Determination of Hexavalent Chromium in Water Samples by Digital Image Colorimetry

50 mL of water sample, 0.5 mL of 50% (volume fraction) sulfuric acid solution, 0.5 mL of 50% (volume fraction) phosphoric acid solution and 2.0 mL of 2.0 g·L^-1 DPCI solution were taken and placed into a 125 mL separating funnel, and the mixture was shaken well. After the solution was kept under steady condition for 5 min, 2 mL of 5.0 g L^-1 sodium dodecyl sulfate solution together with 2 mL of n-butanol was added. After shaking, 5 mL of chloroform was added, and the solution was shaken vigorously for 2 min to form microemulsion. The mixture was kept under steady condition to to have the purple complex formed by DPCI and hexavalent chromium under acidic conditions extracted to the chloroform. The bottom chloroform phase was transferred to the sample cuvette, and a self-built digital imaging colorimetric devicethe was used for digital image colorimetric analysis. A smart phone was selected to collect the photos of the detection area, and G values of the R (red), G (green), and B (blue) values of the analyte picture given by Color Grab software was used to quantify the hexavalent chromium in the water sample. As shown by the results, the linearity range between the mass concentration of hexavalent chromium and the G value of the chloroform layer was found in the range of 0.010-0.160 mg·L^-1, with detection limits (3s/k) of 0.008 mg·L^-1. This method was used for the determination of hexavalent chromium in water samples, and values of RSDs (n=10) were found in the range of 1.0%-4.2%. The spiked recovery test was made, giving recoveries ranged from 95.0%-97.5%. The proposed method was used to analyze the spiked water samples, and consistent results were found for this method and the standard method.