Selectivity coefficient ion exchange

Ion exchange selectivity coefficients for univalent ion and divalent ion-hydrogen ion exchange are given by the equations... 

Cation Hydrolysis. Many metal hydroxides have a low solubility in moderately alkaline solutions arising from the hydrolysis of high alumina zeolites. For instance, most transition metals and magnesium precipitate at pH 9-10 and at this pH range carbonates are likely to precipitate other metals such as caldiun, strontium and cadmium [109]. Such precipitation phenomena can seriously distort the measurements of ion exchange selectivities. In addition, even when the metal concentrations are below the limits of hydroxide precipitation, hydrolysed species, such as MOH+, M(OH)2 (aq) and M(OH )j, often form the majority of the metal species in solution. For the determination of the ion exchange selectivity coefficient, the concentration of free, non-hydrolysed metal cation should be known. If the concentrations of hydrolysed species are used, a too low value may again be obtained for the selectivity coefficient. 

F. W. E. Strelow, An ion exchange selectivity scale of cations based on equilibrium distribution coefficients. Anal Chem., 32,1185,1960. 

We believe that fundamental assessment of the factors important in defining counterion distribution in charged polymers (crosslinked and linear) has been resolved with our interpretation of osmotic coefficient data. By our analysis, ion-solvent Interactions are believed to contribute most importantly in the polystyrene sulfonate-based resins to their ion-exchange selectivity patterns. The agreement obtained between the prediction of these patterns and their observed distribution without need to resort to a single measurement for calibration of the activity coefficient terms in the polymer provides strong support for the validity of the interpretations made. 

A considerable amount of information on ion-exchange selectivity is available in the older literature. This is often in the form of elution orders of various ions or in terms of selectivity coefficients. 

A constant bulk concentration of BF4 in CP and B4 in PVC provides a sufficient driving force to control a local equilibrium at this interface and to maintain there a constant gradient of electrical potential. (It should be noticed that for ion exchange of the same common ion, the selectivity coefficient can be formally given as ... 

The equilibrium constant for this ion-exchange reaction, which is also called the selectivity coefficient, is... 

Ion-exchange isotherms assume different shapes depending on the selectivity factor and the variations in with the level of exchange The rational selectivity coefficient includes the ionic charge and is given by... 

R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni ). Trivalent ferric ions would require three resin sites. As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference, or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction... 

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