If water is 100% pure technically it will have a pH of 7, that is to say neutral. However in reality this is difficult to achieve as the higher the purity of water the more it acts as a solvent and attempts to absorb anything that it comes into contact with.

One example is high purity water is left open to the air, over time will absorb carbon dioxide from the atmosphere resulting in carbonic acid which will results in a slightly acidic solution eventually dropping to an equilibrium of about 5.8 pH.

The more pure the water the greater its ability to absorb therefore resulting in a classic situation of a vicious circle, opposed to less pure water which has more so-called contaminats which tend to buffer the solution with the result that the acidity will be much closer to 7 or even higher.

To be clear when we talk about contaminants in this context it doesn’t necessarily mean the water is unsafe, It simply means there are other chemicals dissolved such as minerals.

Distilling water with commercial equipment will almost completely remove all dissolved minerals such as calcium, magnesium, sodium, fluoride, potassium, iron, and zinc and reduce its electrical conductivity to <2 μS/cm. Typical tap water has electrical conductivity in the range of 200–800 μS/cm. 

An additional factor is the higher the purity of water the more difficult it is to measure the pH especially with off the shelf pH metres. This is due to the fact of the metres measure ions in the water, the more pure the water the fewer irons there will be and the less accurate the reading.
So not only is a pH of 7 difficult to achieve due to absorption of other chemicals into the water especially when exposed to air it is also difficult to achieve an accurate pH reading of high purity water.

The pH isn’t normally critical for most day-to-day applications such as steam cleaners and day to day rinsing of laboratory glassware, however if the pH is critical the water can be treated prior to use to get it as close as possible to the required pH.

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