Flow cell with double slope factor for potentiometric determination of fluoride at low concentrations

A simple double cell unit with 3 ion-selective membranes is described for continuous detection of fluoride at high throughput. The membranes were single crystal fluoride selective types composed principally of lanthanum fluoride; the cell was made from PTFE tubes and Plexiglas. The internal reference solution contained sodium fluoride and chloride with silver-silver chloride inner reference electrodes. The analyte solution, divided into 2 streams, was buffered before entering the cell in parallel with 2 streams of standard fluoride solution. This arrangement increased the slope factor and permitted a sampling rate of 120 per h. The relative standard deviation for natural waters whose fluoride concentrations were below 0.0001 mol per litre was 0.01-0.03.