ANALYTICAL AND LABORATORY EQUIPMENT 9


of 500 microsiemens/cm, some laboratory applications may call for a conductivity of 0.055 microsiemens/cm (a resistivity of 18MΩ-cm). Laboratory water purification systems make use of a number of different technologies, or combinations of technologies, in their operation. One of the most effective modern approaches is reverse osmosis (RO), which uses a semipermeable membrane to remove particles. Te best RO systems can remove more than 98% of dissolved mineral content and over 99% of bacteria from the feed water supply. RO systems generally include a pre-treatment package designed to meet the characteristics of the feed water. Typically, this equipment includes a base-exchange softener to remove hardness that would otherwise scale the membranes. Further protection is provided by passing the water through activated carbon filters, to remove free chlorine and organic contaminants, with any remaining particulates being removed by a fine filter before the pre-treated water enters the RO plant.


Selection criteria When selecting an RO system, labs need to think hard about their water needs, since both volume and quality requirements have a significant impact on the cost and design of the most appropriate system. Tere is little point using expensive 18MΩ-cm water when a lower grade would be sufficient for the application in question. One common polishing


approach uses deionisation, an excellent method of producing up to 18 MΩ-cm. An RO membrane is used for the main purification process, and this water is then passed through a disposable deionisation cylinder or cartridge before final use. Tese cartridges or cylinders use a mixture of resins to remove anionic and cationic


contaminants from the feed water, exchanging them with active hydrogen and hydroxyl ions, which combine to form water molecules. Some laboratory processes


may be particularly sensitive to specific forms of contamination, requiring even higher levels of purity than those defined by the general standards. In these cases, laboratories may need to install additional equipment to produce water at an appropriate level of purity. As well as determining


the size of the purification system required, the lab’s demand patterns will affect its configuration. Central systems use a single, large purification plant, with water distributed through pipework to the entire laboratory. Tis approach is most appropriate when high volumes of a consistent grade of water are required. Point of use (POU) systems, by contrast, use smaller floor, wall or bench mounted equipment to supply the specific water requirements of each lab. Tis approach is useful for lower volume applications, or where some activities have


highly specific water purity requirements. Point of use purification systems must also compete with other equipment for valuable lab space, however. Between these two extremes, a number of alternative configurations are possible, for example a ‘floor by floor’ approach that uses smaller versions of the centralised systems distributing water locally via pipework, or the installation of point of use polishing equipment to generate higher purity water for specific applications.


Operation and maintenance As soon as water leaves the purification system, it is exposed to potential sources of contamination. Chemicals, particulates, gases and bacteria can enter from the air in the laboratory or from containers used to store or transport the water to the point of use. As a result, proper selection of equipment and correct operating procedures are every bit as important to the final quality of the water as the


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Water is one of the key raw materials used in the lab


Suez offers lab-specific tools such as the Select Fusion


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