pH standards at various temperatures; aqueous solutions of acid potassium phthalate.

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,