Use and Abuse of pH Measurements

Abstract

The development of the concept of pH from its invention by Sorenson in 1909 to the present-day National Bureau of Standards' pH scale is reviewed. The liquid junction potential problem involved in the use of the pH meter is discussed in detail. The magnitude of the error due to the junction potential depends upon the pH, ionic strength, nature of the solutes, nature of solvent, presence of colloids, temperature--in fact, anything that affects the mobilities of charged particles. Whether the junction potential error can be ignored depends upon the use to which the pH measurement is put. Some estimates from the literature as to the probable magnitude of this error are presented and discussed. Under certain relatively simple conditions an approximate relationship may be considered to exist between the pH meter reading and the hydrogen ion concentration. Under other conditions, the ph meter reading must be considered as simply a reproducible reference point having no theoretical significance. Under some conditions, the meter reading cannot even be considered as a reproducible reference point. These points are developed in detail.