How long is pH constant in a water sample?

[nim6us]

When I would do my weekly testing in the past I took each sample directly from the tank before I ran the test. However I saw someone testing on a stream and he got a 1L jug and filled it full so he could take the sample to his desk and do it all there. Which is way more convenient!

However today as I was testing I wondered how long the pH would remain constant in the jug on my desk before it started shifting up/down. I read a couple sources that said 2 hours is the standard time, but none of the sources cited they were using saltwater.

So I'm curious, does anyone know if it's 2 hours, or perhaps different?

 

[arking_mark]

Following and hoping @Randy Holmes-Farley will provide insight.

My guess is it depends. pH can be calculated based on Alk and CO2 and several other factors including temp of the sample.

A 20 degree temp drop is roughly a -0.03 impact on pH.

If the sample is sealed with no air then I don't think CO2 will change. If open to air it will slowly balanced CO2 with the air which could raise or lower pH depending on if the air was higher or lower in CO2. If aerated, then the pH will more quickly do this.

I know for a cup aeration test, I usually aerate for 1hr to reach equilibrium with the air. But that might be overkill.

Worst case scenario might be sampling the water when CO2 in the water is lowest due to photosynthesis and then taking it to a cold stuffy high CO2 room to measure it after it has equalized with that rooms level of CO2.

Assuming your actual Alk is at 8, pH 8.3, and temp is 78 and you move the sample to a 58 degree room with 1000ppm CO2 and let it equalized with the air CO2, your reading will be about 7.9.

 

[Malcontent]

Well, when you open a bag of fish the pH supposedly changes instantly but for some reason this takes longer. Lol.

 

[Bj’s Reef]

In the waste water industry we have 15 minutes to run ph test after the sample has been collected. Past that time the sample is invalid because of the potential for a ph shift. So I apply that logic when I’m s testing my tank.

 

[nim6us]

I think I'll go with that more conservative estimate and make my pH test the first one I complete when I start testing.

 

[arking_mark]

Well, when you open a bag of fish the pH supposedly changes instantly but for some reason this takes longer. Lol.

Fish in a bag produce CO2 and ammonia. The CO2 lowers pH. At lower pH the ammonia is less toxic to fish. If you aerate the water with the fish you can quickly bring up the pH which may bring the ammonia to a toxicity level that is harmful. That is why we we typically add tank water to the bag to acclimate the fish as this slowly raises the pH while diluting the bag water. Some shippers include ammonia binders to dose during acclimation and many shippers have adjusted their recommended acclimation process to just match temp and salinity before adding the fish.