Everyday water is usually a good conductor of electricity because it contains dissolved salts, like cations and anions, which an electric current can travel through. Deionised water, on the other hand, is a poor conductor of electricity because of the purification process it undergoes.
What is Deionised Water?
Deionised (DI) water is a purified water product that has had all of its ionic impurities removed via a process of deionisation. This involves using ion exchange resins to replaced charged cations and anions with hydrogen (H+) and hydroxide (OH–) ions. These then combine to form pure H2O.
How do you Know Deionisation has Been Successful?
Successful deionisation can be tested by running an electric current through the water. If there is no electric current, then deionisation has been successful. This is the first indication as to why deionised water is not very conductive; its whole purpose revolves around its lack of ion content.
Ions and Conductivity
Ions in water are important for conductivity because they behave like stepping stones which the electric current can essentially hop between. You can measure the conductivity of water by looking at its TDS (total dissolved solids) content:
- The TDS of water is a measure of how many ions there are in a solution
- The conductivity of water is a measure of the ionic activity in a solution
These definitions give us an insight into the corresponding relationship between ions in a solution and the conductivity of a solution. Since an electric current needs ions to transport it, the conductivity of water increases in direct proportion to the increasing concentration of ions in water, and vice versa.
Therefore, the concentration of ionic substances in water determines the conductivity of that type of water. Tap water, which hasn’t been purified, is very conductive because it contains a lot of dissolved salts. Deionised water, however, as well as nearly all types of purified water products, is not a good conductor because of its lack of salts.
Why is Deionised Water a Poor Conductor?
The motion of electrically charged particles is what causes electric currents. This shows that conductivity is strongly dependent on the number of electrons available in any given state of matter. Metals, for example, are excellent conductors because they are comprised of a large number of free electrons.
In water, an electric current called ionic conduction is produced by a net motion of charged ions. However, when the hydrogen and hydroxide ions in deionised water bond to form pure H2O, they share all of their free electrons between themselves. It is this absence of free electrons that makes deionised water more of an insulator than a conductor.
Is Deionised Water Entirely Non-Conductive?
While its scarcity of ions makes deionised water a poor conductor of electricity, it is still slightly conductive. Most deionised water products used in high-purity industrial processes have a:
- Resistivity of 18.2 megaohm
- Conductivity of 0.055 microsiemens
Deionised water is always slightly conductive because of two main factors. The first is that if left over time, water will naturally begin to self-ionise. When this happens, it forms positively charged hydronium (H3O+) and negatively charged hydroxide (OH–) ions. The way deionised water reacts with the air is the second reason as to why it is slightly conductive.
Reactions with Air
When pure water comes into contact with the air, carbon dioxide begins to dissolve in it. The CO2 is then able to combine with the pure H2O to form a slightly acidic solution of carbonic acid (H2CO3) that is also weakly conductive:
2H2O + CO2 → H2O + H2CO3 → H3O+ + HCO3–
This reaction is also why many types of purified water, including demineralised and distilled water, can corrode things like steel over time. By dissolving carbon dioxide, the solution becomes slightly acidic and eventually eats away at the metal.
This is also why purified products like deionised water don’t have a neutral pH. Instead, the formation of carbonic acid makes them slightly acidic.
How to Avoid This
The pH, corrosiveness and conductivity of deionised water, or any purified water product, can be controlled by continually degassing it. This is a process that removes any dissolved gases, like carbon dioxide, and ensures the water stays as pure as possible.
At ReAgent, our deionised water has a resistivity of 1/conductivity m-ohms and a conductivity of <1µS/cm. All of our products are backed by a 100% quality guarantee, and our deionised water is so pure it is even used in the world’s first SuperSonic car. Order online today or contact us for more information.