Determination of chromium(vi) in natural waters by the sorption of chromium-diphenylcarbazone with xad-2 resin

The sorption of the chromium(III)-diphenylcarbazone complex (Cr-DPC) with XAD-2 has been investigated, for use in the separation of Cr(VI) from Cr(III) species in natural waters. Cr-DPC is formed from the reaction of Cr(VI) with diphenylcarbazide, but Cr(III) species give no reaction in aqueous solution. The addition of sodium chloride or sodium beta-naphthalenesulphonate markedly enhances the sorption. The Cr-DPC sorbed on XAD-2 can be almost completely eluted with organic solvents, especially methanol, but about 5% of it is irreversibly sorbed and cannot be eluted. The excess of diphenylcarbazide and some of the organic matter in natural waters are also sorbed on XAD-2 but most of this can also be eluted with methanol. Organic matter which interferes with measurement of the absorbance of Cr-DPC can almost all be removed by extraction with chloroform. By use of these techniques, Cr(VI) in sea-water has been determined by the standard-addition method. Although about 50 litres of sea-water are necessary for the analysis, organic and colloidal Cr(III) species do not interfere.