Measurements were made of the potential difference between hydrogen electrodes and silver-silver-chloride electrodes in aqueous solutions of 72 different phthalate-chloride mixtures at 13 temperatures from 0° to 60° C. By described experimental and mathematical procedures, the second dissociation constant of o-phthalic acid and related thermodynamic quantities were evaluated, and pH values were assigned to 217 solutions, including some containing no potassium chloride. The addition of the chloride lowers the pH of the solutions principally because of an increase in the ionic strength.The second dissociation constant may be computed at each temperature by the equationwhere T=t °C+273.16.The pH values of the solutions may be computed for ratios of dipotassium phthalate to acid potassium phthalate from 1 to 2, and for ionic strengths, /10, from O. to 0.5, by the equation• V /10 /10, in which A and B are constants dependent upon the temperature, and {3 is a constant dependent upon the temperature, salt concentration, and buffer ratio. The addition of potassium chloride lowers the pH of the solutions in accordance with the equation pH=(pH)o+almKCI+a2m2KCl+a3mSKCl, where (pH)o is the value at any temperature for a solution containing no potassium chloride and al, a2, and aa are constants.These solutions with or without potassium chloride are useful as pH standards from 4.87 to 5.72 and vary only slightly with changes in temperature. They have good buffer capacity and may be readily prepared from pure materials. The NBS Standard Samples 84a and 84 b of acid potassium phthalate or Standard Samples of comparable purity are recommended for preparation of the solutions/ together with carbonate-free potassium hydroxide of high grade and distillea water of pH 6.7 to 7

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Measurements were made at 5-degree intervals from 0° to 60° C of the potential difference between hydrogen electrodes and silver-silver-chloride electrodes in 45 aqueous solutions that contained various amounts of a-phthalic acid, acid potassium phthalate, and potassium chloride. The first dissociation constant of a-phthalic acid and related thermodynamic quantities were evaluated, and pH values were assigned to each of the 45 solutions.The The separation of the carboxyl groups in a-phthalic acid was found to be 1.66 A by means of an equation r elating this distance with the ratio of the two dissociation constants of a-phthalic acid. This value is higher than 1.53 A obtained from X-ray measurements of the carbon-carbon distance in the diamond and 1.43 in the benzene molecule.

A method is described to differentiate between completely and incompletely dissociated acids by means of measurements of the emf of galvanic cells without liquid junctions. Hydrogen and silver-silver-chloride electrodes and solutions of the acid, its sodium or potassium salt, and sodium or potassium chloride are used. The activity coefficients of hydrochloric acid in such mixtures are calculated by the equation relating the emf to the activity of hydrochloric acid in known concentrations of the acid and salts. If the mean values of the activity coefficient of hydrochloric acid in the mixtures are higher than those predicted by the limiting law of Debye and Hiickel, the acid is then considered to be completely dissociated. If they are lower, the acid is incomplet ely dissociated. By this method it was found that the sulfonic acid group in p-phenolsulfonic and 4-chlorophenol-2-sulfonic acids may be regarded as completely dissociated into sulfonate and hydrogen ions at temperatures of 10°, 25°, 40°, and 60° C.. Solutions containing p-phenolsulfonic acid, its sodium or potassium salt, and sodium or potassium chloride and solutions conta ining 4-chlorophenol-2-sulfonic acid, its sodium salt, and sodium chloride are s uitable for use as pH standards. Their pH values range from 1.2 to 2.5 and vary only slightly with temperature changes. Directions for their preparation a re given.The significa nce of t he complete dissociation of the sulfonic acid group in the calculation of the ioniza tion constants and pH values of sulfonate buffers and indicators is di scussed in t erms of the t automeric relations and the theory of color changes of indicat ors.