Double distilled water
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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 dissolved carbon dioxide (CO2) that is absorbed from the atmosphere and which forms carbonic acid (H2CO3) in water. 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.
