R. D. LANIER
3992
Activity Coefficientsof Sodium Chloride in Aqueous Three-Component Solutions by Cation-Sensitive Glass Electrodes’”
by R. D. Lanierlb Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
(Received June 16, 1966)
~_________
Activity coefficients of NaCl in aqueous three-component solutions were measured a t 25’ a t ionic strength 1-6 with cation-sensitive glass electrodes us. silver-silver chloride electrodes. The second salts were MgC12, CaC12, SrC12, BaC12, NaC2H302,NaC104, NaN03, and NazS04. Agreement with available literature values is satisfactory. Except possibly a t low NaCl contributions to total ionic strength, Harned’s rule appears to apply. An approximate form of Brfinsted’s rule allowed in most cases good estimates of activity coefficients of NaCl a t trace concentration in solution of a second electrolyte from the activity coefficients of two-component solutions of the two solutes a t the ionic strength in question.
I n recent experiments,2 , 3 a cation-sensitive glass electrode silver-silver chloride cell was used to measure activity coefficients in water and water-organic mixtures. This paper reports measurements of NaCl activity coefficients in aqueous solutions containing a second electrolyte having one ion in common. Included were salt pairs with alkaline earth chlorides and pairs with Na+ in common. Other alkali metal chlorides were not included because the response of the glass electrode to them is comparable to its response to Xa+. The ionic strength range of this study is 1-6 m. R e ~ e n t l ysimilar ,~ measurements were reported for four of the present mixtures a t an ionic strength of 0.7. Measurements were carried out at constant ionic strength in order to test the applicability of Harned’s rule5 log
712
= log
710
- cy1212
Experimental Section 1. Potential Measurements. A vibrating reed electrometer, in conjunction with a Rubicon or a Leeds and Northrup K-3 potentiometer, was used for measurements of e.m.f.6 2. Electrodes. The glass electrode (Cationic 39137) was obtained from Beckman Instruments, Inc., and its
(1)
which states that a12 is a constant at a given ionic strength. The symbols in eq. 1 and other symbols used in this paper are for the most part those used by Harned and are defined below. I is ionic strength, = l / 2 Zmizi2, m being concentration in moles/kg. of H20, and the summation being over all ions. Subscript 1 refers to NaCl and 2 to the other solute. yij is the mean ionic activity coefficient of solute i in a three-component solution also containing solute j . yio is the activity coefficient of a two-comThe Journal of Physical Chemistry
ponent solution of component i a t the total ionic strength under discussion. yoi is the activity coefficient of component i in the presence of j , extrapolated to zero concentration of i. 4i is the osmotic coefficient of a two-component solution containing solute i. zi is the charge on ion i. Pij is the coefficient of a term quadratic in Ij added to eq. 1 for cases where a linear relationship is not adequate. vi is the number of moles of ions/niole of component i.
(1) (a) Research sponsored by The Office of Saline Water, U. S. Department of the Interior under Union Carbide Corporation’s contract with the U. S. Atomic Energy Commission; (b) Southwest Missouri State College, Springfield, Mo. (2) R. D. Lanier, J. Phys. Chem., 69, 2697 (1965). (3) M. M. Shul’ta and A. E. Parfenov, Vestn. Leningr. Univ., Ser. Fiz. i. K h i m , 13, No. 3, 118 (1958). (4) W. F. Fitzgerald, Abstracts, 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. ( 5 ) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolyte Solutions,” ACS Monograph Series, 3rd Ed., Reinhold Publishing Corp., New York, N. Y., 1958, Chapter 14. (6) K . A. Kraus, R. W. Holmberg, and C. J. Borkowski, Anal. Chem., 22, 341 (1950).
ACTIVITYCOEFFICIENTS OF NaC1 IN
AQUEOUSTHREE-COMPONENT SOLUTIONS
performance is described by their literature.’ Reference 2 gives further data on the performance of this electrode in water-sodium chloride and water-organicsodium chloride solutions. The silver chloride electrodes were of the thermal types; AgZO and AgC103 paste were fired on Pt wires a t 500-600°. These electrodes were shorted together and allowed to equilibrate in S a c 1 solution, which was initially at 80’ and which cooled to room temperature overnighkg The performance of the AgCl electrodes was satisfactory (rapid, accurate response to a change in C1concentration), if the solution being tested had been previously saturated with AgC1, except a t very low chloride concentrations. 3. Reagents. Reagent grade salts were used without further purification. Laboratory distilled water, which had been run through a deionization column, was used to prepare solutions; from measurements of e.m.f. of dilute solutions (
