# My Two-Pence-Worth on  CO2, kH, gH & TA



## GPHowell (22 Sep 2011)

Hi All,

I'm new to this forum (which I like a lot btw) and also new to "planted aquaria" although I have been keeping community-style aquaria for 15 years or so. I'm also a research chemist by profession and have an interest in the "water chemsitry" aspects of the hobby.

Having taken a look through these and other forums, I thought I'd add my opinions on a couple of topics. As a qualifying statement, I've drunk a bottle of rioja whilst typing this post so if there are any errors, take it easy on me!

So, here we go...

When we add CO2 into water, the* kH value will technically increase* (albeit by a vanishingly small amount); *there is no possible mechanism for it to decrease*. *The pH will drop* and the *TA (total alkalinity) and gH will necessarily remain unchanged*.

For example, let's take a litre of pure water (pH = 7; kH = gH = TA = 0)

if we add CO2 (g) to a level such that we have 15ppm dissolved CO2:

_concentration CO2 = [CO2] = 0.015 g/L = 0.015/44 = 3.4 x 10^-4 M_

Based on the various equilibrium constants (pKa values) for CO2, carbonic acid and bicarbonate:

_approx 10% of these dissolved CO2 molecules will react with a water molecule to generate a molecule of carbonic acid

approx 99% of this H2CO3 will dissociate into H+ & HCO3- ions

virtually none of the HCO3-- ions will dissociate into H+ & CO3-- ions_

*in numerical terms:*

_when [CO2] = 3.4 x 10^-4 M; [H2CO3] = 5.7 x 10^-7 M; [HCO3-] = [H+] = 1.2 x 10^-5 M; [CO3--] = 5.6 x 10^-11 M_




*So, what effect has this had on pH, kH, gH & Total Alkalinity?*

We've generated 1.2 x 10^-5 M of protons (H+ ions) so the pH is now:

_-log 1.2 x 10^-5 = pH 4.9 (we can ignore the negligible number of protons generated from CO3-- formation)_ 

We've generated the same number (concentration) of HCO3- ions and these are the only species that contribute to kH (we can once again ignore the CO3-- since it is negligible in comparison).

If [HCO3-] = 1.2 x 10^-5 M then the corresponding [CaCO3] hardness (for kH calculation) is:

_half x 1.2 x 10^-5 = 6.0 x 10^-6 M

this equates to an increase of 0.6 ppm or 0.03 kH_

So, you'd need the best test-kit in the universe to detect the increase in kH... and even then you wouldn't detect a change because kH test-kits actually measure total alkalinity (TA); _this has been excellently covered in the sticky-post at the top of this forum board._

*We can see from the sums above that the various ions we have generated all contribute to total alkalinity (TA), but overall:*

_[HCO3-] + [CO3--] - [H+] = 0 _

and, hence, the *TA is necessarily unchanged*.




Similarly, we haven't added any divalent cations (we've only added a gas to the system),* gH is also necessarily unchanged*.   



So, hopefully you agree with the above?  But obviously, our aquaria aren't filled with pure water, so, what difference does all the dissolved sulfate, bisulfate, phosphate, chloride, magnesium, calcium, & potassium make to the calculations above?

The answer is: _very little_.

*The pH value we calculated above of 4.9 is unlikely to be reached when we add CO2* to our water because the various dissolved anions will have a buffering effect and a lot of the protons generated will be "mopped-up" and the pH reached will typically be more in the region of 6-6.5.

*What about the kH?* It doesn't matter what's dissolved in your aquarium water, the kH will remain essentially unchanged. This is because the only species we're adding that is capable of removing bicarbonate or carbonate is carbonic acid and for every molecule of HCO3- or CO3-- we remove we generate another molecule of HCO3-... and the kH value is restored (in practice it is raised very slightly).

*And the TA (Total Alklalinity)?* Again, we may remove some borate or phosphate ions via the generation of carbonic acid but these are all replaced in stoichiometric fashion with bicarbonate and the TA remains unchanged, although the composition (speciation) may change.

*The gH is similarly unchanged* since (as we stated earlier) we're not adding any divalent cations...




In the way of final comments, I would argue that kH and gH are essentially defunct and archaic measurements. 

The former (kH) is used to express the buffering ability of the aquarium water except it doesn't! You could buffer an aquarium with borate or phosphate and it would be very resistant to pH change but would have a kH of zero. Total alkalinity is a much more sensible measurement... and this is what your kH test kits measure anyway.

The latter (gH) is used to express 'hardness' or the total amount of dissolved ionic media... except it only measures divalent cations. Saturated brine solution (aqueous NaCl) has a gH of zero but your freshwater fish certainly won't like it...

So, maybe you don't agree with some of the statements I've made or maybe I've made some mistakes; either way I'd be interested to discuss further or listen to alternative suggestions...


PS- All of the background info is readily availble... Wikipedia is probably the best source if you want something remotely readable. If you look at the articles on Carbonic Acid, Carbonate Hardness, Alkalinity, pKa & Acidity, Water Hardness you'll find pretty much everything you need...


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## dw1305 (23 Sep 2011)

Hi all,


> So, hopefully you agree with the above?


 I think we do, we have been down this route many times before, but it is an area where the wider fish keeping world really struggles.


> I would argue that kH and gH are essentially defunct and archaic measurements.


 Again I think we agree with this, you wouldn't start from here, but we have arrived at this point and there isn't much we can do about it. The problem comes because people now have the potential to change their "water" (really a dilute solution of various solutes) in many different ways and often they don't really understand what they are doing, or why they are doing it.

I'd probably include pH as a less than useful measure as well, particularly for very dilute solutions.  The problem is that concepts like pH and buffering aren't easy to explain to a non-scientific audience. 

I would suspect most high tech. planted tank keepers don't measure any parameters other than a CO2 proxy by using a drop checker with bromothymol blue pH indicator. <http://www.ukaps.org/drop-checker.htm>. 

I spend a lot of time explaining water chemistry on some of the other forums that I belong to (mainly soft water cichlid and Loricariid based), and basically I tell people to:

Get some plants
Maximise oxygenation
Reduce BOD and ensure that you have a large biological filtration capacity.
Throw away all their test kits and buy a reasonable quality low range conductivity meter. 

Have a look here for a practical example  <http://www.plecoplanet.com/forum/showthread.php?t=8904> & an article <http://plecoplanet.com/?page_id=829>

cheers Darrel


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