International Journal of Chemical and Pharmaceutical Review and Research

ISSN No: 2395-3306 Int. J. Chem. Pharm. Rev. Res. Vol (1), Issue (2), 2015, Page. 1-12 Research Article International Journal of Chemical and Pharmaceutical Review and Research www.ijcprr.com/browse-journal Tetrabutylammonium bromide (Bu4NBr), Sodium tetraphenylborate(nabph4) and Sodium bromide (NaBr) in Acetonitrile(1) + Water(2) mixtures at (288.15, 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15)K A Conductance Study Ngasepam Bhogenjit Singh and Gopal Chandra Bag* Department of Chemistry, Imphal College, Imphal-795 001, Manipur, India A R T I C L E I N F O Article history: Received 6 May 2015 Accepted 4 June 2015 Available online 14 June 2015 Keywords: Electrolytic conductance, Shedlovsky equation, ion-association, acetonitrile- water, solvation. A B S T R A C T Molar conductances of Bu4NBr, NaBPh4 and NaBr have been measured in acetonitrile(1) + water(2) mixtures containing 0, 20, 40, 60 and 80 mass% acetonitrile at temperatures ranging from 288.15K to 318.15K at an interval of 5K. The conductance data have been analyzed by Shedlovsky method to obtain Ʌ and KA values of the electrolytes. The ionic contributions to the limiting molar conductance have been estimated using tetrabutyl ammonium tetraphenyl borate (Bu4NBPh4) as the reference electrolyte. Thermodynamic quantities for the ion-association process have been derived along with Walden product. All these electrolytes are highly associated in the acetonitrile rich region of the solvent system within the studied temperature ranges. It has been found that solvated radii (ri) of Bu4N+ and Ph4B-decreases with increasing acetonitrile percentage while that of Na+ and Br- show an increase with acetonitrile concentration. Specific interactions of the ions with the solvent molecules have been found to exhibit a profound influence on their mobilities besides the effect of viscosity and permittivity of the media. IJCPRR All rights reserved. 1. Introduction Frequently, investigations were made for the binary mixture of water and some organic solvents in varying compositions.binary mixture of acetonitrile with water is also one of the commonly studied mixtures which is non-ideal and constitutes three different regions3-5. It may affect significantly the ion-pair formation and solvation of ions. Studies on the transport properties of electrolytes in different solvent media are of great importance in obtaining information as to the solvation and association behavior of ions in solutions1. The conductometric method is well suited to investigate the ion-ion and ion-solvent interactions in electrolyte solutions. From the concentration dependence of the electrolyte conductivity, different quantities strongly influenced by solvent properties can be derived. Their changes with mixed solvent composition may thus reflect the change in solvent structure and ion-solvent interactions2. Conductometric studies on a number of electrolytes have been carried out in acetonitrile(1) + water(2) mixture. There are only a few studies on Bu4NBr, NaBPh4 and NaBr in a very limited range of temperature and composition in this solvent system1. The present paper reports the molar conductivities of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate(nabph4) and sodium bromide (NaBr) in acetonitrile(1) + water(2) * Corresponding author. E mail address: gopalbag53@gmail.com Present address: Assoc. Prof., Department of Chemistry, Imphal College, Imphal-795 001, Manipur, India. 1

mixtures with an acetonitrile mass percentage of 0, 20, 40, 60 and 80 at seven temperatures from 288.15K to 318.15K with an interval of 5K. In our reports, we included an extended work in acetonitrile rich region and broader range of temperatures so that a clear trend can be observed. The observed data were treated in order to obtain Ʌ (limiting molar conductivity) and (ion association constant) which further enables to calculate the Walden product (Ʌ η )and other thermodynamic properties (ΔG, ΔH and ΔS ) for the process of ion association. The precise temperature-dependent single-ion conductivities were also obtained because such data are practically very less in mixed-solvent media. 2. Experimental HPLC grade acetonitrile (SRL) was used without further purification. Double distilled water of specific conductance of ~3 10-6 Scm -1 at 298.15K was used throughout the experiments. Conductance measurements were carried out on Systronics-306 Conductivity Bridge using a dip-type cell of cell constant 1.0cm -1 having an accuracy of ±0.1%. Solvent mixtures were prepared by mass percent and that of electrolyte solutions for the conductance runs were prepared in molarities using a balance (Mettler Toledo, Model No. ME204) of 0.0001g accuracy. All the gathered permittivity, viscosity and density values were taken from the literature 6-15 and by graphic interpolation of the reported values. They are enlisted in Table 1. All the salts were SRL AR grade. Tetrabutylammonium bromide(bu 4NBr) and sodium tetraphenylborate (NaBPh 4) were recrystallized from acetone and then dried under vacuum. Sodium bromide (NaBr) was recrystallized from water and dried in vacuum for 72 hours immediately prior to use. Measurements were made in a refrigerated water bath maintained within ±0.01K of the desired temperature. The details of the experimental procedure have been described elsewhere 16. Due correction was made for the specific conductance of the solvent at all temperatures., where D is the permittivity of the medium; η the viscosity co-efficient of the medium. The degree of dissociation (τ) is related to S(Z) by the equation, f ± is the mean activity coefficient of the free ions and was calculated using eqn. (2) where and (2) R is the maximum centre to centre distance between the ions in the ion-pair. There exists at present no precise method 17 for determining the value of R. In order to treat the data in the present system, the R value is assumed to be R =a + d, where a is the sum of crystallographic radii of the ions and d is the average distance corresponding to the side of a cell occupied by a solvent molecule. The distance d is given by 18 d = 1.183 (M/ρ) 1/3 Å where M is the molecular mass of the solvent and ρ is the density of the solution. For mixed solvents M is replaced by mole fraction average molecular weight. 3. Results and Discussion 3.1 Evaluation of Λ 0, and Λ 0η o The measured conductance data of all the electrolytes have been analyzed by using Shedlovsky equation: (1) Where Ʌ is the molar conductance at a concentration C mol.dm -3, Ʌ o, the limiting molar conductance and, the observed association constant. The other symbols are given by x 1 is the mole fraction of acetonitrile of molecular weight M 1 and x 2 that of water of molecular weight M 2.The values of association constant () and the limiting molar conductance (Ʌ o) have been calculated using eqn. 1 by an iterative procedure 19. All the calculations have been carried out on IBM PC. The results of Ʌ 0 and for NaBr, NaBPh 4 and Bu 4NBr at different acetonitrile-water compositions and temperatures are collected in Tables 2-4. 2

Table 1: Relative permittivity, density and viscosity of water-acetonitrile mixture at different acetonitrile mass percent and temperatures. %ACN : 0 20 40 60 80 288.15K 82.00 c 73.85 c 63.44 c 53.10 c 44.82 c 0.9992 f 0.9669 e 0.9250 e 0.8791 e 0.8301 e.01138 g.01230 b.01090 b.00890 b.00700 b 293.15K 80.14 c 72.05 c 62.00 c 52.20 c 43.92 c 0.9983 f 0.9643 a 0.9212 a 0.8724 4 0.8271 4.01002 g.01100 b.00990 b.00810 b.00560 b 298.15K 78.33 c 70.49 c 60.61 c 51.05 c 43.10 c 0.9971 f 0.9617 d 0.9174 d 0.8690 d 0.8213 d.00890 g.00978 d.00865 d.00663 d.00464 d 303.15K 76.50 c 68.85 c 59.30 c 50.07 c 42.20 c 0.9957 f 0.9573 a 0.9114 a 0.8634 4 0.8169 4.00797 g.00870 b.00800 b.00650 b.00450 b 308.15K 74.84 c 67.33 c 58.07 c 48.88 c 41.38 c 0.9941 f 0.9551 h 0.9040 e 0.8584 e 0.8105 e.00719 g.00780 b.00720 b.00590 b.00430 b 313.15K 73.15 c 65.90 c 56.79 c 47.98 c 40.64 c 0.9923 f 0.9582 j 0.8943 j 0.8541 i 0.8065 i.00652 g.00700 b.00650 b.00540 b.00410 b 318.15K 71.51 c 64.34 c 55.44 c 46.87 c 39.74 c 0.9902 f 0.9489 h 0.8946 e 0.8519 e 0.7989 e.00596 g.00640 b.00590 b.00500 b.00380 b a - calculated values, b reference 1, c reference 2, d reference 3, e reference 6, f reference 14, g reference 15, h reference 16, i reference 4, j reference 7 3

Table 2: Λ o Scm 2 mol -1 and dm 3 mol -1 values of NaBr at different acetonitrile- water compositions and temperatures. %ACN T/K 288.15 293.15 298.15 303.15 308.15 313.15 318.15 121.669 267.834 0 20 40 60 80 134.002 227.762 150.992 231.919 164.228 239.316 177.226 208.160 192.748 214.810 216.09 217.30 107.333 256.490 117.426 269.034 133.026 277.854 147.526 289.258 165.623 298.703 188.812 316.742 201.271 161.844 103.854 297.042 116.978 299.392 128.328 305.921 143.087 251.600 161.264 314.818 187.412 338.198 199.596 393.622 106.957 433.278 119.832 460.272 142.396 479.744 139.050 496.344 153.370 523.378 174.501 536.834 191.394 227.164 133.302 580.779 149.099 613.898 167.316 625.322 183.017 648.872 199.146 683.207 212.186 721.357 243.243 738.846 Table 3: Λ Scm 2 mol -1 and KA dm 3 mol -1 values of Ph4BNa at different acetonitrile-water compositions and temperatures. T/ K %ACN 0 20 40 60 80 288.15 293.15 298.15 303.15 308.15 313.15 54.621 17.174 62.238 20.744 70.227 23.684 77.977 28.994 87.067 31.080 96.900 33.727 49.462 8.502 57.033 27.891 65.891 9.463 75.938 42.128 83.452 46.073 92.311 59.806 52.906 30.234 59.728 31.539 67.240 42.664 75.084 51.754 82.528 36.091 91.215 42.092 63.614 47.080 71.064 49.451 80.458 66.942 89.488 81.099 98.048 94.724 108.698 96.860 92.442 50.802 106.730 148.391 114.162 203.540 125.016 217.651 133.722 189.929 139.322 116.294 106.902 103.612 104.929 121.019 147.628 4

318.15 39.346 66.111 89.866 117.929 126.620 Table 4: Λ Scm 2 mol -1 and KA dm 3 mol -1 values of Bu4NBr at different acetonitrile-water compositions and temperatures. %ACN T/K 288.15 293.15 298.15 303.15 308.15 313.15 318.15 76.37 4.08 0 20 40 60 80 86.85 5.74 97.09 9.79 107.16 9.42 116.62 8.62 127.07 8.21 138.72 7.18 64.767 7.571 73.840 14.771 83.470 19.264 92.016 17.208 105.248 20.788 115.219 24.446 128.370 38.734 68.272 12.011 76.890 16.588 89.109 23.749 98.504 28.631 108.950 31.364 118.417 57.601 131.610 64.652 73.578 14.29 86.368 20.937 91.846 5.306 101.606 31.818 110.362 41.888 126.344 62.852 135.292 75.803 94.582 17.157 104.212 21.631 113.330 32.599 123.750 69.496 133.134 71.041 142.958 86.417 154.311 99.013 The variation of Ʌ 0 with temperature (Fig. 1) and solvent composition is similar for Ph 4BNa and Bu 4NBr. Throughout the composition ranges, trends in Ʌ 0 values show a minimum in the 20%acetonitrile region. However for NaBr, the decrease is up to 40% in (288.15-298.15)K temperature range and above 298.15K, it decreases further to 60% and then increases. Figure 1: Plot of Ʌ vs T for NaBr, Ph4BNa and Bu4NBr at 40% acetonitrile-water mixture Ph4BNa NaBr and Bu4NBr. These trends clearly depict the dependence of Ʌ o of electrolytes to the viscosity of the solvents. Viscosities of acetonitrile(1) + water(2) show a maxima in 20% acetonitrile (Table 1) region, the effect of which is manifested in the molar conductance values of all the electrolytes where a drop in the Ʌ o values is observed in this particular region. Ʌ o values of all the electrolytes increase with increasing temperature which is attributed to the decrease in viscosities of the solvent mixture. A comparison of Ʌ o values of all the electrolytes show that Ʌo of NaBr>Bu 4NBr>NaBPh 4. This trend can be explained based on the differences in the ionic mobilities of the ions contributing to the total conductivity of the respective electrolytes. In most of the composition and temperature range, the association constant () values follow the order: NaBr>NaBPh 4 Bu 4NBr. Minimum association occurs in aqueous medium for all the electrolytes and increases with increasing acetonitrile percentage and temperature. The association constant for Bu 4NBr in pure water is practically negligible (i.e. < 10) at all the temperatures and hence the trends in their values is not important 20. In water, it almost exists as free ions 5

in the studied range of temperatures. Ghosh and Das 1 also reported a very low value of of all the electrolytes in two compositions (viz. 0.20 & 0.40 volume fractions of acetonitrile in water) at (308.15, 313.15 & 318.15)K. Table 5: Calculated λ Scm 2 mol -1 values of,, and ions at different acetonitrile-water compositions and temperatures. %ACN : 0 20 40 60 80 288.15K 4.822 3.564 8.954 15.628 27.768 4.506 3.331 8.368 14.606 25.952 75.034 61.436 59.318 57.950 66.814 50.115 46.131 44.538 49.008 66.490 293.15K 7.800 6.950 10.152 19.435 31.967 7.290 6.495 9.488 18.164 29.876 80.398 67.345 66.738 66.933 72.245 54.948 50.538 50.240 52.900 76.854 298.15K 8.436 8.443 14.484 15.459 31.105 7.884 7.891 13.536 14.448 29.071 89.668 75.579 74.625 76.387 82.225 62.344 58.000 53.704 66.010 85.091 303.15K 10.810 10.559 15.766 26.902 33.986 10.100 9.868 14.734 25.142 31.763 97.910 82.148 82.738 74.704 89.764 67.880 66.070 60.350 64.346 93.253 308.15K 13.677 11.929 15.618 28.450 34.999 12.782 11.148 14.596 26.589 32.710 105.263 94.100 93.332 81.912 98.135 74.285 72.304 67.932 71.459 101.012 313.15K 16.142 9.675 11.486 31.294 36.232 15.086 9.042 10.734 29.247 33.862 113.559 106.177 106.931 95.050 106.726 81.814 83.269 80.481 79.451 105.460 318.15K 15.264 15.875 19.096 33.556 30.339 6

14.266 14.836 17.847 31.361 28.354 126.888 113.534 112.514 101.736 118.079 92.636 88.776 94.195 89.658 119.274 Table 6: Calculated ri Å values of,, and ions at different acetonitrile-water composition and temperatures. %ACN : 0 20 40 60 80 288.15K 14.964 18.721 8.402 5.895 4.218 16.016 20.098 8.991 6.308 4.515 0.960 1.085 1.268 1.590 1.754 1.438 1.445 1.689 1.879 1.762 293.15K 10.486 10.732 8.159 5.210 4.581 11.232 11.484 8.732 5.574 4.901 1.018 1.106 1.241 1.512 2.026 1.489 1.475 1.648 1.914 1.905 298.15K 10.918 9.927 6.550 8.008 5.682 11.681 10.622 7.002 8.559 6.083 1.028 1.109 1.270 1.619 2.149 1.478 1.446 1.765 1.874 2.077 303.15K 9.512 8.932 6.502 4.691 5.362 10.199 9.557 6.961 5.018 5.738 1.051 1.147 1.238 1.688 2.030 1.516 1.426 1.698 1.961 1.954 308.15K 8.342 8.817 7.295 4.886 5.452 8.922 9.425 7.802 5.230 5.832 1.084 1.117 1.220 1.697 1.943 1.535 1.454 1.676 1.944 1.888 313.15K 7.794 12.112 10.992 4.852 5.518 8.333 12.974 11.748 5.193 5.908 1.108 1.103 1.180 1.598 1.874 1.537 1.407 1.568 1.911 1.896 318.15K 7

9.011 8.071 7.282 4.886 7.118 9.647 8.632 7.794 5.230 7.614 1.084 1.128 1.235 1.612 1.828 1.485 1.443 1.475 1.830 1.809 Table 7: Thermodynamic parameters for the association of Bu4NBr in different acetonitrile-water composition and at different temperatures. %ACN T/K 0 20 40 60 80-3.36-4.84-5.95-6.37-6.80 288.15 56.24 129.07 166.58 163.84 189.53-4.26-6.56-6.84-7.41-7.49 293.15 58.32 132.71 166.76 164.60 188.63-5.65-7.33-7.85-4.13-8.63 298.15 62.02 133.07 167.35 150.84 189.30-7.21-7.17-8.45-8.72-10.68 303.15 66.12 130.34 166.57 163.48 192.95-5.52-7.77-8.82-9.56-10.92 308.15 59.57 130.18 165.08 163.57 190.58-5.48-8.32-10.55-10.78-11.61 313.15 58.50 129.86 167.96 164.84 189.73-5.30-9.67-11.02-11.44-12.15 318.15 57.00 132.06 166.81 164.34 188.46 Comparable large sizes and similar charge distribution of Bu 4N + and Ph 4B - lead to comparable extent of association, which are lower than NaBr in the corresponding temperature and composition of the solvent. It may be attributed to the lower charge densities of Bu 4N + and Ph 4B - owing to their large sizes as compared to the ions of NaBr. The increasing association with increasing temperature and acetonitrile percentage in the medium is well explained by the decrease in the dielectric constant of the medium. For NaBPh 4 and Bu 4NBr,the decreasing compatibility of solvent molecules (acetonitrile) to interact with the large Bu 4N + and Ph 4B - ions with increasing acetonitrile percentage, leading to strengthening of short-range attraction between the counterions and thereby contributing to the increase in the association constant is worth mentioning here 22. 8

3.2 Limiting ionic molar conductance (λ i) and effective ionic radius (r i) Limiting ionic molar conductivities of all the ions were determined by resolving the experimentally determined Ʌ values for corresponding electrolytes on the basis of Bu 4NBPh 4 assumption 1 using the following relationships Ʌ (Bu 4NBPh 4) = λ (Bu 4N + )+ λ (Ph 4B - ) -------(3) --------- (4) using the salt tetrabutylammonium tetraphenylborate (Bu 4NBPh 4) as the reference electrolyte and 5.35Å, 5.00Å are the crystallographic radii of Ph 4B - and Bu 4N + ion, respectively. The Ʌ values of Bu 4NBPh 4 have been obtained by combining those of NaBr, NaBPh 4 and Bu 4NBr using Kohlrausch s additive rule: Ʌ (Bu4NBPh4)= Ʌ (Bu4NBr) +Ʌ (NaBPh4) Ʌ (NaBr)--(5) The calculated λ values for Bu 4N +, Ph 4B -, and at various acetonitrile-water compositions and temperatures are reported in Table5. In mixed solvent media, the limiting ionic equivalent conductances decrease in the order: λ Br- >λ Na+>λ Ph4B- λ Bu4N+ at each temperature indicating that sizes of ions as they exist in solutions follow the order < < Ph 4B - Bu 4N +. It is observed that the limiting molar ionic conductances of and decrease from 0% (pure water) to 40%acetonitrile up to 298.15K while it decreases up to 60% acetonitrile at higher temperatures. Whereas, for Ph 4B - and Bu 4N + ions, a decrease in individual ion conductance occurs up to 20%acetonitrile and then increases with increasing acetonitrile percentage in all the solvent compositions and at all the experimental temperatures. A similar trend had been reported earlier 1. The trend indicates the influence of specific interaction of the ions with the solvent media on their mobilities. The corresponding effective ionic radii (r) have been calculated using Stokes 23 law radius.the trends show that Bu 4N +, Ph 4B - and ions are solvated both in aqueous medium and in solvent mixture throughout the whole composition range, though the extent of solvation and its variation with percentage of acetonitrile are different for different ions. For Bu 4N + and Ph 4B -, the extent of solvation decreases with increasing acetonitrile percentage which reflects the weak interaction of these ions with acetonitrile which was mentioned earlier. is more solvated in aqueous medium than in 20% acetonitrile-water mixture; the extent of solvation further increases with increase in acetonitrile percentage. Water being a stronger solvator of than acetonitrile 24 the decreasing concentration of water in 20% acetonitrile decreases the solvation of the ion leading to a decrease in the effective ionic radii. Protophobic nature, capability to behave as a ligand by acetonitrile 25 altogether with comparatively small size of with respect to Bu 4N + results in increasing interaction between and acetonitrile molecules. The lower r i values of than the corresponding crystallographic radii may be explained by the effect of polarization of the ion by which it loses its spherical shape, the extent of which decreases with increasing acetonitrile content 22,25. The ion-solvent interactions can be studied by analyzing the variation of Walden product (Ʌ η ) with solvent composition which indicates the change in total solvation 26. The variation of the Walden product (Ʌ η ) with acetonitrile percentage at 303.15 K for all the electrolytes is shown in Fig. 2. As the fraction of acetonitrile increases, smaller ions are solvated stronger but the differences become smaller. (6) where r is expressed in Å, η o in poise and λ o in S cm 2 equiv -1. The Stokes radii of,bu 4N +, Ph 4B - and at different temperatures and compositions are listed in Table6.The sizes of the ions are greater than their corresponding crystallographic radii except for the bromide ion where it is less than its crystallographic Figure 2: Plot of λ η vs acetonitrile mass percent for NaBr, Ph4BNa and Bu4NBr at 30 o C NaBr, Bu4NBr and Ph4BNa. 9

Table 8: Thermodynamic parameters for the association of Ph4BNa in different acetonitrile-water composition and at different temperatures. %ACN T/K 288.15 293.15 298.15 303.15 308.15 313.15 318.15 0 20 40 60 80-6.81-5.12-8.16-9.22-9.41 93.78 110.68 128.64 115.71 185.36-7.38-8.11-8.41-9.51-12.18 94.16 118.97 127.30 114.69 191.66-7.84-5.56-9.30-10.42-13.18 94.10 108.42 128.15 115.83 191.78-8.48-9.42-9.94-11.08-13.56 94.66 119.39 128.16 116.10 189.89-8.80-9.81-9.18-11.66-13.44 94.16 118.70 123.62 116.09 186.41-9.16-10.65-9.74-11.91-12.38 93.79 119.48 123.40 115.02 180.05-9.71-11.08-11.90-12.62-12.80 94.06 118.97 128.25 115.45 178.55 Table 9: Thermodynamic parameters for the association of NaBr in different acetonitrile-water composition and at different temperatures. %ACN T/K 288.15 293.15 0 20 40 60 80-13.39-13.28-13.64-14.54-15.24 2.54 5.98 5.97 6.38 6.08 55.29 66.86 68.06 72.62 74.01-13.22-13.63-13.90-14.94-15.64 2.54 5.98 5.97 6.38 6.08 53.79 66.90 67.78 72.74 74.11 10

-13.50-13.94-14.18-15.30-15.96 298.15 2.54 5.98 5.97 6.38 6.08 53.80 66.83 67.61 72.72 73.92-13.81-14.28-13.93-15.64-16.32 303.15 2.54 5.98 5.97 6.38 6.08 53.92 66.84 65.66 72.65 73.88-13.68-14.60-14.74-16.04-16.72 308.15 2.54 5.98 5.97 6.38 6.08 52.62 66.78 67.20 72.74 73.98-13.98-14.99-15.16-16.36-17.13 313.15 2.54 5.98 5.97 6.38 6.08 52.76 66.96 67.48 72.64 74.12-14.23-13.45-15.80-14.35-17.47 318.15 2.54 5.98 5.97 6.38 6.08 52.72 61.08 68.44 65.16 74.02 3.3 Evaluation of thermodynamic parameters The free energy change (ΔG o ) for the association process is calculated from the relation ΔG o = -RTln (7) The heat of association (ΔH o ) is obtained from the slope of the plot of log vs 1/T(Fig. 3) and the entropy change ΔS o is then calculated from the Gibbs-Helmholtz equation, ΔG o =ΔH o -TΔS o (8) Figure 3: Plot of logkavs 1/T for NaBr, Ph4BNa and Bu4NBr at 60% acetonitrile-water mixture NaBr, Bu4NBr and Ph4BNa. The values of these thermodynamic functions for the association of Bu 4NBr, Ph 4BNa and NaBr are given in Tables 7, 8 and 9, respectively. It is expected that more strongly solvated ions will be less inclined to association. This is confirmed by the order of the ΔG o values: Bu 4NBr <NaBPh 4< NaBr. All the values are uniformly descending with increasing acetonitrile content, indicating a greater degree of association at lower relative permittivity. An observation of ΔH o values for all the salts show that only a significant entropy gain can make an endothermic reaction spontaneous. In this case, all the possible structuremaking processes accompanying the ionic association are more than compensated for by weakening of the solvation shells. The positive ΔS values of all the salts make the (-TΔS o ) term greater than the ΔH o term in eqn (8), making the process of ion-association favourable. 4. Conclusion In mixed solvent media, the limiting ionic equivalent conductances decreases in the order: λ Br- >λ Na+>λ Bu4N+ λ Ph4B- at each temperature indicating that sizes of ions as they exist in solutions follow the order: < <Bu 4N + Ph 4B -. The values of the Stokes radii also follow the same order. Another interesting observation is that the limiting ionic equivalent conductances of Bu 4N + and Ph 4B - ions first decrease up to 20% ACN and then increase with increasing % of ACN. The λ values for and ions decrease from 20% to 40% acetonitrile mixture and then increase at all temperatures. This indicates that besides the relative permittivity and viscosity of the media the specific interaction of the ions with the solvent media has a profound influence on the mobilities of the ions. 11

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