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.
In this post:
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.
The pH of distilled water
Pure distilled water generally has a pH of 7, which means it’s technically neutral on the pH scale. Substances with a pH below this are classed as an acid, whereas those with a pH above 7 are a base or alkali.
An example of an acid in everyday life is vinegar, which has a pH of around 2. Baking soda, however, is an alkaline compound because it has a pH of about 9.
Although water undergoes spontaneous ionisation into hydronium and hydroxide ions, the pH level remains neutral because the ions form very low concentrations and the reaction is reversible:
H2O + H2O ⇌ H3O+ + OH−
At precisely 24.87 °C and zero ionic strength, the ionisation constant of pure water, or Kw, is equal to 1.0×10−14. Since the reversible equal dissociation of water can be computed as the negative natural log of Kw, its value is approximately 14 at 25 °C. This is also equal to the total of potential hydronium ions and potential hydroxide ions. That means the following relationships are constant at standard conditions:
- Kw = 1.0×10−14
- pKw = −log10 Kw = 14
- pKw = pH + pOH
Therefore, if the potential hydronium and hydroxide ions are equal, the neutral level would be half, or 7 in value. The theoretical levels of the pH scale can be extended to negative infinity and positive infinity. However, in practical terms, the acidity or basicity of solutions don’t go beyond the 0 to 14 pH scale.
It’s worth noting that perfectly 100% pure water is almost impossible to obtain and sustain – even in a highly controlled laboratory environment. There will always be some impurities, such as dissolved air. Some miniscule impurities may also come from the container of the processed water. Consequently, the pH of distilled water may fluctuate slightly, albeit by a very negligible amount.
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.
In medicine
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.
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.
