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Mystery pH rise?

T

Tomo

Member
Joined
4 Aug 2009
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50
Hello,

Can anybody explain why a bucket of tap water (nothing else in the bucket) has a pH of 7.2 initially, and then 24 hours later a pH of 7.4? The colour change in the test is quite considerable. I was going to place some natural slate I bought in the bucket to test if it raised the pH or not, before I put it into my tank. I don't know whether to do this now as the pH of the water has gone up on its own! I don't understand why.

Does natural slate raise pH?

Thanks for any help
 
For reasons I never worked out, tap water seems to have more CO2 in it than at equilibrium with air after it comes out. So once the excess has left, the pH rises accordingly.
 
Another thing would be to revise the accuracy of your pH measure... 0.2 change can be a reliable measure with good equipment but probably nothing to trust if you are using pH measure kits commonly available in the hobby.
Edit: sorry, I see you mention it clearly changed the colour... You know the trend, but with this kind of test 0.2 means not very much

Jordi
 
Presence of chlorine in your water will make it slightly acidic and as is gasses off the pH will rise.

I assume you used a laboratory titration test kit to measure pH ? Most hobby strip and liquid test kits are hopeless as they are easily influenced by other salts present in the water.
 
Hi all,
It is like the other posters have said, there isn't really much difference between the two readings. If they weren't measured at the same temperature? it is probably a temperature effect because gases are less soluble at higher temperatures and as well as chlorine (Cl2) outgassing, there is dissolved CO2 to take into account.
  • CO2 is in equilibrium with bicarbonate (HCO3-) and the solubility of gases is governed by temperature (and Henry's Law, which is why the added Cl2 is outgassed). As the temperature rises less CO2 is in solution and the very small proportion of CO2 that turns into carbonic acid (H2CO3) is reduced, raising the pH. This process is why we can use pH drop to estimate they amount of CO2 dissolved in the water.
solubility-co2-water.png

  • Also Like Jordi and "ian_m" says there is actually very little difference chemically between pH7.2 and pH7.4. This is because of the nature of pH, pH is a very strange measurement on a log10 scale. The neural point (neither acid no base) is at the activity of 10-7 H+ ions (and 10-7 OH- ions), at pH8 you have 10 times less H+ ions (10-8) and 10 times more OH- ions, and at pH9 100 times less H+ ions etc. This is an acid:base curve for
sasb1.gif


a "strong acid:strong base" titration from <http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html>, as you can see not much happens to pH until we get near to the equivalence point.

cheers Darrel
 
Last edited:
Chlorine having an impact is something I hadn't come across before. Question then is, how much of an impact does it have? In a quick search, one water company says they aim to have at most 0.5ppm at tap, which is relative terms is even lower than CO2. But I don't know how strong this effect might have. In molar terms, it may be more significant than its mass would suggest.

Follow up question would be, would dechlorinators immediately neutralise the pH effect of chlorine?
 
I wouldn't dwell on the chlorine or the ph difference as long as your water is dechlorinated. Most likely your issue is some co2 trapped in tap water. On my tanks I do 50% water changes straight out of the tap with the massive 0.8 difference in ph without any issues whatsoever, though I put the hose above to splash into the water and outgass immediately most of it. I dechlor directly in the tank while filling it up.
 
Hi all,
Adding chlorine gas (Cl2) should theoretically increase the acidity of the solution (when it reacts with the water, Cl2 + H2O ==> HOCl + HCl (hypochlorous & hydrochloric acid)).

It is the HOCl that is the oxidising agent and has the disinfectant properties, when you add chlorine, sodium hypochlorite bleach etc to water, but HOCl is unstable and will quickly form other compounds.

It is also another equilibrium equation and HOCl is highly reactive, and I assume that the 0.5ppm Cl in our tap water wouldn't have much effect on pH. I'm pretty sure that some of these reactions only occur in light, but I can't remember why.

cheers Darrel
 
Thanks everyone for your quick replies - rather technical for me, but I appreciate what you're all saying. I will just put up with it, and maybe lower the pH with some peat balls - hopefully that'll work!

Anybody know if natural slate raises pH?

Thanks
 
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