What is an alkaline extractant

The extraction reaction mechanism of alkaline extractants is the anion exchange mechanism. The main extractants belonging to this category include primary amine, secondary amine, tertiary amine, and quaternary amine salts. Primary and secondary amine molecules contain N-H bonds, which can be associated by forming N-H... N hydrogen bonds between molecules, but no association occurs between tertiary amine molecules. In amines with the same number of carbon atoms, their boiling and melting points decrease in order of primary, secondary, and tertiary amines. Primary, secondary, and tertiary amines can form hydrogen bonds with water, N... H-O-H, therefore, small molecular weight amines are easily soluble in water. As the number of carbon atoms in amine molecules increases, their water solubility decreases. Large molecular weight amines with a relative molecular weight of 250-600 have low solubility in water and are well soluble in certain organic solvents, making them suitable as extractants; Alkylamines with a molecular weight greater than 600 are mostly solid and have low solubility in diluents, making them unsuitable as extractants.

In amine extractants, due to the presence of hydrogen bonds, intermolecular association occurs, usually occurring not as bimolecular association but as multi molecule association.

The degree of association of various alkaline extractants is not only related to the structure of the extractant, but also to the properties of the diluent. The order of difficulty in the association of fatty amine salts with external anions for sulfates is primary amine salt, secondary amine salt, tertiary amine salt, and quaternary amine salt. Alkyl groups have branched secondary amines, which hinder the formation of hydrogen bonds and therefore have less association ability. The effect of the polarity of diluents on the degree of association of alkaline extractants follows the pattern that the smaller the polarity of diluents, the greater the degree of association. After the formation of amine salts from amines and acids, if the concentration of acid in the aqueous phase is high, a reaction occurs where amine salts and acids form 1/1 ionic complexes, which is called the addition reaction of amine salts.

The alkalinity of fatty amines is stronger than that of inorganic amines, which is due to the induction effect of alkyl donating electrons after replacing the hydrogen of fatty amines, resulting in an increase in charge density on amino nitrogen atoms. In aqueous solutions, the alkalinity of tertiary amines is lower than that of secondary amines, which is due to the steric hindrance effect. The shielding effect of the three alkyl groups on amino nitrogen atoms increases, thereby hindering the approach of protons and nitrogen atoms.

The order of alkaline strength of long-chain amines is primary amine>secondary amine>tertiary amine, and the order of selectivity for extracting metal anions is tertiary amine>secondary amine>primary amine. Amines are weak bases and can only be extracted in acidic solutions, while quaternary ammonium salts are strong alkaline extractants that can be extracted in acidic, neutral, or even alkaline solutions.

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