Extraction of alkali halides from their aqueous solutions by crown ethers

Potassium and sodium halides can be extracted from their aqueous solutions by dibenzo-18-crown-6 and dicyclohexo-18-crown-6 dissolved in protic solvents, which solvate the anions effectively. Both separate ions and ion pairs are extracted, the order of extractability among the halides being fluoride > iodide > bromide > chloride, apparently reflecting the solvation of the anions. The solubility of the crown ethers in a series of solvents are correlated with the solvent properties. The results are summarized in a prescription for selecting the best crown ether/solvent combination for effective extraction of the alkali halides.