Lab water: distilled, deionised, reverse osmosis and ultrapure

Deionised water has had its dissolved ions removed by ion-exchange resin; distilled water has been boiled and condensed, leaving salts behind. Reverse osmosis forces water through a membrane, and ultrapure (Type 1) water combines these steps with UV light and fine filtration. Tap water carries dissolved salts, organic matter, particles, microbes and gases. Each purification method removes some of these and leaves others behind. This guide explains the four common types of lab water, how their purity is measured, and which one suits which job.

Terms are defined in the glossary.

What is in tap water

Purification methods are easier to compare once you know what they are trying to remove. Tap water contains:

• Ions: calcium, magnesium, sodium, chloride, sulfate, bicarbonate and others. These make water hard and let it conduct electricity.
• Organic compounds: natural material from soil and plants, plus traces from pipes and treatment.
• Particles and colloids: fine sediment, rust and silica.
• Microorganisms: bacteria, and the fragments they leave, such as endotoxin.
• Dissolved gases: oxygen, and carbon dioxide, which forms carbonic acid and lowers the pH.

No single method removes all five well. That is why lab water systems combine several.

Distilled water

Distillation boils water and condenses the steam. Salts, particles and most microbes stay behind in the boiler, because they don't evaporate.

• What it removes well: dissolved salts, particles, bacteria and most non-volatile organic matter.
• What can carry over: gases and volatile organic compounds that boil near water, such as some solvents. Carbon dioxide redissolves as the water cools, so distilled water is often slightly acidic, around pH 5.5–6.
• Drawbacks: stills use a lot of energy and need regular descaling. Glass stills give cleaner water than metal ones.

Double-distilled water has been through the process twice, which lowers the trace contamination further.

Distilled water is also sold for irons, car batteries and steam appliances. That water is fine for its purpose but isn't made or tested to a laboratory grade.

Deionised water

Deionisation (DI) passes water through beds of ion-exchange resin. One resin swaps positive ions such as calcium and sodium for hydrogen ions (H⁺). Another swaps negative ions such as chloride and sulfate for hydroxide ions (OH⁻). The H⁺ and OH⁻ combine to form water.

• What it removes well: dissolved ions. A fresh mixed-bed cartridge can bring water close to 18 MΩ·cm.
• What it doesn't remove: uncharged organic molecules, particles and bacteria. Resin beds can even grow bacteria if water sits in them, and the resin itself can shed organic fragments.
• How it fails: when the resin is used up, ions start to break through. A resistivity meter on the outlet shows when to change the cartridge.

So deionised water can have very low conductivity and still contain organic contamination. Conductivity alone doesn't tell you it is clean.

Deionised water vs distilled water

In practice many labs use both: deionised or reverse osmosis water as the everyday supply, and an ultrapure system for sensitive work.

Is deionised water the same as distilled water?

No. Both have most dissolved salts removed, but by different routes. Distillation also leaves behind bacteria and most organic matter, while deionisation removes only charged particles (ions), so deionised water can still carry organic contamination and bacteria.

Is RO water deionised?

Not strictly. Reverse osmosis removes most ions with a membrane rather than ion-exchange resin, and it leaves a small fraction behind. That is why lab systems usually follow RO with a deionisation stage.

Reverse osmosis water

Reverse osmosis (RO) pushes water under pressure through a membrane with pores too small for most dissolved salts, organic molecules, particles and bacteria.

• What it removes: typically 95–99% of dissolved ions, and most organic matter, particles and microbes in one step.
• What it leaves: a small fraction of ions and dissolved gases. RO water is cleaner than tap water but well short of ultrapure.
• Where it fits: as a cheap first stage. It takes most of the load off the deionisation cartridges that follow, so they last much longer.

Home RO units for drinking water use the same principle. A laboratory RO system is built and monitored to a stated output quality.

Ultrapure water

Ultrapure water comes from a polishing system fed with RO, DI or distilled water. A typical system chains several steps:

1. Activated carbon to remove chlorine and some organic matter.
2. Mixed-bed ion exchange to remove the remaining ions.
3. UV lamp (often at 185 nm) to break down trace organic molecules.
4. 0.2 µm filter at the outlet to remove particles and bacteria.

The output is measured continuously. It is usually given as resistivity (18.2 MΩ·cm at 25 °C) and total organic carbon (TOC), often a few parts per billion. "Milli-Q" is a brand name for one maker's ultrapure systems that is often used for ultrapure water in general.

Why it goes off quickly

Ultrapure water is so free of ions that it pulls carbon dioxide out of the air as soon as it is exposed. The carbon dioxide forms carbonic acid, and within minutes the resistivity falls well below 18 MΩ·cm. It also picks up ions and organic matter from containers. So ultrapure water is drawn fresh at the point of use and not stored in bottles for later.

How water purity is measured

Resistivity and conductivity are measured at, or corrected to, 25 °C, because both change with temperature. Very pure water is also hard to measure the pH of, because it conducts so poorly: see pH meters for how labs deal with that.

Water grades and standards

Several standards set out grades of laboratory water. They use different names but follow the same idea: the purest grade for the most sensitive methods.

ASTM Type I to IV water

Limits as listed by Puretec; check the current edition of the standard before relying on them. The types are labels, not a strict ladder: Type III has a higher resistivity limit than Type II but a looser organic carbon limit.

"Type 1 water" is common shorthand for ultrapure water, borrowed from these grading schemes. "Milli-Q water" means the same thing in many labs.

Which water for which job

Ghost peaks are a good reason to take the water grade seriously. In reversed-phase HPLC, organic contaminants in the water stick to the column at the start of a run and then wash off as the solvent strength rises. They show up as peaks that belong to no compound in the sample. A blank run, with no sample injected, shows whether they come from the water.

Purified water is not sterile water

None of the waters above is sterile unless it has been sterilised afterwards and kept in a sealed container. A 0.2 µm filter removes bacteria at the outlet, but the water can pick them up again from the container or the air. For the difference between sterile water and bacteriostatic water, and when a preservative is used, see bacteriostatic water vs sterile water.

Related guides

For making up buffers with purified water, see buffers and pH. For why trace contaminants matter to an analysis, see reading a chromatogram. For sterile handling, see aseptic technique and autoclaving. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.