Water is used in almost every laboratory task, from rinsing glassware to preparing solutions. Ordinary tap water contains dissolved minerals, organic matter, particles and microorganisms. Any of these can interfere with sensitive work. Proteins and other biological molecules are especially sensitive to contaminants and to changes in acidity.
Pure is not the same as sterile
- Purity is about what is dissolved or suspended in the water. Distilled water has been boiled and condensed. Deionised water has had its mineral ions removed. Ultrapure water has had almost everything removed and is the highest grade.
- Sterility is about living microorganisms. Water is sterilised with heat or by passing it through a very fine filter. Water can be very pure without being sterile, and sterile without being very pure.
With or without a preservative
Some sterilised water has a small amount of preservative added. The preservative slows microbial growth, so a container can be accessed more than once. Preservative-free water has no such protection and is normally treated as single-use once opened.
Slightly acidic water is sometimes used for materials that do not dissolve well at neutral acidity. Even pure water becomes mildly acidic when left open, because it absorbs carbon dioxide from the air.
Storage and handling
- Keep containers sealed, at controlled room temperature and away from light
- Write the date on a container when it is first opened
- Use clean technique every time a container is accessed
- Discard preservative-free water after a single use
- Never use water that looks cloudy or contains particles
This article is for general educational purposes only.
Frequently asked questions
What is the difference between purity and sterility?
Purity describes dissolved and suspended contaminants. Sterility describes the absence of living microorganisms. They are separate properties.
What is deionised water?
Water that has had its dissolved mineral ions removed.
Why is a preservative sometimes added?
To slow microbial growth so that a container can be accessed more than once.
Why is preservative-free water single-use?
Without a preservative there is nothing to stop microorganisms growing once the container has been opened.
How should laboratory water be stored?
Sealed, at controlled room temperature, away from light, and dated when first opened.
Is this medical advice?
No. This article is general educational information only.