The Properties of Aqueous Ions

The electrostatic interactions of the cations and anions making up an elecfiolyte in aqueous solutions compete with the thermal movement of all the particles in the solution, ions and water molecules, and are screened by the high dielecfiic permittivity of the water. The overall interactions, involving ion hydration and effects of ions on the water structure, in addition to ion-ion interactions and those of the hydrogen bonded network of water, are quite complicated. Approximations have to be applied in order to handle the resulting behavior of the ions theoretically or by means of computer simulations. 

The simplest approximation is the restrictedprimitive model that considers the ions as charged conducting spheres dispersed uniformly in a continuum fluid made up of a compressible dielectric. The ions are characterized by their charges (sign and magnitude) and sizes (radii), and are assumed to be spherical. The solvent, whether single or a mixture, is characterized by its permittivity, compressibility, and 

The more common process that can be carried out experimentally is to dissolve in water an entire electrolyte, consisting of a matched number of cations and anions to produce a neutral species. Conditions can be chosen for approximating infinite dilution as a limit of extrapolation from low, finite concentrations. This limit corresponds to the dissolution of an infinitesimal amount of electrolyte in a finite amount of water or a mole of electrolyte in a huge amount of water. It is then possible to deal with the molar quantities pertaining to the aqueous electrolyte at infinite dilution. One must still devise some means to deduce from the measured quantities those pertaining to the individual ions. 

The problem of the validity of methods for obtaining the so-called absolute property values of individual aqueous ions was discussed by Conway (1978) and more recently by Marcus (2008a) and by Hiinenberger and Reif (2010). These issues are treated in the following sections dealing with the properties of aqueous ions. 

It is important to bear in mind the consequences of the electric charge on the ion in aqueous solutions. The electric field at the boundary between the ion and its hydration shell is huge, of the order of many GV m For instance, the field right 

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The chemistry of plutonium ions in solution has been thoroughly studied and reviewed (30,94—97). Thermodynamic properties of aqueous ions of Pu are given in Table 8 and in the Uterature (64—66). The formal reduction potentials in aqueous solutions of 1 Af HCIO or KOH at 25°C maybe summarized as follows (66,86,98—100) ... 

These solutions have similar properties and are called acid solutions. The common species in the solutions is the aqueous hydrogen ion, H+(ag), and the properties of aqueous acid solutions are attributed to this ion. We shall investigate these solutions in Chapter 11. 

In studying the properties of solutions of substances such as HC1 and HN03, Arrhenius was led to the idea that the acidic properties of the compounds were due to the presence of an ion that we now write as H30+ in the solutions. He therefore proposed that an acid is a substance whose water solution contains H30+. The properties of aqueous solutions of acids are the properties of the H30+ ion, a solvated proton (hydrogen ion) that is known as the hydronium ion in much of the older chemical literature but also referred to as the oxonium ion. 

Many tables of values of standard changes of enthalpy of formation list values for individual ions, particularly in aqueous solutions. In order to do so, an arbitrary definition must be introduced because the properties of individual ions in solution cannot be determined. We consider an electrolyte Mv+Av which is completely ionized in the infinitely dilute solution. We choose this solution to be the standard state for the enthalpy. The enthalpy of this solution per mole of solute, H, is given by... 

Figure 2.12 illustrates schematically the essential features of the thermodynamic formulation of ACT. If it were possible to evaluate A5 ° and A// ° from a knowledge of the properties of aqueous and surface species, the elementary bimolecular rate constant could be calculated. At present, this possibility has been realized for only a limited group of reactions, for example, certain (outer-sphere) electron transfers between ions in solution. The ACT framework finds wide use in interpreting experimental bimolecular rate constants for elementary solution reactions and for correlating, and sometimes interpolating, rate constants within families of related reactions. It is noted that a parallel development for unimolecular elementary reactions yields an expression for k analogous to equation 128, with appropriate AS °. 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

These observations clearly point to an electrostatic interpretation of the properties of aqueous lyophobie dispersions. Thus early attempts to understand coagulation by electrolytes related it to the adsorption of counter-ions and the neutralisation of the surface charge, a view supported by the empirical observation that coagulation often occurs when the /eta-potential has been reduced to some critical value around 30 mV. However, a quantitative theory has to be based on the more general concept of the electrical double layer and of the influence of electrolyte concentration on its properties. 

A similar ordering of the properties of these ions results from a consideration of phenomena such as the surface tensions of aqueous electrolytes, ionic adsorption at surfaces, ester hydrolysis, and the precipitation of albumin. 

The availability of the HKF model makes simple the demonstration of a feature of single ion properties that we have explained previously. By convention the properties of aqueous anions are taken to be those of the corresponding strong acid, unless there are complicating factors. This means, for example, that the properties of the chloride ion are those of aqueous HCl, and the properties of the sulfate ion are those of aqueous H2SO4. The conventions involved in this were discussed in 17.6.1. 

PURPOSE OF EXPERIMENT Study properties of aqueous ions representing multiple oxidation states of several transition elements, and determine the identity of two vanadium ions by redox titrations with Mn04 . 

Knowledge of the properties of hydrated ions is of fundamental importance to understand the chemistry of these ions in aqueous solutions. The fact that many f-element salts which have relatively large lattice energies are fairly soluble in water is a reflection of the strength of the interactions between the metal cations and water molecules. In turn, this strong hydration competes with complexation by a ligand as the process of complexation involves the displacement of one or more water molecules by a ligand. 

The properties of ions in solution depend, of course, on the solvent in which they are dissolved. Many properties of ions in water are described in Chapters 2 and 4, including thermodynamic, transport, and some other properties. The thermodynamic properties are mainly for 25°C and include the standard partial molar heat capacities and entropies (Table 2.8) and standard molar volumes, electrostriction volumes, expansibilities, and compressibilities (Table 2.9), the standard molar enthalpies and Gibbs energies of formation (Table 2.8) and of hydration (Table 4.1), the standard molar entropies of hydration (Table 4.1), and the molar surface tension inaements (Table 2.11). The transport properties of aqueous ions include the limiting molar conductivities and diffusion coefficients (Table 2.10) as well as the B-coefficients obtained from viscosities and NMR data (Table 2.10). Some other properties of... 

It is clear from our observations that no one method will be sufficient to resolve structure at the required level of detail around all hydrated species. Instead one must rely on a full complement of diffiaction and other techniques including computer simulation to answer the many and outstanding questions regarding the degree to which ions specifically influence the properties of aqueous electrolyte solutions. 

In Chapter 14 (Solutions and Their Physical Properties), we have added a section to describe the standard thermodynamic properties of aqueous ions. We use the concepts of entropy and chemical potential in Chapter 13 to explain vapor pressure lowering and why gasoline and water don t mix. 

The presence of chloric(I) acid makes the properties of chlorine water different from those of gaseous chlorine, just as aqueous sulphur dioxide is very different from the gas. Chloric(I) acid is a strong oxidising agent, and in acid solution will even oxidise sulphur to sulphuric acid however, the concentration of free chloric(I) acid in chlorine water is often low and oxidation reactions are not always complete. Nevertheless when chlorine bleaches moist litmus, it is the chloric(I) acid which is formed that produces the bleaching. The reaction of chlorine gas with aqueous bromide or iodide ions which causes displacement of bromine or iodine (see below) may also involve the reaction... 

The properties of several representative liquid-based ion-selective electrodes are presented in Table 11.3. An electrode using a liquid reservoir can be stored in a dilute solution of analyte and needs no additional conditioning before use. The lifetime of an electrode with a PVC membrane, however, is proportional to its exposure to aqueous solutions. For this reason these electrodes are best stored by covering the membrane with a cap containing a small amount of wetted gauze to... 

Sa.lts Salting out metal chlorides from aqueous solutions by the common ion effect upon addition of HCl is utilized in many practical apphcations. Typical data for ferrous chloride [13478-10-9] FeCl2, potassium chloride [7447-40-7] KCl, and NaCl are shown in Table 9. The properties of the FeCl2-HCL-H2 0 system are important to the steel-pickling industry (see Metal SURFACE TREATMENTS Steel). Other metal chlorides that are salted out by the addition of hydrogen chloride to aqueous solutions include those of magnesium, strontium, and barium. 

The activity of the hydrogen ion is affected by the properties of the solvent in which it is measured. Scales of pH only apply to the medium, ie, the solvent or mixed solvents, eg, water—alcohol, for which the scales are developed. The comparison of the pH values of a buffer in aqueous solution to one in a nonaqueous solvent has neither direct quantitative nor thermodynamic significance. Consequently, operational pH scales must be developed for the individual solvent systems. In certain cases, correlation to the aqueous pH scale can be made, but in others, pH values are used only as relative indicators of the hydrogen-ion activity. 

Prussian Blue. Reaction of [Fe(CN)3] with an excess of aqueous h on(Ill) produces the finely divided, intensely blue precipitate Pmssian Blue [1403843-8] (tetrairon(Ill) tris(hexakiscyanoferrate)), Fe4[Fe(CN)3]. Pmssian Blue is identical to Turnbull s Blue, the name which originally was given to the material produced by reaction of [Fe(CN)3] with excess aqueous h on(Il). The soHd contains or has absorbed on its surface a large and variable number of water molecules, potassium ions (if present in the reaction), and h on(Ill) oxide. The h on(Il) centers are low spin and diamagnetic h on(Ill) centers are high spin. Variations of composition and properties result from variations in reaction conditions. Rapid precipitation in the presence of potassium ion affords a colloidal suspension of Pmssian Blue [25869-98-1] which has the approximate composition KFe[Fe(CN)3]. Pmssian Blue compounds are used as pigments in inks and paints and its formation on sensitized paper is utilized in the production of blueprints. 

Thermodynamic. Thermodynamic properties of Pu metal, gaseous species, and the aqueous ions at 298 K are given in Table 8. Thermodynamic properties of elemental Pu (44), of alloys (68), and of the gaseous ions Pu", PuO", PuO" 27 PuO 2 (67) have been reviewed, as have those of aqueous ions (64), oxides (69), haUdes (70), hydrides (71), and most other compounds (65). 

The extraction of metal ions depends on the chelating ability of 8-hydroxyquinoline. Modification of the stmcture can improve its properties, eg, higher solubility in organic solvents (91). The extraction of nickel, cobalt, copper, and zinc from acid sulfates has been accompHshed using 8-hydroxyquinohne in an immiscible solvent (92). In the presence of oximes, halo-substituted 8-hydroxyquinolines have been used to recover copper and zinc from aqueous solutions (93). Dilute solutions of heavy metals such as mercury, ca dmium, copper, lead, and zinc can be purified using quinoline-8-carboxyhc acid adsorbed on various substrates (94). 

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