J . Chem. SOC., Furaday Trans. I , 1982, 78. 3249-3261 Ion-Ion-Solvent Interactions in Solution Part 5.4nfluence of Added Halide, Change in Temperature and Solvent Deuteration on Ion Association in Aqueous Solutions of Nitrate Salts BY RAY L. FROST Chemistry Department, Queensland Institute of Technology, Brisbane, Australia 400 1 AND DAVID W. JAMES* Chemistry Department, University of Queensland, Brisbane, Australia 4067 Received 2 I st December, I98 1 Band-component analysis of the band due to symmetrical stretching of the nitrate ion in aqueous solutions has been used to study association equilibria. Variation of anion composition at constant ionic strength indicates that the association equilibria are weakly dependent on the nature and concentration of counter-ions. Variation of ionic strength at constant anion concentration produced large changes in the associated species.Association was found to be dependent on both the competitive association equilibria with other anions and of the structural disturbance of the solution. The measurement of the concentration of associated species as a function of temperature in solutions of NaNO, showed that the formation of both solvent-separated ion pairs and contact ion pairs is favoured by a reduction in temperature. Values of the enthalpy change associated with the equilibria *HI AH Na+ (aq) + NO; (aq) Na+ . H,O . NO; (aq) e Na+ . NO; (aq) were found to be AHl = -42.2 kJ mol-l and AH, = -64.56 kJ mol-l. In a comparison of association equilibria in H,O and D,O solutions of LiNO,, NaNO, and KNO, the association equilibria were all altered in a way compatible with the solvent D,O having weaker hydrogen bonding and aquating properties than H,O The effect of ionic strength on association equilibria in solution has been assumed to be small in any but dilute ~olutions.l-~ It is assumed that the ionic medium becomes ‘saturated’ and ionic strength is not an important variable at high concentrations.Different electrolytes have been classified as structure making and structure breaking according to their presumed influence on water s t r u c t ~ r e , ~ although this classification may not be appropriate.6 It is to be expected that different electrolytes will produce different effects on the solvent structure and may also have competing equilibria. Thus for a series of solutions having the same ionic strength the association process may be significantly different.In this paper the influence of ionic strength and different counter-anions is reported for association equilibria in aqueous solutions containing LiNO,, NaNO, and KNO,. Studies of the temperature dependence of the association equilibria in solution should yield useful thermodynamic information on solution processes. The limitations of Raman spectroscopy in this context have been recently reviewed.’ A possible major problem which has not been discussed previously is the assignment of spectroscopic components to particular solution species. An incorrect assignment will obviously lead to erroneous results and possibly values of AH and A S having the wrong sign.In spite of these shortcomings the spectroscopic method, with at least a possibility of 32493250 I 0 N-I ON-SO L V E N T I N ’I’ E R A C T I 0 N S identifying particular solution species, has an advantage over bulk measurements where the nature of the associated species is relatively undefined.6 We report in this paper a study of the temperature dependence of the components of the band due to the symmetrical stretching vibration of the nitrate ion in solutions of NaNO,. Water and D,O have been used interchangably as solvents and although attempts have been made to estimate differences in the strength of hydrogen bonding and deuterium bonding the results have not established a clear di~tinction.~ We examine the influence of D,O on the association equilibria for solutions of LiNO,, NaNO, and KNO,.EXPERIMENTAL D,O was used as supplied by the Australian Atomic Energy Commission (99.5%). The alkali halides were recrystallised twice from water, oven dried for 48 h at 110 O C and stored over P,O,. All other experimental and analytical details were the same as previously described.8 RESULTS AND DISCUSSION ASSOCIATION OF MNO, I N THE PRESENCE o’F MX For all solutions studied the correlation functions and theta functions showed strong similarity to those for the corresponding solution in the absence of halide ion. The variations were those expected if variations of component intensities occurred. The spectra were analysed in terms of the same components as were found for the nitrate solutions with the intensity of all components allowed to vary and the band parameters of bands other than the dilute-solution band allowed to vary.The first study was carried out for mixtures of NaNO, and NaCl at constant ionic strength (total concentration “a+] of 5 mol drn-,) and this was followed by a series of studies in which [NO;] was kept constant at 1 mol dm-, and halide was added up to a total concentration ([M+]) of 6 mol drn-,. Ion association at a constant cation concentration was studied for mixtures of sodium nitrate and sodium chloride with “a+] of 5 mol dm-,. The results are collected in table 1 and fig. 1. The relative intensities of the three bands for concentrations of NaNO, from 1 to 5 rnol dm-, are shown in parentheses. It is clear that association between sodium and nitrate does not follow the concentration of nitrate but varies around the values for 5 mol dm-, sodium nitrate.Apparently the association equilibria are strongly controlled by the concentration of water in the solution, which at 5 mol dm-, salt is of the order of 9 molecules of water per molecule of salt. TABLE 1 .-RELATIVE INTEGRATED INTENSITIES OF COMPONENT BANDS FOR NaNOJNaCl AT 5 rnol dmP3 Na+ concentration/ mol dm-3 aquated-nitrate solvent-separated contact NO; c1- ion ion pair ion pair Kl K2 5 0 0.15 0.25 0.52 2.2 2.2 4 1 0.25 (0.25) 0.23 (0.34) 0.52 (0.38) 1.5 2.26 3 2 0.23 (0.32) 0.30 (0.40) 0.46 (0.25) 1.2 1.53 2 3 0.23 (0.38) 0.29 (0.48) 0.47 (0.13) 1.2 1.62 1 4 0.15 (0.42) 0.36 (0.58) 0.47 (0.01) 2.2 1.30R. L. FROST AND D. W. JAMES 0 325 1 F- 1 I 1 I I 1 1 L 1 I I 1 2 3 4 5 concentration/mol drn-3 FIG.1 .-Variation in component-band intensity with anion composition at constant Na+ concentration ( 5 mol dm-3). (a) Aquated-nitrate band; (b) solvent-separated-ion-pair band; (c) contact-ion-pair band. The changes in the concentration of the three species (as indicated by component- band intensities) reflect the influence of the chloride ion competing in the association processes. As the concentration of chloride increases the concentration of solvent- separated ion pairs increases while the concentration of contact ion pairs decreases. This is compatible with contact ion pairing being favoured for the chloride ion over the nitrate ion. In addition, if the nitrate ion is more strongly hydrated than the chloride ion in a mixed solution the solvent-separated ion pair will be favoured for the nitrate ion.TABLE 2.-RELATIVE BAND INTENSITIES FOR BAND COMPONENTS IN SYSTEMS MNO,/MX ~~ solvent-separated- aquated-ni tra te band ion-pair band contact-ion-pair band electrolyte, 4 shape 4 shape mj shape V V V concentration/mol dm-, /cm-' /cm-' ratio area /cm-l /cm-' ratio area /cm-' /cm-' ratio area e.m.s.LiNO,, 1 $0 LiC1,l .O 2.0 3.0 4.0 5.0 6.0 7.0 LiBr, 1 .O 2.0 3.0 4.0 5.0 LiI, I .O 2.0 3.0 4.0 5.0 NaNO,, 1 .O NaC1,O.O 1 .o 2.0 3.0 4.0 LiNO,, 1 .O LiNO,, 1 .O - - 1047.6 3.397 - - - - 1047.6 3.397 - - 0.644 0.636 0.352 0.197 0.150 0.1 15 0.036 0.723 0.475 0.422 0.214 0.086 0.760 0.651 0.566 0.403 0.055 0.389 0.384 0.356 0.278 0.156 4.97 4.94 4.87 5.07 4.88 5.02 5.49 4.86 5.07 5.28 4.77 4.86 5.12 5.07 5.19 4.97 5.05 3.55 3.43 3.28 3.67 3.69 0.756 0.750 0.718 0.712 0.658 0.734 0.621 0.713 0.756 0.813 0.695 0.690 0.73 1 0.726 0.705 0.696 0.675 0.90 0.814 0.755 0.729 0.728 0.345 0.354 0.643 0.663 0.619 0.572 0.574 0.247 0.518 0.553 0.547 0.612 0.220 0.321 0.412 0.460 0.570 0.595 0.439 0.430 0.435 0.496 - - - 8.95 8.90 7.70 7.48 - - 7.95 7.55 7.65 - - 7.85 7.75 7.75 8.1 1 5.00 5.93 5.29 4.87 - - - 0.647 0.647 0.606 0.526 - - 0.735 0.769 0.706 - - 0.696 0.695 0.690 0.857 0.908 0.893 0.903 0.841 0.00 1 0.001 0.001 0.135 0.232 0.293 0.390 0.001 0.00 1 0.024 0.226 0.367 0.00 1 0.001 0.056 0.141 0.375 0.001 0.167 0.200 0.282 0.36 1 1.38 1.30 8.30 0.362 0.69 0.366 2.02 1.429 1.291 6.244 0.016 0.964 1.64 2.75 0.81 1.78 1.63 0.08 0.06 0.04 0.01 0.13NaN0,,4.0 3.0 2.0 1 .o NaNO,, 1 .O KNO,, 1 .O KNO,,l .O KNO,, 1 .O KNO,, 1 .O NaC1,l .O 2.0 3.0 4.0 NaI, 1 .O 2.0 3.0 4.0 KF, 1 .O 2.0 3.0 4.0 KC1,l .O 2.0 3.0 4.0 KBr, 1 .O 2.0 3 .O 4.0 KI, 1 .O 2.0 3.0 4.0 5.0 1047.6 - 3.397 0.852 - - - 1047.6 - - - 3.397 0.852 - - 1047.6 - - 1047.6 - - - 3.397 0.852 - - 3.397 0.852 0.245 0.230 0.23 1 0.145 0.546 0.392 0.259 0.155 0.520 0.391 0.304 0.164 0.509 0.463 0.223 0.157 0.494 0.417 0.384 0.226 0.453 0.486 0.43 1 0.344 0.345 1049.1 3.55 3.56 - 3.56 - 3.56 1049.0 3.56 3.39 3.81 3.60 - - - - 0.90 0.90 0.90 0.90 0.902 0.782 0.719 0.769 0.234 0.307 0.296 0.361 0.420 0.416 0.540 0.48 1 1052.6 4.22 4.54 - 4.43 - 4.72 1052.5 3.26 4.67 5.12 - 4.79 1049.7 3.397 3.397 - 3.397 3.397 3.43 - 3.447 - 3.52 3.60 3.50 - 3.43 3.49 - 3.67 3.79 3.86 3.48 .- 3.61 3.78 - - - - - - - - - - - - __ 0.71 1 0.679 0.672 0.650 0.756 0.906 0.675 0.679 0.826 0.823 0.81 1 0.795 0.799 0.806 0.8 17 0.784 0.837 0.800 0.759 0.820 0.782 0.724 0.824 0.810 0.759 0.519 0.456 0.467 0.491 0.032 0.178 0.189 0.354 0.450 0.596 0.684 0.825 0.477 0.527 0.765 0.823 0.497 0.574 0.619 0.750 0.442 0.507 0.563 0.643 0.650 1.629 1.223 1.145 0.723 0.64 0.08 0.02 0.09 1.26 2.46 1.09 2.52 0.90 1.20 1.19 0.9 1 0.620 1.210 0.327 0.719 1.13 2.05 1.45 1.65 0.633254 I ON-I ON-SO L V EN T INTER ACT I 0 N S For solutions where the concentration of nitrate is kept constant and halide is progressively added, the results may be compared with nitrate solutions containing the same cation concentrations.Results for relative band intensities for baild components are collected in table 2 and these will be discussed according to cation.The results for solutions having potassium cations are summarised in fig. 2. Because of the limited solubility of potassium nitrate comparison of association behaviour in the presence and absence of halide is not possible over all concentrations. The addition of one mole of potassium halide does not change the relative concentration of free and associated nitrate ion. The increase of cation concentration produces considerable reduction in the calculated association quotient. However, there are competing association equilibria in the halide system and also a change in free water and so these association quotients have little absolute significance. The changes in relative intensities of band components do show systematic trends which can be interpreted in terms of solution interactions.In solutions containing fluoride ions the aquated nitrate is reduced relative to those containing iodide, with the tendency being directly related to anion size. A complementary trend is seen for the associated species. Association between the potassium ion and halide ion will be more likely with a larger anion and so the observed trend can be understood in terms of competition for cations between the two anionic species. Solutions of NaNO, (1 mol drn-,) with progressively increasing halide ion concen- tration have been analysed and the results are summarised in fig. 3. At 1 mol dm-, added halide the aquated nitrate is favoured over the associated species and as the concentration of halide increases the solvent-separated ion pairs becomes favoured at the expense of the aquated nitrate ion while the contact ion pair remains lower than for solutions containing only nitrate solute. The variation in the contact ion pairs shows anion dependence, the influence being greater for iodide than for chloride.This is in agreement with the observations in solutions of potassium salts and emphasises the competitive equilibria for contact ion pairs with the largest most polarizable anions preferentially forming the contact ion pairs. The variations seen for the aquated nitrate species and the solvent-separated ion pairs may be due to the disturbance of the water structure by the added halides, both of which can be considered structure breakers due to the large anion size.The disturbances to the water structure facilitate the formation of free aquated ions and as the solute concentration increases the concentration of solvent-separated ion pairs with the nitrate remains high because formation of contact ion pairs takes place preferentially to the halide anion. Similar but more pronounced variations are observed for the systems containing lithium cations, as illustrated in fig. 4. The concentration of the aquated nitrate ion is enhanced by the presence of halide and this enhancement is directly dependent on anion size, being most pronounced for the iodide ion. The concentrations of the solvent- separated ion pairs with nitrate are initially depressed but then are enhanced by the presence of halide ions, with the effect being inversely dependent on the size of the halide ion.Finally, the concentration of contact ion pairs is depressed by the presence of halide with the chloride producing the greatest change. These results may again be understood in terms of competing equilibria and disturbance of the water structure. When the network association between water molecules is disturbed by large anions the formation of aquated cations (and anions) will be enhanced. Thus in the presence of iodide ions there is a pronounced increase in free aquated nitrate ions which occurs at the expence of the formation of solvent-separated ion pairs. At higher concentrations where contact-ion-pair formation is expected such species are formed preferentially with the large polarizable anion and the formation of contact ion pairs with nitrate is depressed. This in turn causes an increase in the proportion of solvent-separated ion pairs, this increase being dependent on the size of the halide ion.R.L. FROST AND D. W. JAMES 3255 0.6 0.4 0.2 0.6 x u w? E e, .- + .s 0.4 aJ .- Y cd - 2 0.2 0.6 0 . 4 0.2 [Na+]/mol dn1-3 FIG. 3.-Variation in component-band with anion composition in the presence of sodium cations. ( a ) Aquated-nitrate band ; (b) solvent-separated-ion-pair band ; (c) contact-ion-pair band. (-) NaNO, ; (- - - -) 1 mol dm-3 NaNO, +x mol dm-3 NaX.3256 I 0 N-I 0 N-S 0 L V E N T INTER ACT I ON S O.( 0.1 0 . i 0 . € h c U .- U .5 0.4 Q) .- Y - m Q) 0.2 0.6 0 .I 0.2 / b l \ 4- -. Cl-,’,/-- ‘ / / /’// / / / A- / ( C l [ Li+l/mol dm-3 FIG.4.-Variation in component-band intensity with anion composition in the presence of lithium cations. (a) Aquated-nitrate band; (b) solvent-separated-ion-pair band; (c) contact-ion-pair band. (-) LiNO, ; (----) 1 mol dm-3 LiN03+x mol dmP3 LiX.R. L. FROST AND D. W. JAMES 3257 The bandwidths and band shapes give evidence of increased perturbation caused by the presence of added halide. The bandwidth is broader and the band shows greater Gaussian character in the presence of halide for both the solvent-separated- ion-pair band and the contact-ion-pair band. The increased width in the solvent- separated-ion-pair band indicates a greater symmetry perturbation leading to more rapid dephasing. This may come from a change in the relative orientation of the anion and cation or possibly from a long-range environmental disturbance due to the additional ionic species.The increased width for the contact-ion-pair band corresponds to a reduction in vibrational relaxation from ca. 1 to 0.65 ps. This increased width is best attributed to a long-range environmental perturbation and the associated increase in Gaussian character is compatible with a change to a more rigid structural character in the solution. This would agree with the greater population of more strongly bound ion pairs due to the presence of halide ion. INFLUENCE OF TEMPERATURE ON THE ASSOCIATION EQUILIBRIA It has been suggestedg that the influence of temperature on the concentration of free and bound species may be quite small.Two studies agree that the formation of inner-sphere complexes ZnNOl is favoured by an increase in temperature,l0? l1 and in solutions of MgSO,, for the formation of contact ion pairs, a AH of ca. 12 kJ mol-l was obtained.12 A study of so1utions13 of Ca(NO,), led to a value of AH = 1.1 kJ mol-1 for a complex which is probably an inner-sphere complex. A solution of sodium nitrate (2 mol dmP3) was studied over the temperature range - 5 to 65 "C and the resulting profiles for the band due to the symmetric stretching vibration were component-band analysed. Three band components were required to describes the profile and these had temperature-independent shape and half-width. The variation in intensity of these three components is shown in fig. 5, from which it is clear that both the solvent-separated species and the contact pair are favoured as the temperature decreases.Although the equilibrium constants calculated from the intensities of band com- ponents are not related to any standard state, estimates of the enthalpy and entropy changes for the equilibria may be obtained from the representation in fig. 6. The 0. .- c 0 . v1 K 2 0 . .- 0 . 0 . 0 . 0 . I 0 10 20 30 40 50 60 temperature/OC' FIG. 5.--Variation in component-band intensity with temperature for 2 mol dm-3 aqueous NaNo,. (a) Aquated-nitrate band ; (h) solvent-separated-ion-pair band; (c) contact-ion-pair band.3258 4 3 2 1 o n 0 1 * - 2 3 4 ION-I ON-SO LVEN T I N TE R A C T I ON S FIG. 6.-Variation of K , with 1/T for the results in fig. 5. obtained values of AH, = -42.2 kJ mol-1 and AH, = - 64.6 kJ mol-l are remarkably larger than expected on the basis of previous studies.l27 l3 The values for the entropy changes of ASl = - 142 J K-l mol-1 and AS2 = - 294 J K-l mol-l indicate that there is a pronounced decrease in entropy on formation of associated species.The results for AH and A S are large in comparison with previous estimates but the change in free energy for the association equilibrium is still quite small because of the balance of energy and entropy terms. Thus at 296 K the value for AG is zero; at higher temperatures AG is positive and the aquated nitrate is favoured while at lower temperatures AG is negative and the solvent-separated ion pair is favoured. The association process involves changes to the total aquation energies which are relatively small (if changes in water-water interactions are excluded).These changes involve the disruption of the solvent sphere about both anion and cation due initially to the disturbance resulting from sharing of a water molecule and secondly to the removal of a solvent to allow direct anion-cation interaction. The major energy changes can be associated with the coulombic interaction between the two ions. This interaction can be visualised as the approach of an anion and a cation to an initial stable separation corresponding to the solvent-separated ion pair and then to a separation corresponding to the contact ion pair. Calculation of such ion-pair binding energies is rendered very difficult because of the planar nature of the nitrate ion and the uncertainty about the stereochemistry of the approach. Assuming an equatorial approach of the cation along a nitrogen-oxygen bond axis the expected Na-N distance would be ca.350 pm.14 If the ions are given central point charges the ion-pair binding energy would be ca. 400 kJ mol-l. The combination of AH, and AH, of - 106 kJ mol-l is much less than this but the relativity of values indicates that the large values of AHl and AH, obtained are not unreasonable on the basis of the expected ion-pair interaction energies.R. L. FROST AND D. W. JAhlES 3259 1 . 0 0.8 0.6 0 . 4 0 . 2 0 . c 0.8 A 4- .& g O * € W c W c) ..-, .- % 0 . 4 - 2 0.2 0 . c 0.8 0.6 0 . 1 0 . 2 0 A '\ \ B / C 1 2 I 6 8 cation concentration/mol dm-3 FIG. 7.-Comparison of component-band intensities in solutions in H,O (---) and D,O (-).A, KNO,; B, NaNO,; C, LiNO,; (a) aquated-nitrate band; (b) solvent-separated-ion-pair band; ( c ) contact-ion-pair band; (d) ion-aggregate band; (e) aquated-ion-pair band. Band parameters for the various bands with the half-width (cm-I) and shape ratio. LiNO,/D,O: band 1 , 3.40, 0.85; band 2, 5.1, 0.84; band 3, 5.6, 0.79; band 4, 6.8, 0.76. NaNO,/D,O: band 1, 3.40, 0.85; band 2, 4.0, 0.90; band 3, 4.5, 0.70; band 4, 9.5, 0.68. KNO,/D,O: band 1, 3.40, 0.84; band 2, 3.5, 0.90; band 3, 6.7, 0.76.3260 I 0 N-ION-SO LVE N T INTER ACT I 0 N S ASSOCIATION I N D,O AS SOLVENT The difference between hydrogen bonding and solvent properties of D20 and H,O is undoubtedly small and in most instances the two solventscan be used interchangeably.Estimates of the difference in hydrogen-bond strength between the two have been inconcl~sive.~ While the time correlation functions for the v1 band of NO, in solutions of LiNO,, NaNO, and KNO, are significantly different in H,O and D20, the theta functions in the two solvents are very similar. The energy band profiles in D20 were analysed assuming that they contained the same number of bands as the aqueous- TABLE 3.-EQUILIBRIUM QUOTIENTS OF GROUP I NITRATES IN H 2 0 AND D20 electrolyte concentration/ mol dm-3 K K2 LiNO, in H,O LiNO, in D20 NaNO, in H 2 0 NaNO, in D,O KNO, in H,O KNO, in D,O RbNO, in H,O 0.5 1 .o 2.0 4.0 6.0 8.0 10.0 0.5 1 .o 2.0 4.0 6.0 8.0 10.0 0.5 1 .o 2.0 4.0 6.0 8.0 0.5 1 .o 2.0 4.0 6.0 0.5 1.0 2.0 3.0 0.5 1 .o 2.0 3.0 1 .o 2.0 3.0 1.79 1.34 0.586 0.862 0.552 2.200 0.926 0.605 2.465 3.81 1 6.6 53.89 115.9 2.74 3.28 I .69 1.34 3.50 8.40 1.5 1.37 0.729 1.38 3.74 11.22 - - - - - - - - - - - 0.0026 0.9 14 0.49 1 5 1 .oo 1.49 1.78 2.776 - - - 0.120 0.500 3.53 2.86 - - 0.269 1.122 3.26 4.35 - - 0.1178 0.843 1.06 1.046 1.95 1.85 1.88 0.440 0.430 0.639 0.582 1.0375 0.9457 0.8205R.L. FROST AND D. W. JAMES 326 I solution bands. In addition, the parameters from the aqueous-solution bands were used as starting parameters in the analysis. The results of the analysis are summarised in fig. 7. The band parameters for the solutions of NaNO, and KNO, are similar in H,O and D,O. However, parameters of band components for solutions of LiNO, in D20 are significantly different from those in H,O.For both the solvent-separated-ion-pair band and the contact-ion-pair band the parameters indicate a more Gaussian shape and a greater half-width in D,O solution. This implies that the perturbation of the nitrate ion is greater in the presence of D,O, which could be due to the influence of the Li+ ion being less ‘moderated’ by the D,O. The component-band intensities (fig. 7) are appreciably different in D,O from those observed in H,O and this difference is emphasised in the equilibrium quotients listed in tableg. For both NaNO, and KNO, solutes the values of the association quotients in D20 are much less than the values in H,O. If the hydrogen bonding in D,O was significantly weaker than in H,O aquation of the cations could be expected to disturb the structure more and this, together with the weaker hydrogen bonding to the nitrate ion, will destabilise the solvent-separated ion pair and will allow the entropic driving force to favour the free aquated ions.The differences in association quotients for LiNO, solute show more complex variation. In dilute solution in D20 the values are much smaller than in H,O, showing that the free aquated ions are again favoured. However, above 2 mol dm-, the association species become more favoured than in water. The formation of associated species in water was influenced by the strong hydration of the lithium ion which moderated the strong polarizing power of the cation. If aquation by D,O is less strong than that by H,O, then this moderating influence would be reduced and the influence of the small cation would be evident.In this regard the k , values for 2 mol dm-, solutions of LiNO,, NaNO, and KNO, in D,O follow the inverse of the order of cation size, i.e. follow the cation polarizing power. The changes in association quotients are all compatible with hydrogen bonding and aquation by D,O being weaker than that by H,O. Note that because of the small values for AG reported in the previous section the changes in bonding and aquation forces could be quite small to produce the changes observed. We thank the Australian Research Grants Committee for grants enabling the purchase and maintenance of the Raman spectrometer. D. E. Irish and M. H. Brooker, in Advances in Infrared and Raman Spectroscopy, ed. R. J. H. Clark and R. E. Hester (Heyden, London, 1976), vol. 2, p.212. J. Nixon and R. A. Plane, J . Am. Chem. SOC., 1962, 84, 4445. J. T. Bulmer, T. G. Chang, P. J. Gleeson and D. E. Irish, J . Solution Chem., 1975, 4, 969. J . Bjerrum, Trans. R. Inst. Technol., Stockholm, 1972, 69, 248. G. E. Walrafen, in Water: A Comprehensive Treatise, ed. F. Franks (Plenum Press, London, 1972), vol. I , p. 151. P. G. Wolynes, Annu. Rev. Phys. Chem., 1980, 31, 345. R. L. Frost and D. W. James, J. Chem. Soc., Faraday Trans. I , 1982, in press. M. H. Brooker, J . Chem., SOC., Faraday Trans. I , 1975, 71, 647. I 1 A. T. G. Lemley and R. A. Plane, J. Chem. Phys., 1972, 57, 1648. l 3 R. E. Hester and R. A. Plane, J . Chem. Phys., 1964, 40, 41 1 . l4 D. W. James, Aust. J . Chem., 1966, 19, 993. ’ B. E. Conway, Ionic Hydration in Chemistry and Biophysics (Elsevier, New York, 1981), p. 552. l o S. A. Al-Baldawi, M. H. Brooker, T. E. Gough and D. E. Irish 1982, 78, 3235. 1202. R. M. Chatterjee, W. A. Adams and A. R. Davis, J. Phys. Chem., 1974, 78, 264. (PAPER 1/2001)