Second ionization constant and related thermodynamic quantities for malonic acid from 0 degrees to 60 degrees C

Abstract

The second ionization constant of malonic acid in aqueous solution was determined at 0° to 60° C from measurements of the electromotive force of galvanic cells without liquid junction. Solutions containing sodium acid malonate, sodium malonate, and sodium chloride were employed for these determinations.From the values of the ionization constants, the closest distance of approach, or the so-called diameters, of the ions in the solution was calculated for the different temperatures. These ionic diameters, the electromotive forc e, and the known molalities were then employed to calculate pH values of the solutions. The ionization constant at the different temperatures was determined from the experimental data by three different methods, using a least-square calculation in each. The values are different for the various temperatures, and may be computed for temperatures from 0° to 60° C, inclusive, by the equation 1053.08 log K 2= --T -+20.3223 log T-0.05838 T+0.0000236 T2-37.1402.Equations were formulated to express the variation of pH as a function of the ionic strength of the solution, to give the heat of ionization of the acid malonate ion, and to give the difference in the specific heats of the ions and the undissociated acid malonate ion at each temperature. The change in free energy and entropy for the ionization of the acid malonate ion were also calculated for each temperature. All of these quantities are of importance in arriving at explanations for the variation of hydrogen-ion activity with temperature.It has been found that solutions containing equal concentrations of sodium acid malonate, sodium malonate, and sodium chloride, each varying from 0.001 to 0.044 molal, have pH values ranging from 5.272 to 5.761 at 0° to 60° C. These solutions are suitable for use as pH standards.

The second dissociation constant of p-phenolsulfonic acid and pH values of phenolsulfonate-chloride buffers from 0 degrees to 60 degrees C

“…The calculation of the second dissociation constant, K 2 , of a weak acid and the pH values of solutions of the acid mixed with its salts from emf measurements of hydrogen-silver-chloride cells has been outlined in earlier publications from this Bureau [2,6,7] . Mention should be made, however, of the assumptions upon which these calculations are based.…”

“…the following equation 2 is obtained: (6) where pK~ is the negative logarithm of the "apparent dissociation constant." Upon extrapolation to infinite dilution.…”

Dissociation constants and pH-titration curves at constant ionic strength from electrometric titrations in cells without liquid junction: titrations of formic acid and acetic acid

“…A value of 3.8 A was found to give the b est representation of the activity coefficient of hydrochloric acid in mixturcs of primary phosphate, secondary phosphate, and sodium chloride at room temperature [12]. In sodium acid malonate-diRodium malonatc·sodium chloride buffers, 4.75 A gives the proper expression for the activity coefficient [6]. In unpuhlishcd measurements in this Bureau, W. J. Bamer has found the value of 3.76 A to apply both to m ixtures of phthalic acid, potassium acid phthalate, and potassium chloride, and to mixtures of acid potassiuUl phthalate, dipotassium phthalate, and potassium chlor ide.…”

“…When an accuracy within 0.003 unit is desired, the emf method described in earlier papers from this Bureau [6,7,12) should be employed. The improved titration should be regarded rather as a rapid method of establishing di ssociation constants and practical buffer standards for pH control where an uncertainty of 0.02 unit is permissible.…”

“…6 As a mean value, therefore, 4 A has been chosen for the computation of the pH values. The contribution of the activity coefficient to the pH at an ionic strength of 0.1 is, then, assumed to be 0.114 unit.…”