What’s the Difference Between Distilled and Ultrapure Water?

by Kate Onissiphorou

Although distilled water is often synonymous with ultrapure water, the two are technically different products. Distilled water is a product of simple evaporation and condensation, while ultrapure water is produced through a relatively complex multi-staged process, which includes distillation. What’s more, distilled water is safe to drink, whereas ultrapure water is not.

Distilled water explained

What is distilled water?

Distilled water is simply the product of the distillation process. This means it has been heated to boiling point, evaporated and then condensed in a separate container.

The main reason for distilling water is to remove impurities, including germs. See the illustration of a simple laboratory distillation apparatus below. A graphic showing the distillation process

How is distilled water made?

Distilled water is made through the simple process of evaporation and condensation. This typically involves the following steps:

  1. The water in the distillation flask heats up until it boils and starts evaporating 
  2. The vapour then passes through the condenser and cools down
  3. The vapour condenses and collects at the other end of the condenser tube. The water collected in the receiving flask is distilled.

Ultrapure water explained

How is ultrapure water made?

Unlike distilled water, ultrapure water undergoes other steps aside from distillation. These additional processes include reverse osmosis, deionisation, carbon filtration, and exposure to ultraviolet light. They’re often performed in multiple passes to ensure a high level of purity. Flow chart showing the process of making ultrapure water

Ultrapure water is processed to make it as close as possible to the theoretical and truly ‘pure’ water that has no other molecular content other than dihydrogen oxide or oxidane (IUPAC name), which is composed of two hydrogen atoms and one oxygen atom. 

Water purity can also be measured in terms of its resistivity to an electric current. The calculated electric resistivity ratio of theoretically pure water at 25.0°C is 18.24 MΩcm. This level of purity is virtually impossible to attain and sustain. The presence of the smallest amount of impurities in the container and surrounding air can cause the ultrapure water to become less pure.

Is pure water and distilled the same?

Although pure water and distilled water are chemically identical in terms of their atomic composition per molecule, they contain different amounts of non-water molecules. Pure or ultrapure water has almost zero impurities, whereas distilled water may still contain significant amounts of impurities such as dissolved gases and minerals.

When should ultrapure water be used?

Due to its high purity, ultrapure water is very close to the theoretical zero impurity content of water. This extreme purity is necessary for a variety of analytical purposes, while its high sensitivity makes it ideal for very precise measurements and comparisons.

Ultrapure water has applications in microelectronics, photovoltaic power production, and pharmaceuticals. Some scientific methods that use ultrapure water include:

  • High performance liquid chromatography Vials on a tray inside the injection section of a High Performance Liquid Chromatography (HPLC) machine
  • Gas chromatography – mass spectrometry
  • Polymerase chain reaction
  • Culturing cells of mammals in the laboratory
  • Clinical analysis for diagnostic purposes
  • Graphite furnace atomic absorption spectroscopy
  • Trace analysis
  • Liquid chromatography – mass spectrometry

When should distilled water be used instead?

Distilled water is much easier to prepare than ultrapure water. It can be made using standard distillation apparatus in a school chemistry laboratory or even at home using a kettle.

Distilled water is generally free from living germs and large molecular impurities, including minerals. Although distilled water is not as pure as ultrapure water, it can still be used in some laboratory experiments such as preparing assays or solutions. It’s also useful for cleaning laboratory glassware.

Unlike ultrapure water, distilled water is safe to drink as it doesn’t compete with the minerals in your cells. Other uses of distilled water are most prevalent in the home, and include the following:

  • Steam irons
  • Aquarium water
  • Watering plants
  • Preparing infant milk
  • Car cooling systems Person adding distilled water from a bottle to a car battery
  • Some medical devices

Summary: distilled vs. ultrapure water

Distilled water is prepared through simple distillation and is safe to drink. Ultrapure water, however, involves multiple types of treatments including reverse osmosis and exposure to ultraviolet rays. It’s also dangerous to consume.

ReAgent's 25l packsize of ultrapure waterUltrapure water tends to be used for sensitive laboratory applications and industrial processes, whereas distilled water is typically used for simpler experiments and chemical preparations.

If you’re a UK-registered business, visit our online shop to buy ultrapure water from one of the UK’s leading chemical suppliers. We also sell distilled water in a range of container sizes so you’re sure to find the right product for your needs.


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