CO2 relationship to KH

[Miccoh Mendoza]

Hi everyone,

I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?

 

[ian_m]

Ignore the chart, you are heading down the path of poor plants and maybe even fish death.

Either aim for a green drop checker, which works regardless of tanks water kH or a pH drop of 1 unit from CO2 start to lights on, again completely independent of tank water kH. Job done, no unreliable test kits required.

 

[dw1305]

Hi all,

Hi everyone,

I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?

The CO2 ~ carbonate ~ pH equilibrium is a bit strange. The total amount of inorganic carbon doesn't change, it just changes form (in the graph below).


When you add CO2 you change the pH equilibrium values, but you don't change the alkalinity.

Either aim for a green drop checker, which works regardless of tanks water kH or a pH drop of 1 unit from CO2 start to lights on, again completely independent of tank water kH. Job done, no unreliable test kits required

<"I'm not a CO2 user">, but I agree with @ian_m you really need a <"drop checker with 4dKH solution and bromothymol blue pH indicator">.

cheers Darrel

 

[ceg4048]

I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?

Just aim for a pH drop of anywhere from 0.5 to 1.0 depending on the amount of light intensity you have. Start the injection early enough so that the drop is achieved by the time the lights turn on. As mentioned by Ian and Darrel, use a dropchecker to give you a visual guide to the CO2 level.

Cheers,

 

[REDSTEVEO]

Did I read that correctly, KH is 24, and PH is 8.2 to 8.4??? Regardless of CO2 that sounds like really hard water, hard enough to keep lake Malawi cichlids or even marine fish.

What is driving the PH and the KH so high. If you've got fish in there good luck with them,

If I've misread the figures fair enough.

 

[Andy Pierce]

The CO2 ~ carbonate ~ pH equilibrium is a bit strange. The total amount of inorganic carbon doesn't change, it just changes form (in the graph below).

It's even stranger than that. The total amount of inorganic carbon does change - it increases very sharply with increasing pH, AND the relative distribution of carbonate species also changes. The Bjerrum plot above only reflects that latter point.

The above shows the total amount of DIC (light blue curve) sharply increasing at pH 6 and higher. Note the Y axis is plotted on a log scale. There are both relatively easy to digest and more rigorous mathematical descriptions of the concept available.

 

[dw1305]

Hi all,

The total amount of inorganic carbon does change - it increases very sharply with increasing pH, AND the relative distribution of carbonate species also changes. The Bjerrum plot above only reflects that latter point.

This is when a <"little learning is a dangerous thing">, and I patently need a bit more. I just wish my <"inorganic chemistry was better">. I'm not an added CO2 user, and this was all new to me when I joined UKAPS. The only problem is that its fifteen years on and I still don't know what I'm talking about.



@Andy Pierce Is this why the pH rises well above pH 8 when you <"deplete the CO2 via photosynthesis">? I've already been disabused of the idea that oxygen is a base (it isn't, and I should have realised that).

cheers Darrel

 

[hax47]

Hi all, The CO2 ~ carbonate ~ pH equilibrium is a bit strange. The total amount of inorganic carbon doesn't change, it just changes form (in the graph below).
When you add CO2 you change the pH equilibrium values, but you don't change the alkalinity.

It's even stranger than that. The total amount of inorganic carbon does change - it increases very sharply with increasing pH, AND the relative distribution of carbonate species also changes.

Although these two statements may look contradictory, they are both true. If one changes the CO2 level, the KH won't change - given the concentrations of other buffers are negligible compared to KH/bicarbonate. If one changes the pH with NaOH or HCl, for example, while having equilibrium with the same CO2 levels, then there will be a change in carbonate species' concentration.

 

[John q]

It's even stranger than that. The total amount of inorganic carbon does change - it increases very sharply with increasing pH, AND the relative distribution of carbonate species also changes.

, then there will be a change in carbonate species' concentration.

All intresting stuff. But... what effect does this have on the plants; negative/positive?
Asking for a friend 😀

 

[dw1305]

Hi all,

......... Asking for a friend 😀

I also have friend who is interested in the outcome of this one.

Cheers Darrel

 

[hax47]

All intresting stuff. But... what effect does this have on the plants; negative/positive?
Asking for a friend 😀

Well, I am not an expert in that part 🙂
The original question was whether the CO2 would change the KH. It should not, but more CO2 is probably good for plants. If you dose an acid to reduce the pH, the KH will be changed permanently (with a temporal CO2 increase).

To add a bit more detail to the CO2 injection:

This is the reaction when CO2 is injected and HCO3- is produced:

CO2 + H2O <----> H+ + HCO3-

Intuitively, it would seem that the more CO2 you add, the more HCO3- will be produced. Thus, it should increase the KH. However, it has to be noted that H+ concentration is in a different concentration range than the HCO3-. So when 0.001 mM H+ is created in water with a pH of 7, then pH would decrease to 6, while HCO3- concentration increases with 0.001 mM. That is about a 0.002 dKH unit increase, which is usually negligible.
But then, if there is another buffer (phosphate or humic acids) in relatively high concentration, compared to HCO3-, that "sinks" the created H+ in the above reaction, the HCO3- concentration would increase significantly (since the equilibrium in the above equation will be reached anyways). The KH increase depends on the capacity of these other buffers (how much H+ they can bind/"sink" at the given pH). But I guess the alkalinity, what we measure when determining the KH, would remain the same even in this case . ?

 

[John q]

Well, I am not an expert in that part 🙂

Double like for the answer 👍

 

[sparkyweasel]

what effect does this have on the plants;

what kind of plant?

 

[Andy Pierce]

This is the reaction when CO2 is injected and HCO3- is produced:​

CO2 + H2O <----> H+ + HCO3-
Intuitively, it would seem that the more CO2 you add, the more HCO3- will be produced. Thus, it should increase the KH.

The confusion here arises over the definitions of KH, carbonate hardness, carbonate alkalinity and total alkalinity. What people usually measure (by titration) is total alkalinity: a measure of all species that can accept a proton (H+). If all you have are carbonate species in water (which in an aquarium is usually pretty close to true):

total alkalinity = [HCO3- bicarbonate] + 2x[CO3-- carbonate] + [OH- hydroxide] - [H+ free protons]​

You multiply the carbonate by two because each carbonate ion can accept two protons. Free OH- and H+ are set by the pH (or more accurately they are the pH). OH- can accept a proton and become H2O. H+ is like alkalinity anti-matter: a proton you don't need to provide in your titration measurement because it's already there; that's why it needs to be subtracted from total alkalinity.

So what happens if you inject CO2 gas? As described by hax47's chemical reaction, some of the CO2 is converted to H+ and to HCO3-. This does increase the number of carbonate species molecules (the HCO3- piece) but it does not change total alkalinity because it also creates the exact same number of H+ species and bicarbonate and H+ balance each other out.

 

[Andy Pierce]

All intresting stuff. But... what effect does this have on the plants; negative/positive?
Asking for a friend 😀

Don't know. 😀
The good news is you only need to measure two parameters and everything else is determined for you: atmospheric CO2 (you can make some assumptions about this) and pH (because in an open-to-atmospheric CO2 system the pH determines the equilibrium concentration of all carbonate species). It's very well established that dissolved CO2 makes a huge positive difference to plants generally - that's why we inject CO2 gas. To my mind it's much less well established whether pH and everything that goes with it (KH) is positive/negative for plants and if so for which ones. I have been vaguely convinced by @_Maq_ and others that high pH (and thereby high KH) is overall "bad"(ish?) for many (most?) plants. Meh.

@_Maq_ this would be an easy experiment for you to run (please oh please oh please) using inert substrate: identical tanks that differ only in KH (which you can easily control). As described above these will have identical dissolved CO2 concentrations which will make interpretation of results straightforward.

 

[dw1305]

Hi all,

So what happens if you inject CO2 gas? As described by hax47's chemical reaction, some of the CO2 is converted to H+ and to HCO3-. This does increase the number of carbonate species molecules (the HCO3- piece) but it does not change total alkalinity because it also creates the exact same number of H+ species and bicarbonate and H+ balance each other out.

Well at least <"I got that bit right">.

cheers Darrel

 

[hax47]

@_Maq_ this would be an easy experiment for you to run (please oh please oh please) using inert substrate: identical tanks that differ only in KH (which you can easily control). As described above these will have identical dissolved CO2 concentrations which will make interpretation of results straightforward.

+1 vote to that.

 

[_Maq_]

an easy experiment for you to run (please oh please oh please) using inert substrate: identical tanks that differ only in KH

I've performed such an experiment a few years ago, before having entered UKAPS. Only two species: Ammannia crassicaulis and Ludwigia glandulosa. While the former coped with alkalinity (and thus pH above 7) very well, the latter suffered from inability to acquire iron, and probably other transition metals, too.
From left to right:
A - low alkalinity, low pH, low nutrients
B - low alkalinity, low pH, high nutrients
C - moderate alkalinity, pH above 7, low nutrients
D - moderate alkalinity, pH above 7, high nutrients

 

[John q]

All intresting stuff. But... what effect does this have on the plants; negative/positive?

While the former coped with alkalinity (and thus pH above 7) very well, the latter suffered from inability to acquire iron, and probably other transition metals, too.

In an attempt to answer my own question I'll offer this answer.

Kh in and of itself is of little consequence for aquatic plants. However, the ph resulting from said levels of Kh plays an important factor in the health of these plants, more than likely due to nutrient availability/uptake?

 

[_Maq_]

Kh in and of itself is of little consequence for aquatic plants. However, the ph resulting from said levels of Kh plays an important factor in the health of these plants, more than likely due to nutrient availability/uptake?

Mayaca, and especially Tonina fluviatilis are considered rather demanding by CO2 users. For me, they grow like weeds. A possible answer is that while CO2 users can reach acidic range thanks to elevated level of CO2, I can't. My water features almost zero alkalinity, and Toninas grow quite well in water near neutral.
This question is difficult to prove experimentally. In low-tech, keeping water acidic means that alkalinity must be very low. I can't increase alkalinity and keep the water acidic at the same time. On the other hand, in high tech, if you change alkalinity and want to keep pH constant, you must adjust CO2 injection significantly. Then it's hard to tell what is the cause of the results observed. It would require quite a many carefully balanced comparative tests.