REPORT FOR ANALYSTS riched with alcohol. Likewise, t h e p H m e t e r demonstrates t h e high acidity of alcoholic hydrochloric acid as compared with t h e aqueous solution. I t has been claimed t h a t a compari son of acidities in pure ethanol and pure water is subject t o a liquid-junc tion error of as much as 2 p H units. Nevertheless, it is believed t h a t a de termination of relative acidity in a non aqueous medium m a y be successful, as long as t h e composition of t h e solvent does not change. If relative measure
ments of this t y p e are indeed possible, the availability of a series of reference standards for a variety of nonaqueous and mixed solvents, whose p H values were consistent with t h e aqueous scale, would link all of these measurements t o a common basis. Unfortunately, no independent practical means of estab lishing these reference values has been proposed. I n contriving such a p H scale, we would be motivated, perhaps falsely, b y a desire for formal consistency with t h e
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ANALYTICAL CHEMISTRY
procedures t h a t are successful in aqueous systems. One m u s t remember, how ever, t h a t t h e p H meter with its aque ous standards can furnish reproducible numbers in m a n y partially aqueous and nonaqueous media. A plurality of standards can be justified only if it provides results of increased usefulness. Behavior of acids and bases in water and some other leveling solvents is sub ject t o certain generalizations t h a t do not apply to all other solvents. Acidbase interactions a r e influenced not only b y t h e so-called strengths of these substances b u t b y such factors as di electric constant, association, a n d steric effects. Hence, it is unlikely t h a t a measurement of a conventional proton activity relative to water would be of great practical value. T w o acids of different structures a n d charge types would probably n o t participate t o t h e same extent in a reaction with a given base, even though t h e p H values of their solutions were identical. N o r would a single acid react t h e same a t identical p H values in different solvents, for t h e extent of a chemical reaction is deter mined b y factors t h a t do n o t necessarily influence t h e electrometric measure m e n t . Where indicator reactions are concerned, one is accustomed t o a t tribute these differences in reactivity t o salt effects and medium effects. I n view of these limitations, it seems unwise t o a t t e m p t t o relate acid-base phenomena in nonaqueous a n d partially aqueous media t o a scale which a t best has no exact theoretical meaning, or t o apply t h e t e r m p H t o these media in other t h a n its operational sense. W h e n t h e qualitative indication afforded b y t h e operational p H will n o t suffice, t h e analyst m a y resort t o t h e H a m m e t t acidity function or t o t h e electrometric function, p w H . Both functions relate t h e acidity t o t h e aqueous s t a n d a r d s t a t e ; both are free from t h e experi mental uncertainties of t h e liquid junc tion; a n d both permit a n escape from the fundamental ambiguity with which the p H value is beset. Summary T h e p H measurement is a useful tool for t h e analyst, providing h i m with re producible numbers in both aqueous and nonaqueous media. I t s interpre tation, however, in terms of hydrogen ion activity or concentration, can be a t t e m p t e d only for certain aqueous sys tems of relatively ideal composition. It should n o t be applied t o partially aqueous or nonaqueous media at the present time. Literature Cited (1) Bates, Roger G., "Electrometric pH Determinations," Chap. 4, Wilev, New York, Ν . Υ., 1954.
