Tap Water & PH - A Call to the Experts" - The Planted Tank Forum
 
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post #1 of 9 (permalink) Old 10-20-2014, 11:18 PM Thread Starter
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Tap Water & PH - A Call to the Experts"

I have a situation that is bothering me. I understand that water out of the tap can contain more than normal amounts of atmospheric CO2 and that you should let it set for a day to measure the "true" PH. Well yesterday I measured the water out of the tap with a PH Meter and it was 7.9. I let it set for a day and when I measured it again it was 7.1. I would think the PH should go up if anything.

The meter is freshly calibrated and after that reading I put it in 7, 4 & 10 calibration fluid and it read perfectly. I drew another glass this morning and it was 8.1 out of the tap, I will retest tomorrow to see if my reading today was a fluke.

Does anyone have an explanation that can educate me.

Thanks

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post #2 of 9 (permalink) Old 10-21-2014, 12:06 AM
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I'm no expert, but there are a few things I can think of...


The biggest one is that CO2 acidifies water.

Tap water doesn't have to contain more CO2 than atmospheric equilibrium, it could contain much less.

Water with little or no CO2 in it, which is possible for your tap, will go down in pH after absorbing atmospheric CO2.

Water with a lot of CO2 in it, which is also possible for your tap, will go up in pH after eliminating excess CO2.

Last edited by mattinmd; 10-21-2014 at 12:09 AM. Reason: added non-expert status.
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post #3 of 9 (permalink) Old 10-21-2014, 12:18 AM
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Hmm...interesting.

I have two thoughts.

One, your glass wasn't completely clean (not to slight your dish cleaning abilities, haha) and there was something acidic left over that dissolved after sitting overnight.

Two, your tap water has less CO2 than the atmosphere, contrary to what you have heard, and has absorbed some from the air, lowering the pH.

Otherwise, depending on the hardness of your water, there may be some other complicated chemistry going on...

Old tanks, all torn down:
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Hope to get a new tank (or two) up soon...
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post #4 of 9 (permalink) Old 10-21-2014, 12:36 AM Thread Starter
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I'm not taking offence to your statement, but the glass was clean and I even rinsed it well before I took the first reading. The same thought crossed my mind so I rinsed the glass well again and used the same glass. If there is enough residue of something acidic to lower the ph I think I will throw out the glasses. Oh and it is glass, not plastic that could easily absorb and hold something.

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Originally Posted by ChemGuyEthan View Post
Hmm...interesting.

I have two thoughts.

One, your glass wasn't completely clean (not to slight your dish cleaning abilities, haha) and there was something acidic left over that dissolved after sitting overnight.

Two, your tap water has less CO2 than the atmosphere, contrary to what you have heard, and has absorbed some from the air, lowering the pH.

My vote is with this, it will defend my dish washing ability.

Otherwise, depending on the hardness of your water, there may be some other complicated chemistry going on...
Bump: I'm going with the less CO2 theory unless someone comes up with something else. Thanks

Quote:
Originally Posted by mattinmd View Post
I'm no expert, but there are a few things I can think of...


The biggest one is that CO2 acidifies water.

Tap water doesn't have to contain more CO2 than atmospheric equilibrium, it could contain much less.

Water with little or no CO2 in it, which is possible for your tap, will go down in pH after absorbing atmospheric CO2.

Water with a lot of CO2 in it, which is also possible for your tap, will go up in pH after eliminating excess CO2.

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post #5 of 9 (permalink) Old 10-21-2014, 12:48 AM
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Ditto: Tap water can go either way: more CO2, so it loses some and pH rises, or less CO2, so it gains some and pH drops.

Repeating the test is a good idea.
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post #6 of 9 (permalink) Old 10-21-2014, 01:50 AM
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And now that you have fallen into the trap of asking about your PH and finding how easy it is to skew the readings, do you really want to spend much time or worry about the PH? I find my African cichlids have little concern about PH. They do like the GH/KH steady, though.
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post #7 of 9 (permalink) Old 10-21-2014, 03:39 AM Thread Starter
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I don't feel like I am being trapped, but I do believe that PH can be one of several indicators that can alert you if something has changed in your tank. I have read enough to know that I can't get an accurate CO2 level but it is one more collection of data that will help keep me stable. My question was about my tap water, not CO2 in the tank. I watch things like my KH & GH along with the behavior of the livestock and plants.

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Originally Posted by PlantedRich View Post
And now that you have fallen into the trap of asking about your PH and finding how easy it is to skew the readings, do you really want to spend much time or worry about the PH? I find my African cichlids have little concern about PH. They do like the GH/KH steady, though.
Bump: Well I repeated the test and the tap water started out at a PH of 8.1 and 12 hours later the PH was 7.12. So if my dish washing skills are adequate, I have CO2 challenged tap water.

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post #8 of 9 (permalink) Old 10-21-2014, 04:49 AM
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Smile Water District, Well Water...

Hi,

Call your local water district, ask to speak to a chemist and ask.

It seems it may be water treatment stuff, but that big of a difference somebody understands.

I do not think it is at all unreasonable to understand our tap water and its effect on our livestock, and, well, ourselves.

If you share your location, we may be able to offer some insights.

Respectfully,
Joe
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If your Guru tells you otherwise and you are true believer that is okay I will not argue the point.
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I pretty much limit what I say to things that anyone can easily verify from recognized references or by direct observation. If it is my opinion I say so.

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post #9 of 9 (permalink) Old 10-21-2014, 05:32 AM
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It isn't unusual for tap water to be high in pH out of the tap, and go down when exposed to the air for a day or two. Water companies add chemicals to raise the pH well above 7, so the water won't erode copper piping or lead soldered joints in the piping. Those chemicals don't stay in the water in the original form once the water is exposed to the air for some time. I have read several articles on the internet where the chemicals used are discussed, but, not being a chemist, I don't remember what they are. Try researching with google to see if you can find out.

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