Double distilled water (also abbreviated ddH2O) is water prepared by double distillation.
Double distilled water is used often in the laboratory when single distillation of water is not of sufficient purity for some research applications.
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.
Molecular biology labs often use water purification systems such as Milli-Q or Millipore systems creating ultra pure water for usage.
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.
(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.
