pH drops with water aging: what is the explanation?

[MattArmstrong]

Has anybody seen credible and scientific explanations of why "Seattle" tap water drops in pH after sitting/aging for a day or so?

I asked this on another forum and didn't get great answers. I'm curious if wafishbox folks know the answer, especially since this pertains to the soft "Seattle" water that many of us have.

Many people that "age" their tap water before water changes observe a pH rise. Most often this is explained by off gassing of CO2. I'm finding that my water drops from 8.0 -> 7.5. What might explain this? Infusion of CO2 from the ambient air?

My municipal water treatment facility (Seattle Public Utilitie's Tolt facility) uses Lime (CaO) and CO2 to buffer the water up to KH 2. From some reading, I think these two combine into the other carbonates that make up KH. This might imply that the water comes out my tap with less CO2 than would be the case if it were in equilibrium with air -- if a lot of it combines with the CaO to form things like Calcium Carbonate (CaCO3).

Just a guess. I've not seen this discussed much.

 

[popss]

all city water drops PH when ROE,d , being colder as we in winter water hold allot more gas, to get a true reading of what your water PH will be 24 hours later do a bucket test, fill a 5 gallon bucket and put a aerator in it, test it 24 hours later and you will have your PH base line.
not sure where the PH goes up instead of down from.

As a side note wave! My wife works at told middle school.

 

[popss]

not sure what fish you are running, but unless running black water fish the PH swing should not be an issue.

 

[MattArmstrong]

Yep. Seems few really know why our tap pH swings down when aged. I certainly don't (I only have my CO2 absorption theory, backed by no science).

pH drops in RO makes some sense if KH is being reduced.