution (2 to G ml) \vcrc taken in the Iquitl ccl1 and sufficicnt.conccntratr.4 hydrochloric acid was adcled to mzuntain the norrnahty hctwccn I .5 to 2.5. About 2 ml of mercuric chloride solutton (saturated at room tcmpcrnturc) wxc also nddcd The reductant solution was lntroducod from a so-ml mlcroburctte and the progress of the titration was followed with the absorptiomctcr. The second end-pomt was obtamcd at it comparattvelp high acidity whrch was maintatnccl by adding sufficient hydrochloric acid at the first end-point to ~ncreasc the mlrmahty from 2 ta 6, Direct titrations of the ioclntc: wcrc thus carrred out with arscmc(IIf), thzosulphatc and hydrazrnc, References p.