# Rex\verse Osmosis Problem (?)



## t2000kw (Jul 29, 2006)

I have a new RO and unit with a deionizer (about 3 weeks old).

I never tested the pH of the output until recently but it is very high, like almost 9.

I did test the TDS and it is low on the RO, like 21, and much lower on the DI output, like 2. 

What could be going on here?

The TDS reading tells me that it's taking the minerals out of the water and suggest that I should expect a near neutral pH reading.

Since there would be nearly zero minerals in the water, its buffering capacity should be near zero, and that pH reading would not indicate that it would raise the pH of the tank by much, if at all.

I'm open to suggestions here. 

Would it be worthwhile to mix 1/2 tap at a known pH and 1/2 RO and see if it lowers the pH of the mixed water any?

I'll check this board later on this evening and tomorrow.


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## goalcreas (Nov 20, 2006)

How are you testing your PH, your test kit could have gone bad, or your PH probe could need calibration.
Or you could have thought of this already and this information is of no use to you.


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## t2000kw (Jul 29, 2006)

goalcreas said:


> How are you testing your PH, your test kit could have gone bad, or your PH probe could need calibration.
> Or you could have thought of this already and this information is of no use to you.


I only have a test kit, no probe, except for the TDS meter.

I suppose I could buy another pH test kit refill for pH but you would think that it would read less than the tap water. I can taste the lack of minerals in the RO water. I'll think about that tomorrow.

Thanks for the suggestion.

I'd like to get a pH meter sometime but I would need to learn a bit more about them. If the probes don't last long, I'd just stick with test solutions.

I should have some pH papers somewhere that I used to use for beermaking to test the mash pH. If I can find those, it might give me a ballpark idea of where the pH is at, though I think I'd have to use the wide range strips since I on;y had those and the low range ones.


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## BryceM (Nov 6, 2005)

Extremely dilute solutions like RO/deionized water are quite difficult to test for pH. I'm absolutely certain that hobby-grade test kits won't do it correctly. Even pH probes have "issues".

pH meters rely on conductance of electricity through a water solution. When calibrating a pH meter, the reading zero's in very quickly since the calibration fluids are VERY concentrated. Typical aquarium water is much more dilute and it takes much longer to come to equilibrium. RO water is a very poor conductor of electrical charge, making it almost impossible to get a true reading.

Incidentally, pure RO water often gives false reading for NO3, and PO4 too - bascially for the same reason.


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## t2000kw (Jul 29, 2006)

Just noticed that I typed too fast for the title of this post. Well, it will get more attention that way, won't it?


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## goalcreas (Nov 20, 2006)

FYI, IME ph probes don't often go bad, just need to be re-calibrated with soultion periodically. However thay can go bad, but not so often that they are not worth having.


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## t2000kw (Jul 29, 2006)

I'll put it on my wish list.

Now, if they'd only make an electronic chemical tester for ammonia, nitrites, nitrates, and maybe phosphates. Sample a bit of water, pour it in the tester, and read the digital results or LEDs to show the range of each.


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## Squawkbert (Jan 3, 2007)

(meaningless, but if you insist)...To test RO/DI water with a probe, add just a drop or two of saturated KCl to a 50 or 100mL sample (should be anywhere between 5 & 7). Don't bother trying to test pH of it with a kit. All you'll see is the pH of the reagents in the kit.

A better indication of how pure your RO/DI is would be a condictivity test. 

You can also test for total organic carbon, but your aquarium couldn't care less about a few ppm of styrene monomer making it through the final filter.


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## Laith (Sep 4, 2004)

t2000kw said:


> I'll put it on my wish list.
> 
> Now, if they'd only make an electronic chemical tester for ammonia, nitrites, nitrates, and maybe phosphates. Sample a bit of water, pour it in the tester, and read the digital results or LEDs to show the range of each.


They do. They are colorimetric testers. LaMotte and others make them.

Expensive!


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