Speciation of chromium in natural waters by micropumping multicommutated light emitting diode photometry

A simple and sensitive multicommutated flow procedure, implemented by employing a homemade light emitting diode (LED) based photometer, has been developed for the determination of chromium (VI) and total chromium in water. The flow system comprised a set of four solenoid micro-pumps, which were assembled to work as fluid propelling and as commutating devices. The core of the detection unit comprised a green LED source, a photodiode and a homemade flow cell of 100 mm length and 2 mm inner diameter. The photometric procedure for the speciation of chromium in natural waters was based on the reaction of Cr (VI) with 1,5-diphenylcarbazide. Cr (III) was previously oxidized to Cr (VI) and determined as the difference between total Cr and Cr (VI). After carrying out the assays to select the best operational conditions the features of the method included: a linear response ranging from 10 to 200 μg l−1 Cr (III) and Cr (VI) (r = 0.999, n = 7); limits of detection of 2.05 and 1.0 μg l−1 for Cr (III) and Cr (VI), respectively; a relative standard deviation lower than 2.0% (n = 20) for a typical solution containing 50 μg l−1 Cr; a sampling throughput of 67 and 105 determinations per hour for total Cr and Cr (VI), respectively, and recovery values within the range of 93–108% for spiked concentrations of the order of 50 μg l−1.