Direct detection of acidity, alkalinity, and pH with membrane electrodes.

An electrochemical sensing protocol based on supported liquid ion-selective membranes for the direct detection of total alkalinity of a sample that contains a weak base such as Tris (pK(a) = 8.2) is presented here for the first time. Alkalinity is determined by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane-sample interface depletes owing to diffusion limitation is related to sample alkalinity in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alkalinity (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ determination of these characteristics in complex samples such as natural waters.