Hydroxyapatite-based electrode: a new sensor for phosphate

Hydroxyapatite, prepared by precipitation from a calcium hydroxide suspension with phosphoric acid at 60 °C and a pH of 9, was used to develop a potentiometric sensor for phosphate. The electrodes were made in two ways: firstly, by mixing hydroxyapatite powder with an epoxy polymer and graphite; and secondly, by sealing porous hydroxyapatite sticks to the extremity of a glass tube. The first sensor showed a very small linear range of response (6.0 × 10–5 to 2.0 × 10–4 mol l–1) to log [phosphate] at pH = 5.0, with a slope of 55 mV decade–1 of phosphate concentration and a detection limit of 4.0 × 10–5 mol l–1. The second showed a higher linear response range from 5.0 × 10–5 up to 5.0 × 10–2 mol l–1, with a slope of 33 mV decade–1 and a detection limit of 2.5 × 10–5 mol l–1, under the same conditions. The response time was about 25 s in the first case and 40 s in the second. The effects of interfering ions were investigated and the Kpot values for Cl–, NO3–, SO42–, ClO4–, acetate and citrate were determined, showing only a small interference with the electrode response. The powder electrode is not stable but the stick has good stability.