pH values of aqueous solutions of acid potassium phthalate, National Bureau of Standards Standard Sample 84a, are given for concentrations from 0.001 to 0.2 m and for temperatures between 0° and 60° C.They were calculated from the values of the two dissociation constants of o-phthalic acid and the activity coefficients of the acid phthalate and phthalate ions.The pH values range from 3.88 to 4.42.They may be computed for any temperature from 0° to 60° C, inclusive, by the equation pH=A*/T+B+CT or by the equation where A*, B, C, and k are constants whose numerical values are dependent upon the molality of the solutions, pH15 is the pH value at 15° C, and T = tOC+273.16.A 0.05-m solution of acid potassium phthalate, the one used most extensively in the calibration of pH assemblies, has a pH value of 4.005 at 25° C. Its pH value at any temperature from 0° to 60° C, inclusive, may be computed by the equation pH = 1336.08/T -5.2678+0.016069T,or by the equation pH= 4.000+ 0.0000479 (t-l5)2.The pH values of a 0.05-m solution of acid potassium phthalate prepared with Standard Samples 84a and 84b were found to agree within 0.001 pH unit at the various t emperatures.Directions are included for preparations of the solutions on a molal and a molar scale of concentration.A critique of the method of evaluating the pH values and comparisons with older values reported for 0.05-M and 0.2-M solutions are given.
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Hamer et al. (1946) studied this question.