Question about KH & CO2

rrkss

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May 7, 2006
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When I test the kH of my degassed water, it tests at 4.0 with a pH of 7.4. When I run the same test on my CO2 rich water it tests at 3.0 with a pH of 6.4. Why would the kH drop by an entire degree when CO2 is added but return to 4 as soon as I run an airstone to degass the water?
 

VaughnH

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Re: Question about KH & CO2

That's an interesting question, and unless we can find a reason that does not involve the KH actually changing, our whole two PH method of measuring CO2 is down the tubes! When I experimented with it I didn't see a KH change.
 

N_E

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Jun 16, 2005
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Re: Question about KH & CO2

What resolution did you use when you tested it?
If you have a whole degree as resolution in theory you could have 3.5 and depending on how exact the drops are you get either 4 or 3.
 

rrkss

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Re: Question about KH & CO2

It is tested at 1 degree resolutions. If I take some tank water and test it, it takes 3 drops. If I take the same tank water, run an airstone through it and test it, it takes 4 drops. If I take tap water and test it, it takes 4 drops.
 

quenton

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Re: Question about KH & CO2

rrkss said:
It is tested at 1 degree resolutions. If I take some tank water and test it, it takes 3 drops. If I take the same tank water, run an airstone through it and test it, it takes 4 drops. If I take tap water and test it, it takes 4 drops.

Since you have not taken anything out or added anything, I do not see how the KH could possibly change. I would have to go along with the previous response that its just test kit issues. I have found that drop sizes from the bottles often vary a bit.

To try a scientific method -- can you get someone else to reproduce your results :)
 

rrkss

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Re: Question about KH & CO2

This is not a one time occurance. I've redid the test out of utter confusion about 6 times on different days. The only thing I can think of is the indicator in the kH test kit relies on pH. Since the pH of my tank water is 1 lower than the airated tank water, that could cause the problem.
 

quenton

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Mar 14, 2006
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Re: Question about KH & CO2

rrkss said:
This is not a one time occurance. I've redid the test out of utter confusion about 6 times on different days. The only thing I can think of is the indicator in the kH test kit relies on pH. Since the pH of my tank water is 1 lower than the airated tank water, that could cause the problem.

That is something I never thought of -- I am not a chemist, but it seems more reasonable than the actual KH changing!

Where is tom when you need him ;)
 

N_E

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Jun 16, 2005
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Re: Question about KH & CO2

If you test it with higher resolution I'm most certain that you will find that the change is not 1 degree, its probably much less.
You can increase the resolution by adding twice, three or four times the amount of water and with each step the resolution will increase with the double (0.5, 0.33 or 0.25 degrees).
 

Tom Barr

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Re: Question about KH & CO2

Resolution becomes rather poor beyond 1/2.
Without a spect, about 4-8ppm.

Regards,
Tom Barr
 

VaughnH

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Re: Question about KH & CO2

When I play around with this I use twice the water amount. I dip out two samples in the little test tube, dumping them into a very narrow champaign flute. Then I count each drop as .5 in KH degrees. Even then I can sometimes see that the color change isn't quite all the way one time and fully changed another time - I think that means the actual number of drops is 5 the first time and 4 1/2 the second time (for example). In a way we are using a tape measure while trying to measure the thickness of paper. But, it's all we have.

I am more concerned with the inaccuracies in PH than in KH. Since the measured amount of CO2 is much different for .8 difference than for 1.0 difference, and most PH measurments are only good to +/- .1, therefore +/- .2 for the difference between two measurements, this method doesn't give a really accurate measure of ppm of CO2. (A .2 error in the difference is a factor of 1.6 in ppm of CO2. So, if we think we measured 30 ppm, it might actually be 48 or 19.) But, again its the best we have.
 

rrkss

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May 7, 2006
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Re: Question about KH & CO2

I will run the test again on 10 mL of water instead of 5 and see what results I get tonight.