EQUIVALENT, in chemistry, the proportion of an element which will combine with or replace unit weight of hydrogen. When multiplied by the valency it formerly gave the atomic weight ; in 1905, however, the standard for the calculation of atomic weights was changed from hydrogen = 1 to oxygen = 16. (See ATOMIC WEIGHTS; and CHEMISTRY.) In a more general sense the term "chemical equivalent" is used to denote quantities of elements, compounds or radicals which combine with, displace or in any way cause the migration of unit weight of hydrogen. This more extended conception of the term is fundamental in vol umetric analysis and indicates the amounts of each reagent to be taken for a normal solution, as, for example, NaOH, HC1,