Double distilled water: Difference between revisions
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'''Double distilled water''' (abbreviated "ddH<sub>2</sub>O" 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]]. |
'''Double distilled water''' (abbreviated "ddH<sub>2</sub>O" 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]]. |
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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 |
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. |
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Modern molecular biology labs typically use water purification systems such as [[Milli-Q]] to create ultra-pure water. |
Modern molecular biology labs typically use water purification systems such as [[Milli-Q]] to create ultra-pure water. |
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Revision as of 05:26, 4 August 2007
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.
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.
Modern molecular biology labs typically use water purification systems such as Milli-Q to create ultra-pure water.
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.