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Deionized vs. Distilled Water

July 12th, 2010

By Gregory Salata, Ph.D., Kelso, WA

Deionized vs. Distilled Water

Many sampling programs include collection and analysis of an equipment blank to ensure there is no contribution of contaminants from the sampling equipment and associated process. To establish that sample collection procedures are contaminant free, an equipment blank is often collected. Equipment blanks are collected by passing water through the sample collection apparatus or utensil and collecting the water into the appropriate containers. To ensure that the water itself is contaminant free, the laboratory will supply the field crew with deionized (DI) water.

Also known as demineralized water, DI water is prepared by passing a water source through specially-manufactured ion exchange resins, which bind to and remove minerals (i.e. salts). The minerals include both cations and anions including (but not limited to) sodium, calcium, iron, copper, chloride, sulfate, and bromide. Ion-exchange systems are commonly used as residential water softeners, normally as cation exchange systems to remove hardness. More sophisticated systems, such as the one used at Columbia Analytical Services, incorporates a reverse osmosis membrane, a carbon pre-treatment cartridge to remove organic analytes, and a UV post treatment to kill bacteria. DI water is dispensed throughout the laboratory through a segregated plumbing system to specially marked faucets.

In instances where no DI water is available, commercially available distilled water may be used as a substitute. Distilled water is produced by boiling the water and then condensing the steam, leaving residual material (salts, etc.) behind. Because most commercially available distilled water is stored in plastic containers, care must be taken if the water is to be used for semi-volatile and/or volatile organic analysis due to phthalate contamination or elevation of volatile organics due to diffusion through the plastic.

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5 Responses to “Deionized vs. Distilled Water”

  1. John Says:

    I purchased a gallon of distilled water from Walmart and I tested the PH. The PH was >4. I thought steam distilled water was supposed to be neutral. I can start my car with this PH number.

  2. Bob Says:

    Distilled water starts out neutral but when exposed to CO2 will form a small amount of carboxylic acid. It is a weak acid and because distilled water has no buffering capacity so it is negligible.

  3. SAM Says:


  4. robert Says:

    what is the quality difference distilled vs DI? I have a company using induction machines that have been using DI but the rep says need distilled?

  5. admin Says:

    Hello Robert!

    The quality of deionized water depends on the procedures involved in the deionization process. Water can be put through the simplest deionization process which will remove both cations and anions from it, but perhaps not non-ionized organic analytes, bacterias or viruses that may be present. The deionized water used in laboratories such as ours undergo additional processing that addresses these constituents. Distilled water uses the inability of cations, anions and other analytes to volatize with the water as it turns to steam. The higher temperature will also kill most of the bacteria and viruses that may be present. So water from a simple deionization process may not be as free of analytes like non-polar organics, bacterias and viruses as water from a simple distillation process. You could ask your DI water vendor to give you information about their processes to ascertain whether your DI water meets the standards your induction vendor requires.


    Dee O’Neill
    ALS | Environmental

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