What is a neutral extractant

Neutral extractants can be subdivided into neutral oxygen extractants, neutral phosphorus extractants, neutral sulfur extractants, and amide extractants.

Neutral oxygen-containing extractant

Neutral oxygen-containing extractants mainly refer to compounds such as alcohols (ROH), ethers (ROR ′), ketones (RCOR ′), and esters (RCOOR ′). The electronic density of the oxygen atom in the ligand of the extractant and the dipole moment of the molecule are the main factors determining the extraction ability of such extractants. Therefore, their extraction ability increases with the enhancement of their Lewis alkalinity. Among the four types of compounds including alcohols, ethers, ketones, and esters, only alcohol molecules contain - OH. Due to the presence of - OH, alcohol molecules form hydrogen bonds and undergo their own association reactions, resulting in higher boiling points than the other three types of neutral extractants. Neutral oxygen-containing extractants all contain oxygen atoms, so they can undergo hydrogen bonding and association with water molecules, and have a certain solubility in water. Alcohols, ethers, ketones, and esters can form salts in concentrated strong acids. These salt cations can not only bind to inorganic acid radicals, but also to metal anions, enabling them to extract many substances. The main reason why these extractants can extract metals is that they can form complexes with metals and enter the organic phase for extraction. [1]

Neutral phosphorus extractant

Neutral phosphorus extractants refer to compounds in orthophosphate molecules where the three hydroxyl groups have been completely esterified or replaced. Any hydrocarbon group directly connected to phosphorus atoms, that is, those with carbon phosphorus bonds (C-P), are called phosphine, while those without carbon phosphorus bonds are called phosphine. On these extractants, there is a phosphoryl group (Å P=O), which is a functional group responsible for extraction.

Neutral phosphorus containing extractants associate with water molecules to form an association compound, and due to the ability of phosphoryl oxygen atoms to provide lone pair electrons and combine with protons to form salt. The ions formed by neutral phosphorus containing extractants can combine with metal anions to form salts and be extracted into the organic phase. This type of extractant can also be extracted into the organic phase by coordinating the oxygen atom of the phosphoryl group with the metal to form a neutral complex. Because the stability of phosphate esters is poorer than that of phosphonate esters, hydrolysis reactions occur when these extractants come into contact with inorganic acids for a long time. The order of influence of various inorganic acids on the hydrolysis reaction of neutral phosphorus extractants is HI>HBr>HCl>HNO3>H2SO4; In addition, as the temperature and concentration of acid in the solution increase, the hydrolysis rate of these extractants increases. In neutral phosphorus containing extractants, with the decrease of alkoxy groups in the molecule and the increase of alkyl groups, the order of extraction ability increase is: (RO) 3PO<RPO (OR) 2<R2P (O) OR<R3PO. The spatial effect of alkyl also has an impact on the extraction ability of this type of extractant. Generally, in extractants with the same number of alkyl carbon atoms, with the increase of branch chains, the spatial hindrance increases and the extraction ability significantly decreases.

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