Water has been rightly termed as a Universal solvent because it can dissolve most materials. It finds extensive applications in the laboratory which include:
- Media for dissolving compounds
- As Blank solution in quantitative analysis
- Washing of laboratory glassware
- Cooling operations
- Mobile phase component in HPLC analysis
It is not possible to use the same quality of water for all laboratory applications. The universal solvent character unfortunately makes water susceptible to contamination from different sources and different purification practices need to be adopted to achieve the required purity levels for different laboratory requirements.
Tap water is extensively used for primary cleaning of laboratory glassware and for cooling purpose in condensers. Unfiltered tap water can contain innumerable impurities such as inorganic cations like Ca, Mg, Na, Al, Fe, Cr, Pb, etc , anions like chlorides, sulphates, bicarbonates, organic contaminants, low volatility organic compounds, and microbial contaminants. Tap water if used universally in lab without further treatment can lead to erroneous results. Further it can also reduce life span of instruments due to corrosion where it is used for cooling through heat transfer.
Distilled water is purified water obtained by evaporation of water and condensation of the vapour. Vaporization is achieved by heating the water conventionally in a heating mantle and condensing the steam in a clean receptacle. Theoretically distilled water should be of highest purity but organic impurities having lower or similar boiling points also get carried over. Distilled water so obtained is commonly stored for later use. During storage it can get contaminated by the container through leaching so long storage should be avoided and it should be used at the earliest opportunity.
Distilled water is used for secondary cleaning and rinsing of glassware and also as a blank solution in spectroscopic observations.
Deionized water has similar or even higher purity than distilled water. Deionization is achieved by passing through a mixed cation and anion exchange resin bed. However. deionization does not remove suspended matter, pyrogens or bacteria. Ion exchange resin beds turn out to be favourable habitats for particles and suspended microbes which can find their way into the deionized water stream. It is therefore necessary to subject the deionized water stream through filtration and activated carbon treatment to remove organic impurities. In the final stage exposure to UV radiation can eliminate trace organics at 185nmand inactivate microorganisms at 254nm. The demineralized water should have resistivity around 18megohm-cm at 250C to comply with Type-1 ultrapure quality.
Reagent Grade Water
Several manufacturers provide bottled purified water to meet different analytical requirements.For large throughput laboratories it may not be a viable option to procure water from such sources and it would be economical to install a commercially available water purification facility.
Laboratory grade water specifications have been defined by global bodies such as ASTM(D1193 and D5196), ISO (3696), Clinical and laboratory standards institute(C3-A4).Almost all most national level standard bodies also specify water grades for laboratory use.
Traditionally highest purity waterType-I is referred to as ultrapure water followed by Type-II and Type-III in order of descending purity by ASTM. ISO has laid down similar categories but refers to them as Grades instead of Types.