One of the most remarkable properties widely used in chemical analysis is the ability of a metal ion solution or a chemical substance to change its color under the effect of external factors, most often chemical reactions. Such reactions are known as color reactions. Color reactions of practical value have been found for almost all inorganic ions. It would hardly be an exaggeration to say that until recently almost half of all analyses, particularly in important branches of industry and in agriculture, were based on colorimetric and, later, photometric (or spectrophotometric) techniques.