Ionization constant of boric acid and the pH of certain borax-chloride buffer solutions from 0 degrees to 60 degrees C

Abstract

The ionization constant of boric acid was determined by the use of the elec- tromotive force of cells without liquid junctions. Hydrogen and silver-silver- chloride electrodes were immersed in borax-sodium chloride solutions. To reduce tbe possibility of formation of polyborates, dilute solutions were used. The emf were measured at 5-degree intervals over the temperature range 0° to 60° C. The negative common logarithm of the ionization constant (pK) of boric acid over the temperature range 0° to 60° C may be represented by the equation pK=2237.94/T+0.016883T-3.305, where where T is the absolute temperature. The data indicate that the mean s.ctivity coefficients of the ions of sodium chloride and of sodium borate do not differ appreciably. The pH values for the solutions studied and for rounded values of the concentration are tabulated as functions of temperature and ionic strength. These solutions, which range in pH values from 8.934 to 9.465, can be used as standards in the calibration of glass-calomel and other electrometric pH equipment. A discussion is given of the significance of the quantity ai (commonly called the "distance of closest approach" of the ions) and its importance in the calculation of pH value~, especially when the buffer ion is not univalent. The changes in free energy, beat content, entropy, and beat capacity that accompany the ionization of 1 mole of boric acid are listed.