pH Standards of High Acidity and High Alkalinity and the Practical Scale of pH

The practical scale of pH is defined in terms of the electromotive force of the galvanic Pt; H2 (g), solution | satd. KC1 | standard, H2 (g); Pt. This potential is usually obtained as the difference of two electromotive force values for a cell with glass and calomel electrodes, one of which is a calibration with a standard of known pH. Appropriate corrections must be applied if the glass electrode does not respond to changes in hydrogen-ion activity in exactly the same manner as the hydrogen electrode. However, there is no simple means of correcting pH measurements for the potential differences at the junctions of the solution and the standard with the solution of potassium chloride. These errors are sufficiently large in highly acid and highly alkaline solutions to render uncertain the interpretation of measured pH in these regions, in spite of the fact that reasonably accurate standards of hydrogen-ion activity are available at intermediate pH values. The purpose of this study was twofold: (a) to determine the extent of aberration of the practical pH scale near its ends, and (b) to select new standards that might improve the accuracy of pH measurements and facilitate their interpretation over the entire practical scale, with particular attention to the regions of high acidity and high alkalinity. The results indicate that pH obtained by adjustment of the meter with the present standards (phthalate, pH 4.01 at 25° C; phosphate, pH 6.86; borax, pH 9.18) will usually be low by at least 0.02 to 0.05 unit above pH 11, while errors as great as 0.03 unit, either positive or negative, are not uncommon below pH 2.5. The following additional standards were selected to supplement the three presently available: (1) 0.01-M potassium tetroxalate— pH 2.15 at 25° C; (2) potassium hydrogen tartrate (saturated at room temperature)—pH 3.56; (3) 0.025-M sodium acid succinate, 0.025-M sodium succinate—pH 5.40; (4) 0.025-M sodium bicarbonate, 0.025-M sodium carbonate—pH 10.02; and (5) 0.01 M trisodium phosphate—pH 11.72. The choice was based on a comparison of pH derived from cells with and without liquid junction in a study of 41 promising standard solutions. The pH on the practical scale was determined at 25° C, and electromotive force measurements of hydrogen-silver chloride cells without liquid junction were made at 0°, 10°, 25°, and 38° C.