The useful role of ion exchange is to separate a solution at a low initial concentration C into a small volume of concentrate and a much larger volume of depleted solution. In favourable cases the concentration Ce of the target species in the regeneration effluent approaches the regenerant concentration Cr. The concentration factor achieved is then given approximately by:

defined by:

one cation from a mixture to be selectively concentrated by a large factor.

Weak acid resins tend to be more selective than strong acid. Table 1 also lists some selectivity coefficients (not constants), defined by:

One usually wants to separate a single valuable species from the other constituents of the initial solution, and the ion exchanger must then be selective for the target species. (Separation can also be achieved by selective desorption, but selective adsorption is preferable because more of the available resin capacity is occupied by the target ion.) The affinity of a resin for a single ion M, under given conditions, can be represented by a distribution coefficient, defined by:

^ amount of M on unit mass of resin amount of M in unit volume of solution

Selectivity between two ions can be expressed by a separation factor, defined by:

where CT represents the concentration of the target and CU that of an unwanted species, while Ce refers to the regeneration effluent and Cd to the depleted solution.

Common strong acid resins show a small degree of selectivity between singly and doubly charged cations, and even within these categories. Table 1 shows some selectivity values from the literature for a typical strong acid cation exchanger. These are rational equilibrium constants for the reaction:

where AM is the activity (on the rational scale) of Mn+ bound to the resin phase and aM the activity (molal scale) of the salt MXn in the solution phase. ANa and a^a are the activities of the analogous sodium species. The observed values are too small to allow for adsorption of divalent cations Mn + on a carboxyl resin initially in the Na + form. Here Mm is the molal concentration of Mn + in the resin phase and mM its molality in the solution phase. Although K' and K are only roughly comparable, the carboxyl resin is clearly more selective among these four divalent cations.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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