Choosing a Laboratory Water System

Reagent-grade bottled water is rated to its quality specification at the time of bottling or, in some cases, when the bottle is opened. Once the manufacturer’s seal is broken water begins absorbing CO2 present in the headspace of the container. Water stored in improperly-sealed containers can rapidly pick up amines, aldehydes, alcohols and other common laboratory chemicals in the air.

Laboratory managers rightly bristle at the prospect of purchasing and maintaining two water systems – one for ultrapure water and a second for routine operations and/or to feed the ultrapure system. Single-unit, standalone systems that deliver both grades of water provide higher value and convenience.

A cost-effective water system, regardless of the quality of its output, should use the least expensive feedstock water possible, which is normally tap water. Systems that require a separate source of purified water to feed ultra-purification add an additional layer of cost consumption.

Nevertheless, some food laboratories, whether through custom or other circumstances, find themselves with two separate systems.

Distillation, deionization and reverse osmosis are typical methods for generating pure-grade laboratory water. Stills employ holding tanks in which water sits for hours or days, whereas deionizers feed directly into the ultrapure stage. Carbon dioxide uptake during storage is known to lower product water pH, which affects acid-base titrations and other types of analysis. Blanks effectively will cancel the effect of pH and other chemical changes provided blank readings are obtained within minutes of analytical runs, an unlikely scenario for busy analytical laboratories.

Distilled water is considered pure because most of the particulates, pathogens, or ions present in the feedstock water end up in the distillate. However, ions, particles, and colloids may sweep into the vapor phase and condense along with product water. Although distilled water is pure enough for routine laboratory work, it is best used immediately after producing it, since the relatively high condensate temperature promotes growth of microorganisms.

Atmospheric carbon dioxide easily dissolves in any water on storage. Given the shortcomings of distilled water, it cannot be recommended as feedstock for ultrapure water systems.

Deionization, another common route to pure water and feedstock for ultrapure water generation, also suffers from drawbacks. Although acceptable as a source of “as-is” laboratory-grade water, deionized water does not efficiently remove organic contaminants from common feedstock water. Considering that deionization commonly uses tap water as its source, and most tap waters contain relatively high levels of organic contaminants, deionization is not the most appropriate choice for feeding an ultrapure water system for food laboratories.

By contrast reverse osmosis removes dissolved and suspended species such as particles, bacteria, viruses and macromolecules (e.g. sugars, proteins) with molecular weights above 150 to 250 daltons by simple size exclusion. Microorganism proliferation is far less likely after reverse osmosis, which occurs at ambient temperature. Reverse osmosis removes 95 percent or more of all contaminants likely to be present in tap water. In so doing, reverse osmosis generates pure-grade water suitable for routine laboratory use and as feedstock for ultrapure water production.

Most dedicated ultrapure water systems are not designed to use tap water as feedstock, necessitating the purchase of additional purified water systems or the use of expensive bottled reagent water. Some ultrapure water systems employ built-in electrodeionization, deionization or reverse osmosis pretreatment systems. Others require a stand-alone pure water source to feed into it. Due to poor synchronization of output between purified and ultrapure systems, most dual-system arrangements require a holding tank to store purified water.

Dedicated systems that utilize potable water sources tend to be incapable of generating intermediate-grade purified water. As a result, end-users must bear the cost of using ultrapure water for routine operations.