①Neutral extraction agents, such as alcohols, ketones, ethers, esters, aldehydes, and hydrocarbons. They can directly dissolve the extracted components (such as carbon tetrachloride for extraction of iodine); or first form a solvent complex with the extracted features (such as removing uranyl nitrate tributyl phosphate). ②Acid extractant, such as carboxylic acid, acid phosphate, etc. The extractant exchanges its hydrogen ions for metal cations in the feed solution during extraction, such as extracting indium with bis-2-Ethylhexyl phosphate. Sometimes the hydrogen ions in the extractant are replaced with appropriate metal ions to prevent the raffinate's acidity from increasing and affecting the equilibrium relationship of extraction. ③ Chelating extractant is also an acidic extractant. Two functional groups form a compound with a chelate ring with the ion to be extracted and release hydrogen ions. This type of extractant has better selectivity, such as LIX63 (aryl hydroxy oxime compound) to extract copper. ④ Amine extractants, mainly tertiary amines, and quaternary ammonium salts. The former combines with the free acid in the feed liquid to achieve extraction, such as extracting chromic acid with trioctylamine; the latter uses its anions in exchange for the anions in the feed liquid to extract. Besides, in the petroleum refining industry, the BIX method (see Aromatics Extraction) is used to separate aromatics and alkanes in aqueous solutions of glycols. In exceptional cases, liquefied ammonia, propane, sulfur dioxide, and molten salts are also used as extractants. The solvent used for stripping is called a stripping agent. For the back extraction of organic extracts, pure water or aqueous solutions of acids, alkalis, and salts are usually used.