Double distilled water

Double distilled water (abbreviated "ddH₂O" or "Bidest. water") is prepared by double distillation of water. It is used, among other things, when single distillation does not lead to sufficiently pure water for some applications in biochemistry or trace analysis.

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Bidest water is used when pure, sterile water is essential. Whereas distilled water is enough for most chemical reactions, a molecular biologist attempting to create sterile, enzyme-free media might use bidest. water to ensure sterility.

The theoretical pH of distilled water is 7.0. In practice, however, most distilled water will have a pH that is slightly acidic (less than 7.0) due to the presence of carbon dioxide (CO₂) that is absorbed from the atmosphere. Dissolved carbon dioxide reacts slowly with water to give the bicarbonate and hydronium ions.

CO₂ + 2H₂O HCO₃^- + H₃O⁺

(carbonic acid, H₂CO₃, is only formed in strongly acid solutions). During distillation, the dissolved CO₂ will be driven out of the liquid. However, during condensation the water will re-absorb the CO₂ again resulting in a pH that is less than 7.0.

See also

• Distilled water
• Deionized water