If you’ve performed one of our experiments you’ve probably seen “Distilled or Deionized Water” listed as a supply requirement. If you’ve ever wondered “Why do I need this?” or “What’s the difference?” then you’ve come to the right post.
Lab Water 101
Just like restaurants (sparkling or still) and grocery stores (mineral, alkaline, electrolyte, favored, etc.) most labs have multiple water types. These range from general use water (type III) for cleaning dishware to highly pure water (type I+) for sensitive analysis like spectrometry. There are four lab water types:
- Feed or Raw Water (Type VI) – Tap water whose contents vary widely depending on its source.
- RO or Primary Grade Water (Type III) – Feed water that has been forced through a semipermeable membrane by reverse osmosis.
- Pure or General Laboratory Grade Water (Type II) – RO water that has undergone additional treatments such as deionization or distillation.
- Ultrapure Water (Type I) – Pure water that has gone through additional ‘polishing’ treatments.
These lab water types are based on purity which is the total number of contaminants in a sample. Common contaminants include inorganic ions, organic compounds, bacteria, endotoxins, nucleases, particles, and gases. Because water is such a powerful solvent – it’s sometimes even called ‘the universal solvent’ – there are many such dissolved substances in untreated water even when the water appears clear.
Contaminants are removed by a variety of treatments. One of the most common treatments is reverse osmosis through a semipermeable membrane. During this process, water is forced through a membrane that allows the passage of water but blocks bacteria, larger molecules like glucose, and some ions like Na+. To move the water through the membrane and against its concentration gradient, external pressure is applied. Two other common treatments – distillation and deionization – are discussed in the next section.
Once a water sample has been treated its purity needs to be measured. There is no one way to measure water purity. Instead, scientists use a range of measurable properties including electrical conductivity, resistivity, organic compound levels, oxidizable matter content, biological contamination, turbidity, absorbance, and even the amount of residue left after complete water evaporation.
The allowable measures for the four water types are shown below.
Distilled and Deionized Water
Most of our experiments use distilled or deionized water. Both are pure or Type II waters and both are used extensively in labs particularly when preparing buffers, making microbiology media, and performing basic biochemistry analysis. Because these waters are so similar the terms are sometimes used interchangeably. However, the two water types are different – they are produced using different methods (deionization and distillation) and these methods remove different water impurities.
During deionization water is exposed to an electrically charged resin. This resin attracts and binds various ions which can then be removed from the water. Because the majority of water impurities are dissolved in salt this process removes not only ions but also many other contaminants. It also removes all charge from the water. This lack of charge creates a water blank that will assume the chemistry of whatever product is added to it. This is why deionized water is a good solvent for most buffers – even if additional ingredients create a new charge the buffer began at a known zero.
Distillation is one of the oldest water purification methods known and remains one of the most popular and effective today. During distillation, water is boiled and the steam produced is then cooled and collected in a new container. This process removes nearly all minerals, many chemicals, and most bacteria. What it does not remove are highly volatile organic compounds whose boiling temperatures are similar to or lower than water.
For the purposes of most lab experiments, both distilled and deionized water will create robust and reliable results. You can make distilled water using one of these guides “How to Make Distilled Water at Home for Free” and “How to Distill Water”. You can also purchase both types at most grocery stores in the water aisle.
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