Field analysis of Cr(<scp>vi</scp>) in water samples by using a smartphone-based ultralong absorption path reflection colorimetric device

Hexavalent chromium is highly toxic and regarded as one of the main pollutants in the environment. Sophisticated laboratory instruments are commercially available for the in-field determination of chromium. It is highly desirable to develop affordable, portable, sensitive and reliable methods for on-site Cr(VI) detection. For field analysis, we designed a smartphone-based absorbance device with an ultralong absorption path to improve sensitivity. The intensity of the transmission light was detected by the illumination sensor in the smartphone. The absorption path was enlarged to a maximum of 83.5 cm by 41 reflection cycles. The intensity of the reflected light in an empty colorimetric cell can be serviced as the internal references to correct the possible baseline drift due to light source intensity decay. The absorbance measured by the proposed device obeys the Lambert–Beer law in the range of 0–1.1. This device was used to determine Cr(VI) in water samples on-site by using 1,5-diphenylcarbazide as a colorimetric reagent, with the linear range of 1–18 μg L−1 and limit of detection of 0.26 μg L−1. This simple absorbance device offers the advantages of cost-effectiveness, ease of implementation, portability, sensitivity and reliability for field analysis.