Metal extraction is usually referred to as hydrometallurgy and it deals with extraction of metals from acidified aqueous solutions. For this purpose the aqueous solution, that is the raffinate phase, is contacted with an organic extract phase. However, since metal ions are insoluble in organic solvents, the latter must contain a water-insoluble complexing agent which will react with the metal and extract it into the organic phase in the form of a metal complex. The reaction is reversible and the metal is re-extracted from the organic phase in the stripping process usually by an aqueous phase suitably modified to move the reaction equilibrium in the appropriate direction. This modification is usually attained by adjustment of pH of the solution. The insolubility of the complexing agent in water is very important from process considerations, since solubility would mean not only losses of the agent but its presence in the stripping solvent would introduce difficulties in subsequent processing, e.g. electrowinning. Nevertheless, the agent must contain a hydrophilic group to allow it to pick up the metal ion but this effect must be counteracted by a large hydrophobic chain to retain insolubility.