Central Composite Design Applied to the Optimization of a Triiodide Polymeric Membrane Electrode based on Triiodide-Piroxicam Ion Pair

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In this study, the application of response surface methodology (RSM) and central composite design (CCD) for modeling and optimization of the influence of some variables on the performance of a new PVC membrane triiodide ion-selective electrode (ISE) is discussed. This triiodide sensor is based on triiodide-piroxicam ion-pair complexation. Four main variables, namely, the PVC, plasticizers, ion pair amounts, and pH value were changed during CCD. A total of 27 experiments (27 electrodes) were designed. The electrodes were prepared by incorporating the ion-exchanger in a PVC matrix plasticized with 2-nitrophenyl octal ether without any additives. The slopes of the calibration curves of the constructed electrodes were collected as the response. In order to optimize the conditions, mathematical model equations were derived by computer simulation programming applying the least squares method. The equations that were second-order response functions representing the slope of the calibration curve were expressed as functions of four operating parameters of electrode fabrication. In order to gain a better understanding of these variables effects on electrode performance, the models were presented as 3-D response surface graphs. The influence of foreign ions on the electrode performance was investigated. The optimized membranes demonstrate Nernstian response for triiodide ions over a wide linear range from 1.0 × 10−2 to 5.0 × 10−6 mol L−1 with a limit of detection of 2.0 × 10−6 mol L−1 at 25°C. The electrodes could be used over a wide pH range, 3–8, and have the advantages of ease of preparation, good selectivity, fast response time, long lifetime (over 3 months), and small interferences from hydrogen ion.