pH values of acid-salt mixtures of some aromatic sulfonic acids at various temperatures and a criterion of completeness of dissociation

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

Section: Complete Methodsmentioning

confidence: 93%

Section: Complete Methodsmentioning

confidence: 99%

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

Wj¹,

Sf²

1945

“…If a equals the molality of the acid and b the molality of added alkali, then for any mixture of the two (13) (14) By the use of these equations and eq 3 and 8, eq 15, (15) (16) obtained by the elimination of mH2Ph, mHPh, and mph in eq 3, 8, 10, and 11, is satisfied for the solutions of any composition and concentration. However, solution of eq 15 requires a knowledge of mH, the activity coefficients, K …”

Section: Complete Methodsmentioning

confidence: 99%

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

Wj¹,

Gd²,

Sf³

1945

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.

“…The "cutoff" positions of many buffer compounds of about O.OI-M concentration have already been determined in l-cm absorption cells. 4 In order to prevent distortion of the spectrophotometric curves by specific salt errors, it is desirable to keep the ionic strength of each buffer as low as possible, without sacrifice of buffering capacity. In earlier published work on the acid range of metacresolsulfonphthalein in the visible region [8], it was shown that the intermediate and complete transformation of this indicator could be accomplished simply by the use of solutions of hydrochloric acid of concentrations ranging from about 10-3 to 2 M. Provisional measurements of several other indicators in the visible and in the ultraviolet have now been obtained by use of phthalates, benzoates, citrates, phosphates, and phellolsulfonates, in addition to hydrochloric acid and sodium or potassium hydroxide.…”

Section: Selection Of Buffersmentioning

confidence: 99%

“…The dissociation of the two groups of p-phenolsulfonic acid has also been recently studied by electromotive-force methods [3,4]. The esters of p-hydroxybenzoic acid are representative of compounds difficulty soluble in water.…”

mentioning

confidence: 99%

Ultraviolet spectra and dissociation constants of p-hydroxybenzoic acid, methyl, ethyl, n-butyl, and benzyl p-hydroxybenzoate and potassium p-phenolsulfonate

Ee¹,

Mr²

1945

The apparent first and second dissociation constants of p-hydroxybenzoic acid, the constants for the phenol group of methyl, ethyl, n-butyl, a nd benzyl p-hydroxybenzoate, and potassium p-phenolsulfonate have been determined from ultraviolet spectral data. The compounds have wide absorption bands located in the region of 200 to 400 mIl, and the differences between the transmittancy curves for their ionic and molecular forms are great enough to p ermit the measurement of several intermediate steps in the transformation at various pH values. The buffers used to · control the dissociation were acetate/3, phosphates, and borate's, all of low ionic strengths. Esterification of t he carboxyl group of p-hydroxybenzoic acid apparently increases the dissociation of the phenol, and pK* decreases from 9.3 for the second dissociation of the acid to 8.3 for the esters. The thermodynamic dissociation constants are estimated.