Ions, association

The presence of ion association in polymer electrolytes was alluded to in Section 6.2.1. Here we will consider the nature of the clusters and the 

The formation of solvent separated ion aggregates is largely determined 

Both X-ray diffraction and Raman spectroscopy have been applied in an attempt to probe the nature of ion association. The latter technique relies on monitoring changes in the vibrational modes of the anions, particularly CFjSOj or CIO. , anions as their immediate environment changes. It is important to note that only contact ion clusters will be 

The eventual upturn of the mean ionic activity coefficients as the electrolyte concentration increases, described in Equation 7.11 by the linear term Cm and in Equations 7.6 and 7.7 by higher powers of the concentration, can be interpreted in several ways. A concept that differs from that involving hydration numbers. Equations 7.14 and 7.15, is in terms of the association of ions of opposite charges. In fact, ion association competes with the solvation of the ions and in certain cases an ion of opposite charge may replace some of the solvent in the solvation shell of a given ion. 

The overall association constant is =K, +K,K.+K,K.K. Most methods for studying ion association (such as conductivity or potentiometry, see below) provide values only for or K, but some methods (Section 1.12 and volumetric data according to Hemmes [25]) are able to distinguish between the three kinds of ion pairs. 

Qualitative views of ion association have been derived from the trends dealt with above of activity coefficients of series of ions with a common counterion. The trends for at 1 m, being RbCl Rbl and CsCl Csl, may be compared with the opposite, increasing trends of the lighter alkali metal halides, MCI MBr MI, see Table 7.1. 

FIGURE 7.3 The hydration numbers h(c) of aqueous salts at 25°C obtained from isothermal compressibility data for LiCl (-0-) and NaOH (- -). At the right-hand side are shown the BET parameters r (number of water-binding sites per formula unit of the salt) of these salts (large filled symbols) (From Ref. 21 with permission from the pubhsher, ACS). 

FIGURE 7.4 The Eigen-Tamm scheme for stepwise formation from free solvated cations X and solvated anions of 2SIP ion pairs, then SIP ion pairs, and finally CIP ion pairs, with elimination of solvent molecules from the solvation shells of the ions (From Ref. 24 by 

It is important to remember that reactions such as the above do not invalidate our choice of a strong electrolyte standard state. We repeat that thermodynamics does not tell us what is happening in solution on a molecular (ionic) level. The observation we make is that 7 deviates more for mixtures of 2 2 electrolytes than for solutions of 1 1, 1 2, and 1 3 electrolytes, and we attribute this deviation to ion association. But we can handle these deviations perfectly well in our treatment without assuming ion association. We can stay with our strong electrolyte standard state and accept as a fact of life that, at a given low concentration, j deviates more from unity than for other mixtures. 

An analogy can be made with nonelectrolyte mixtures. For (nonpolar+ nonpolar) mixtures, near-ideal solution behavior occurs and activity coefficients given by 

In a similar manner, standard states can be chosen for electrolytes that take into account molecular association. We call this the weak electrolyte standard state, and some method must be employed to determine the extent of association. As an example, we usually treat nitric acid HN03 as a strong electrolyte so that in solution 

But there is evidence that, at least in more concentrated solutions, ion association occurs. That is 

For example, Raman spectroscopy techniques show vibrational transitions that can be attributed to the HN03 molecule as well as to the NO ion.11 By using the intensities of the Raman bands to measure the concentrations of the species in solution, an equilibrium constant IQ can be written 

It is found that in many cases experimental values of conductances do not agree with theoretical values predicted by the Onsager equation (see Equation (4.18)) and that mean ion activity coefficients cannot always be properly predicted by the Debye-Huckel theory. It was suggested by Bjerrum that, under certain conditions, oppositely charged ions of an electrolyte can associate to form ion pairs. In some circumstances, even association to the extent of forming triple or quadruple ions may occur. The most favourable situation for association is for smaller ions with high charges in solvents of low dielectric constant. Hence such phenomena occur to a usually small extent in water. 

Association leads to a smaller number of particles in a system and associated species have a lower charge than non-associated ones. This will obviously serve to diminish the magnitudes of properties of a solution which are dependent on the number of solute particles and the charges carried by them. 

Bjerrum s basic assumption was that the Debye-Huckel theory holds so long as the oppositely charged ions of an electrolyte are separated by a distance q greater than a certain minimum value given by 

When the ion separation is less than q, ion pairing is regarded as taking place. Equation (2.51) may be derived from a consideration of the Boltzmann dis- 

The number of i-type ions in such a shell is given by 

For strong electrolytes, the activity of molecules cannot be considered, as no molecules are present, and thus the concept of the dissociation constant loses its meaning. However, the experimentally determined values of the dissociation constant are finite and the values of the degree of dissociation differ from unity. This is not the result of incomplete dissociation, but is rather connected with non-ideal behaviour (Section 1.3) and with ion association occurring in these solutions (see Section 1.2.4). 

Arrhenius also formulated the first rational definition of acids and bases  

An acid (HA) is a substance from which hydrogen ions are dissociated in solution  

A base (BOH) is a substance splitting off hydroxide ions in solution  

This approach explained many of the properties of acids and bases and many processes in which acids and bases appear, but not all (e.g. processes 

---

Camp P J and Patey G N 1999 Ion association and condensation in primitive models of electrolytes J. Chem. Phys. 

The concentration of anionic surfactants at the sub-ppm level in natural waters and industrial waters are determined spectrophotometrically. The anionic surfactants are extracted into a nonaqueous solvent following the formation of an ion association complex with a suitable cation. 

Size Isomers. In solution, hGH is a mixture of monomer, dimer, and higher molecular weight oligomers. Furthermore, there are aggregated forms of hGH found in both the pituitary and in the circulation (16,17). The dimeric forms of hGH have been the most carefully studied and there appear to be at least three distinct types of dimer a disulfide dimer connected through interchain disulfide bonds (8) a covalent or irreversible dimer that is detected on sodium dodecylsulfate- (SDS-)polyacrylamide gels (see Electroseparations, Electrophoresis) and is not a disulfide dimer (19,20) and a noncovalent dimer which is easily dissociated into monomeric hGH by treatment with agents that dismpt hydrophobic interactions in proteins (21). In addition, hGH forms a dimeric complex with ( 2). Scatchard analysis has revealed that two ions associate per hGH dimer in a cooperative... 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

Because of fluoride ion associated renal impairment, the duration of anesthesia using methoxyflurane must be limited (51,52). 

A chelate compound may be either a neutral molecule or a complex ion associated with the appropriate counterions to produce electroneutraUty. 

Unloaded silica does not recover HPA from aqueous solution. The surface of silica gel modified with quarternary ammonium salts (QAS) gets anion-exchange properties. The aim of the work is the elaboration of solid-phase reagents on the base of ion associate of HPA with QAS immobilized onto silica surface for the determination of phosphoms and organic reductants. Heterocyclic (safranine and lucigenine) and aliphatic (trinonyloctadecyl ammonium iodide and tetradecyl ammonium nitrate) compounds have been examined as QAS. 

Two techniques for sorption-spectroscopic determination of ascorbic acid have been proposed. The first one is the recovery by silica modified with tetradecyl ammonium nitrate of blue form of molibdophosphoric HPA in the presence of vitamin C. And the second one is the interaction between the ascorbic acid in solution and immobilized on silica ion associate of molibdophosphoric acid with lucigenine. The detection limits of vitamin C are 0.07 and 2.6 mg respectively. The techniques were successfully applied to the determination of ascorbic acid in fmit juices. 

Realization of many photometric reactions in water-organics mediums often leads to substantial increasing their sensitivity and selectivity. However, the description of extraction of ion-associates (lA) of basic dyes from water-organic mediums practically is absent in scientific literature. 

In this work hybrid method is suggested to determine anionic surfactants in waters. It is based on preconcentration of anionic surfactants as their ion associates with cationic dyes on the membrane filter and measurement of colour intensity by solid-phase spectrophotometry method. Effect of different basic dyes, nature and hydrophobicity of anionic surfactants, size of membrane filter pores, filtration rate on sensitivity of their determination was studied. Various cationic dyes, such as Methylene Blue, Crystal Violet, Malachite Green, Rhodamine 6G, Safranin T, Acridine Yellow were used as counter ions. The difference in reflection between the blank and the sample was significant when Crystal Violet or Rhodamine 6G or Acridine Yellow were used. 

SOLID-PHASE SPECTROPHOTOMETRIC AND TEST DETERMINATION OF CATIONIC SURFACTANTS ON PAPER FILTERS AS ION ASSOCIATE WITH BROMPHENOL BLUE... 

In this work hybrid method is suggested to determine cationic surfactants in water. It is based on preconcentration of cationic surfactants in the some of ion associates with acidic dyes on the paper filter and measurement of color intensity by solid-phase specdophotomenic method or visual comparison. 

In the present work, the technique of XO and MTB immobilization onto silica gel in the form of its complexes with Fe(III) and Bi(III) respectively were found. The acid - base and chemical-analytical characteristics of solid-phase reagents were examined. The optimal conditions of quantitative recovery of Pb(II) and Zn(II) from diluted solutions, such as acidity of aqueous phase, the mass of the sorbents, the volume of solutions and the time of equilibrium reaching, were found. The methods of and F" detenuination were based on a competitive reactions of Zr(IV) with immobilized MTB and or F". Optimal conditions of 0,0 and F" determination in solution using SG, modified ion associates QAS-MTB (pH = 1,5, = 5-10 mol/1). 

When diazomethane is slowly added to excess lactam, the anions formed can interact with unreacted lactam by means of hydrogen bonds to form ion pairs similar to those formed by acetic acid-tri-ethylamine mixtures in nonpolar solvents. The methyldiazonium ion is then involved in an ion association wdth the mono-anion of a dimeric lactam which is naturally less reactive than a free lactam anion. The velocity of the Sn2 reaction, Eq. (7), is thus decreased. However, the decomposition velocity of the methyldiazonium ion, Eq. (6a), is constant and, hence, the S l character of the reaction is increased which favors 0-methylation. It is possible that this effect is also involved in kinetic dependence investigations have shown that with higher saccharin concentrations more 0-methylsaccharin is formed. 

Furthermore, about 1920 the idea had become prevalent that many common crystals, such as rock salt, consisted of positive and negative ions in contact. It then became natural to suppose that, when this crystal dissolves in a liquid, the positive and negative ions go into solution separately. Previously it had been thought that, in each case when the crystal of an electrolyte dissolves in a solvent, neutral molecules first go into solution, and then a certain large fraction of the molecules are dissociated into ions. This equilibrium was expressed by means of a dissociation constant. Nowadays it is taken for granted that nearly all the common salts in aqueous solution are completely dissociated into ions. In those rare cases where a solute is not completely dissociated into ions, an equilibrium is sometimes expressed by means of an association constant that is to say, one may take as the starting point a completely dissociated electrolyte, and use this association constant to express the fact that a certain fraction of the ions are not free. This point of view leads directly to an emphasis on the existence of molecular ions in solution. When, for example, a solution contains Pb++ ions and Cl- ions, association would lead directly to the formation of molecular ions, with the equilibrium... 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

An alternative to the formation of neutral metal chelates for solvent extraction is that in which the species of analytical interest associates with oppositely charged ions to form a neutral extractable species.6 Such complexes may form clusters with increasing concentration which are larger than just simple ion pairs, particularly in organic solvents of low dielectric constant. The following types of ion association complexes may be recognised. 

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

Those in which solvent molecules are directly involved in formation of the ion association complex. Most of the solvents (ethers, esters, ketones and alcohols) which participate in this way contain donor oxygen atoms and the coordinating ability of the solvent is of vital significance. The coordinated solvent molecules facilitate the solvent extraction of salts such as chlorides and nitrates by contributing both to the size of the cation and the resemblance of the complex to the solvent. 

Spectrophotometric methods may often be applied directly to the solvent extract utilising the absorption of the extracted species in the ultraviolet or visible region. A typical example is the extraction and determination of nickel as dimethylglyoximate in chloroform by measuring the absorption of the complex at 366 nm. Direct measurement of absorbance may also be made with appropriate ion association complexes, e.g. the ferroin anionic detergent system, but improved results can sometimes be obtained by developing a chelate complex after extraction. An example is the extraction of uranyl nitrate from nitric acid into tributyl phosphate and the subsequent addition of dibenzoylmethane to the solvent to form a soluble coloured chelate. 

Discussion. The method is based upon the complexation of boron as the bis(salicylato)borate(III) anion (A), (borodisalicylate), and the solvent extraction into chloroform of the ion-association complex formed with the ferroin. 

The method has been applied to the determination of boron in river water and sewage,16 the chief sources of interference being copper(II) and zinc ions, and anionic detergents. The latter interfere by forming ion-association complexes with ferroin which are extracted by chloroform this property... 

DETERMINATION OF SILVER BY EXTRACTION AS ITS ION ASSOCIATION COMPLEX WITH 1.10-PHENANTHROUNE AND BR0M0PYR0GALL0L RED... 

Discussion. Silver can be extracted from a nearly neutral aqueous solution into nitrobenzene as a blue ternary ion association complex formed between silver(I) ions, 1,10-phenanthroline and bromopyrogallol red. The method is highly selective in the presence of EDTA, bromide and mercury(II) ions as masking agents and only thiosulphate appears to interfere.8... 

R is the distance parameter, defining the upper limit of ion association. For spherical ions forming contact ion pairs it is simply the sum of the crystallographic radii of the ions a — a+ + a for solvent-shared and solvent-separated ion pairs it equals a + s or a + 2s respectively, where s is... 

In contrast to bilateral triple-ion formation, unilateral triple-ion formation may also occur in solvents of high permittivity, when ion-pair association is increased by noncoulombic specific ion-ion interactions in solvents of low basicity such as PC or AN. Exclusive formation of anionic tripleions [A-C+A-] ", is observed in these solvents when large organic molecular anions A interact with small cations such as Li + or H+. For example, in contrast to lithium acetate in DMSO [97], where ion association is moderate, ion association as well as unilateral triple-ion formation is observed in the solvent PC [105] due to the much lower basicity of this solvent, (see Table 2)... 

Selective Solvation of Ions and Competition Between Solvation and Ion Association. 

The most spectacular feature of a conductivity-concentration function is its maximum, attained for every electrolyte if the solubility of the salt is sufficiently high. For electrolytes which do not show strong ion association, the maxima can be understood on the basis of the defining equation of specific conductivity at the maximum [205], yielding... 

---