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