A STUDY OF THE HYDROXIDE ION INTERFERENCE ON SEVERAL FLUORIDE ION-SELECTIVE ELECTRODE MEMBRANES

A study of the response characteristics of some fluoride selective ele ctrodes based on three lanthanide fluoride single crystal membranes (L nF(3); Ln = La, Ce and Nd) is reported. There is little difference bet ween the response of these three membranes in buffered fluoride soluti ons except that CeF3 shows a slightly faster response and a slightly l ower detection limit. The effect of the hydroxide ion on the response to fluoride is examined and is shown to be particularly severe for the CeF3 membrane. It is demonstrated that hydroxide ion interference on these membranes is due to the formation of hydroxo-complexes which for m within a gel layer at the membrane surface. A mechanism is proposed for the response of the electrodes to the hydroxide ion.