# Why doesn't CO2 injection affect KH? technical



## titan97 (Feb 14, 2005)

Ok smarties. I'm a biochemist by education. I understand the Henderson-Hasselboch equation just fine. Equilibriums, no problem. But I can't firgure this one out.

1) If CO2 is dissolved in water, some if it turns to H2CO3, right?
2) Some of that H2CO3 will rapidly (at near neutral pHs) dissociate partially to H+ and HCO3-, right?
3) And further, some of that HCO3- will again dissociate (slightly at neutral pHs) to H+ and CO3--, right?
4) And don't our KH test kits test for HCO3- and CO3--?

It may just be a matter of scope and concentration, as I haven't had the inclination to work out the math just yet. But it would seem apparent that if you add CO2, you will get some increase in HCO3- and CO3--. LeChatlier's Principle and The Common Ion Effect would seem to show that an increase in CO2 would push the reaction towards the CO3--. I would agree with this because we see the end effect as a drop in pH (an increase in [H+]).

So, in summary, if an increase in CO2 causes an increase in [H+], why doesn't it also affect the KH?
 
-Dustin


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## RTR (Oct 28, 2005)

Figure out the math - it is a pure concentration issue. The dissociated bicarb and carb are way behind the decimal if you are working in degrees KH, still behind the decimal in ppm. It is present and real, but we will not kit measure it shy of anesthetic levels of CO2 and beyond.


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## plantbrain (Jan 23, 2004)

The part that is messing with you, CO2 is a *weak acid*, not HCL, etc, which will consume the HCO3(KH) and release CO2 from the KH.

Some folks tired to add H2SO4/HCL to lower their KH's in the past, not recommended! In marine water, the H2CO3:CO2 ratio is 1:400
So we generally ignore the H2CO3 and call it all CO2 [aq].

Adding loads of CO2, will not reduce KH.........you can change the pH, by adding more CO2, but once you stop adding the extra CO2, the KH is still the same the pH moves back up as the the CO2 excess is degassed to equilibrium. You increased the total carbon, by adding more CO2, but you have not added more HCO3/CO3. That stays the same unless you add a *strong acid* like HNO3 etc.

I forget the FW ratio but it's somewhat similar.

You might find this useful:
See the Bjerrum plot for carbonate species:

http://www.ldeo.columbia.edu/~sanpisa/OceanSed project/bjerrum.htm

Downloadable as a spreadsheet:

http://www.cas.vanderbilt.edu/geol320/spreadsh.htm

More on the basics and Test kit measurements:
http://www.thekrib.com/Plants/CO2/alkalinity.html#3

I have an ISE meter that can read units of Alk also.
Non bicarb alkalinity will throw off some test kits also.
Most aquarium kits test total alk.

Alkalinity Calculator:

http://or.water.usgs.gov/alk/faq.html

Some standard methods:

http://www.norweco.com/html/lab/test_methods/2320bfp.htm

http://www.ldeo.columbia.edu/~sanpisa/OceanSed project/carbonatechem.htm

That should keep you busy and out of trouble,

Regards, 
Tom Barr


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## titan97 (Feb 14, 2005)

Ok, so let me get this straight. This is probably more for the benefit of myself, than anyone else, but I digress.

1) I understand that most of our dissolved CO2 is CO2(aq), not H2CO3.
2) Carbonic acid is a weak acid and only partially dissociates.
3) The partial dissociation of the H2CO3 will have a much greater effect on pH than on KH, due to the scale of the concentrations. ie, It doesn't take much H+ from the H2CO3 to change the pH, but it takes much more HCO3- to affect the KH.

Doing the math (finally):

ph 7 = 1x10^-7 M H+
KH 10 = 107.1ppm CO3-- = 1.78x10^-3 M CO3--

Already we see a difference of x10^4. It would take 1000 times more HCO3- than H+ to get an equivilent change in KH.

I get it.

Thanks to RTR and especially Tom for the info and great links.

-Dustin


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## plantbrain (Jan 23, 2004)

Well, it helps you know more about chem than most, and also to see what was at the core of what was bothering you too

These are old discussions from the past that were often brought up, but seldom discussed these days with a new crowd.

Regards, 
Tom Barr

www.BarrReport.com


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## titan97 (Feb 14, 2005)

I suppose that the change in interests happens to most geek-gone-popular topics. I would think that in the beginning, the interest in planted aquariums was small, and a niche group of people really tried to figure it out. At that time, it was more of a techinical exercise. Looking back into the archives and other sources, you can see different experiments and philosophies were around to try and explain algae control and plant growth. It wasn't that long ago that the PO4-free system was en vogue. But as planted aquaria become more popular, I think that people will care less about WHY things work a certain way, and instead focus on WHAT works now. The geeks have solved many of the pressing issues that now allow us novices to easily grow plants. It may be a little sad to see the techincal aspect of the hobby going by the wayside, but I'm glad there are still several of you forefathers that have put in the legwork.

-Dustin


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## plantbrain (Jan 23, 2004)

OoooooOOOOooooo!

I mentioned the CO2 mist thing and I've been getting a good poo pooing lately
But I do not care, I know it works.

Just like the PO4 dosing that folks poo pooed me said at first.
But I had a different group poo pooing me then.

I obviously know how to test O2 and CO2 in water, and know what good/better submersed growth looks like, but some folks think I'm nuts..........

But many thought I was nuts before, so that is nothing new except the crowd today is new......not me............I'm the same I was then.

I know what good growth looks like. 

Regards, 
Tom Barr


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