Optimization of a new polymeric chromium (III) membrane electrode based on methyl violet by using experimental design

Plackett–Burman and Box–Behnken designs were applied as experimental design strategies to screen and optimize the influence of membrane ingredients on the electrode performance. A new poly(vinyl chloride) membrane sensor for Cr(III) based on methyl violet as an ionophore was planned. The major variables to find a model for achieving the best Nernstian slope as response were: PVC, plasticizers, methyl violet, KpClTPB, pH, conditioning time and internal solution concentration. Plackett–Burman design was used to screen the main factors and Box–Behnken response surface was led to find a model for optimizing the response. The optimized membrane electrode shows a Nernstian slope for chromium (III) ions over a wide linear range from 1.99 × 10−6 to 3.16 × 10−2 mol L−1 and a slope of 19.5 ± 0.1 mV decade−1 of activity. It would be successfully applied in the pH range from 3.5 to 6.5 with detection limit of 1.77 × 10−6 mol L−1 (0.092 mg L−1). The response time of the sensor is about 8 s and the membrane can be used for more than 6 weeks without any deviation. The relative standard deviations (R.S.D.) for six replicate the measurements of 1.0 × 10−4 and 1.0 × 10−3 mol L−1 of Cr(III) were 3.2 and 3%, respectively. The electrode revealed comparatively good selectivity with respect to many cations including alkali earth, transition and heavy metal ions. The electrode was successfully used as an indicator in the potentiometric titration of Cr(III) with EDTA and was also applied to the direct determination chromium (III) content of spiked water and soil samples.