Monovalent ions

This means that the potential some distance away appears to follow Eq. V-13, but with an apparent value of AkTjze, which is independent of the actual value. For monovalent ions at room temperature this apparent would be 100 mV. 

The relative measurement error in concentration, therefore, is determined by the magnitude of the error in measuring the cell s potential and by the charge of the analyte. Representative values are shown in Table 11.7 for ions with charges of+1 and +2, at a temperature of 25 °C. Accuracies of 1-5% for monovalent ions and 2-10% for divalent ions are typical. Although equation 11.22 was developed for membrane electrodes, it also applies to metallic electrodes of the first and second kind when z is replaced by n. 

Gathodically Colored Inorganic Films. The generalized cathodic, monovalent ion-iasertion reaction for inorganic thin films is... 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved soHds using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaCl solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallisa tion at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

Leading Examples Electrodialysis has its greatest use in removing salts from brackish water, where feed salinity is around 0.05-0.5 percent. For producing high-purity water, ED can economically reduce solute levels to extremely low levels as a hybrid process in combination with an ion-exchange bed. ED is not economical for the produc tion of potable water from seawater. Paradoxically, it is also used for the concentration of seawater from 3.5 to 20 percent salt. The concentration of monovalent ions and selective removal of divalent ions from seawater uses special membranes. This process is unique to Japan, where by law it is used to produce essentially all of its domestic table salt. ED is very widely used for deashing whey, where the desalted product is a useful food additive, especially for baby food. 

Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose hmits the practical concentration of whey to about 20 percent total sohds, about a 4x concentration fac tor. (Membranes do not tolerate sohds forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinaiy whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

When the oxidation product is an /i-type oxide like ZnO, the conditions are reversed (Fig. 1.78). If a monovalent ion like Li enters the oxide layer in place of Zn one free electron (eo) is destroyed. But the product n(Zn 0)n(eo) is fixed by the reaction governing the non-stoichiometry of ZnO. Hence n(Zn O), the concentration of interstitial Zn ions, increases, and the oxidation rate, which depends upon the concentration of these ions in the oxide in equilibrium with metallic Zn, increases. 

The question arises as to how the B-coefficient for any solute is to be divided among the cation and anion. Before attempting to solve this problem, we could, of course, begin by assigning an arbitrary value to some species of ion (we could, for example, assign the value zero to K+) then the values to be assigned to all the other monovalent ions can readily... 

Therefore, the Henderson approach117 for monovalent ions of the same sign for calculating ed can be applied ... 

If Eqs. (28) and (32) are inserted into Eq. (30) the mixed potential of the ion-selective membrane for an ion exchange process of only monovalent ions of the same sign is obtained ... 

Though solid electrolytes for multivalent ions offer the advantage of a larger charge transfer, their conductivities are much lower than those of monovalent ions at ambient temperature because of a higher activation enthalpy for the ionic motion... 

The selectivity of ion exchange Ks can easily be determined experimentally for the simplest systems of exchange between monovalent ions. The value of Ks may be used for analysis of thermodynamic functions AG°, AH0 and AS0, of sorption selectivity... 

Because of their lower charge densities, monovalent ions are exchanged less efficiently, so chloride and sodium ions in particular tend to slip through the resin beds. Where twin-bed DI is employed,... 

The nature of the Debye-Hiickel equation is that the activity coefficient of a salt depends only on the charges and the ionic strength. The effects, at least in the limit of low ionic strengths, are independent of the chemical identities of the constituents. Thus, one could use N(CH3)4C1, FeS04, or any strong electrolyte for this purpose. Actually, the best choices are those that will be inert chemically and least likely to engage in ionic associations. Therefore, monovalent ions are preferred. Anions like CFjSO, CIO, /7-CIC6H4SO3 are usually chosen, accompanied by alkali metal or similar cations. 

Influence of the Monovalent Ions. The addition of ammonium salts retard the course of the deesterification by changing the reaction rate of hydrolysis and ammonolysis. By increasing the concentration of the ions the conversion of the ester groups is reduced from 83.3 % for 0,01 M to 62.8 % for 0.2 M (without added salt this value is 84.05 %), and the ratio hydrolysis ammonolysis is changed correspondingly from 53.8 76.2 to 37.3 62.7 (without added salt this ratio is 57.6 42.4). 

Table 2. Thermal behaviour of the gels. Temperatures [°C], where minimal storage moduli were observed on reheating. Concentrations of ions in mmol / lOOg gel, [V " ] = added bivalent ion, [Z ] = added monovalent ion, [C1 ] = 2.5, 2[V+ ]+[Z+]=2.5 or [Na ]+[K ]=2.5.

Exchange between monovalent ions pectin A73 pectin A64 pectin A43 ... 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

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