Differential flow-injection potentiometry with double sensitivity using one ion-selective membrane

Differential flow-injection potentiometric determinations of nitrate with one ion-selective membrane fixed in a symmetrical flow-through cell and one reference electrode on each side of the membrane are described. The measured potential depends on the difference of the measuring ion activity on both sides of the membrane only. By connecting both sides of the flow-through cell with a tube and the sample contacting both sides of the membrane subsequently, double sensitivity is achieved. By varying the length of the connecting tube either separate or overlapping peaks are observed. As only the difference of the peak maxima is necessary for the determination of the concentration, this analytical method shows good reproducibility and is independent from drift or noise of the baseline. In this way, the nitrate content of tap water and spinach is determined and compared to ion-chromatography.