pH standards at various temperatures: aqueous solutions of acid potassium phthalate

Abstract

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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