Purified water is water that has been physically processed to remove impurities and is commonly used as laboratory water, where the purity of water used is essential to the outcome of laboratory processes. There are a large number of methods that are commonly used for laboratory water purification.
Distilled water and deionized water are the most familiar forms of purified water. Distillation involves boiling the water and then condensing the steam into a clean container, leaving most solid contaminants behind. Distillation produces very pure water, but also leaves behind a leftover white or yellowish mineral scale on the distillation apparatus. This requires the frequent cleaning of the apparatus. Distillation does not guarantee the absence of bacteria in water unless reservoir and the bottle are both sterilized before being filled. Once opened, there is again a risk of bacterial contamination; because of these flaws, distilled water is not used as laboratory water for high-sensitivity applications.
Deionization is a physical process of laboratory water purification which uses specially manufactured ion exchange resins to filter out mineral salts from the water. Because the majority of water impurities are dissolved salts, deionization produces water at a high purity level that is generally similar to distilled water except this process is faster and it eliminates the scale buildup problem. Unfortunately deionization does not significantly remove uncharged organic molecules, viruses or bacteria, except by incidental trapping in the resin. Deionization also does not remove hydroxide or hydronium ions from water. These are the products of the self-ionization of water to equilibrium; therefore removing them would cause the removal of the water itself.
Laboratory water can also be produced by other laboratory water purification processes. These include reverse osmosis, carbon filtration, microporous filtration, ultrafiltration, ultraviolet oxidation or electrodialysis. More recently, a combination of some of these processes has been utilized to produce water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt).
Purified water has many uses, largely in science and engineering laboratories and industries. A modern purification system will combine all of the above technologies (except distillation) to produce reagent grade water. Reagent grade water is defined by the College of American Pathologists, the American Society of Testing and Materials and the National Committee for Clinical Laboratory Standards as being Type I, Type II, Type III or Type IV, in terms of its specific conductance, specific resistance, silicate, bacterial count and pH (Type III and Type IV water only). The parameters for highly purified laboratory water and the laboratory water purification processes specified by each of these organizations is similar, but not identical.
Type I is considered to be the purest type available, but regardless of which organization’s water quality standard is used, even Type I water may require further purification depending upon the specific laboratory application. For example, water that is being used for molecular biology experiments needs to be DNase or RNase-free. This requires special additional treatment or functional testing. Water for microbiology experiments must be completely sterile, which is usually accomplished by autoclaving.
Laboratory water used to analyze trace metals may require laboratory water purification which results in the elimination of trace metals to a standard beyond that of the Type I water standard.