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Ion exchange
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<p>Ion exchange is a reversible interchange of one species of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid. Ion exchange is used in softening or demineralizing of water, purification of chemicals, and separation of substances. </p><p>Ion exchange usually describes a process of purification of aqueous solutions using solid <a href="/facts/Polymer/eF32E50K">polymeric</a> <a href="/facts/Ion-exchange_resin/z6KwlG8K">ion-exchange resin</a>. More precisely, the term encompasses a large variety of processes where ions are exchanged between two <a href="/facts/Electrolyte/4YE8bIVa">electrolytes</a>. Aside from its use to purify drinking water, the technique is widely applied for purification and separation of a variety of industrially and medicinally important chemicals. Although the term usually refers to applications of synthetic (human-made) resins, it can include many other materials such as soil. </p><p>Typical ion exchangers are <a href="/facts/Ion-exchange_resin/z6KwlG8K">ion-exchange resins</a> (functionalized porous or <a href="/facts/Gel/t7eStnK0">gel</a> polymer), <a href="/facts/Zeolite/SNrXy4v2">zeolites</a>, <a href="/facts/Montmorillonite/PxKtefgA">montmorillonite</a>, <a href="/facts/Clay/xnwouEkB">clay</a>, and <a href="/facts/Soil/5Ddpr3QM">soil</a> <a href="/facts/Humus/vlhws4I0">humus</a>. Ion exchangers are either cation exchangers, which exchange positively <a href="/facts/Electric_charge/xxsBfmlB">charged</a> ions (<a href="/facts/Cation/wrbwhF3z">cations</a>), or anion exchangers, which exchange negatively charged ions (<a href="/facts/Anion/wrbwhF3z">anions</a>). There are also <a href="/facts/Amphoterism/2UrXEsyl">amphoteric</a> exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions is often performed in <i>mixed beds</i>, which contain a mixture of anion- and cation-exchange resins, or passing the solution through several different ion-exchange materials. </p> <p>Ion exchangers can have binding preferences for certain ions or classes of ions, depending on the physical properties and <a href="/facts/Chemical_structure/r5rT1Lj4">chemical structure</a> of both the ion exchanger and ion. This can be dependent on the size, charge, or structure of the ions. Common examples of ions that can bind to ion exchangers are: </p> <ul><li>H+ (<a href="/facts/Hydron/pjypwafP">hydron</a>) and OH− (<a href="/facts/Hydroxide/zW0wxpWD">hydroxide</a>).</li> <li>Singly charged monatomic (i.e., monovalent) ions like <a href="/facts/Sodium/eYuSPu2V">Na+</a>, <a href="/facts/Potassium/5lcumVgo">K+</a>, and <a href="/facts/Chloride/m9NUaUDR">Cl−</a>.</li> <li>Doubly charged monatomic (i.e., divalent) ions like <a href="/facts/Calcium/hf252CC0">Ca2+</a> and <a href="/facts/Magnesium/XyIhd2FG">Mg2+</a>.</li> <li>Polyatomic <a href="/facts/Inorganic_compound/TfFfzxSz">inorganic</a> ions like <a href="/facts/Sulfate/yuWVxsUf">SO2−4</a> and <a href="/facts/Phosphate/FDGVCxUG">PO3−4</a>.</li> <li><a href="/facts/Organic_base/jjaVBLpR">Organic bases</a>, usually <a href="/facts/Molecule/N9ljSfFq">molecules</a> containing the <a href="/facts/Functional_group/GXkSPsjM">functional group</a> of <a href="/facts/Ammonium/jYm8wc1n">ammonium</a>, −N+R2H.</li> <li><a href="/facts/Organic_acid/2Pfb26Lp">Organic acids</a>, often <a href="/facts/Molecule/N9ljSfFq">molecules</a> containing −COO− (<a href="/facts/Carboxylate/vEDekXRr">carboxylate</a>) functional groups.</li> <li><a href="/facts/Biomolecule/DGvFzN7R">Biomolecules</a> that can be ionized: <a href="/facts/Amino_acid/WDxYwBny">amino acids</a>, <a href="/facts/Peptide/C4ltc7UG">peptides</a>, <a href="/facts/Protein/Jxmagu3E">proteins</a>, etc.</li></ul> <p>Along with <a href="/facts/Absorption_(chemistry)/XGeB64bC">absorption</a> and <a href="/facts/Adsorption/ARH9peYM">adsorption</a>, ion exchange is a form of <a href="/facts/Sorption/nWDhtFkL">sorption</a>. </p><p>Ion exchange is a <a href="/facts/Reversible_reaction/V3Afejgb">reversible process</a>, and the ion exchanger can be <i>regenerated</i> or <i>loaded</i> with desirable ions by washing with an excess of these ions. </p>