International Organization for Standardization specification for water for laboratory use ISO 3696: 1987

This standard covers three grades of water as follows:

Grade 1

Essentially free from dissolved or colloidal ionic and rganic contaminants. It is suitable for the most stringent analytical requirements including those of high performance liquid chromatography （HPLC）. It should be produced by further treatment of grade 2 water for example by reverse osmosis or ion exchange followed by filtration through a membrane filter of pore size 0.2?m to remove particle matter or re-distillation from a fused silica apparatus.

Grade 2

Very low inorganic, organic or colloidal contaminants and uitable for sensitive analytical purposes including atomic absorption spectrometry （AAS） and the determination of constituents in trace quantities. Can be produced by multiple distillation, ion exchange or reverse osmosis followed by distillation.

Grade 3

Suitable for most laboratory wet chemistry work and reparation of reagent solutions. Can be produced by single distillation, by ion exchange, or by reverse osmosis. Unless otherwise specified, it should be used for ordinary analytical work.