Some Definitions

A broad definition of ion exchange is that it is the transfer of ions across a boundary; this would then cover movement of ions from one liquid phase to another. This is too broad a base for the purpose of this article, which will restrict itself to those exchanges of ions that occur between a liquid phase and a solid (organic or inorganic) that is insoluble in that liquid. A simple representation of the process when univalent cations are being transferred is given in the chemical equation [I] below:

Here M~A#+ represents a solid carrying a negative charge ('solid anion', sometimes described as a 'fixed ion') neutralized by the A+ ions inside its structure. The AC ions are replaced by BC originally in the solution phase (normally aqueous). The subscripts 'c' and 's' refer to the solid and solution phase, respec tively. The process must be totally reversible to fit the strict definition of ion exchange. However, in practice interference from other nonreversible events may occur. Examples of disruptive influences that may have to be faced are the imbibition of salt molecules, precipitation reactions, chelating effects, phase changes and surface sorption. Some of these will be mentioned later.

An equivalent stoichiometric equation can be written for the anion exchange process, as in eqn [2]:

Now M carries a positive charge ('solid cation' or 'fixed ion') and X and Y are exchanging anions moving reversibly between solid and liquid phases. The ion pairs, A, B and X, Y are called 'counterions'. An ion which is mobile and has the same charge as that of the solid exchanger is called a 'co-ion'.

The extent to which an exchanger can take up ions is called its 'capacity'. In the case of an organic resin exchanger, this can be related to the number of fixed groupings that have been introduced into the polymer as part of its synthesis to create ion exchange properties. These are known as 'ionogenic' groups and are either ionized, or capable of dissociation into fixed ions and mobile counterions. In an inorganic exchanger the ionogenic nature of the solid matrix arises from the presence of positive or negative charges on the solid (usually on an oxygen ion). These charges are a consequence of metal cations in the exchanger that are in nonexchangeable sites. Examples of these will be discussed later.

Recent workshops on ion exchange nomenclature have suggested that the ion exchange capacity is expressed as the concentration of ionizable (ionogenic) groups, or exchange sites of unit charge, per gram of dry exchanger. The units of concentration should be millimoles or milliequivalents per gram. This definition can be taken as the theoretical capacity - Q0.

The workshops also prefer the term 'loading' to describe the capacity experienced under the specific experimental conditions at which the ion uptake is being observed. This can be higher or lower than the theoretical capacity. Higher capacities can arise from electrolyte imbibition or surface precipitation, and lower capacities often arise in inorganic exchangers when all the sites of unit charge are not accessible to the ingoing ion. These circumstances will be considered later.

The suggested definition of loading is the total amount of ions taken up per unit mass, or unit volume, of the exchanger under clearly defined experimental conditions. The concentrations again should be given in millimoles or milliequivalents, but with the option to relate this to mass or volume. An appropriate symbol would be QL.

It should be noted that this is a new approach, differing from the IUPAC recommendations of 1972, and is felt necessary because of the new interest in inorganic exchangers whose properties do not fit the IUPAC concepts.

The definition of capacity associated with column use remains unchanged. The 'breakthrough capacity' (Qb) of a column is still best defined according to the IUPAC definition as the practical capacity of an ion exchanger bed under specified experimental conditions. It can be estimated by passing a solution containing the ion to be taken up through the column and observing the first appearance of that ion in the column (bed) effluent, or when its concentration in the effluent reaches a convenient, arbitrarily defined, value. QB can be expressed in units of mil-limoles, or milliequivalents, of wet, or dry, exchanger using volumes or mass as appropriate.

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Solar Panel Basics

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