Picrate ion determination using a potentiometric sensor immobilized in a graphite matrix

The characteristics, performance, and application of an electrode, namely, Cu|Hg|Hg2(Pic)2|graphite, where Pic stands for picrate ion, are described. This electrode responds to picrate with a sensitivity of 56.8 ± 0.5 mV/decade over the range 2.5 × 10−5 to 1.0 × 10−1 M at pH 5.7-8.0 with a detection limit of 1.3 × 10−5 M and an ionic strength of 0.5 M, adjusted with NaClO4. The inverse solubility product (Ks−1) for picrate precipitates with a range of cations was also estimated. The electrode shows easy construction, fast response time (about 25 s), low cost, and excellent response stability (lifetime > 6 months, in continuous use). The proposed sensor shows high selectivity towards picrate ion over many hydrophilic and lipophilic anions and exhibits a non-Hofmeister selectivity sequence, which is an improvement over most of the methods reported so far. The electrode was successfully applied to the potentiometric determination of picrate ions and indirect determination of some pharmaceuticals such as quinidine through precipitation reactions with picrates. The results obtained by using this electrode compared very favorably with those given by the official standard methods.