Distilled, Deionised or Purified: What Is Actually Being Sold

Distilled water has been boiled and its vapour condensed. Deionised water has been passed through ion-exchange resin. 'Purified' means one of two very different things: a pharmacopoeial water made in a controlled, monitored system, or, on most retail labels, nothing more specific than 'treated'. The three processes remove different contaminants and leave different ones behind, so the word on the label tells you the route, not the result. In markets without a laboratory-supply chain the more pressing problem is that the word frequently does not describe the route either.

The pillar of this cluster covers where to find better water when Type I is out of reach. This piece is narrower: decoding what is on the shelf, running a few checks on an unfamiliar bottle, and deciding whether it is adequate for a given job. The grade limits themselves are in the reference treatment of laboratory water grades and are not repeated here.

The three terms and what each process removes

Deionisation deserves the plainest warning. A resin bed can deliver water with excellent resistivity while that water carries organic contamination and a live bacterial population, because a resistivity reading reports ionic content and nothing else ^[4]. The standards themselves have moved away from process names for this reason: the current ASTM specification allows each reagent-water type to be produced by any technology provided the measured limits are met ^[2]. A buyer should make the same move — stop asking how the water was made and start asking what it measures.

Why local labelling frequently does not match the standard definitions

Most water sold as distilled or deionised in retail outlets was never made for a laboratory. It is made for lead-acid batteries, steam irons, vehicle cooling systems, humidifiers and cosmetics, where the only property anyone cares about is low mineral content. In those markets the words are product categories rather than specifications. Small bottlers decant from drums into whatever containers they have; containers are refilled; and 'purified drinking water' is water treated to a potable standard, which is about the health risk of drinking it, not about contamination that shows up in a laboratory result.

Even correctly graded laboratory water makes a narrower claim than buyers tend to assume. ISO 3696 specifies its grades for the analysis of inorganic chemicals and states that it does not apply to water for biological or medical analysis ^[1], so no grade under it is a sterility claim. And the pharmacopoeial meaning of 'purified' attaches to a qualified, monitored production and distribution system, not to the contents of a bottle ^[3]. A retail label that uses the same word makes neither claim.

Simple checks on an unfamiliar product

The checks below are cheap, and each one can reject a bottle. None can accept one for analytical or biological work, for reasons given after the list.

1. Read the label for a named standard or grade, a manufacturer, a lot, a date and a stated purpose. A bottle labelled for batteries is telling you what it was made for.
2. Inspect it. ISO 3696 requires laboratory water to be a clear, colourless liquid on inspection ^[1]; any tint, haze, surface film or particle is a rejection.
3. Smell it on opening. Any odour points to organic contamination or microbial growth.
4. Measure conductivity immediately on opening, with a meter checked against a reference solution, and compare the reading with the limit for the grade you need. Inexpensive dissolved-solids pens estimate from conductivity; they are adequate for a screen, not for a certificate.
5. Dry a few drops on a clean glass slide. A visible ring or spot means dissolved or suspended solids; ISO 3696 includes residue after evaporation among its test methods ^[1], and the slide is a crude version of it.
6. Repeat on a second bottle from the same lot and on a bottle from a later purchase. Consistency between bottles is information in its own right.

What none of these checks see is organic carbon at the levels analytical work cares about, bacteria, or endotoxin — the contaminants a resistivity or conductivity reading is blind to ^[4]. A bottle that passes all six is best described as not obviously wrong. That is enough for many jobs, and not enough for the ones in the last rows of the table below.

Industrial and automotive distilled water, and why it is not equivalent

Battery water is made to keep minerals out of a lead-acid cell. It may well be genuinely distilled, and it may measure respectably on a conductivity pen. It is still made in bulk, held in tanks, filled into thin polymer containers that are often reused, and frequently stored on a forecourt in direct sun. There is no microbial control, usually no lot number, and no container specification. It is a reasonable feed for a water bath or a humidity reservoir after the checks above. It is not a reagent, not a blank, and not a substitute for anything that has to be sterile.

Container material and leaching

Water of any grade leaches from whatever holds it: glass gives up sodium, boron and silica, and polymers give up organics and additives, faster in heat and sunlight and faster still from low-grade or recycled plastic. A bottle of clean water in the wrong container becomes a bottle of something else, and the reference on how container material interacts with what it holds sets out what each material contributes. Pharmaceutical guidance treats the material and design of storage vessels as part of water quality control rather than an afterthought ^[3].

• Prefer rigid containers from a laboratory or pharmaceutical source, sealed at filling.
• Reject any container that has visibly been refilled, or that once held something else.
• Keep stock out of direct sun and off hot floors.
• Never decant into a container that has held detergent or solvent, however well it was rinsed.

Deciding what is adequate for a given task

The split between first and final rinse is where most laboratories quietly lose control, because the final rinse decides the detergent and mineral residue left on glassware, and it is tempting to use the same cheap water for both. Keep the cheap water for the jobs it can do, spend on documented water for the ones it cannot, and record which was used where.