What water type should I use?

What’s the difference Between Type I, II, III & IV Water?

Each water type has its own level of purity. All types of water from Type I water (ultrapure) to Type IV water (semi-pure) is definedby the ASTM International* standard to have certain characteristics of resistivity (measured in Ω-cm) or conductivity (measured in µS/cm), which reveals inorganic impurities. Type I water is furthermore defined by its TOC (total organic compounds) purity, measured in parts per billion.


Using the right water type in your laboratory is important. To avoid wasted work, ruined results or sullied samples, make sure your laboratory is using a water type with the appropriate purity level for your particular application.

Don't go overboard and use water that's too pure for your needs, either; that mistake could cost cash and resources better spent on research.

Click the infographic for an explanation of the differences among laboratory-grade types of water, including common applications to which they're conducive, and available dispensing methods.

Contaminants in water can include particulates, dissolved organics, dissolved inorganics, microorganisms and/or pyrogens. To the varying degrees mentioned, the following key water types have such contaminants removed to produce water of application-specific quality**.

Water Types

Type I - Ultrapure, Type I water is defined by the American Society for Testing and Materials (ASTM) as having a resistivity of >18 MΩ-cm, a conductivity of <0.056 µS/cm and <50 ppb of Total Organic Carbons (TOC). What does that mean? Type I water is truly ultrapure and a requirement for analytical labs. Ultrapure water is a necessity for applications such as HPLC, gas chromatography, cell culturing, tissue culturing, mass spectrometry and any endeavor involving trace elemental laboratory instrumentation, among others.

Type II - ASTM defines Type II water as having a resistivity of >1 MΩ-cm, a conductivity of <1 µS/cm and <50 ppb of TOCs. Essentially, Type II water is cleaner than Type III/RO water but not ultrapure like Type I. Type II water is a good choice to feed to instruments and clinical analyzers because there is less calcium buildup, and it can also be used as feed water to a Type I system. Additional example applications include electrochemistry, sample dilution, radioimmunoassay and media preparation.

Type III - ASTM defines Type III water as having a resistivity of >4 MΩ-cm, a conductivity of <0.25 µS/cm and <200 ppb of TOCs. Less pure than Type I and II water, Type III water removes 90-99% of contaminants. It is produced using a Reverse Osmosis (RO) system directly from tap water, and is the starting point for many laboratory applications including glassware rinsing, media preparation, and other non-critical laboratory applications. In addition, Type III water is another reasonable choice as a feed water source to a system that produces Type I, ultrapure water.

Type IV - ASTM defines Type III water as having a resistivity of 200KΩ and a conductivity of <5µS/cm. Type IV water is most generally produced by RO and is used as feed water to a Type I or Type II deionized (DI) system.

Dispensing Methods & Available Systems

For large laboratories, such as university or pharmaceutical research buildings, large water purification units are often necessary. To meet the varying needs of the researchers utilizing multifunctional laboratory spaces, a substantial water purification system may produce RO, DI or Type III water that is then held in a reservoir tank for a specified period of time until it is needed. (This feed water can also be pumped to locations within laboratories with large-scale water requirements.) At that point, smaller units—some portable and benchtop in size—can be utilized to deliver the water to the necessary purity level of individual labs or even individual researchers. Ideal for smaller laboratories without large quantity water needs, some benchtop units can even deliver Type I ultrapure water directly from tap water.

Of note when choosing a water system for your laboratory are the volumes and qualities of water your applications will necessitate. Other important considerations include filter costs, ease of use and available water storage and system space, if applicable.

* ASTM International (formerly the American Society for Testing and Materials)

** For more information on water types, refer to ASTM D1193-91.

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