Continuous-flow determination system based on null-point potentiometry using a nitrate ion-selective membrane

A new automated potentiometric analysis system for nitrate was developed. This system is based on the principle of null-point potentiometry. On either side of a nitrate ion-selective PVC membrane were the flowing sample and standard nitrate solutions. The concentration of the latter was automatically matched with that of the sample solution by controlling the dilution rate of a basic standard solution so that the membrane potential reached the `null-pointʹ. The determination without using a calibration graph was possible. The precision of five independent determinations was within 8% in the concentration range above 1 × 10−5 mol dm−3. This system was applied to rain water analysis.