Lab water grades explained: deionised, distilled and reagent grade water
by Labdrop
Lab water is graded by how few impurities it contains. The two standards most labs refer to are ISO 3696, which sets grades 1 to 3, and ASTM D1193, which sets types I to IV. Conductivity is the quickest way to compare water against them: the fewer dissolved ions left in the water, the lower its conductivity.
ISO 3696 grades
| Grade | Maximum conductivity at 25 °C | Typical use |
|---|---|---|
| Grade 1 | 0.1 µS/cm | The most demanding analysis, such as HPLC |
| Grade 2 | 1.0 µS/cm | Sensitive analysis, such as atomic absorption |
| Grade 3 | 5.0 µS/cm | Most wet chemistry and preparing reagent solutions |
ASTM D1193 types
| Type | Maximum conductivity at 25 °C | Minimum resistivity at 25 °C |
|---|---|---|
| Type I | 0.056 µS/cm | 18 MΩ·cm |
| Type II | 1.0 µS/cm | 1.0 MΩ·cm |
| Type III | 0.25 µS/cm | 4.0 MΩ·cm |
| Type IV | 5.0 µS/cm | 0.2 MΩ·cm |
Both standards also set limits for other tests, such as oxidisable matter, silica and residue after evaporation. Conductivity alone does not make water a particular grade, so check every limit in the standard your method names.
Conductivity and resistivity
They measure the same thing from opposite ends. Resistivity is the reciprocal of conductivity, so water with a conductivity of 0.5 µS/cm has a resistivity of 2 MΩ·cm. Ultrapure Type I water reaches 18.2 MΩ·cm, the theoretical limit for pure water at 25 °C.
Deionised or distilled?
Deionised water has its dissolved ions removed by ion exchange. Distilled water is boiled and the steam condensed, which leaves most dissolved solids behind. Both are used in labs and either can reach a given grade. What matters is whether the water meets the limits your method sets.
Is deionised water sterile?
No. Deionisation removes ions, not microorganisms. Where a method needs sterile or preserved water, it will say so.
Our deionised water
Our deionised water, joining the range shortly in 5 L and 25 L, has a conductivity below 0.5 µS/cm and under 1 ppm total dissolved solids. That is within the conductivity limit for ISO 3696 grade 2 and ASTM type II. The full specification is on the product page and in our lab water range.
Labdrop products are sold for laboratory use only. Not for human or veterinary use.