Ternary Solution Mutual Diffusion Coefficients and Densities of Aqueous Mixtures of NaCl and Na₂SO₄ at 298.15 K for Six Different Solute Fractions at a Total Molarity of 1.000 mol·dm^-3

Abstract

Isothermal mutual diffusion coefficients (interdiffusion coefficients) were measured for ternary aqueous mixtures of NaCl and Na2SO4 at a constant total molarity of 1.000 mol·dm-3 and 298.15 K. Measurements were performed using Rayleigh interferometry with computerized data acquisition at NaCl molarity fractions of z 1 = 1, 0.90, 0.75, 0.50, 0.25, and 0. Densities of the solutions were measured with a vibrating tube densimeter. At all ternary solution compositions, one cross-term diffusion coefficient has negative values whereas the other has positive values. These measurements supplement our earlier results at 0.500 mol·dm-3. Both main-term diffusion coefficients are significantly smaller at 1.000 mol·dm-3 than at 0.500 mol·dm-3 at any fixed value of z 1, whereas both cross-term coefficients are shifted in a positive direction. Trace diffusion coefficients D*(Cl-) and D*(S ) were extrapolated from these results for the Cl-(aq) ion in 1.0 mol·dm-3 Na2SO4(aq) and for the S (aq) ion in 1.0 mol·dm-3 NaCl(aq). Values of D*(Cl-) in Na2SO4(aq) and in NaCl(aq) were found to be essentially identical, as were D*(S ) in these same two electrolytes, provided the comparisons were made at the same volumetric ionic strengths.

Mutual Diffusion Coefficients and Densities at 298.15 K of Aqueous Mixtures of NaCl and Na2SO4 with NaCl Solute Fractions of 0.9500, Trace Diffusion Coefficients of SO42- in NaCl(aq), and Trace Refractive Index Increments and Partial Molar Volumes of Na2SO4 and NaCl

“…14,32 These diffusion measurements were made with the high-quality Gosting diffusiometer 29 and with automated "real time" recording of the fringe positions. [16][17][18][19] The movement of the photodiode array is now controlled with a 550 MHz Pentium 3 Dell Computer. 19 At each overall solution composition (constant total molarity), diffusion experiments were performed with four solution pairs having essentially the same average con-Journal of Chemical and Engineering Data, Vol.…”

“…However, as noted previously, we believe the true uncertainties are about four times larger than these statistical values. [10][11][12][13][14][15][16][17][18][19][42][43][44] More realistic estimates of the (D ij ) V errors may be made using various subsets of the diffusion experiments. 44 These error estimates use the (D ij ) V values calculated from the four possible three-experiment subsets of the R 1 values at each constant 〈C h T 〉 value.…”

“…1,2 We and our collaborators have been measuring properties of aqueous solutions of the various soluble salts present in seawater and natural brines. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, we reported precise mutual diffusion coefficients of NaCl(aq) and mutual diffusion coefficients and osmotic coefficients of Na 2 SO 4 (aq) at 298.15 K, 3,6,7 and a thermodynamic equation of state for Na 2 SO 4 (aq) from freezing temperatures to 423.65 K. 5 In 1995 we began a systematic investigation of the mutual diffusion coefficients and densities/partial molar volumes of {(z 1 )NaCl + (1 -z 1 )Na 2 SO 4 }(aq) at 298.15 K, and we have reported our results at total concentrations of (0.500, 1.000, and 1.500) mol‚dm -3 over the NaCl composition fraction range z 1 ) 0.25 to 0.90, [16][17][18] and at z 1 ) 0.90 to a total concentration of 5.007 mol‚dm -3 . 19 Because of solubility limitations at 298.15 K, 1.500 mol‚dm -3 is very close to the maximum concentration for which diffusion coefficient measurements can be made over the full range of z 1 values.…”

“…Diffusion data for our other 14 reported compositions of {(z 1 )NaCl + (1 -z 1 )Na 2 SO 4 }(aq) were measured using this automated Rayleigh mode. [16][17][18][19] The measurements at constant total molarities of (0.5, 1.0, and 1.5) mol‚dm -3 were used to derive D tr (SO 4 2-) values in NaCl(aq) and D tr (Cl -) values in Na 2 SO 4 (aq) at these three concentrations. [16][17][18] However, measurements at higher concentrations of (2.0 to 5.0) mol‚dm -3 were limited to mixtures with z 1 ) 0.90, 19 so trace diffusion coefficients could not be derived.…”

Predicting Sulfate-Mineral Solubility in Concentrated Waters

“…Pitzer's equations can be used to estimate solution densities (e.g. Monnin 1989;Obsil et al 1997) and mutual diffusion coefficients (Albright et al 1998). These models require data on binary and ternary mixtures, from which physical properties of more complex mixtures can be calculated, similar to the approach used to calculate ion activities in complex mixtures through Equations (3) and (4).…”