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

Summary

One of the most complex establishments of standards in quality control is that for the measurement of acidity, the pH. Not only is this complexity linked with the environment or the matrix considered (solutions from chemical laboratories or industries, biophysiological fluids, sea waters, estuarine waters, freshwaters, acid rains, etc.) but also with the solvent type (water, nonaqueous solvents, aqueous-organic solvent mixtures). The results are distinct pH scales which, for each solvent considered, are articulated on one reference value standard (pH_RVS) plus a group of primary standards (pH_PS) and/or operational standards (pH_os), as specified in recent IUPAC recommendations. Such specifications ensure that the above standards be determined according to the same electrochemical principles and procedures and be accurate typically to ±0.002 in pH. However, the acquisition and availability of such standards, though rapidly expanding, are hitherto limited to a few nonaqueous solvents or aqueous-organic solvent mixtures. Within this context, the determination of pH_RVS in ethylene glycol/water mixtures, based on electromotive force measurements of the cell Pt|H₂|RVS Buffer + KCl|AgCl|Ag|Pt over a range of temperatures and solvent compositions is here described. Anyway, 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 standards, whithin acceptable reliability limits, for hitherto unexplored solvent media has been recently assessed in terms of such qualifying physicochemical quantities as solvent composition, dielectric constant and temperature. Status, applications and problems related to the above points are here analysed.