Metformin‐selective polyvinyl chloride (PVC) membrane electrode based on the metforminium phosphomolybdate ion pair

The (1:1) (phosphomolybdate : metforminium) ion pair was used as the ion exchanger in preparing conventional plastic membrane electrodes for metformin (Mf) and their performance characteristics were studied. The electrode exhibited a linear response with a Nernstian slope over a relatively wide concentration range. The calibration-plot slope was constant for up to 5 days with continuous soaking at 57 mV/concentration decade at 25°C; it subsequently decreased as the time of soaking increased, reaching 42mV/decade after 20 days. The change in pH did not affect the electrode performance within the range 3.0–10.5. The standard electrode potentials were determined at different temperatures and were used to calculate the isothermal coefficient of the electrode (0.000695 V/°C). The electrode displayed good selectivity for Mf with respect to a large number of inorganic and organic cations. The standard addition method and potentiometric titration were used to determine Mf in pure solutions and in pharmaceutical preparations.