It is the assumption that because distilled water has been purified, it has a neutral pH of 7. But this is not always the case because distilled water is very rarely 100% pure, and even more rarely has a pH of exactly 7.
The Basics of pH
Loosely standing for the power of hydrogen, the pH scale is a measure of the concentration of hydrogen ions in a given solution. As the pH increases, it indicates that the hydrogen content has decreased. This shows that the pH scale is the negative logarithm of hydrogen ion concentration.
Acids and Bases
A pH reading of 7 is exactly neutral. A solution with a pH lower than 7 is considered acidic while a solution with a pH higher than 7 is considered alkaline. The further away from 7 the reading is, the more acidic or alkaline it is.
The Bronsted-Lowry Theory of acids and bases defines acid as a compound that releases free protons in water. A base, on the other hand, is a compound that accepts protons. Examples of a strong acid and alkali are hydrochloric acid and sodium hydroxide. If either of these chemicals or similar are added to a solution, the pH will change dramatically.
Does Distilled Water Have a Neutral pH?
With all of this in mind, then, it would make sense if distilled water had a neutral pH of 7. The distillation process removes nearly all of its ionic, mineral and organic impurities, making it one of the purest forms of water available. But just because it is classified as purified water, this doesn’t mean that distilled water is pure H20.
The pH of distilled water is actually slightly acidic. This is because as soon as distilled water comes into contact with the air, carbon dioxide gas dissolves in it and creates a dilute solution of carbonic acid. This is also why demineralised water, another purified water product, reacts poorly with metals like steel.
When carbonic acid is formed, hydronium (H3O+) ions are released throughout the solution. This has the same effect as an influx of free hydrogen ions, which would decrease the pH reading:
2H20 + CO2 → H20 + H2CO3 → H3O+ + HCO3-
Very pure distilled water that has had little contact with air will have a pH just below 7, usually 6.9. But this does not mean that distilled water is an acidic substance. Its acidity is only very slight, and this is further shown by the fact that normal rainwater actually has a pH of around 5.6 and is still safe to touch.
Can the pH of Distilled Water Change?
While the pH of this product will change depending on how long it has been exposed to air, there are other ways that the pH reading can change.
Depending on how long it has been exposed to air, the pH reading of distilled water can range anywhere between 5.5 and 6.9. If it has been left open to the air, the pH can even fall just below 5.5. This happens because distilled water has had all of its salts removed during distillation, meaning that it has a very poor buffering capacity.
Temperature also affects pH readings. As temperature increases, so does the ionisation of the water. This, in turn, increases the concentration of hydrogen ions in the solution, making the pH more acidic.
Even though distilled water isn’t neutral unless it’s being tested straight from the distiller, it still has the closest pH to our blood. For this reason, it is often used by doctors in IVs or injections to help administer life-saving medicines.
How Do You Measure the pH of Distilled Water?
Purified water products like distilled or deionised water do not often have a strong ionic presence if any at all. This can be especially challenging when trying to measure the pH of distilled water because there are not enough ions in the solution for the pH electrode to function properly.
This means that the pH reading of distilled water is not always accurate, and you can tell that this is happening if the readings are drifting between numbers. One way to tackle this is to add a few drops of potassium chloride (KCl) to the solution.
The presence of KCl makes the water more conductible. This is because potassium chloride is an ionic compound that contains K+ and Cl- ions. Adding this to distilled water before pH testing will not affect the reading but it will make it more stable and likely to give trustworthy results. You can also use sodium chloride (table salt) to increase the conductivity of distilled water.
For comprehensive information about distilled water, check out our Complete Guide to Distilled Water resource.
At ReAgent, our distilled water products are backed by a 100% quality guarantee and are internationally recognised for their uncompromising purity. Order in a range of pack sizes today or get in touch for free technical advice.
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