Hydroxyapatite-based electrode: A new sensor for phosphate

Hydroxyapatite, prepared by precipitation from a calcium hydroxide suspension with phosphoric acid at 60 degrees 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 x 10(-5) to 2.0 x 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 x 10(-5) mol l(-1). The second showed a higher linear response range from 5.0 x 10(-5) up to 5.0 x 10(-2) mol l(-1), with a slope of 33 mV decade(-1) and a detection limit of 2.5 x 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 K-pot 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.