ASTM Purity Classes for Laboratory Water Explained

Not every laboratory application requires the same water purity. The international ASTM standard (D1193) divides laboratory water into three purity classes – from Type I to Type III – creating a uniform standard that laboratories worldwide can rely on.

Type I – Highest Purity

Type I water reaches a resistivity of up to 18.2 MΩ·cm with a TOC value (total organic carbon) below 10 ppb. This purity class is required for particularly sensitive applications, such as HPLC analysis, cell culture, and molecular biology procedures, where even the smallest contamination can distort results.

Type II – High Purity

With a resistivity of at least 1 MΩ·cm, Type II water is suitable for general laboratory analysis as well as the preparation of reagents and buffers – anywhere high, but not the very highest, purity is required.

Type III – Medium Purity

Type III water reaches at least 0.05 MΩ·cm and is often used as a pre-filtration stage for Type I water, for rinsing glassware, or for less critical laboratory work.

A Practical Note

In practice, people sometimes also speak of “Type IV” – however, this is not an official ASTM category, but an informal term for raw or pre-treatment water. Which purity class your laboratory actually needs depends on the specific application – we design our laboratory water systems individually for the right ASTM class.