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ION EXCHANGE METHODS IN ANALYTICAL CHEMISTRY
Ion exchange is now one of the recognized processes of chemical engineering. It has been applied to the separation processes of quantitative analysis.
General Principles. By ion exchange wemean the exchange of ions of like sign between a solution and a solid insoluble body in contact with it. For such an exchange to be possible, the solid must contain ions of its own. The solid (called the ion exchanger)must have an open, permeable molecular structure, so that ions and solvent molecules can move freely in and out. Many substances, both natural and artificial, have ion exchanging properties.
In analytical work we are primarily interested in the synthetic organic exchangers. These have a high capacity for holding ions and they are not broken down by acids or alkalies, they have a relatively simple composition.
Acids, bases and salts, when dissolved1 in water, yield ions. Ions are atoms or radicals which carry an electric charge. Ionization is the process by which ions are formed from neutral atoms or molecules. Ions have structural completeness but lack electrical balance. Their properties are entirely different from those of the atoms from which they are formed.
Compounds that conduct electricity in solution or in a molten state are called electrolytes. Those substances that do not conduct electricity under such conditions are called non-electrolytes. The greater the number of ions in a solution, the better its ability to carry an electric current. Highly ionized solutions are strong electrolytes and good conductors of electricity. Many covalent compounds have molecules with unsymmetrical electrical field. It is believed4 that the shared electrons move closer to the atoms of such molecules. This results in a partial positive charge on one part of the molecule and a partial negative charge on another part of the molecule. Such compounds are called polar covalent compounds and the molecule is called a dipole. Water is such a compound. The polar covalent compounds which form ions when dissolved in water do so by reacting with water. Electrovalent compounds are composed of ions. Water dipoles exert an attractive force on these ions, weakening their bonds and causing dissociation. Crystals of electrovalent compounds are made up of ions. In molten conditions, these compounds are electrolytes. By means of electrolysis active metals may be obtained from such compounds.
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