Comments on potentiometric pH titrations and the relationship between pH-meter reading and hydrogen ion concentration

Abstract

It is emphasized and shown that the concept of pH is more complicated than might be thought (and to some extent also unsatisfactory); there are (at least) three pH scales in general use and it is the aim to make this fact recognized. These scales are: (1) an activity scale, where the hydrogen ion activity is measured based on NBS or similar standards by carefully eliminating the liquid-junction potentials of the electrode system via experimental determinations; (2) a practical scale, which has unintentionally developed by convenience over the past ca. 30 years, is based on now generally available combined glass electrodes together with NBS (or related) buffers used for calibration; and (3) a concentration scale which uses strong acids and/or bases for calibration and defines the pH-meter reading in terms of −log[H +]. Scale (2) is clearly the one least well defined, yet it is also the one most widely used. If a ‘pH’ is measured for a given constant H + concentration in the three scales, its value decreases in the order (1) > (2) > (3). Scales (1) and (3) may be converted into each other by using the single ion activity coefficient of H +, e.g., at 25°C and at ionic strengths of 0.1 and 0.5 M the differences correspond to 0.11 and 0.15 log unit, respectively. The conversion term from scale (2) to (3) corresponds at 25°C and an ionic strength between 0.1 and 0.5 M to about 0.03 log unit. It is evident that any acidity constant, i.e. p K a value, determined for a given system (HA ⇌| A − + H +) is affected to the same extent; hence, the mentioned conversion factors have to be applied if P K a values determined in different scales are to be compared or used. It may be added that many workers believe that combined glass electrodes measure the hydrogen ion activity and that they are working in sale (1), yet this is not the case, they are actually working in scale (2). Moreover it is also barely (or not at all) recognized that the values in scale (2) are in fact closer to those of scale (3) and not to those of scale (1), as is often assumed. Some general comments regarding potentiometric pH titrations and the determination of equilibrium constants (i.e., p K a values and stability constants of metal ion complexes) are also made, and the advantages of different titration procedures are discussed and pitfalls are pointed out.