324 Acta Chim. Slov. 2006, 53, 324–330 Scientific Paper The Electric Conductivities of Aqueous Solutions of Rubidium and Cesium Cyclohexylsulfamates, Potassium Acesulfame and Sodium Saccharin† Darja Rudan-Tasič1, Cveto Klofutar1, Marija Bešter-Rogač2* 1Biotechnical Faculty, University of Ljubljana, SI-1000 Ljubljana, Slovenia. 2Faculty of Chemistry and Chemical Technology, University of Ljubljana, SI-1000 Ljubljana, Slovenia. † Received 24-04-2006 Dedicated to the memory of Prof. Dr. Davorin Dolar Abstract The electric conductivities of aqueous solutions of rubidium and cesium salts of cyclohexylsulfamic acid, potassium acesulfame and sodium saccharin were measured from 5 °C to 35 °C (in steps of 5 °C) in the concentration range 0.0003 < c/mol dm-3 < 0.01. Data analysis based on the chemical model of electroyte solutions yielded the limiting molar conductivity ?? and the association constant KA. Using the known data of the limiting conductivities of rubidium, cesium, sodium and potassium ions the limiting conductivities of the cyclohexylsulfamate, accesulfame and saccharin ions were evaluated. Total dissociation of the investigated salts in water and negligible hydration of anions are evident. Key words: electrolyte conductivity, electrolyte solution, cyclohexylsulfamates, acesulfame, saccharin, chemical model 1. Introduction Saccharin, potassium acesulfame and the salts of cyclohexylsulfamic acid are widely used as non-caloric sweetening agents in foods, beverages and pharmaceuticals.1 In our previous study the conductivity2 of aqueous solutions of some cyclohexylsulfamates was studied. It has been found that cyclohexylsulfamates as salts are completely dissociated in water solutions. The cyclohexylsulfamate anion turned out as weakly hydrated due to its hydrophobicity, whereas the hydration of cations depends on their charge densities. The obtained results were in agreement with the volumetric properties3 and viscosities4 and confirmed the suggestion that sweetness is a complex interplay of structural and solution properties. On the other hand, it was recognized, that the sulfamate function is essential for cyclamate sweetness although the cation seems to have some effect on the sweet taste.5, 6 The behaviour of an ion in a solvent depends on the ion-ion and ion-solvent interactions. It is to be expected that the taste of a sweet substance could be interpreted by an understanding of these interactions in the medium. Because there is a lack of the literature information on the properties of accesulfame and saccharin anion in the solutions we extended our investigations with precise conductivity measurements on potassium acessulfame and sodium saccharin dilute aqueous solutions. However, aqueous rubidium and cesium cyclohexylsulfamate solutions were also investigated in order to complete the series of the mentioned salts of monovalent cations. The obtained data were treated in the framework of the low concentration chemical model (lcCM).7 2. Experimetal 2.1 Materials Rubidium (RbCy) and cesium cyclohexylsulfamate (CsCy) were obtained by careful neutralization of cyclohexylsulfamic acid (HCy, purchased from Sigma) with the corresponding base (Fluka or Merck). The purity of the salts was checked after repeated recrystallizations from water by analysis of the elements C, H and N (Perkin Elmer, 2400 Series II CHNS/O Analyzer) and also by the ion exchange of the cations with the hydrogen ion (DOWEX, Type 50 WX8); a purity of 99.9% for all the salts was determined. Sodium saccharin (Sacharin-Na, Sac-Na) was purchased from Merck (dihydrate purum, ? 99.0%). Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of Acta Chim. Slov. 2006, 53, 324–330 325 By thermogravimetric analysis (Mettler Toledo TGA/ SDTA 851e) the amount of crystal water 1.55 ± 0.01 H2O in the Sac-Na was found. The Fluka product potassium acesulfame (Acesulfame-K, Ace-K, puriss, > 99.0%) was used without further purification. The salts were kept in a vacuum desiccator over P2O5. Demineralized water was distilled in a quartz bidistillation apparatus (Destmat Bi18E, Heraeus). The final product with specific conductance of less than 6 • 10-7 S cm-1 was distilled into a flask permitting storage and transfer of the solvent into the measuring cell under an atmosphere of nitrogen. The stock solutions were prepared by weighing salt and water. 2.2 Thermostat The high precision thermostat used in the laboratory experiments has been described previously.8 It can be set to each temperature of a temperature programme with a reproducibility of less than 0.003 °C. 2.3 Conductivity Measurement The conductivities of dilute solutions were determined with the help of a three-electrode measuring cell, described elsewhere.9 The cell was calibrated with dilute potassium chloride solutions.10 At the beginning of every measuring cycle the cell was filled with a weighed amount of water. After measurement of the solvent conductivity at all temperatures of the programme, a weighed amount of a stock solution was added using a gas-tight syringe and the temperature programme was repeated. From the weights and the corresponding solution densities d, the molar concentrations c were determined. A linear change of d with increasing salt content for diluted solutions was assumed, d=ds +Din, where d is the density of the solvent (water) and m is the molonity of the electrolyte (moles of electrolyte per kilogram of solution). The densities of the solutions were determined by the method of Kratky et al.11 by use of a Paar densimeter (DMA 60, DMA 601 HT) at 25 °C combined with a precision thermostat. As usual the density gradient D is considered to be independent of temperature, see Table 1. The measuring procedure, including corrections and the extrapolation of the sample conductivity to infinite frequency, is described in the literature.10 The measured conductivity data of all investigated salts are given in Table 1 as a function of the temperature independent molonities. They can be converted to the temperature-dependent molarities by use of the relationship c = md. Taking into account the sources of error (calibration, titration, measurements, impurities), the specific conductivities are accurate to within 0.1%. 3. Data Analysis The analysis of conductivity data in the framework of the low concentration chemical model (lcCM) given in Ref. (7) and the literature quoted there, uses the set of equations — = A°° - S^jac + Eacln(ac) + Jjtxc + J2(0.0) 2 (1) a K \-a ca2y'l -^: y'± = exp *q 2 k = 16nN Aqac; q KA = 4?iNA \r exp %KEE0kT r kT dr (2a-b) (2c-d) (3) where A and A°° are the molar conductivities at molarity c and infinite dilution, (1-a) is the fraction of oppositely charged ions acting as ion pairs, and KA is the equilibrium constant of the lcCM with an upper association limit R; y'+ is the corresponding activity coefficient of the free ions, (y'+ ) = y'+ y'_, k is the Debye parameter, eo is the proton charge, e is the relative permittivity of the solvent, eo is the permittivity of a vacuum and T the absolute temperature. The other symbols have their usual meaning. W is a step function for the potential of mean force between cation and anion due to non-Coulombic interactions. The coefficients of Eq. (1) are given in Ref . (7). The limiting slope S and the parameter E are evaluable when the solvent data are available. The coefficients J1 and J2 are functions of the distance parameter R, representing the distance to which oppositely charged ions can approach as freely moving particles in solution. Analysis of the conductivity data of associated electrolytes are carried out by setting the coefficients S, E and J1 of Eq. (1) to their calculated values7 and then usually using three-parameter fits to obtain the limiting values of molar conductivity A°°, the association KA and the coefficient J2 by non-linear least squares iterations. A three-parameter evaluation is reduced to a two-parameter procedure for non-associating electrolytes,9 where usually the coefficient J2 is also fixed. The input data for the calculation of the coefficients are the known solvent properties used in the literature12 and the distance parameter R. The lower limit a of the association integral is the distance of closest approach of cation and anion (contact distance) a = a + + a– calculated from the ionic radii of the cations7 a+ = 0.098, 0.133, 0.149 and 0.165 nm for Na+, K+, Rb+ and Cs+ respectively. Cyclohexylsulfamate, saccharin , a Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of 326 Acta Chim. Slov. 2006, 53, 324–330 and acesulfame anions have a shape far from spherical. The radii for saccharin and aceslufame anions were estimated from van der Waals radii of the atoms given by Bondi.13 Obtained values 0.327 and 0.315 nm for saccharin and acesulfame anion respectively are in good agreement with radii from X-ray diffraction measurements (0.355 nm for saccharin and 0.339 nm for acesulfame anion). The later ones were used in the further procedure. For cyclohexylsulfame anion we used the value of a– = 0.176 nm which was estimated for sulfamic acid assuming that the cyclohexyl radical does not change its interionic distance between the proton and the basic oxygen atom in the zwitterion structure of sulfamic acid.14 From extended investigations of electrolyte solutions in amphiprotic hydroxylic solvents (water, alcohols) it is known that the upper limit of association is given by an expression of the type R = a+ n ? s, where s is the length of an oriented solvent molecule, n is an integer, n = 0, 1, 2,... Here, s is the length of an OH-group, dOH and s =dOH = 0.28 nm. In our previous work only slightly dependence of the association constants by the choice of the distance parameter R was observed. Here for all the systems investigated n = 2 was chosen in order to encompass three types of ion pairs: contact, solvent shared and solvent separated ion pairs. Table 1: Experimental molar conductivities of the investigated sweeteners in watera T 278.15 283.15 288.15 293.15 298.15 303.15 308.15 10 x m A RbCy, D = 0.1290 26123 65.808 75.011 84.623 94.612 104.987 115.668 126.577 51673 65.487 74.656 84.254 94.230 104.535 115.221 126.181 78251 65.253 74.418 83.965 93.921 104.198 114.824 125.780 09083 65.061 74.178 83.705 93.610 103.852 114.468 125.408 41094 64.921 74.011 83.510 93.379 103.603 114.198 125.121 82113 64.717 73.800 83.255 93.097 103.309 113.859 124.729 19430 64.552 73.598 83.036 92.847 103.031 113.543 124.372 67868 64.375 73.396 82.810 92.595 102.727 113.227 124.018 07203 64.253 73.242 82.625 92.387 102.512 112.971 123.654 57864 64.084 73.065 82.422 92.133 102.226 112.707 123.352 CsCy, D = 0.1690 20232 65.911 75.064 84.567 94.578 105.327 115.542 125.283 41746 65.470 74.593 84.098 94.038 104.528 114.266 125.183 66172 65.097 74.164 83.625 93.459 103.498 113.602 124.045 91796 64.766 73.779 83.166 92.858 103.088 113.428 124.201 17636 64.531 73.490 82.793 92.533 102.696 113.000 123.749 40136 64.313 73.254 82.583 92.301 102.424 112.694 123.408 64177 64.127 73.040 82.345 92.041 102.124 112.384 123.043 89557 63.985 72.881 82.148 91.837 101.880 112.136 122.762 17077 63.849 72.710 81.975 91.614 101.656 111.888 122.090 52123 63.688 72.542 81.781 91.385 101.407 111.385 121.483 Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of 4. Results and Discussion Figure 1 shows a comparison of the experimental data for the sodium saccharin aqueous solutions given in Table 1 and the results of the lcCM calculations executed using Eqs. (1-5) under the assumption n = 2 for Eqs. (4 and 5), encompassing three types of ion pairs: contact ion pairs, solvent-shared and solvent-separated ion pairs. All other investigated systems show similar dependence. In Figure 2 the conductivity data for sodium saccharine, potassium acesulfame together with rubidium and cesium cycloheyxysulfamate aqueous solutions at 298.15 K are presented. —o-—-o——o—-a c1/2/ mol1/2dm-3/2 Figure 1. Molar conductivities of aqueous solutions of sodium saccharin from 278.15 K to 308.15 K (in steps of 5 K) in the concentration range 0.0003 < c/mol dm-3 < 0.01; full lines: lcCM calculations. 278.15 K 0.02 0.04 0.06 0.08 Acta Chim. Slov. 2006, 53, 324–330 327 Table 1: Contunued T 278.15 283.15 288.15 293.15 298.15 303.15 308.15 io3x m Ace-K, D = 0.0949 0.26345 67.704 77.274 87.356 97.833 108.673 119.822 130.854 0.61191 67.097 76.610 86.635 97.008 107.805 118.611 130.044 0.94979 66.571 76.050 86.029 96.339 106.985 117.902 129.022 1.28621 66.313 75.754 85.652 95.899 106.544 117.520 128.851 1.64720 66.076 75.509 85.338 95.587 106.191 117.147 128.447 2.07266 65.863 75.237 85.071 95.286 105.878 116.785 128.018 2.49919 65.661 74.821 84.837 94.992 105.550 116.426 127.623 2.97005 65.469 74.831 84.558 94.717 105.206 116.056 127.226 3.49614 65.275 74.569 84.288 94.420 104.878 115.701 126.619 4.08285 65.079 74.378 84.101 94.193 104.654 115.111 125.631 Sac-Na, D = 0.0936 0.40135 49.407 57.070 65.170 73.697 82.603 91.902 101.048 0.80063 48.960 56.556 64.575 73.015 81.849 90.888 100.391 1.24622 48.628 56.159 64.125 72.498 81.138 90.251 99.746 1.76948 48.303 55.748 63.687 72.028 80.715 89.738 99.147 2.28232 48.053 55.491 63.351 71.611 80.256 89.279 98.475 2.92215 47.767 55.158 62.962 71.183 79.786 88.480 97.672 3.55777 47.539 54.904 62.681 70.855 79.360 87.893 97.161 4.19230 47.351 54.687 62.431 70.564 79.068 87.927 97.129 4.85544 47.167 54.442 62.179 70.289 78.747 87.569 9 6.729 5.57112 46.981 54.263 61.917 70.008 78.426 87.218 96.337 ~ aUnits: m, mol kg-1;T, K; ?, S cm2 mol-1; D, kg2 dm-3 mol-1 110 105 100 95 90 85 80 0.00 0.02 0.04 0.06 0.08 c 1/2 / mo 1/2 dm -3/2 Figure 2. Molar conductivies of the rubidium (o) and cesium (•) cyclohexylsulfamate, potassium acesulfame (O) and sodium saccharin (A) in water at 25 °C. In Table 2 the limiting conductivities and association constants calculated by using the lcCM are gathered. The values of the association constants are very low: KA = 5-6 for aqueous solutions of the cesium cyclohexylsufamate and KA = 2-3 in all other systems investigated. All the investigated salts could be regarded as completely dissociated in water solutions (“strong-electrolytes”). Whereas the temperature coefficient dKA/dT is usually positive for the alkali salts water solutions, no evidence for the temperature dependence of the association process in the inestigated systems was found. Table 2. Limiting molar conductivities ?? and association constants KA of rubidium and cesium cyclohexylsulfamates, sodium saccharin and potassium acesulfame in watera T A~ Ka A~ Ka RbCy CsCy R = 0.885 R = 0.901 278.15 66.55 ± 0.02 1.0 ± 0.7 66.54 ± 0.06 6.2 ± 0.8 283.15 75.89 ± 0.02 1.3 ± 0.5 75.81 ± 0.06 6.1 ± 0.7 288.15 85.65 ± 0.03 1.0 ± 0.6 85.46 ± 0.07 5.8 ± 0.6 293.15 95.82 ± 0.03 2.0 ± 0.7 95.55 ± 0.09 5.9 ± 0.8 298.15 106.33 ± 0.02 0.9 ± 0.4 106.21 ± 0.17 6.8 ± 1.2 303.15 117.18 ± 0.08 1.0 ± 0.4 116.46 ± 0.15 4.5 ± 1.0 308.15 128.42 ± 0.18 3.3 ± 0.3 127.13 ± 0.19 3.3 ± 1.2 Sac-Na Ace-K R = 1.014 R = 1.032 278.15 50.23 ± 0.03 3.0 ± 0.3 283.15 58.02 ± 0.04 3.0 ± 0.3 288.15 66.27 ± 0.05 3.1 ± 0.3 293.15 74.94 ± 0.05 3.1 ± 0.3 298.15 83.99 ± 0.07 3.1 ± 0.3 303.15 93.37 ± 0.14 3.2 ± 0.6 308.15 102.90 ± 0.14 3.1 ± 1.0 68.27 ± 0.09 77.96 ± 0.10 88.18 ± 0.09 98.76 ± 0.10 109.74 ± 0.12 120.99 ± 0.11 132.57 ± 0.14 3.5 ± 0.6 3.1 ± 0.7 3.0± 0.5 2.8 ± 0.7 2.7 ± 0.7 2.6 ± 0.5 2.8 ± 0.6 a Units: T, K; ??, S cm2 mol-1; KA, dm3 mol-1; R, nm 75 70 Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of 328 Acta Chim. Slov. 2006, 53, 324–330 Combining the limiting ion conductivities A°° of Table 2 and the known limiting values of cations15 A°°(M+ ) , M+= Na+, K+, Rb+ and Cs+ ( Table 3) A. °°(T, A ) = A°° (T, MA) - X°°{T, M ) (4) yields the limiting anion conductivities ??( A- ) for cyclohexylsulfate, saccharin and acesulfame anions and their temperature dependence; see Table 4. Table 3. Densities and viscosities of pure water and conductance of ions in watera limiting T dsb \d\rf A"*(Na+)c X~ (K+)c X~ (Rb+)c /T(Cs+)c 278.15 0.99997 1.5192 30.30 283.15 0.99970 1.3069 34.88 288.15 0.99910 1.1382 39.72 293.15 0.99821 1.002 44.81 298.15 0.99704 0.8903 50.15 303.15 0.99565 0.7975 55.72 308.15 0.99404 0.7195 61.53 46.72 53.03 59.61 66.44 73.50 80.76 88.20 50.12 56.63 63.44 70.51 77.81 85.30 92.94 50.00 56.47 63.18 70.12 77.26 84.59 92.10 a Units: T, K; ds, kg dm-3; r\, Pa s; A™, S cm2 mol-1 b Ref.12 c Ref. 15 Table 4. Limiting conductances of cyclohexylsulfamate, saccharin and acesulfame ion in water as a function of temperaturea X" (Cy") X" (Sac) X" (Ace) RbCy CsCy Literature Sac-Na Ace-K 278.15 16.44 16.54 16.25 19.93 21.54 283.15 19.26 19.34 19.08 23.14 24.93 288.15 22.21 22.28 22.05 26.55 28.57 293.15 25.31 25.43 25.21 30.13 32.32 298.15 28.52 28.95 28.47 33.84 36.24 303.15 31.88 31.87 31.86 37.64 40.23 308.15 35.48 35.03 35.37 41.44 44.37 a Units: T, K; ??, S cm2 mol-1 The values obtained from the measurements on the rubidium and cesium cyclohexylsulfamate solutions are in good agreement with the data published recently2 whereas for saccharin and acesulfame anions no reported data were found in the literature. From the Walden rule7 X°{T)rj{T) Feo\z\ 6jtr (5) the hydrodynamic radii r could be estimated (F is the Faraday constant and z the ionic charge). Figure 3 represents the corresponding Walden products A°° (T) t](T) as a function of temperature for all investigated ions. All hydrodynamic radii are collected in Table 5. As already discussed 2,16 comparison of the values of the hydrodynamic radii and the crystal radii of cations shows large differences for Li+ and Na+ ions, whereas the ion-size parameters of K+ are close together. Cs+ and Rb+ ions, however, exhibit perceivable lower values of the hydrodynamic radii. An inspection of the Table 5 275 280 285 290 295 300 305 310 T/K Figure 3. The temperature dependence of the Walden products of cyclohexylsulfamate (?), acesulfame (O) and saccharin anion (A) in water. Table 5. Hydrodynamic radii, r, of ions in water from Walden’s rule as a function of temperaturea T Na K Til + Kb Cs Cy" Sac- Ace- 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.178 0.180 0.181 0.182 0.183 0.184 0.185 0.115 0.118 0.121 0.123 0.125 0.127 0.129 0.107 0.111 0.113 0.116 0.118 0.120 0.122 0.108 0.111 0.114 0.117 0.119 0.121 0.124 0.332 0.328 0.326 0.324 0.323 0.322 0.322 0.270 0.271 0.271 0.271 0.272 0.273 0.275 0.250 0.251 0.252 0.253 0.254 0.255 0.257 a Units: T, K; r, nm b Ref.2 reveals, together with the published data for Li+, a well known and unequivocal order of hydration values for monovalent cations Li+ > Na+ > K+ > Rb+ > Cs+ although there is considerable disagreement over the actual values.17 As shown before2 the obtained hydrodynamic radii for the cyclohexylsufame anion are in reasonable agreement with the reported crystal radius of the anion (rcry = 0.37 nm) and with the value obtained from volumetric properties, rh = 0.334 nm at 298.15 K4 and no explicit hydration could be assumed. On the other hand the obtained radii for saccharin and acesulfame anions are distinctly different from their van der Waals and crystal radii: a-/r = 1.20 and 1.24 for saccharin and acesulfame anion at 298 K respectively. Similar was observed for the smaller tetraalkylammonium ions (a+/r= 1.69, 1.42, 1.15 for Me4N+, Et4N+, Pr4N+ respectively7). Thus, the hydrophobicity of the organic anions seems to predominate in their intrinsinic hydrophilic/ hydrophobic balance. Walden rule treats the ionic migration as a movement of a rigid spherical ion through viscous continuum therefore no further information on the molecular scale transport process could be estimated. 35 30 25 r T Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of Acta Chim. Slov. 2006, 53, 324–330 329 The temperature dependence of limiting conductivity yields Eyring’s enthalpy of activation of charge transport18 lnX° + — lnds 3 ?H* RT + B (6) where B is the integrations constant. Values ?H* = 16.762, 14.982, 14.568, 14.395, 18.342, 17.356 and 17.051 kJ/mol for Na+, K+, Rb+, Cs+, Cy-, Sac- and Ace- respectively (Figure 4) show that the molar ionic enthalpy of activation for the anions of all sweeteners are higher than the values for the cations. 0.0034 0.0035 1/T/ K-1 2 Figure 4. Plot of In X" +—In ds as a function of 1/T for Cs+ and Rb+ (®), cyclohexylsulfamate (?), acesulfame (O) and saccharin anion (A). From the slope the activation energy of the ionic movement, AH*, is obtained. It has been shown that the ionic migration in a non-structures solvent is a solvent property mainly and that the difference in the mobilities of ions is simply the result of different ion sizes.19 The observed differences in the Eyring’s enthalpy of activation of charge transport in the investigated systems could be also ascribed to the differences in the ion sizes. In water additional strong hydrophobic interactions are presented, resulting not only in the size parameters. Therefore it could be assumed that, for the jump of the cyclohexylsulfamate, saccharinate and acesulfame anions to a prepared vacancy in the solvent - or to produce such a vacancy – a higher energy is required than for the cations investigated in this work. This could be explained by the repulsion of water molecules by the hydrophobic sites of the anions. The observed order of the molar ionic enthalpy of activation for the cations Li+>Na+>K+>>Rb+> Cs+ agrees with the order of hydration values for these cations and could be explained by the energy needed for the desolvation and rearrangement of water molecules in the vicinity of the ion and it depends on the expressed hydration. 5. Conclusion Investigated sweeteners, i.e. rubidium and cesium cyclohexylsulfamates, potassium acesulfame and sodium saccharin are completely dissociated in water solutions. The anions are weakly hydrated due to its hydrophobicity, whereas the hydration of cations depends on their charge densities. 6. Acknowledgements The authors are grateful to Mr. Miha Goličič for technical assistance at conductivity measurements, Dr. Romana Cerc Korošec, Dipl. Chem., for thermo-gravimetric measurements and Prof. Ivan Leban for providing us the X-ray diffraction data. Financial support by the the Slovenian Research Agency through Grants No. P1-0201, P4-0121 and J1-6653 is gratefully acknowledged. 7. References 1. L. O’Brien-Nabors, R.C. Gelardi, Alternative sweeteners: an overwiev in Alternative Sweeteners, Second Ed., Revised and expanded, Marcel Dekker Incorporated, New York, 1991. 2. D. Rudan-Tasic, T. Župec, C. Klofutar, M. Bešter-Rogač, J. Solution Chem. 2005, 34, 631–644. 3. D. Rudan-Tasic, C. Klofutar, Food Chem. 2004, 84, 351–357. 4. D. Rudan-Tasic, C. Klofutar, J. Horvat, Food Chem. 2004, 86, 161–167. 5. W.J. Spillane, A.C. Ryder, M.R. Walsh, P.J. Curran, D.G. Concagh, S.C. Wall, Food Chem. 1996, 56, 255–261. 6. G.G. Birch, K.A. Haywood, G.G. Hanniffy, M.C. Coyle, J.W. Spillane, Food Chem. 2004, 84, 429–435. 7. J. Barthel, H. Krienke, W. Kunz, Physical Chemistry of Electrolyte Solutions-Modern Aspects, Steinkopf/ Darmstadt, Springer/New York, 1998. 8. M. Tomšič, M. Bešter Rogač, A. Jamnik, R. Neueder, J. Barthel, J. Solution Chem. 2003, 31, 19–31. 9. J. Barthel, R. Wachter, H.-J. Gores, in: B. E. Conway and J. O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, New York: Plenum Press, Vol. 13, pp.1–79, 1979. 10. J. Barthel, F. Feuerlein, R. Neueder, R. Wachter, J. Solution Chem. 1980, 9, 209–219. 11. O. Kratky, H. Leopold, H. Stabinger, Z. Angew. Phys. 1969, 27, 273–277. 12. M. Bešter Rogač, R. Neueder, J. Barthel, J. Solution Chem. 1999, 28, 1071–1086. 13. A. Bondi, J. Phys. Chem. 1964, 68, 441–451. 14. C. Klofutar, M. Luci, H. Abramovič, Physiol. Chem. Phys. Med. NMR 1999, 31, 1–8. 4.5 4.0 3.5 3.0 2.5 0.0032 0.0033 0.0036 0.0037 Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of 330 Acta Chim. Slov. 2006, 53, 324–330 15. H.S. Harned, B.B. Owen, The Physical Chemistry of 18. Electrolyte Solutions, 3rd edn., Reinhold, New York, 1958, p. 233. 19. 16. Y. Xu, H-K. Yan, J. Solution Chem. 1993, 22, 919–226. 17. R. A. Robinson, R. H. Stokes, Electrolyte Solutions, 2nd ed. Butterworths, London, 1970, p. 57. Povzetek Izmerili smo električno prevodnost razredčenih vodnih raztopin rubidijevega in cezijevega cikloheksilsulfamata, natrijevega saharina in kalijevega acesulfama v temperaturenem območju med 5 in 35 °C v območju koncentracij med 0.0003 < c/ mol dm3 < 0.01. Na osnovi kemijskega modela smo določili vrednosti molskih prevodnosti pri neskončnem razredčenju, A°°, ter konstane asociacije ionov, KA, v posameznem sistemu. S pomočjo znanih vrednosti limitnih prevodnosti kationov smo ocenili limitne prevodnosti cikloheksilsulfamatnega, saharinovega in acesulfamovega aniona. Ugotovili smo, da je delež ionskih parov v raztopini zanemarljiv in preiskovanim elektrolitom v vodnih raztopinah lahko pripišemo popolno disociacijo v celotnem obravnavanem temperaturnem območju ter zanemarljivo hidratacijo anionov. S.B. Brummer, G.J. Hills, J. Chem. Soc. Faraday Trans. 1961, 5, 1816–1837. F. Barreira, G.J. Hills, J. Chem. Soc. Faraday Trans. 1968, 64, 1359–1375. Rudan-Tasič et al. The Electric Conductivities of Aqueous Solutions of