Extractants

1.4.2 Extractants

During the years 1900 to 1940, solvent extraction was mainly used by the organic chemists for separating organic substances. Since in these systems, the solute (or desired component) often exists in only one single molecular form, such systems are referred to as nonreactive extraction systems; here the distribution ratio equals the distribution constant.

However, it was also discovered that many organic substances, mainly weak acids, could complex metals in the aqueous phase to form a complex soluble in organic solvents. A typical reaction can be written

M z + ( a q ) + z H A ( a q  o r  o r g ) →← M A z ( o r g ) + z H + ( a q ) ( 1 . 6 )

which indicates that the organic acid HA may be taken from the aqueous or the organic phase. This is an example of reactive extraction. It became a tool for the analytical chemist, when the extracted metal complex showed a specific color that could be identified spectrometrically. The reagent responsible for forming the extractable complex is termed the extractant.

The industrial use of solvent extraction of inorganic compounds grew out of the analytical work. As both areas, analytical as well as industrial, needed both better extractants and an understanding of the reaction steps in the solutions in order to optimize the applications, theoretical interpretations of the molecular reactions in the solutions became a necessity, as will be described in later chapters.

The increased use of computer graphics for modeling molecular structures and chemical reactions has opened a path for the synthesis of tailormade extractants. Thus the future promises new varieties of extractants with highly selective properties for the desired process.

Soure: Solvent Extraction Principles and Practice, Revised and Expanded edited by Jan Rydberg