Total capacity of an ion exchange resin:

The total capacity of an ion exchange resin is the number of ionic (or potentially ionic) sites per unit weight or volume of resin. The dry weight total capacity is usually expressed in milliequivalents per gram of anhydrous resin. Scientifically, that is commonly expressed as meq/ g dry H⁺ or Cl^- form. The wet volume capacity is the number of sites per unit volume of the water swollen resin. The performance of an ion exchange resin is generally based on volume and the wet volume total capacity is the theoretical or maximum capacity which the resin can show in any aqueous ion exchange application. It may be expressed in milliequivalents per milliliter.

The net number of sites which are utilized in a given volume of resin in a given cycle in known as the operating capacity of the resin in that particular cycle. It may be expressed in the same terms as total capacity or as a percent of total capacity.

There is another term which is known as useful capacity which is the capacity when equilibrium is not attained. It depends on experimental conditions viz. ion exchange rates etc. There is another capacity which is known as breakthrough (dynamic) capacity which is utilized in column operation. That depends on operating conditions. There is also a capacity known as sorption capacity which is the amount of solute taken up by sorption rather than ion exchange per specified amount of the exchanger.