Colourimetry

A sample is preserved in the field with mineral acid. The sample aliquot is mixed with diphenylcarbazide in an acid solution. A red-violet colour is produced. (The reaction is very sensitive as the absorbancy index per gram of chromium is approximately 40,000 at 540 nm). The absorbance is measured and compared to identically-prepared standard and blank solutions, ensuring the blank is subtracted, at a wavelength of 540 nm.
Note: If the solution is turbid, take an absorbance reading before adding the diphenylcarbazide reagent and correct the absorbance reading of the final coloured solution.