Water for Analytical Purposes

1. Distilled Water

This is produced by vaporizing water and condensing it in a purer state. It is mainly used for preparing reagents but may also be required for other laboratory operations such as rinsing an analyte, transferring a test material as a slurry, as a calibration standard or analytical blank and for cleaning of apparatus. Unless specifically indicated, water meeting the requirements for Purified Water derived by other means of purification could be equally suitable where the use of distilled water is recommended.

2. Freshly Distilled Water

Also known as “recently distilled water”, it is the article produced in a similar manner as Distilled Water but is to be used shortly after its generation. This implies the need to avoid endotoxin contamination, or any other adventitious forms of contamination from the air or containers that could arise with prolonged storage. It is used for preparing solutions for subcutaneous test animal injections and also for preparing specific reagent solutions.

3. Deionized Water

This is produced by an ion-exchange process in which contaminating ions are replaced with either H’ or OH ions. It is used primarily as a solvent for preparing reagents and for the other aforementioned laboratory operations where distilled water is used. In this case too, Purified Water derived by other means of purification could be equally suitable where deionized water is specified.

4. High Purity Water

This is water that is prepared by distilling previously denoised water, and then filtering it through a 0.45-µm membrane. This water should have an on-line conductivity of not more than 0.15 µS/cm at 25°. It may be used where the use of ammonia-free water is specified.

5. Carbon Dioxide-free Water

This is Purified Water that has been vigorously boiled for at least 5 minutes, then cooled and protected from absorption of atmospheric carbon dioxide. Most of the uses of this grade of water are either associated as a solvent in pH-related or pH-sensitive reagents or determinations. It is also used in certain optical rotation tests and in the tests for appearance of solution. In addition to boiling, deionization could be an effective process for removing carbon dioxide.

6. Water BET

This is usually Water for Injections which may have been sterilized. It is free from a level of endotoxin that would yield any detectable reaction or interference with the lysate used in the test for Bacterial endotoxins.

Read also:

• Milli-Q vs. Elix Water Systems
• Comparison Between Deionized Water, Reverse Osmosis Water and Distilled Water

Resource Person: Susmita