Flow injection potentiometric determination of phosphate in waste waters and fertilisers using a cobalt wire ion-selective electrode

Flow injection potentiometric (FIP) determinations of dihydrogenphosphate (H2PO4–) in fertilisers and waste waters were undertaken using a cobalt wire phosphate ion selective electrode (ISE). The cobalt wire electrode was used in the FIP determination of phosphate in a carrier containing 4 × 10–2 mol l–1 potassium hydrogenphthalate at pH 5. The FIP technique was validated for fertiliser and waste water samples against standard spectrophotometric methods of analysis. The experimental data demonstrated that FIP is a highly selective and accurate technique for the determination of phosphate in fertilisers [i.e., the FIP and spectrophotometric data compare to within a few per cent (relative)]. A significant chloride interference occurs in waste water samples; however, this problem can be eradicated by using the chloride selectivity coefficient, and chloride concentrations determined independently using ion-selective electrode potentiometry. The discrepancy between corrected FIP and spectrophotometric data for phosphate in waste waters is generally ±5% (relative). X-ray photoelectron spectroscopic and electrochemical impedance spectroscopic (EIS) results suggest that the cobalt oxide surface film of the electrode is dissolved in phosphate media, facilitating the corrosion process that regulates the response of the cobalt wire electrode. It has been found that the EIS response of the cobalt wire electrode is dependent on both the pH and phosphate content of the solution. A charge-transfer reaction mechanism, and accompanying Nernst equation, have been proposed to explain the phosphate response of the cobalt wire electrode.