Some CO2 Relevant Data - Fertilizing - Aquatic Plant Central

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Old 05-23-2006, 06:04 PM   #1 (permalink)
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Default Some CO2 Relevant Data

Yesterday I found some time, had my pH probe freshly calibrated, and did a couple of experiments. I took a tank sample of water, let it sit on the counter for several hours, measuring pH periodically, to see how long it takes for the pH to stabilize. And, I mixed up a bit of sodium bicarbonate in distilled water to get some KH, let the water sit on the counter and measured pH periodically to see at what ppm of CO2 it would settle at. The data is plotted on a couple of graphs on the attached jpg. I'm really puzzled about what it all means. Any ideas?
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Old 05-23-2006, 06:58 PM   #2 (permalink)
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Example of KH preventing major ph swing?
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Old 05-24-2006, 04:35 AM   #3 (permalink)
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Nice correlation with CO2 degassing and time there . You have verified the advice we're all giving about letting your water sit out for 24 hours to measure the degassed pH as accurate. The pH variation you see at 18+ hours, is a measure of the limit of the pH probe you're using, apparently +/- 0.1 unit.

I remember reading somewhere in the dark ages of my earlier college years, that pure water, which is basically what the RO water you're using is, is hard to pH because of the lack of ions (therefore electrical conductivity) in the water. I am surprised by the high pH of a pure sample of RO water with a kh of 4. We need some chemists to chime in here.
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Old 05-24-2006, 07:25 AM   #4 (permalink)
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I added a tiny amount of sodium bicarbonate to the sample so there would be ions in it. It takes a ridiculously tiny amount to reach a 4.5 KH. My probe is supposed to be accurate to 0.01, but I am quite sure it isn't, so I usually round off the numbers to the nearest .1, but didn't in this case.

What troubles me the most about this is that the distilled water with bicarbonate in it only climbed to about 1.0 in ppm of CO2 and did that relatively quickly. That suggests that our 24 hour outgassed tank samples have about 1 ppm CO2 in them and not 3, which means the 1.0 drop in pH is only 10 ppm of CO2 in the tank. Another test for another day would be to repeat that one, but first run some CO2 bubbles in it to raise the CO2 to well above 3 ppm, then see how low it drops in 24 hours. We have to use RO or distilled water for this to avoid having other sources of alkalinity in the water, making the KH/pH/CO2 relationship accurate.

In any case, at this point I don't think we know how much CO2 is in our tanks. So, increasing it to where the fish start to have problems, then backing off a bit is probably the only reasonably accurate method we have. I hope I am wrong about this.
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Old 05-24-2006, 08:10 AM   #5 (permalink)
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I remember reading on the Krib sometime about the fact that there is debate about the actual atmospheric concentration of CO2 in a degassed water sample. What we need is someone with access to a blood gas monitor to take a sample of their tap water, or distilled, which has sat out at atmosphere for 24 hours and measure CO2 content. Seems to me that would settle this once and for all. Any med students or docs out there who could swing this?
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Old 05-24-2006, 08:36 AM   #6 (permalink)
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Quote:
Originally Posted by Bert H
I remember reading on the Krib sometime about the fact that there is debate about the actual atmospheric concentration of CO2 in a degassed water sample. What we need is someone with access to a blood gas monitor to take a sample of their tap water, or distilled, which has sat out at atmosphere for 24 hours and measure CO2 content. Seems to me that would settle this once and for all. Any med students or docs out there who could swing this?
Have asked a chemistry professor friend of mine here at Lehigh if they have any such thing (or equivalent). And if so, could it be used for such? If so, I will let you know. Also to get the parameters for the experiment...

-Jason

P.S. I loooove playing with tech gadgets...
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Old 06-15-2006, 04:21 AM   #7 (permalink)
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Jason, any follow up on this?
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Old 06-15-2006, 09:10 AM   #8 (permalink)
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One of the members of Tom Barr's site did some testing of outgassing of tank water recently. He found that even after 48 hours the pH was slowly rising, indicating either that evaporation was raising the KH or that CO2 continued to outgas. As a result, I tried some testing to see if you could reach a stable pH by blowing into a water sample for a reasonably short time. It appears that you can do so if you blow bubbles in the water for 5 minutes or so. That suggests another awkward, inconvenient way to test for CO2, but it is still going to be a very crude measurement. I think the basic problem is that the ppm of CO2 depends on an exponential - ten raised to the power of a pH difference. And, that really compounds errors in measuring pH for any reason.

I have decided that for now I am just going to use the maximum amount of CO2 I can without distressing the fish too much.
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Old 06-15-2006, 06:43 PM   #9 (permalink)
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Look how long it takes to change the CO2 concentration. CO2 on a light timer must produce long time delays and large pH swings.
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