Basic ionization constant of metacresolsulfonphthalein; pH values and salt effects

Abstract

Spectral transmittancy values at room temperatures approximating 25° C were obtained for 1.25 X 10-5 M metacresolsulfonphthalein in water and in 0.001-to 8-M hydrochloric acid. In water the indicator exists in the yellow form (Y), and hydrogen ions (H) from acids convert it into the red form (R). The molar concentrations of each form of the indicator in various concentrations of hydrochloric acid were computed from the spectrophotometric data. The dissociation constant (Kb) for the reaction of the indicator with hydrochloric acid is given by the equation in which M and j, with appropriate subscripts, represent the molarities and activity coefficients, respectively. The value of Kb is 1.98 X 10-2 and that of -log Kb or pK2 is 1.703 ± 0.005. The indicator is useful over the pH range 0 to 3. CONTENTS Page

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

“….fJf havior for pure salts of sulfonic acids, which may be attributed ill part to the presence of "ion pairs" arising from the resonant, polar structure of the phenolsulfonates [8,9,32]. The value of at found here is nearly equal to Bjerrum's [23] critical value and may indicate, although not conclusively, that "ion pairs" are present.…”

“…Furthermore, a precise study by Sager, Keegan, Schooley, and Acree of the spectrophotometric curves for a number of the sulfonphthaleins at widely different pH values [9] shows that the absorption index bands characteristic of the sulfonate salt are not changed, within experimental errors, by addition of hydrochloric acid up to 0.5 M. An alternative possibility that the ionized and nonionized sulfonic acid group have exactly the same absorption index bands is of doubtful validity in view of the fact that the ionized and molceular forms of carboxylic acids, urazoles, quinonephenolates, and quinaminone indicators, etc., were found to give quite different bands.…”

“…where A and B are constants whose numerical values depend upon various natural constants, {3 is an empirical constant expressing in part the interactions of the nonpolar hydrogen and chloride ions with the polar phenolsulfonate ions [8,9,32] analogous to Kirkwood's treatment of ampholytes [20], al is the so-called closest distance of approach of the ions; and J.1. is the ionic strength of the mixtures.…”

“…The fractional transformation, (x , of the indicator or buffer into bivalent ions, and the change in color,2 with increase in neutralization with alkali and corresponding rise in pH are represented byeq 1, , The p-phenolsulfonates and indicators give absorption index bands in the ultraviolet that are as different for acid ann alkaline solutions as are the visible bands for indicators. The molar absorption indices are used as a measure of the concentrations of the molecules and ions of these organic compounds [6,8,9]. The equations apply regardless of the number of uni-and bivalent electromers or electronic states giving the corresponding index bands [S, p. 220].…”

ph of aqueous mixtures of potassium dihydrogen phosphate and disodium hydrogen phosphate at 0 degrees to 60 degrees C

“…The purpose of this investigation was twofold: to determine the pH of a series of phosphate buffer standards, and to study tbe effect of sodium chloride on the thermodynamic properties of the phosphate buffer system. The study of salt effects on the behavior of indicators and buffers has been the subject of earlier communications from this Bureau (2,3,4,5].…”