Distilled water and demineralised water both refer to water that has been purified, and share similar uses.
It’s not surprising, then, that the two are often confused or considered interchangeable. However, this is an erroneous assumption as distilled and demineralised water have many diverging properties.
The main differences lie in the purification processes which create discrepancies between the two. This means that one should not be exchanged for the other. In this post, we delve into the characteristics that separate these similar substances.
In this post:
The Water Purification Processes
Demineralised Water Purification Process
Demineralised water (also known as deionised water) undergoes a process that removes all of its minerals and ions. This is achieved by passing the water through ion exchange resins. These resins replace the cations and anions that are present in the water with hydrogen and hydroxide, which come together to form pure water.
This process is a form of deionisation, and is a chemical reaction. Distillation, on the other hand, occurs through separation.
Distilled Water Purification Process
To yield a purified sample using distillation, water is boiled at extremely high temperatures until it begins to vaporise, leaving behind the impurities. The vapour is collected and eventually condensed so that it converts back to a liquid state. This becomes the distilled water.
While distillation is extremely effective at removing impurities, it can be put through the process again in order to obtain an even purer sample. This is referred to as double distillation.
The Effectiveness of Demineralisation vs. Distillation
Because of the different process these waters undergo, it means that distilled water and demineralised water similarly have varying degrees of effectiveness when it comes to removing impurities.
Demineralised water is likely to still contain organic chemicals like bacteria or salt. This is because deionisation occurs when ion exchange beds replace the positively and negatively charged molecules. Since organic particles are uncharged, they are unaffected by deionisation and will remain in the water.
Distillation, on the other hand, removes nearly all impurities as it relies on selective evaporation and condensation. Unwanted molecules are easily separated from the water as they do not vaporise.
This is because the vast majority of contaminants have higher boiling points than water, meaning that they get left behind when the water vaporises. Therefore, the vapour that is collected and condensed will produce a purer sample than demineralised water obtained from deionisation.
However, just as distilled water can undergo double distillation to yield an even purer sample, demineralised water can also be purified further. The organic chemicals and impurities that are left behind following deionisation can be removed by passing the water through activated charcoal filters. These absorb carbon-based, meaning organic, impurities by trapping them in their huge surface area.
As well as this, while distilled water tends to be more effective at removing impurities, deionisation leaves behind less scale than distillation, meaning that demineralised water has a much cleaner production.
Uses of Demineralised & Distilled Water
Demineralised water and distilled water share many common uses. This is because they have both been purified, and their lack of contaminants makes them ideal for use in steam irons, aquariums, and as a solvent to prepare solutions.
However, while they do have some common usages, they shouldn’t be considered interchangeable without knowledge of their specific reactions. Here are some uses of demineralised water and distilled water:
1. Motor Vehicles
Both demineralised and distilled water are used in lead batteries and to top up the coolant system in engines. This because, since they have both had their ions and contaminants removed, they improve the longevity of these systems by preventing corrosion.
With that being said, it is worth noting that demineralised water is more corrosive than distilled water when put in contact with certain substances. While it has excellent compatibility with most metal materials, such as aluminium or stainless steel, demineralised water has severe corrosive effects on materials like carbon steel, copper, and cast iron.
Corrosion occurs when demineralised water comes into contact with carbon dioxide in the air. This is because the deionisation process makes it reactive.
While DI water usually has a pH of 7, when the carbon dioxide dissolves in the water it produces Carbonic Acid. This pushes the pH closer to 5.6. It is this slight acidity that is likely to cause corrosion with particular metals.
Unlike this, distilled water is a lot purer as nearly 99.9% of its contaminants are successfully removed. It is therefore not as reactive as demineralised water when interacting with metals.
2. Laboratory Testing
When it comes to laboratory testing, cleaning laboratory equipment, and many other industrial applications, purified water is always the go-to.
This because it won’t leave behind any build-up of minerals, and its purity means that it’s a great steriliser. Distilled and demineralised water are mostly interchangeable in these circumstances.
The advantage that demineralised water has is that it is a lot more cost-effective to produce. This is because distillation requires a lot more electricity than deionisation. Unless the application specifically requires distilled water because of its high purity, demineralised water is preferred in laboratories because it is cheaper to obtain.
