Expression as equivalent weights/liter, that is, normal concentration, is based on a definition that is related to the type of reaction in which the solution constituents are involved.
Single substance has two different equivalent weights because of its involvement in two different types of reaction
Three methods of definition of equivalent weight are commonly encountered in water and wastewater chemistry are:
Equivalent weight based on ion charge
Equivalent weight based on acid-base reactions
Equivalent weight based on oxidation-reduction reactions