Status and problems of standardization of pH scales for controls in different media - Reference value standards in ethylene glycol/water mixed solvents

One of the most complex establishments of stds. of quality control is that for the measurement of acidity, the pH. Not only is this complexity linked with the environment or the matrix considered (solns. from chem. labs. or industries, biophysiol. fluids, seawaters, estuarine waters, freshwaters, acid rains, etc.) but also with the solvent type (water, nonaq. solvents, aq.-org. solvent mixts.). The results are distinct pH scales which, for each solvent considered, are articulated on one ref. value stds. (pHRvS) plus a group of primary stds. (pHPS) and/or operational stds. (pHOS), as specified in recent IUPAC recommendations. Such specifications ensure that the above stds. be detd. according to the same electrochem. principles and procedures and be accurate typically to +0.002 in pH. However, the acquisition and availability of such stds., though rapidly expanding, are hitherto limited to a few nonaq. solvents or aq.-org. solvent mixts. Within this context, the detn. of pHRVS in ethylene glycol/water mixts., based on emf. measurements of the cell Pt|H2|RVS buffer + KCL|AgCl|Ag|Pt over a range of temps. and solvent compns. is here described. The comparability of pH scales in different solvent media (and even in different environments) depends on the uncertain determinability of the primary medium effect upon the H+ ion. Finally, the predictability of the above stds., within acceptable reliability limits, for hitherto unexplored solvent media has been recently assessed in terms of such qualifying physicochem. quantities as solvent compn., dielec. const. and temp. Status, applications and problems related to the above points are analyzed.