Deionised water is one of the most popular purified water products on the market. But it doesn’t stay pure forever, and there are several factors that can affect how long deionised water keeps for.
Undergoing a process that removes its charged ions, deionised water is completely free of ionic and mineral impurities. While water never officially goes ‘bad,’ there are certainly ways to prolong the shelf life of this purified product.
Reactions with the Air
One major factor that can affect the shelf life of deionised water is its reaction to the environment, specifically with carbon dioxide in the air.
When this product is exposed to air, carbon dioxide quickly dissolves in it. This forms a weak solution of carbonic acid, releasing positively charged hydronium ions and negatively charged bicarbonate ions. Deionised water will then:
- Become slightly conductive
- Become slightly acidic with a low pH
- Contain charged ions
Therefore, in order to make deionised water keep as long possible, it should be kept in a tightly sealed container which should only be open for the amount of time it takes to obtain the amount of deionised water you need.
A method called degasification can be used on purified water products that have been contaminated in this way. This process removes all dissolved gases, including carbon dioxide, from the water. Degasification is one way to extend the life of your deionised water, but it isn’t always the most convenient solution.
Storing Deionised Water
The unique physical properties of deionised water mean that it must be stored under proper conditions. Since it is likely to absorb carbon dioxide over time, it is important to protect its integrity as much as possible if you want it to keep for a while.
The packaging used to store this grade of water also has a huge impact on how long it keeps for. The storage you use will either prolong the life of DI water or make it deteriorate quicker, so it is important that you get it right.
- Glass containers are recommended for small amounts of DI water. This is because they are chemically inert and, unlike plastic containers, the deionised water won’t be able to leach any chemicals from it. However, for industrial quantities, glass containers aren’t the most convenient option as they can easily break if dropped.
- HDPE containers are the ideal packaging choice for large-scale quantities of DI water. High-Density Polyethylene containers are corrosion resistant, have limited exposure to light and are also able to maintain a cool temperature. Like glass, they are chemically inert and provide excellent conditions.
- Metal containers are not recommended for storing purified water as they are not chemically inert. DI water can easily react with and absorb metal compounds. When carbon dioxide dissolves into deionised water, the resulting carbonic acid solution is also able to corrode the metal, creating even higher levels of contamination.
As well as carbon dioxide, deionised water is easily able to combine with other compounds around it, whether they are in the packaging or the air. This causes contamination which directly affects its shelf life. This can be avoided by taking extra precautions when selecting your storage options:
- Always ensure the packaging is tightly sealed whenever you are not using deionised water. This will prevent any more carbon dioxide from dissolving in the solution.
- Store deionised water away from sunlight. Ideally, deionised water should be stored in a dark room. HDPE containers are a great way to protect against sunlight.
- Deionised water should be stored under cool temperatures instead of being left in a warm environment. HDPE containers are also able to maintain cooler temperatures.
Ultimately, if you store it in a good-sized, protective container under proper conditions, deionised water will keep much longer than if you were to leave it in a bottle near an open window.
Sometimes, no matter how hard we try, things can’t last forever. This is the case with deionised water, and the reason for it is self-ionisation.
Also known as autoionisation or auto-dissociation, the self-ionisation of water is a reaction that occurs in all aqueous solutions, including pure water. It involves one water molecule donating a proton to another water molecule, forming hydronium (H3O+) and hydroxide (OH–) ions.
- One water molecule deprotonates and becomes a negatively charged hydroxide ion. Deprotonation occurs when a water molecule loses the nucleus of one of its hydrogen atoms.
- Another water molecule is then immediately protonated by the hydrogen nucleus (H+) and becomes a positively charged hydronium ion. Protonation occurs when a proton is transferred from one water molecule to another.
This reaction is an example of autoprotolysis, a process in which a proton is transferred between two identical molecules. One of the molecules acts as an acid and releases a proton. The other acts as a base and accepts the proton. This also shows how water is amphoteric.
While deionised water contains an extremely small percentage of water molecules that would transfer protons, self-ionisation still occurs and will inevitably have an impact on how long deionised water keeps for.
Therefore, even if DI water is stored under the proper conditions and never introduced to air, it will still be unlikely to keep forever as a pure solution because of its capacity to self-ionise.
For comprehensive information about deionised water, check out our Complete Guide to Deionised Water resource.
ReAgent stocks deionised water in a wide range of pack sizes and even gives you the choice of customised labelling. All of our products are backed by a 100% quality guarantee so that you can buy with confidence. Order online today or contact our sales team for more information.
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