Hexavalent chromium quantification in solution: Comparing direct UV-visible spectrometry with 1,5-diphenylcarbazide colorimetry

Quantification of Cr(VI) in an aqueous solution is conducted by direct UV-visible spectrophotometry based on the yellow coloring of the chromate ion. Measurements show that absorption follows the Beer-Lambert law over a wide range of concentrations. At pH below the pK(a) of 6.4 (HCrO4-/CrO4-2), the absorption maximum lies at 350 nm wavelength and the linear range spans from 0.5 to 100 mg Cr(VI)/L; above the pK(a) (pH 6.4), the absorption maximum is 373 nm and linearity occurs in the range of 0.5-25 mg/L. The wide range of validity of the Beer-Lambert law is advantageous for the measurement of concentrated samples. The standard method of analysis of aqueous Cr(VI) is by colorimetry with the 1,5-diphenylcarbazide (DPC)-Cr(VI) complex. This method, although very sensitive, bears a narrow range of linearity from 0 to 0.8 mg Cr(VI)/L. It is shown that when analyzing Cr(VI) solutions with concentrations in the range of 30-500 mg/L, the DPC method gives inaccurate results and relative standard deviations of 20-50%. This is due to high dilution factors. On the contrary, the direct method performs with high accuracy. Relative standard deviation is only 0.5% at 500 mg Cr(VI)/L. The direct method is fast, reliable, and nondestructive for the sample. The direct method is recommended for the quantification of Cr(VI) at concentrations greater than 1 mg/L. (C) 2018 Academie des sciences. Published by Elsevier Masson SAS.