Distilled vs. Deionized Water: Which High-Purity Water is Right for Your Application?

Most comparisons argue about which method is "purer". That question has no general answer. Below is what a real deionized water specification limits, how to read it, why its pH is not 7, and which water the jobs people actually ask about call for.

What is the difference between distilled and deionized water?

They are not the same water, though for many jobs either one will do. They are made differently:

• Distilled water is boiled, and the steam is condensed back into liquid. Dissolved minerals stay behind in the boiler.
• Deionized water (DI water, or deionised water in British spelling) passes through ion-exchange resins that swap dissolved positive and negative ions for hydrogen and hydroxide ions, which combine into water.

Both remove the dissolved minerals that leave scale and spots. Where they differ is in what else gets through and how consistently, and that depends on the equipment and how it is run, not on the name on the label. That is why the useful comparison is between specifications, not methods.

What does a deionized water specification actually limit?

Here are the main limits on the published sample Certificate of Analysis for our Deionized Water ACS Reagent Grade. Our Battery Water is deionized water and carries the same limits on its own sample certificate.

Two things in that table are worth noticing. The organic carbon limit (under 50 ppb, reference 18 ppb) answers the usual objection that deionized water "still has organics in it": a DI water sold to a specification has its organics limited too. And the hardness ions, calcium and magnesium, are limited to 0.1 ppm each, which is the part that matters for batteries, coolant and anything you do not want scaled.

How do conductivity and resistivity relate?

They are the same measurement, inverted. Resistivity in MΩ·cm equals 1 divided by conductivity in µS/cm. Our limit of under 1 µS/cm is the same as over 1 MΩ·cm, and the reference conductivity of 0.47 µS/cm works out to about 2.1 MΩ·cm, which is the resistivity the certificate reports. The lower the conductivity, the fewer dissolved ions are left in the water.

Which is purer, distilled or deionized water?

It depends on the contaminant and on the specification, not on the method. ASTM D1193, the standard specification for reagent water, defines water by measured limits: it specifies four types of water, and adds grades that "specifically address contaminants of microbiological origin". The standard also notes that the letters given to the types "should not be taken as an indication of a progression in water purity". If purity matters to your work, buy against numbers like the ones above, and ask for the certificate for your lot.

Is distilled or deionized water sterile?

Do not assume either one is. Microbial contamination is a separate property: ASTM D1193 treats it with separate grades on top of the water type. Our deionized water certificate sets limits on ions, organics and residue and states no microbiological grade, so do not treat it as sterile. If you need sterile water, or water for a regulated pharmaceutical, medical, food or drinking use, buy water made and labeled for that use. Our Deionized Water is sold as an ACS Reagent Grade laboratory and industrial chemical.

Why is the pH of deionized water not 7?

Our specification allows pH 5–8, and the reference result is 6.4. Water this pure has almost nothing dissolved in it to resist a change in pH, so small amounts of anything it picks up move the reading. The usual culprit is the air: carbon dioxide is soluble in water and forms carbonic acid, a mild acid (PubChem), so an open container of high-purity water drifts below 7. That is normal and does not mean the water is contaminated with minerals; check conductivity if you need to know.

Does deionized water expire?

Water itself does not break down. What changes is purity once the container is opened: it takes up carbon dioxide from the air, as above, and anything from a dirty transfer container or the room. Keep it closed until use, transfer it only into clean containers that are made for chemical storage, and measure conductivity before use if your process depends on it. For a longer look at how fast purity changes after opening, see Deionized Water: How It's Made, Why It Degrades, and How to Buy It.

Should you use distilled or deionized water?

For most jobs that call for "distilled or deionized" water, what matters is that the dissolved minerals are gone, and either one does that. Some equipment makers state a preference or their own water limits, so check the manual first. These guides cover the uses people ask about most:

Buying deionized water or battery water

We sell Deionized Water ACS Reagent Grade and Battery Water in sizes from a quart to 275- and 330-gallon IBC totes; see every deionized water size. The SDS, TDS and sample COA are on the deionized water document page, and a certificate for your lot is free on request.