Second dissociation constant of omicron-phthalic acid and related pH values of phthalate buffers from 0 to 60-degrees C

Abstract

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

First dissociation constant of o-phthalic acid and related pH values of phthalate buffers from 0 degrees to 60 degrees C

“…For example, Scatchard, Hamer, and Wood [2] by means of isotonic measurements obtained values of 1.0146, 0.9918, and 1.0026, respectively, for the activity coefficient of sucrose, urea, and glycerol of 0.1 molality. Extrapolation of their data gives values of 1.0029, 0.9984, and 1.0005 or an average value of 1.0006 for a O.OlS-molal solution, the highest molality of a-phthalic acid used in this work. Hence, the assumption that the activity coefficient of undissociated a-phthalic acid is unity does not introduce an appreciable error in the final results.…”

“…VIII. A previous publication [1] 1 gave values of the second dissociation constant of a-phthalic acid, of related thermodynamic quantities, 1 Figures in brackets indicate the literature referenccs at the end of this paper. 539 and of the pH values of solutions containing add potassium phthalate and dipotassium phthalate with or without potassium chloride at intervals of 5° from 0° to 60° O.…”

Sulfonyl Chloride Kinetics. Part II. Solvolysis of a Series of Benzenesulfonyl Chlorides: An Unexpected Heat Capacity Variation

“…The consequence of the temperature effect on the above temperature dependent competition is clearly illustrated by a consideration of the change in ACpO for AH,' and AH,' as illustrated in the case of o-phthalic acid (59,60). Thus the ACpO value for the second ionization constant for a dibasic acid is normally much more negative than for the first ionization constant, e.g.…”

Resolution of the dissociation constants of d,l-malic acid from 0-degrees-c to 50-degrees-c

“…The closely related function P , h owever, possesses the thermodynamic validity that ag lacks. With the further substitution of ag= P/fCh one obtains which may b e written (12) where (13) and (14) and in which the activit:"T coefficient Lerms are (15) and…”

“…The dissociation constants of a number of weak dibasic acids have nonetheless been determined through variations of this general procedure [6,10,11,12,13] . In this way the dissociation constants of o-phthalic acid (K d K 2= 288) have been resolved [12,13] and the second dissociation constants of succinic acid (K d K 2= 27 ) [141 and d-tartaric acid (K d K 2= 21 ) [151 and the third dissociation constant of citric acid (K 2/K 3= 43 ) [16] have been determined. In the last three instances a buffer ratio, m (neutral salt)/m (acid salt), of 3 was effective in keeping the hydrogen ion concentration low and in reducing the magnitude of the corrections for dissociation equilibria other than the one of primary concern.…”