I would perform an oxidizing digestion (melting the sample with sodium-/potassium carbonate and sodium peroxide) to oxidase the sulfide to sulfate and the organic material to CO2. An aliquote of the dissolved melt id acidified with sulfuric acid and hydrogen peroxide is added. The emerging Chromium(VI) oxide peroxide is extracted into diethyl ether, butan-1-ol or amyl acetate and the blue color can be photometrical measured during the first hours. Of course the exact SOP has to be established before, depending on the concentration range and involved dilutions.

Karl, wouldn't your procedure oxidize Cr3+ to Cr6+?

Do not change the pH of the sample, this will initiate transformations. Use EPA method 218.7 to prepare the complexing reagent- 2 mM of 1,5-diphenylcarbazide. React this reagent with hexavalent chromium and determine its concentration calorimetrically at 530nm. ICP should be able to give you the total Cr metal in your sample. The difference would be an exact measure of trivalent chromium.

You could easily determine the amount of Cr6+ and Cr3+ by X-ray absorption spectroscopy: Cr6+ gives rise to a pronounced pre-edge peak, Cr2O3 (or trivalent Cr in general) does not. So by a quantitative evaluation of the near edge region, you are able to measure both Cr6+ and Cr3+, and even also Cr(0) if present! See the attachment for more details - this is from an older publication of our group, where we studied the decomposition of hexavalent Cr6+ with time resolution. In principle, this can be done with your samples as well!

Best regards, Dirk

Yes, evaluate the pre-edge peak in XANES spectra. Preparate Cr-III and Cr-VI standards and just interpolate between the pre-edge-intensities to quantify your unknown samples.

Erik

Try this one,

USEPA Method 3060 is an alkaline digestion procedure for extracting hexavalent chromium [Cr(VI)] from soluble, adsorbed, and precipitated forms of chromium compounds in soils, sludges, sediments, and similar waste materials

Also, do not use oven to dry your sample since it is reported that sample dried at temp 105 C will lead to more loss of Cr(VI) compare to air dried (Environ Monit Assess (2014) 186:375–382)

ICP (or atomic absorption) measure all kind of chromium together (Cr(III) and Cr(VI).

If you have no Cr(III), this is the simplest way.

If you have chromium(III) and chromium(VI) together, I suggest removing the chromium(VI) content (chromates, dichromates and polychromates together ) with a tertiary anion exchanger resin. The chromium(III) content can be measured in the eluate. In the next step you can elute the fixed chromium(VI) with a strongly acidic non-complexing reducing agent (e..g sulphites/sulfuric acid mixt.), as chromium(3+), and the chromium can be measured with ICP in this 2nd eluate.

If you have no ICP, the Cr(III) content in the two eluates can be measured in other way, e.g. with complexometry as well.