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Re: pH or RO water:

>Question 1: My tap water is pretty much crap for growing plants.
>Its got a pH of about 8.6-8.8 depending on when I measure it. 
>Its also hard as nails.  The RO water I get from it has a 
>measured pH of 7.0-7.2.  Is this measurement wrong?

No, probably not. It doesn't suprise me that the pH dropped.
I used "about the same" in a really sloppy way. With an
RO unit, there will be some readjustments of the pH when you
take out the cations. Sorry.

When you use a DI unit, one that exchanges H+ and OH- for
the anions or cations, you can get large changes of the pH
from neutral when one of the exchange resins is depleted.
You could potentially add a lot of H+ or OH- to your water
and have very high (13-14) or very low (1-2) pH's. I've never
seen this, but a staff scientist at the place I worked before
kept warning me about this because he knew that I took a lot
of water home with me for my fish.

>Question 2: After being in the fish tank for a while with injected CO2,
>the pH dips to about 6.6-7.0 depending on when I measure
>it.  Is this correct, or do I have a screwy test kit?

No, that is expected. You are buffering the RO water before you put
it in the tank. As you add the CO2, it reacts with water and form
carbonic acid, H2CO3, which dissociates into H+ and HCO3- (bicarbonate).
This lowers the pH because of the H+ added to the water. The pH
doens't go to the 5's because you have other buffers in the tank
that prevent that much change. 

I got a nice letter from George Slusarckuk about the technical
problems of measuring the pH of pure  water. I understand his 
comment now, and he is correct. It is purely a technical problem
of not perturbing the pH by performing the measurement, which he
explained to me quite clearly. 

Thanks for your time.

Joe Sachleben
jsachleb at chemistry_ohio-state.edu