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Old 03-11-2007, 11:39 AM   #1
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Does the measured kH change with CO2 injection?

In other words, you have a sample of water and measure the kH and pH. Let's say you get a value for the pH of 7.0 and a kH of 3.0. You then inject CO2 to bring the pH down to 6.5. Will the measured kH change along with pH? If so, in figuring out the CO2 levels in you tank, should you use a sample that has been resting out of the tank for a while to measure kH or one directly from the tank? Does it make a difference?
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Old 03-11-2007, 12:14 PM   #2
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I'm truly only beginning to understand the hardness concept in my aquariums too. I can tell you that I have witnessed my appear KH go up as I started to feed co2. I believe that the co2 actually causes the creation of carbonic acid in the water column. This is what makes the pH appear to fall. Since the KH is referred to as "temporary hardness" or "Alkalinity" I guess that it would make sense that adding acid would effect the water's ability to buffer against change.

There will be much more informed people posting on this issue to help clarify for you (and me too for that matter).
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Old 03-11-2007, 01:37 PM   #3
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Only pH changes as a result of CO2 injection and not KH. This is why you can use the pH KH relationship to measure CO2 levels. The KH should remain the same reguardless of whether or not it comes directly from the tank or is let sit for awhile. Only the pH would change as CO2 gasses out while the water rests.
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Old 03-11-2007, 04:15 PM   #4
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As Purrbox says, only the pH changes with the addition of CO2. The only reason I can think of that your KH would drop is if you neglected water changes for a long time. New, fresh water at water changes replenishes the naturally occuring buffers that keep the KH stable.

Sparky - Do you have anything in your tank like crushed coral or rocks from outside (not purchased at an LFS)? Something in your tank is dissolving a carbonate-based substance into the water. The pH (and KH) rises as the carbonate substance is dissolving into your water. This is dangerous if you're also running a pH meter to control your CO2. The pH probe senses a rising pH, and does not turn the CO2 solenoid off.
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Old 03-12-2007, 06:16 PM   #5
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Strictly speaking, Sparky is right:

CO2 + H2O -> HCO3- + H+

Since HCO3- level is your KH, adding CO2 will increase the KH.....

BUT - the amount of HCO3 produced by the additional CO2 is negligeble in relation to your natural HCO3 level. You are increasing HCO3 level by maybe 0.5 ppm, a level not detectable by your KH kit. In practice, you can say the KH don't change at all with added CO2.

If you actually see a KH change with the hobbyist kits, you should look for causes other than CO2.
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Old 03-14-2007, 09:43 AM   #6
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Ok, I did this test:

Tested KH before a pwc, measured 6 dkh. Did my pwc and got everything back together, measured a 4-5. I left the tank sit without feeding co2 and tested again in the morning and got the same 4-5. I turned the co2 back on and tested in the evening and it was back up to 6. It does not seem to ever go above 6, or under 6 for that matter either.

This has been typical since I started feeding co2. I know it is not much of a change, but it did not happen before starting co2. I cant attribute this to any thing else, the only thing in this tank is eco, plants, snails, and cherry shrimp. I have experienced the same results in my other two planted tanks as well.

This is what I based my answer on.
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Old 03-14-2007, 11:04 AM   #7
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Eco can raise KH. It did to me so I had to replace it. You should test without throwing the water change into the equation, as it takes a while for the KH to rise back up with the buffering.
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Old 03-14-2007, 06:05 PM   #8
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Sparky, what is your pH change with CO2 addition?

A change of KH from 5 to 6 is within the error range of the KH test (mine is +/- 0.5 at best) so the observed change is not significant. Your KH is prob just sitting at the junction between 5 & 6. Suppose the KH is actually 5.5, and your kit is reading that as 5, upping the KH to 5.51 might just be enough to push that to read 6 .....

If all else appears normal, I wouldn't stay up nights worrying about it!

Getting back to the original post ..... the Henderson Hasselbach equation is meant for dynamic systems, so in calculating CO2, you should use a simutaneously measured pair of pH/KH values .... but since any KH change would be small (or not measurable), most people just use their baseline KH & follow the pH only.
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Old 03-14-2007, 07:33 PM   #9
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Interesting thread...
My KH keeps going down all the time (dropping approx. 20ppm per 2 weeks)
Doing EI and weekly water change. So I add baking soda to get it back up there, otherwise
my pH would keep going down below 6,0 with the CO2 injection... and then I would have to
keep readjusting the pH controller, in order not to gas the fish...
All this to say that, well, in my tank also, CO2 seems to affect KH for some reason that I dont understand...
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Old 03-14-2007, 09:12 PM   #10
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Kaaikok - Some plants can use the carbonate found in KH for their carbon source. This will result in a drop in pH. Some plants are especially efficient at obtaining carbon using this method, and can cause a significant drop in the KH. I suspect that this may be what's happening in your tank.
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Old 03-15-2007, 06:40 PM   #11
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Or there might be an organic acid source .... peat, driftwood, rotting vegations, lots of MULM .... all produce acids which will consume buffers & drop the KH.

To pin the KH drop on CO2, you'd have to have a KH drop with CO2 and no KH drop without.
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Old 03-15-2007, 07:29 PM   #12
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Quote:
Originally Posted by jsoong
Strictly speaking, Sparky is right:

CO2 + H2O -> HCO3- + H+

Since HCO3- level is your KH, adding CO2 will increase the KH.....

BUT - the amount of HCO3 produced by the additional CO2 is negligeble in relation to your natural HCO3 level. You are increasing HCO3 level by maybe 0.5 ppm, a level not detectable by your KH kit. In practice, you can say the KH don't change at all with added CO2.

If you actually see a KH change with the hobbyist kits, you should look for causes other than CO2.
That's not totally correct.

Adding CO2 to water makes H2CO3 (carbonic acid) which lowers your pH.
Which then can futher back down, H2CO3 = HCO3- + H+

pH = pKa + log([HCO3-]/[H2CO3])

IF you guys really want to know more about KH (not a very good term to use) or alkalinity, here is some nice info to read through:

http://www.thekrib.com/Plants/CO2/

http://www.thekrib.com/Plants/CO2/kh-ph-co2.html

http://www.thekrib.com/Plants/CO2/alkalinity.html

(prepare for some learning. And I thought KH was easy. LOL)
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