Carbon dioxide-free water is defined in the Reagents, Indicators, and Solutions section of USP–NF as Purified Water that has been vigorously boiled for NLT 5 min, then cooled and protected from absorption of atmospheric carbon dioxide.
Alternatively, this could be Purified Water that has a resistivity of NLT 18 megohm-cm at 25°.
Because the absorption of atmospheric carbon dioxide lowers the pH of high-purity waters, most of the uses of Carbon Dioxide-Free Water are either associated as a solvent in pH-related or pH-sensitive determinations or as a solvent in bicarbonate-sensitive reagents or determinations.
The term “Carbon Dioxide-Free Water” is sometimes used improperly. Besides its use for pH or acidity/alkalinity tests, the purpose for using this water is not always clear. The intention could be to use water that was deaerated (free of dissolved air) or deionized (free of extraneous ions), or even Purified Water with an additional boiling step.
Although boiling is highly effective for removing carbon dioxide as well as all other dissolved gasses, these gases are readily re-absorbed unless the water is protected.
Even with protection, such as use of a stoppered container, re-absorption will occur over time as air will readily transmit through seals and diffuse through most materials.
Deionization is also an efficient process for removing dissolved carbon dioxide.
Carbon dioxide forms ionic bicarbonate in water, and will be subsequently removed by ion-exchange resins.
However, the same problem of carbon dioxide re-absorption will occur after the deionized water is exposed to air.
Also, the deionization approach for creating Carbon Dioxide-Free Water does not deaerate the water or remove other dissolved gases such as oxygen (O2); it only removes carbon dioxide and other ions.
Depending on the application, Purified Water may meet the requirements where Carbon Dioxide-Free Water is called for.
This could also include pH or acidity or alkalinity tests. The pH of a sample of pure Deionized Water is, by definition, 7.0.
When that same sample is exposed to typical environmental atmospheric conditions, the water sample will absorb carbon dioxide and result in a pH range of approximately 5.4–6.2 ([H+] is in the range of 4.0 × 10^–6 M to 6.3 × 10^–7 M).
The added acidity caused by carbon dioxide absorption may be insignificant compared to the material being analyzed.
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