Water purity grades for laboratory use
Water is the most common commodity in any laboratory and finds several uses such as:
- Media for dissolving compounds
- Washing of laboratory glassware
- Coolant in several applications
- Blank media in quantitative analysis
Universal solvent character makes it a dissolution media for dissolving most substances but this unfortunately also leads to its easy contamination from different sources. For this reason several approaches are required to purify water as per the requirements.
Contaminated or untreated water can contain several substances such as suspended particles, dissolved organic or inorganic compounds , dissolved gases, micro organisms, bacteria and viruses.
Untreated Tap Water
Tap water is found to contain inorganic impurities such as Ca, Mg, Fe, Al, Cu, Cr, Pb, etc and anionic impurities such as chlorides, nitrates, sulphates, fluorides, silicates, bicarbonates, etc, low volatility organic compounds and microbial contaminants. Tap water finds use mostly as a primary washing and cleaning agent and as a coolant in laboratory appliances. However, demineralized water with the addition of anti- corrosive agents is recommended in cooling applications.
Distilled water is prepared by boiling tap water and allowing the vapour to condense in a separate container. In the process it gets freed from common impurities but can still contain low volatility organic compounds that distil over along with the water vapour. It finds use in secondary washing and rinsing of laboratory glassware, as a blank in spectroscopic determinations and for preparation of common reagents for titrimetric analysis. Long-term storage is not recommended as the storage container can contribute to its contamination.
De-ionized water is prepared by further purification of distilled water by demineralization and by passing through mixed cation- anion exchange beds. However, freedom from suspended matter, pyrogens and bacteria cannot be guaranteed. Filtration through activated carbon and irradiation by UV light can lead to further purification for most applications involving quantitative studies. Irradiation by UV light is useful for destroying bacteria and viruses.
Several commercial systems are available from reputed manufacturers such as Elga, Milipore,Sartorius,Aurora Biomed,Thermos Scientific and Siemens, which combine several purification techniques such as ion exchange, reverse osmosis, ultra filtration and irradiation by UV light to produce water of very high purity in quantities that can meet the requirements of several sophisticated analytical instruments such asICP, AAS, HPLC, etc.
Water Purity Grades
Laboratory water types are specified by global bodies such as ASTM, ISO and Clinical and Laboratory standards Institute. National standards bodies of most countries specify purity grades as per their prescribed standards. ISO refers to them as grades instead of types.
Type-I water is also referred to as ultrapure water. It is used for detection of sub – ppm and ppb levels of trace metallic impurities using highly sophisticated techniques like ICP – MS and AAS.
Type-II water is used for common laboratory applications such as preparation of reagents, buffers, spectroscopic analysis and for preparation of cells in cultures.
Type-III water is used for common laboratory operations such as washing of glassware, use in humidity and environmental chambers, water baths and for generation of steam in autoclaves and sterilizers.
Purity of water plays an important role in analysis as it is often used as a blank or for dissolving the compounds to be analysed. Even the most sophisticated techniques can lead to unreliable results if water used for analysis is not meeting the required purity standards. It is therefore advisable to run a blank sample to ascertain absence of compound or element of interest in water that is used as solvent media in the analysis.