Like other separation and preconcentration methods, solvent extraction plays a secondary, but very important role in analytical chemistry. Its significance can be judged by the known fact that in many cases, if not in the majority of cases, direct analysis methods do not enable one to determine all the necessary elements in their wide range of concentrations without interference from other components present in the sample. Preliminary separation and preconcentration of elements are the essential stages of analyses, especially when determining small amounts of metals. Among the separation and concentration methods, solvent extraction holds a very prominent place.