Solute dissociation effects

The conductivity also increases in solutions of weak electrolytes. This second Wien effect (or field dissociation effect) is a result of the effect of the electric field on the dissociation equilibria in weak electrolytes. For example, from a kinetic point of view, the equilibrium between a weak acid HA, its anion A" and the oxonium ion H30+ has a dynamic character ... 

The following treatment has been suggested by Shiu et al. (1994) and is reproduced briefly below. The simplest, first-order approach is to take into account the effect of dissociation by deducing the ratio of ionic to non-ionic species I, the fraction ionic x and the fraction non-ionic xN for the chemical at both the pH and temperature of experimental data determination (/D, xID, xND) and at the pH and temperature of the desired environmental simulation (/E, xIE, xNE). It is assumed that dissociation takes place only in aqueous solution, not in air, organic carbon, octanol or lipid phases. Some ions and ion pairs are known to exist in the latter two phases, but there are insufficient data to justify a general procedure for estimating the quantities. No correction is made for the effect of cations other than H+. This approach must be regarded as merely a first correction for the dissociation effect. An accurate evaluation should preferably be based on experimental... 

For an ionic solute dissociating into v ions, the temperature coefficient is 1/v times the right-hand side of Eq. (2.60). Again, it is assumed that the solubility is sufficiently low for the mean ionic activity coefficient to be effectively equal to unity and independent of the temperature. When this premise is not met, then corrections for the heat of dilution from the value of the solubility to infinite dilution must be added to Asoi //°b in Eq. (2.60). 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

Here we discuss a thermodynamic model appropriate to describe effects of strong association in dilute solutions. To have a definite example, consider a dilute electrolyte solution of a salt, say M X, that in solution dissociates to produce cations M of charge qu e and anions X of charge —qx e with aq [ = bqx- The interactions between these ions are composed of short-ranged interactions and long-ranged ionic interactions screened by the dielectric response of the solvent with dielectric constant e, as with r the distance between the ions. If the... 

In (7-5) the three sets of vertical reversible arrows represent reactions of MA (including the ion pair M A ), M, and A with other entities in solution. The effect of these additional equilibria, which is to increase solubility, is the principal subject treated in this chapter. In Sections 7-2 to 7-5 the simplifying assumption is made that solubilities and dissociation constants are such that [MA] is negligible. When the additional equilibria of Equation (7-5) are included, solubility becomes equal to the sum of [MA] + [MA(X)] + [M+] + [M+(Y)],or[MA] -I- [MA(X)] -I- [A ] + [A (Z)]. 

It is assumed that the best catalytic effect can be achieved if the pA a or pA), value of the interphase material is close to 7 [71]. Some weak ion-exchange groups such as tertiary amines, phosphoric acid, carboxylic acids, or pyridine show the required dissociation constant or p ta-values. Certain heavy metal ion complexes, such as chromium(lll)- or iron(Ill)-complexes, provide the required catalytic water dissociation effect. In principle, there are many more suitable metal ions available. The metal ions or complexes are immobilized by either including an insoluble salt in the casting solution of the interface layer between the ion permeable layers or by converting a soluble form by a follow-up treatment [45]. An additional requirement for the catalytic material is to be effective and stable for a long period. It must also remain in the interphase, where it is the most active, for the anticipated lifetime of the membrane. 

An electrolyte in solution dissociates into two (in the case of NaCl) or three (in the case of CaCh) particles, and therefore the colligative effects of such solutions are multiplied by the number of dissociated ions formed per molecule. However, because of incomplete electrolyte dissociation and associations between the solute and solvent molecules, many solutions do not behave in the ideal case, and a 1-molal solution may give an osmotic pressure lower than theoretically expected. The osmotic activity coefficient is a factor used to correct for the deviation from the "ideal behavior of the system ... 

The modified Henry s law coefficient relates the total dissolved S(IV) (and not only SOz H20) with the S02 vapor pressure over the solution. The effective Henry s law coefficient always exceeds the Henry s law coefficient, indicating that the dissociation of a species enhances its solubility in the aqueous phase. 

Figure 3.37 Svante Arrhenius (1859-1927), who proposed the theory of electrolytic dissociations (1887), investigated the viscosity of solutions, the effect of temperature on reaction rates (1889), etc. (Published with permission from the Deutsches Museum, Munich.)...

Alkali halides are compounds with a strong ionic character and without a solvent they are stabilized by the strong electrostatic interaction between the cation and the anion. In the present study we limit attention to the dissociation potentials comparing the curves obtained for the free molecule and for the molecule in water solution. The solvent is treated only as a continuum medium and then in this way we cannot consider the formation of complexes between the ions and the water molecules which instead are extensively studied by means of Monte Carlo (MC) and molecular dynamics (MD) simulations (see for example [13]). Although it could be possible to include some water molecules with the sodium chloride as a more complicated solute, we have preferred to focus attention on the solvent effect on the electronic structure of the simplest solute, this effect being the most important in the next two examples. 

The X-ray analysis all at once also reveals the conformation of the host and the orientation and conformation of the guest in the cavity. Limiting factors are that it is now always possible to grow crystals of sufficient quality and, on the other hand, the results without uncertainties cannot be translated to the different conditions in solution. The orientation of the guest in the crystal lattice of the host can be completely different compared to that in (water) solution. Dynamic effects like complex formation and dissociation processes also should be taken into account. 

The mechanism of the electro-oxidation of SO2 in acidic media can be summarized as a conversion of S(IV) to S(VI), which may be described by a series of reactions in different pathways as shown in Table 14.7. These reactions reveal that changes in speciation are strongly dependent on the extent of dissociation of the reactant and product species. There most likely exist four types of species 804 , HSOj, 803 , and H80. Because the reversible potential is determined by the Nemst equation, the concentration of species present in solution is required, which presents a problem concerning what type of constituents exist in the aqueous solution. The effect of pH should be considered, as different species will dominate in the different pH regions, which directly determines the oxidation mechanisms. 

Arrhenius, Svante Ausust (1859-1927) Swedish physical chemist who, in 1884, was the first to propose that acids, bases, and salts in solution dissociated into ions. His theory of electrolytic dissociation was well before its time and was not scientifically confirmed until the theory of atomic structure was more fully developed. He also worked on reaction rates, and was the first to recognize the greenhouse effect on climate. He was awarded the Nobel Prize in chemistry in 1903. 

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