Capacity:

If we consider an ion exchanger, it could be taken as a reservoir of exchangeable ions. Within the ion exchange operation, it is the counter ions that are put to use. The counter ions content of a provided amount of material is equal to the fixed charges which must be balanced through the counter ions and therefore, is essentially constant. This amounts to the fact that it is independent of particle size and shape and of the nature of counter ions.

Ion exchangers are characterized in a quantitative manner by their capacity. In the common usage, it is defined as the number of ion equivalents in a specified amount of the material. Other than this easy definition is not enough and will have to be qualified. The definition becomes acceptable when the conditions are given. Capacity and related data are primarily used for two purposes, for characterizing ion exchange materials and for use in numerical calculations of ion exchange operations. In the second case, it is more practical to use other definitions or quantitatives which reflect the effect of operating conditions. The different types of capacity are given as under.