|MadSci Network: Chemistry|
Water purity may be measured in various ways. One can attempt to determine the weight of all of the dissolved material ("solute"); this is most easily done for dissolved solids, as opposed to dissolved liquids or gases. In addition to actually weighing the impurities, one can estimate their level by the degree to which they increase the boiling point or lower the freezing point of water. The refractive index (a measure of how transparent materials bend light waves) is also affected by solutes in water. Alternately, water purity can be quickly estimated on the basis of electrical conductivity or resistance — very pure water conducts electricity poorly, so its resistance is high. Distillation entails converting water from the liquid state to the gaseous state and back to liquid again in an apparatus called a "still," comprising a boiling vessel to vaporize the water and a cooling unit ("condenser") to return the water to the liquid state. Most dissolved solids are left behind in an increasingly concentrated solution, so that the boiling point of the liquid water increases. These are said to be "nonvolatile." Substances that vaporize with the water are "volatile." Very elegant stills can selectively condense (liquefy) water from among other volatile substances, but most distillation allows carry-over of at least some volatile substances, and a very little of the nonvolatile material that was carried into the water vapor stream as bubbles burst at the surface of the boiling water. Deionization entails removal of electrically charged (ionized) dissolved substances by binding them to positively or negatively charged sites on a resin as the water passes through a column packed with this resin. Because the resin also collects other dissolved substances that can feed bacteria, it is not unusual to find bacterial growth in a deionization column. However, the water that comes out of one of these is usually very low in conductivity — the solutes that enable electricity to pass have been removed. Water used in my laboratory is first distilled and then deionized. It has very low electrical conductivity, but it may contain a few by-products of the bacteria that grew on the deionizer resin. These can largely be removed with activated carbon, but we have not needed to go this far in recent years. Long ago, we had a commercial ultrapurification apparatus that took distilled water and removed bacteria with a special filter, ions with a resin, and other impurities with activated carbon. The manual that came with the apparatus said (approximately), "The best thing we can tell you about storage of ultrapure water is — don't!" As you supposed, ultrapure water in contact with perfectly clean glass will manage to dissolve substances from the glass surface. The stored water is generally still purer than what comes out of our present treatment system, but not nearly as pure as it was when it emerged from the ultrapurification apparatus. So, that's the story. Distillation and deionization may take different things out of water and yet leave some impurities behind. On the other hand, the purer the water, the more difficult it becomes to store without losing purity. Water we drink need not be as pure as water used for some laboratory purposes, but each laboratory has to adjust its purification system to the intended use of the water — and then devise a monitoring system to ensure that the required purification is being attained.
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