Even though pure water has a pH of 7, there are a few things that can affect the pH of water after or during its passage through a deionization system.
Traces of carbon dioxide in the air have an impact on pH tests. As soon as water comes into contact with traces of CO2, it absorbs carbon dioxide. The absorption of carbon dioxide into the water leads to a chemical reaction that produces carbonic acid which acidifies the water.
Since pure water has no buffering capacity, the addition of any acid will have a significant impact on the pH of the water.
- As an aside, the addition of carbonic acid to pure water can easily lower its pH to 5.6.
Increase in conductivity
Deionized water has a very low conductivity, about 0.055 microsiemens. Fluctuations in the conductivity of deionized water are a precursor to several situations such as
- Absorption of carbon dioxide
- Resin depletion / anion/cation depletion
- System start-up.
Without going into too much detail, during the pH setting, if the water comes into contact with the ambient air, carbon dioxide can dissolve in the water and create carbonic acid. When this acid dissociates, the ions formed increase the conductivity of the water.
On the other hand, when the resin is depleted of either anion, cations or both, an increase in conductivity can be seen. One reason for this is that the radicals released from the resin are conductive ions.
In short, other situations can cause acidification or an increase in the conductivity of deionized water. However, these examples demonstrate why pH changes following a water sample downstream of your deionization system is not problematic.