54The other measurement, based on the Brønsted–Lowry definition, is the acid dissociation constant
(Ka), which measures the relative ability of a substance to act as an acid under the Brønsted–Lowry
definition of an acid. That is, substances with a higher Ka are more likely to donate hydrogen ions
in chemical reactions than those with lower Ka values.
Redox
Redox (reduction-oxidation) reactions include all chemical reactions in which atoms have their
oxidation state changed by either gaining electrons (reduction) or losing electrons (oxidation).
Substances that have the ability to oxidize other substances are said to be oxidative and are known
as oxidizing agents, oxidants or oxidizers.
An oxidant removes electrons from another substance. Similarly, substances that have the ability
to reduce other substances are said to be reductive and are known as reducing agents, reductants,
or reducers.
A reductant transfers electrons to another substance, and is thus oxidized itself. And because it
"donates" electrons it is also called an electron donor.
Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of
electrons may never occur. Thus, oxidation is better defined as an increase in oxidation number,
and reduction as a decrease in oxidation number.