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 electromotive force of cells without liquid junctions. Hydrogen and silver-silverchloride 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 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 a i (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. CONTENTS Page

Comparative liquid-junction potentials of some pH buffer standards and the calibration of pH meters

“…The final product contained less than 0.0002-mole percent of bromides, whi.ch would produce a difference in potential of less than 0.01 mv. The hydrochloric acid used for the preparation of the silver-silverchloride electrodes was also freed from bromides [5]. Although the values for EjI-E'n are independent of the emf and of the nature of the reference electrode, the above precautions were employed in order that the values for Erer+Ej for each buffer would not include a systematic error because of the presence of potassium bromide or air in the reference electrolyte.…”

“…The activity of the chloride ion in the two solutions can be expressed by the following equations: k log aCl1=k pH -(El + E 2) + EO (5) and (6) where EO is the normal potential of the silver-silver-chloride electrode, 0.22238 v at 25° C [9]. Equations 4, 5, and 6 can be combined to give (7) Comparison of the sum of (E1+E2) and (Ere,+E J -E2) with the values for (EI+EJ+Ere,) obtained by direct measurement indicate a consistency of 0.07 mv (0.001 pH unit) in the two sets of data for each buffer.…”

“…Particular care was taken to prevent contamination with atmospheric carbon dioxide. The solutions were transferred when necessary by a vacuum-hydrogen technic [5]. The calcium hydroxide was prepared by carefully washing pure calcium carbonate (low-alkali grade) with distilled water, after which the material was ignited at 1,000° C. The resulting oxide was allowed to react with an excess of water in a stoppered, paraffinlined bottle and the calcium hydroxide washed several times with conductivity water.…”

“…The hydrogen and the silver-silver-chloride electrodes were prepared by methods described previously [5]. Palladinized electrodes were used in solutions of potassium acid phthalate [2].…”

“…[3], phenolsulfonate [4], and borax buffers [5]. The precision of each standard, established by the 1 method of cells without liquid junctions [6], is considered to be 0.002 4 pH unit.…”