Development of a cobalt electrode for the determination of phosphate in soil extracts and comparison with standard methods

A phosphate-sensitive cobalt electrode was evaluated in detecting orthophosphate ions (H₂PO^-₄) in ammonium lactate–acetic acid soil extracts. The dependence of the mixed potential of a cobalt electrode on H₂PO^-₄ concentration was investigated through potentiometry. The mechanism of detection is based on the consumption of a surface cobalt (II) oxide layer to form (Co₃(PO₄)₂), which leads to a concentration-dependent shift of the mixed potential. Two reference electrodes were evaluated: Ag/AgCl (3 M) KCl and a platinum (Pt) wire. A linear response was observed using both reference electrodes. However, application of a Pt wire quasireference electrode increased the linear dynamic response range of the detector from 10–10³ mg L⁻¹ or 10⁻⁴–10⁻¹ M (Ag/AgCl (3 M) KCl) to 0.1–10⁵ mg L⁻¹ or 10⁻⁶–10¹ M. In addition, the response time using the Pt wire was less than 5 min compared to a minimum of 10 min using Ag/AgCl (3 M) KCl. There was close agreement between the response of the phosphate-sensitive cobalt electrode with a standard colorimetric method. As dissolved organic substances can potentially interfere with electrochemical techniques, an investigation into the use of a nonpolar resin for decolorization and removal of organic matter in soil extracts was performed and successfully used. The phosphate-sensitive cobalt electrode was found to be a fast method for the analysis of soil extracts with high sensitivity and selectivity. It has the potential to be developed into a sensor for the in situ measurement of phosphate in various environmental matrices.