Selectivity in extraction:

The selectivity in extraction is generally explained by the compatibility between cation and hole or cavity size of the crown ether. But the ion-in-the whole model has certain limitations. As the number of ring atoms increases the macrocyclic flexibility increases and it may be difficult to define the cavity diameter. The attachment of certain groups affects the donor strength of the oxygen atoms. Also, the nature of the donor atom plays an important role on selectivity. The extent of extraction is affected by the nature and concentration of the crown ether and the counter ion. The nature of the diluent also affects the extractant. Generally, it is preferred to use the solvents of high dielectric constants. Moreover, the solvent should have high solubility for the crown ethers. The complexation with ethers transforms relatively small alkaline earth metal ions into a voluminious organic cation which is more soluble in organic media. As a consequence of this, solubilization of inorganic salt of low polarity results. With anions of highly localized negative charge, the solubilization may be difficult. Therefore, the salts containing hard anions like fluoride or hydroxide can be solubilized with difficulty in solvents like benzene. But soft anions like picrate, iodide or tetraphenyl borate are very suitable. The extraction of metals by crown ethers in the presence of picrates or dye stuffs has the added advantage of employing it as a colorometric method. However, the cost of crown ethers has been a deterrent for their application as extractants on a large scale.