[Wet Thumb Forum]-Question about Co2 Chart

[IUnknown]

I am using The Lamotte 4491 Alkalinity test kit to measure KH in my aquarium. I am using the results to measure the Co2 in the system using the following calculator and a pH probe,
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
My problem is that I initially get a reading, and then the color changes as I let the sample sit (as if the Co2 is being gassed off, and my result changes). What I want to know is if I should use the initial reading to determine my Co2, or should I let the sample sit for an hour to measure my Co2?
Also, Chuck's webpage describes PO4 concentrations as a problem. I was wondering at what levels do they become a problem. I maintain 2ppm throughout the week.
Thanks,
Greg

[IUnknown]

I am using The Lamotte 4491 Alkalinity test kit to measure KH in my aquarium. I am using the results to measure the Co2 in the system using the following calculator and a pH probe,
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
My problem is that I initially get a reading, and then the color changes as I let the sample sit (as if the Co2 is being gassed off, and my result changes). What I want to know is if I should use the initial reading to determine my Co2, or should I let the sample sit for an hour to measure my Co2?
Also, Chuck's webpage describes PO4 concentrations as a problem. I was wondering at what levels do they become a problem. I maintain 2ppm throughout the week.
Thanks,
Greg

[Roger Miller]

Use your initial reading. I don't know why the color would change over time, but the reaction that the kit uses to measure KH is very fast. Anything that happens later is extraneous. CO2 loss won't change the KH, but I don't know what other reaction might be involved.

PO4 is a problem because it can change the KH reading. It has to be fairly high to do that. A quick calculation leads me to believe that it would take something like 10 ppm of phosphate to give 0.5 degrees KH. The problem with phosphate comes up when people add phosphate-based buffers. The buffers produce very high phosphate concentrations.


Roger Miller

[IUnknown]

Thanks for the response Roger.
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
Hi Greg,

Thank you for using LaMotte analytical products. When performing the alkalinity (DKH) test, be sure to take the reading as soon as the sample changes from blue-green to a purple color. This will provide your true DKH reading which you can use with the CO2 calculator. Your observations are correct, the color will change as the sample sits and is exposed to ambient air (due to changes in pH). This reversion of color in the sample is typical of many titration tests, but should be ignored.



Regards,

Technical Service
LaMotte Company

<HR></BLOCKQUOTE>

I guess the pH would make sense for the change in color.

[Roger Miller]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>I guess the pH would make sense for the change in color. <HR></BLOCKQUOTE>Yes it does. The color in a KH test is produced by a pH indicator dye. I wouldn't normally expect CO2 loss to have much effect on a sample where a strong acid is used to force the pH below 4.5, but I suppose it's possible.

Otherwise I might suspect that the pH rebound could be caused by slow-reacting organic acids that buffer the test, but at a much slower rate than the acid + bicarb reaction.


Roger Miller