Distilled vs Demineralised vs Deionised Water: What Australian Labs Need to Know

Distilled water is purified by boiling and condensation. Demineralised water has minerals removed by ion exchange. Deionised water has ions removed by resin beds or electrodeionisation. All three produce water with reduced impurities, but they remove different contaminants and suit different laboratory applications. Using the wrong type can ruin experiments, damage equipment, and compromise accreditation.

This is one of the most common questions we hear from Australian laboratories. The terminology is used interchangeably in many labs, but the differences matter. This guide explains what each type is, when to use which, and the mistakes that lead to failed results.

The Three Types Explained

Distilled Water

Water is boiled, and the steam is collected and condensed back into liquid. This removes dissolved minerals, most bacteria, and many organic contaminants.

Distillation is effective but slow, energy-intensive, and does not remove volatile organic compounds (VOCs) that evaporate and recondense with the water. It also does not remove dissolved gases like CO2, which lowers the pH of the distillate.

Demineralised Water

Minerals (dissolved salts) are removed using ion exchange resins. The resins swap mineral ions (calcium, magnesium, sodium, chloride) for hydrogen and hydroxyl ions, producing water with very low conductivity.

Demineralisation removes ionic contaminants very effectively but does not remove organic compounds, bacteria, or particles. The term "demineralised" is common in Australian and European laboratories.

Deionised Water

Ions are removed using mixed-bed ion exchange resins or electrodeionisation (EDI). The result is water with extremely low ionic content, typically measured by resistivity (up to 18.2 megaohm-cm for ultrapure).

Like demineralisation, deionisation targets ions specifically and does not remove organics, bacteria, or particles on its own. In modern laboratory water systems, deionisation is usually combined with reverse osmosis, UV treatment, and filtration to produce the full range of water grades.

Side-by-Side Comparison

Key takeaway: No single method removes everything. Modern laboratory water purification systems combine multiple technologies (reverse osmosis + deionisation + UV + filtration) to achieve the required grade. The Labconco WaterPro PS is an example of a system that polishes pre-treated water to produce Type I ultrapure water for critical laboratory applications.

Laboratory Water Grades (Type I, II, III)

Australian laboratories follow international grading standards (ASTM, CLSI, ISO) to classify water purity. NATA-accredited labs must demonstrate that their water meets the grade required for their specific assays.

Which Water Type for Which Application?

HPLC and chromatography: Type I only. Organic contaminants create baseline noise and ghost peaks. Even Type II water can produce background signals that mask trace-level analytes.

PCR and molecular biology: Type I only. Metal ions at parts-per-million levels inhibit DNA polymerase. Nuclease-free water is essential for RNA work.

Cell culture: Type I only. Bacteria, endotoxins, and organic contaminants can kill cells or alter experimental outcomes.

Buffer and reagent preparation: Type II is sufficient for most buffers and general reagent preparation. Use Type I when preparing standards for trace-level analysis.

General analytical chemistry: Type II for titrations, pH measurement, and standard solution preparation. Type I for spectrophotometry and flame photometry where ionic contamination affects readings.

Glassware rinsing: Type III is acceptable for final rinses of non-critical glassware. Use Type I or II for final rinses of glassware used in trace analysis. See our guide on how to clean lab glassware properly.

Autoclave feed water: Type III or demineralised water. Using tap water in an autoclave causes mineral scale buildup on the chamber and instruments.

Need help selecting a water purification system? Browse our Labconco WaterPro range or call 1300 501 555.

Common Mistakes with Laboratory Water

• Leaving water stagnant over weekends: Water sitting in a purification system or storage tank over weekends and holidays becomes a breeding ground for bacteria and biofilm. Systems should be set to recirculate, or water should be flushed before use on Monday morning.
• Assuming all purified water is the same: "Distilled", "deionised", and "demineralised" are not interchangeable terms. Each method removes different contaminants. Using deionised water (ions removed but bacteria present) for cell culture will contaminate your cultures.
• Never monitoring water quality: Many labs assume their purifier always produces the right grade. Without regularly checking resistivity, conductivity, and TOC (Total Organic Carbon), quality drift goes undetected until an experiment fails or an audit flags the issue.
• Placing the system in direct sunlight: UV exposure through windows promotes algae and bacterial growth in transparent components. It also causes thermal cycling that degrades seals and tubing. Always place purification systems away from windows.
• Ignoring feed water quality: Australian tap water contains chlorine and fluoride. If the carbon pre-filter is not replaced regularly, chlorine damages the reverse osmosis membrane, leading to expensive repairs. Feed water quality varies by city. Melbourne's soft water puts less load on systems than Perth or Adelaide's harder water.
• Using Type III water where Type I is required: The most expensive mistake in terms of wasted time and reagents. An HPLC run using Type III water will produce unusable chromatograms. Always match the water grade to the application.

Water Purification Systems at John Morris Group

John Morris Group supplies Labconco WaterPro water purification systems in Australia. The WaterPro range includes reverse osmosis pre-treatment, polishing stations for Type I ultrapure water, and HPLC-grade polishing kits for critical analytical applications.

Frequently Asked Questions

What is the difference between distilled and deionised water?
Distilled water is purified by boiling and condensation. It removes ions, bacteria, and particles but not volatile organics. Deionised water is purified by ion exchange resins. It removes ions very effectively but does not remove bacteria, organics, or particles. For laboratory applications requiring both ionic and microbial purity, a combined system using RO, deionisation, and UV is needed.

Is demineralised water the same as deionised water?
They are very similar but not identical. Both remove dissolved minerals/ions using ion exchange. "Demineralised" is the more common term in Australia and Europe. "Deionised" is standard in North American literature. In practice, the terms are often used interchangeably in Australian labs, though technically demineralisation may refer to a broader range of purification methods.

Can I use distilled water for HPLC?
No. Standard distilled water contains volatile organic compounds that co-distil with the water. These organics create baseline noise and ghost peaks in HPLC chromatograms. HPLC requires Type I ultrapure water with resistivity of 18.2 megaohm-cm and low TOC.

What water grade do I need for autoclaving?
Type III (laboratory-grade) or demineralised water is sufficient for autoclave feed. Using tap water causes mineral scale deposits on the autoclave chamber and on the instruments being sterilised.

What does NATA require for laboratory water?
NATA does not publish its own water grade standard. NATA-accredited labs must demonstrate that their water meets the quality specified by the relevant international standard for their assays (ASTM, CLSI, or ISO). Labs must monitor and document water quality as part of their quality management system.

How often should I test my lab water quality?
At a minimum, check resistivity or conductivity daily for critical applications. Monitor TOC weekly or as required by your quality system. Modern systems like the Labconco WaterPro provide continuous real-time monitoring of key parameters.

Where can I buy laboratory water purification systems in Australia?
John Morris Group is an authorised distributor of Labconco WaterPro water purification systems in Australia and New Zealand. We supply systems for Type I, II, and III water production with installation, commissioning, and ongoing service support. Call 1300 501 555 or browse our Labconco WaterPro range online.