Recenltly,l'2 the determination of diffusion coefficients of electrolytes in dilute

Transcription

1 THE DIFFUSION COEFFICIENTS OF THE ALKALI METAL CHLORIDES AND POTASSIUM AND SILVER NITRATES IN DILUTE AQUEOUS SOLUTIONS AT 250* BY HERBERT S. HARNED DEPARTMENT OF CHEMISTRY, YALE UNIVERSITY Communicated April 26, 1954 Recenltly,l'2 the determination of diffusion coefficients of electrolytes in dilute aqueous solutions has been made possible by the utilization of electrolytic conductance for the required determination of concentration at suitable intervals of time. This method is the only one which yields accurate determinations in the very dilute solution range, and therefore the results derived from it are suitable for testing the Onsager and Fuoss3 equations for the concentration dependence of electrolytic diffusion. Since the method of measurement has been described in detail elsewhere, 1 2 only an outline sufficient to define the quantities involved and the calculations will be presented in the sequel. Emphasis will be placed on the actual experimental results as related to theory, and the discussion will be confined to uniunivalent electrolytes in the concentration range from to 0.01 molar. Theoretical Considerations. Fick's first law of diffusion may be expressed by J=cv= -Vc= - MVA, (1) where J is the flow, c the concentration of electrolyte, V the operator del, v the velocity of flow, and D the diffusion coefficient. The theory of irreversible processes4 for mass flow at constant temperature and pressure requires the proportionality of the flow to the gradient of the chemical potential, Vu4, as represented by the term on the right of equation (1). It follows that Ca)1A a) p, (2) If c is expressed in moles per liter, and the equation which related the mean activity coefficient of the electrolyte y, on the molar concentration scale to the relative chemical potential is introduced in equation (2), )= v 1000 RT o I + c 6 In y) (3) is obtained, where v is the number of ions into which the electrolyte disso( iates, R is the gas constant, and T the absolute temperature. The term in parentheses, which results from the gradient of the chemical potential, is denoted the "thermodynamic term," and (on/c) the "mobility term." In dilute solutions, where factors such as change in viscosity may be neglected, the concentration dependence of the mobility has been evaluated by Onsager and Fuoss.3' 5 When an electrolyte diffuses into a solvent, both ions travel with the same velocity, as required by electrical neutrality. In this case, electrophoresis occurs because the ions which move in one direction are replaced by solvent molecules which move in the opposite direction. By the requirement that the forces on 551

2 55252CHEMISTRY: H. S. HARNED PROC. N. A. S. the ions must be balanced by compensating forces on the water molecules, by assuming Stokes's law, which relates the velocity with these forces, and by utilizing the Debye and Hfickell potential of the ion and its atmosphere in the Maxwell- Boltzmann equation for the number of ions of one kind in the presence of another ion, Onsager and Fuoss obtained the following expression for the mobility term: = X x10x0 (jz2xi1o- Z1iX2O)l3.122 X 10-9x (C) V Iz IAO jz1z21 (VI + V2)A02 7o(DT)'/' V/ /Z22X1O + Z12X2$\ X (1 + V A- I 04(Ka). (4) 4o(DT)'/g Here X10 and X20 are the equivalent cation and anion conductances, AO their sum, zi and Z2 their valences, vi and V2 the number of anions and cations into which the electrolyte dissociates, fo the viscosity of the solvent, D its dielectric constant, F the ional concentration or 2CZ12, K the Debye and Huckel reciprocal radius, and a the distance parameter of the order of magnitude of the sum of the crystal radii of the electrolyte. The quantity 4(Ka) is the exponential integral function of the theory, values of which have been tabulated.7 It is to be noted that the thermodynamic term equals unity and the mobility term equals the concentration-independent members on the right when c equals zero. Consequently, equations (3) and (4) yield the limiting equation for the diffusion coefficient Do originally obtained by Nernstj which, for uniunivalent electrolytes in water at 250, becomes 5Do = X 10-10T (X1OX2) (5) The electrophoretic contribution is given by the concentration-dependent terms of equation (4). It is to be noted that these terms have opposite signs. Indeed, for electrolytes such as lithium chloride, in which the cation and anion mobilities differ considerably, the magnitudes of the positive and negative terms are about equal, and their sum is quite small. In this case the concentration dependence of the diffusion coefficient depends entirely on the thermodynamic term in equation (3). The effect of the electrophoretic terms is never greater than 0.5 per cent of the diffusion coefficient at concentrations of 0.01 molar or less. The Conductometric Diffusion Measurement.', 2-The simplest form of cell for an electrolyte diffusing vertically upward is a rectangular parallelepiped of height a, with electrodes at positions which may be most suitably determined from theoretical considerations. The schematic cross-section of such a cell is shown in Figure 1, in which the electrodes are fixed at identical distances t from the top and bottom of the cell. Steady-state diffusion occurs in the x-direction. By a suitable technique, affording the minimum of convection, the diffusion process is started in the cell, and at suitable time intervals the conductance is measured at the top and bottom pairs of electrodes. In this method we rely on measurements toward the latter stages of diffusion, when the concentrations at the top and bottom pairs of electrodes differ so little that it is safe to assume constancy of the diffusion coefficient over this small concentration difference. Under these conditions, Fick's second law for restricted diffusion becomes

3 VOL. 40, 1954 CHEMISTRY: H. S. HARNED 553 with the boundary condition a 2c at = D dx2' (6) - = Oat x = O and x = a. (7) The solution of this equation which satisfies these conditions is where An are the Fourier coefficients. c = E Ae-(n2 21/a2)tCos n + co, (8) n=i a a~~~ FIG. 1.-Verticle cross-section of a cell showing quantities involved in the conductometric method for determining diffusion coefficients. The method of applying this equation has great simplicity, for, by subtracting the concentration of electrolyte at the top from that at the bottom, all the even terms of the series vanish. Further, if the electrodes are placed at a distance t = a/6 from the top and bottom, the third term of the series vanished. As a result, we obtain for the difference of bottom and top concentrations c(t) - c(a- ) - 2Ae-(w2jI/a2)t cosrt + 2Ase- (25r25/a2)f cos 5(9) a a and only the first term on the right has any significance. Further, in dilute solutions, it is safe to replace the difference in concentrations at bottom and top by the difference in conductances (KB - KT), so that the firstorder equation In (KB- KT) = constant (10) is readily obtained, since Al, A5, i, and a are constant. The great simplicity of the method is at once apparent, since a measurement of the depth of the cell, a, and the

4 554 CHEMISTRY: H. S. HARNED PROC. N. A. S. top and bottom conductances at suitable time intervals are the only data required to determine the diffusion coefficient. Finally, since we rely on results obtained when the concentrations do not differ greatly, we obtain the "differential" diffusion coefficient as required by theory. Evidence for the Presence of the Electrophoretic Terms in Equation (4). In Figure 2 values of (DO - D), calculated by equations (3), (4), and (5), for the diffusion of lithium9 and potassium chlorides2' 10 are plotted against the square root of the molar concentration of the electrolyte. The data required for this computation are compiled in Table 1. The middle curve in Figure 2 represents the com- TABLE 1 QUANTITIES EMPLOYED IN THEORETICAL COMPUTATIONS A0 a (A) Li '10 = X 10-3 LiCI 4.0 Na D = NaCI 4.0 K T = KCI 3.5 Rb IzI = 1 RbCI 3.5 Cs IZ21 = 1 CsCI 3.5 Ag Pi = 1 AgNO3 3.5 Cl V2 = 1 KNO3 3.5 NO v = 2 plete calculation for potassium chloride solutions, whereas the top curve, denoted (Do - D) for this (m/c), is the graph of R salt upon neglecting the electrophoretic C 0.t_ terms of equation (4). The difference / K between these two curves represents the magnitude of the electrophoretic influ- 10/ ence. Since the mobilities of the potassium and chloride ions are nearly the LO/same, the second term on the right-hand side of equation (4) is negligible, and only /LUC 0.05 //8 * the last term of this equation contributes */S to the calculation. At 0.01 molar the electrophoretic correction for potassium chloride is 0.5 per cent of the magnitude of its diffusion coefficient. For solutions of lithium chloride, the difference in mobilities of the lithium and chloride ions is such as to cause cancellation of the 5 c~i 0. lnegative and positive electrophoretic FIG. 2.-Comparison of observed and terms of equation (4), so that the comcalculated diffusion coefficients of lithium plete theoretical result is the same as and potassium chloride solutions. The two that obtained by keeping (on/c) constant lower curves represent the complete calculations according to equations (3) and (4). and equal to the first term on the right- The top curve, denoted (Wl/c), represents hand side of this equation. the calculation without the electrophoretic It is apparent from Figure 2 that terms, the last two terms of equation (4). The diameters of the circles equal 0.1 per cent excellent agreement of the experimental of the diffusion coefficient. results with those computed theoretically is obtained. In this figure the diameter of the circles equals 2 in the third decimal place, which corresponds to 0.1 and 0.15 per cent of the diffusion coefficients of potas-

5 VOL. 401, 1954 CHEMISTRY: H. S. HARNED 555 sium and lithium chlorides, respectively. Consequently, these results confirm the validitv of the thermodynamic term in equation (3), as well as the magnitude and sign of the electrophoretic effects. It is important to note that above 0.01 molar concentration, deviations from the theoretical prediction occur.9 Activity Coefficientsfrom Diffusion Data.-In all our previous calculations we computed the thermodynamic term from the best available activity-coefficient data. Since no accurate experimental determinations of these quantities at concentrations -o.' below molar were available, it was necessary to rely on extrapolations by the -o Debye and Huckel equations, the parameters of which had to be determined from " data at higher concentrations. We shall now reverse this procedure and, by assum- co o.u C 0.8 ing the validity of equations (3) and (4) at FIG. 3.-Plot used for computing activity concentrations from 0 to 0.01 molar, com- coefficients according to equation (13). pute the activity coefficients from the diffusion-coefficient data. For the calculation of the (n/c) term, 4.0 A is chosen for the distance parameter, a, for the hydrated electrolytes, lithium and sodium chlorides, and 3.5 A for the other, less hydrated salts. Rearrangement of equation (3) yields whence and RT( - 1 c'= c (11) ln A= D c =, 0CI/ (12) log Ye = 230= J l/ dc'/'. (13) At 25 deg. l ['?In y, lim Lc/2 =- 2 3()= , (14) since the limiting theoretical slope of the Debye and Huckel theory, Se,), is at this temperature. By plotting Y'/c&/2 versus cl/2, evaluation of log y, at suitable concentrations may be easily achieved by graphical integration. Such a graph for potassium chloride solutions is shown in Figure 3. The method is a very sensitive one, since it involves the whole area under the graph. The results of this calculation are recorded in Table 2. The last column contains the bibliographic reference number. Although these results have little significance beyond the third decimal place, we have expressed those for sodium and potassium chloride solutions to the fourth place, in order to compare them with the recent evaluation of this quantity from cells with liquid junction by Shedlovsky," denoted in the table by

6 556 CHEMISTRY: H. S. HARNEDRPROC. N. A. S. NaCl(S) and KCl(S). The agreement between the values derived by those entirely different methods is very encouraging. TABLE 2 ACTIVITY COEFFICIENTS, Y., OF THE ALKALI METAL CHLORIDES AND SILVER AND POTASSIUM NITRATES AT 250 FROM DIFFUSION DATA CONCENTRATION OF ELECTROLYTE REF. LiCl NaCl NaCI(S) KCI KCI(S) RbCl CsCl KNO AgNO For purposes of future comparison and criticism, the diffusion coefficients of these uniunivalent electrolytes obtained at round concentrations from graphs of our conductometric results are recorded in Table 3. The values at zero concentration were computed by equation (5) from the data in Table 1. TABLE 3 DIFFUSION COEFFICIENTS OF ALKALI METAL CHLORIDES AND POTASSIUM AND SILVER NITRATES AT ROUND CONCENTRATIONS AT 250 (3D X 105 [CM.2 SEC. -]) c LiCi8 NaC18 KC19 RbCI12 CaC1i3 KNO:'4 AgNO3U (1.368) (1.612) (1.996) (2.057) (2.046) (1.931) (1.767) * Contribution No from the Department of Chemistry, Yale University. This investigation was supported in part by the Atomic Energy Commission under Contract At-(30-1) H. S. Harned, and D. M. French, Ann. N. Y. Acad. Sci., 46, 267, H. S. Harned, and R. L. Nuttall, J. Am. Chem. Soc., 69, 736, L. Onsager, and R. M. Fuoss, J. Phys. Chem., 36, 2689, L. Onsager, Phys. Rev., 37, 405, 1931; 38, 2265, H. S. Harned, Chem. Rev., 40, 461, 1947; H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions (2d ed.; New York; Reinhold Publishing Corp., 1950), pp , P. Debye and E. Huckel, Physik. Z., 24, 185, Harned and Owen, op. cit., p W. Nernst, Z. physik. Chem., 2, 613, H. S. Harned and C. L. Hildreth, Jr., J. Am. Chem. Soc., 73, 650, H. S. Harned and R. L. Nuttall, J. Am. Chem. Soc., 71, 1460, T. Shedlovsky, J. Am. Chem. Soc., 72, 3680, H. S. Harned and M. C. Blander, J. Am. Chem. Soc., 75, 2853, H. S. Harned and M. C. Blander, ibid. (in press). 14 H. S. Harned and R. M. Hudson, J. Am. Chem. Soc., 73,652, H. S. Harned and C. L. Hildreth, Jr., J. Am. Chem. Soc., 73, 3292, 1951.