Chromium chelation with diphenylcarbazone

Diphenylcarbazone and diphenylcarbazide have been widely used for the spectrophotometric determination of chromium [ 190]. Crm reacts with diphenylcarbazone whereas CrVI reacts (probably via a redox reaction combined with complexation) with diphenylcarbazide [ 191 ]. Although speciation would seem a likely prospect with such reactions, commercial diphenylcarbazone is a complex mixture of several components, including diphenylcarbazide, diphenylcarbazone, phenylsemicarbazide, and diphenylcarbadiazone, with no stoichiometric relationship between the diphenylcarbazone and diphenylcarbazide [192]. As a consequence, use of diphenylcarbazone to chelate Crm selectively also results in the sequestration of some CrVI. Total chromium can be determined with diphenylcarbazone following reduction of all chromium to Crm. 

In this reaction, diphenylcarbazide is oxidized to diphenylcarbazone, while at the same time chromium(VI) is reduced to chromium(III), which forms a chelate complex with diphenylcarbazide. Like the metal-PAR complexes, it is red-purple colored and thus may be photometrically detected at 520 nm. As already mentioned, chromate separation is performed by means of anion-exchange chromatography. The high selectivity of this technique is revealed in Figure 8.51, which shows the analysis of chromium(VI) in a flue gas scrubber solution. Despite high contents of chloride and sulfate, chromium(VI) can be determined in this sample without any interferences. 

This explanation is, however, incomplete since, when the colored reaction product is extracted into isoamyl alcohol or chloroform in the presence of perchlorate, the remaiiung colorless aqueous phase contains half of the chromium (Lichtenstein and Allen, 1959, 1961 Sano, 1962). When studying the reactions of diphenylcarbazide and diphenylcarbazone with various metal cations, Balt and van Dalen (1961) found that diphenylcarbazide only forms metal chelates after its oxidation to dipheitylcaiba-zone. 

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