The Modern Meaning of PH

INTRODUCTION A. Background "Plus ça change, plus c'est la même chose." In a review article published more than 30 years ago, the present author stated " … several scales, all masquerading under the name pH, are in common use. Many investigators are thinking and computing in terms of one definition and measuring a different quantity."1 An improvement toward an understanding of the nature of the practical pH value has been seen in the last three decades, but the precise manner of interpreting these values is still largely a mystery. To emphasize this situation, one has only to survey a dozen modern texts used in the teaching of analytical chemistry. The results of such an examination are summarized in Table 1. As observed earlier … "this state of confusion results in part from the impossibility of determining exactly … the molality and the activity of hydrogen (hydronium) ion for a buffer solution of moderate concentration."1