Deionization

In the world of water there are many applications which require water with a TDS (Total Dissolved Solid) reading of zero. Laboratories, pharmacies, homeopaths, domestic aquariums, battery water, people with certain illnesses all fall in this group. The process employed to remove all ions from water is called deionization or demineralization, the product water often called demin or demi water.

What is deionization?
Water contains dissolved salts in ion form (hence Total Dissolved Solids – TDS). What happens, for example, when a teaspoonful of table-salt (sodium chloride – NaCl) is dissolved in a glass of water? As soon as the salt dissolves it breaks up into positive Na ions and negative Cl ions. This process is called ionisation. In chemistry these ions are expressed as Na⁺ and Cl^-. A positive ion is called a cation and a negative ion an anion.

Deionization is the process whereby all anions and cations are removed by a process called ion exchange, employing mixed bed resin. Mixed bed resin comprises two different resins, a cation resin and an anion resin. The cation resin will exchange all cations (Ca⁺⁺, Mg⁺⁺, Na⁺, etc.) and replace them with a chemical equivalent quantity of hydrogen ions (H⁺), while the anion resin will exchange all anions (HCO₃^-, Cl^-, SO₄⁼, etc.), and replace them with a chemical equivalent quantity of hydroxide ions (OH^-). The hydrogen and hydroxide ions combine to form water: H⁺ + OH^- = HOH or H₂O.

Deionization can be presented as follows

Page Links
•What is deionization?
•Deionization and reverse osmosis
•Regeneration
•In-Line DI 3300 Series




	Deionization and reverse osmosis The phrase demineralised water for deionised water is not quite correct. Deionization does not remove every dissolved substance or contaminant from water. The TDS reading reads zero because all ions are removed. All other contaminants, not ionising remain e.g. sugar, alcohol, benzene and other organic contaminants. This makes reverse osmosis of utmost importance. Reverse osmosis removes a great deal of organic and inorganic pollution, but not all. The small percentage of inorganic ions remaining in purified water is removed by mixed bed resin. Reverse osmosis and distillation remain the best technology to remove inorganic and organic toxins. Regeneration Regeneration of mixed bed resin can not be done as in the case of softening resin. Industrial deionization systems do not use mixed bed resin, but two resin columns: one for cation and one for anion resin. The cation resin is regenerated with a strong acid like hydrochloric acid, (HCl) and the anion resin with a strong base like sodium hydroxide (NaOH). Such a deionization system is not cost-effective for domestic use. Example of industrial deionization



	In-Line DI 3300 Series deionization system from Absolutely Pure Water Absolutely Pure Water markets a system ideal for domestic aquariums, or any other client who needs water with a TDS of zero. An in-line deionization filter which will remove any remaining ions, substitutes the post filter. This is more cost-effective than buying distilled water. Specifications: Diameter: 194 mm Length: 310 mm Maximum flow rate: 250 L/24h Maximum pressure: 6 bar Maximum temperature: 50^oC Resin volume: 0,28 litres Capacity: Depending on TDS of permeate. TDS <8 will produce 500 litres Colour: Between blue and green Exhaustion colour: Light brown After exhaustion this mixed bed resin can not be regenerated. For more information about deionization, please contact us. Sources APS water Services Inc: www.apswater.com SIGMA Product Information Scientific Instrument Services Inc: www.sisweb.com www.freedrinkingwater.com