The two most popular grades of purified water are deionised water and distilled water. Because of their high purity levels, many assume that these products are interchangeable. But this is not always the case because the different purification methods they undergo directly affect how and where these forms of purified water can be used.
The Difference Between Deionised Water and Distilled Water
To understand whether or not these products are interchangeable, we must first understand how exactly they are different. It all comes down to the ways in which they are purified.
Deionised water is produced when raw water is passed through ion exchange resins. In this process, known as deionisation, all charged ions in the water (including Ca2+, Mg2+, HCO3 and No3–) are replaced with hydrogen (H+) and hydroxide (OH–) ions.
When these combine, they form pure H2O. This means that deionised water contains no charged ions, making it a poor conductor of electricity.
However, deionisation only removes charged impurity ions, meaning that organic impurities like bacteria remain in the water since they do not have a charge. Therefore, the overall purity of the final product largely depends on the source water and how much organic matter it contains.
Distilled water undergoes a separation process that extracts pure water vapour from its impurities. This is achieved through distillation, a process that involves heating water past its boiling point so that it vaporises. Meanwhile, the impurities do not vaporise because they have a higher boiling point.
The pure water vapour is then collected and condensed into pure water. Therefore, distilled water is purer than deionised water because distillation removes all impurities, including ions, minerals, heavy metals and organic matter.
While deionised water and distilled water are similar in that they both have undergone a purification process, one cannot always be substituted for the other because of their different purity levels.
Uses and Applications
Since distilled water is purer than deionised water, there are certain applications where the two can’t be used interchangeably. At the same time, these two products do share similar uses and deionised water will often be used instead of distilled water because it is cheaper to obtain or produce.
Both of these water products can be used in the lab where they are sometimes used interchangeably:
- As solvents to prepare various solutions
- In autoclaves to extend the life of the chamber
- To clean glassware without leaving a residue
- To sterilise sensitive equipment
For general laboratory applications like these, deionised water is sometimes preferred because it is more cost-effective to produce. However, if it is a sensitive process that requires the use of high-purity products, distilled water will always be used instead.
The high calcium content in regular tap water can quickly accumulate on and corrode metal parts and surfaces. Therefore, purified water is required for things like:
- Lead-acid batteries
- Cooling systems
- Aircraft engines
- Gas-turbine engines
- Washing mechanisms
While both deionised water and distilled water can be safely used on metal parts in automotive systems, deionised water can be more corrosive than distilled water because it more readily absorbs carbon dioxide from the atmosphere.
When carbon dioxide dissolves in deionised water, it forms a weakly acidic solution of carbonic acid. This not only affects the pH of DI water, but it can also cause corrosion to metal parts, particularly steel. If there are applications that involve long-term contact with metal, distilled water is generally used.
At the same time, certain grades of deionised water can be used in the automotive industry. ReAgent supplies top-quality deionised water to the Bloodhound Project SSC, where it is used in the engine of their SuperSonic car.
In this industry, both types of purified water are often used interchangeably depending on the specifications of the product being produced. When used in pharmaceuticals, purified water is usually referred to as ‘aqua’ and it can commonly be found in:
- Compendial articles
- Analytical reagents
- Active pharmaceutical ingredients and intermediates (APIs)
As with most applications, however, if the product being manufactured has to meet certain quality or purity specifications, distilled water will be used over deionised water.
Purified water has to adhere to a very exact specification in order to be used in the medical industry. High-purity distilled water is often used to carry out certain medical procedures, including:
- To clean wounds in order to protect them from infection
- In irrigation solutions during surgery to prevent the entrance of bacteria
- In mouth rinsing during dentistry so that no bacteria is introduced to any open wounds
- In Constant Positive Airway Pressure (CPAP) machines
Deionised should not be used instead of distilled water in these applications because it contains organic contaminants like bacteria, viruses and other organic impurities. If introduced to an open wound, these could not only cause infection but could also have more serious effects depending on the procedure being carried out.
Which One Should I Use?
If you’re ever not sure about whether you should use deionised water or distilled water for an application, it is always important to take into consideration what the application involves.
In general, deionised water is often used instead of distilled water for applications that do not require high-purity ingredients. But it is always important to do research so that you understand how substituting one product for the other could affect the outcome of your procedure.
For comprehensive information about deionised water, check out our Complete Guide to Deionised Water resource.
At ReAgent, we sell distilled water as well as deionised water. All of our products are backed by a 100% quality guarantee and available in a range of pack sizes. Order online today or contact a member of staff for more information.
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