Why distilled water has a pH of less than 7.0

The reason distilled water has a pH of <7.0 is due to the high purity of the distilled water, it lacks minerals and other contaminants which normally act as a buffer, therefore it tends to absorb COgas from the atmosphere forming carbonic acid and subsequently making the water more acidic.

In theory distilled water has a pH value of 7 meaning it is technically neutral on the acidity scale. This is due to the high purity and lack of minerals and other contaminants which can raise or lower the pH of the water. In theory the purity level of distilled water is so high it must have a neutral acidity reading. However it's not quite that simple!

There are two main reasons why distilled water often has a pH of less than 7. 

1. Absorption of CO2

The reason distilled water has a pH of <7.0 is that it tends to absorb COgas from the atmosphere, when COgas comes into contact with distilled water it begins dissolving forming carbonic acid.
If the distilled water is left unsealed from the environment it can absorb COgas quickly.
Given time the distilled water will reach equilibrium and the pH will stabilise to around 5.8.

2. Electronic Metres

The pH of purified water can be difficult to measure accurately.
Electronic pH metres measure ions in the water to calculate the pH level. Due to the purity of distilled water it lacks ions, this makes it difficult for electronic pH metres to establish an accurate reading. The result can be a fluctuating or even a false reading. 

Tips to maintain the pH level of distilled water

  1. Keep tightly sealed in an airtight container.
  2. Only remove the lid from the container when required and replace it immediately after use.
  3. For repeated use consider decanting from the main container to a smaller bottle such as a lab wash bottle to avoid repeatedly exposing the main container to the atmosphere.
  4. When using or decanting distilled water consider the environment, for example a room with several people in will typically have higher levels of CO2.

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