Limited affect of CO2 In Hard Water ?

[alto]

Depends on type of plants & how much ambient (sun) light tank receives .... you can often tell by looking at the plant leaves - are they already "day open" or still "night closed" (some plants are much more obvious about this than others).
If your tank is running well with your current settings, I'd just leave them as is 🙂

 

[dw1305]

Hi all,

As you can see, there is a relatinship between the chemical form of inorganic carbon and the pH, which also links to the kH, as this parameter represents the total content of carbonates. This mechanism can limit the pCO2 in water, as in fact, the reason why CO2 is so soluble in water is because becomes in HCO3- in most of the total flux of CO2, not being a gas anymore, and then allowing more flux of CO2 to the water until the point of saturation of HCO3-, and then starts the carbonates formation. This whole balance is controlled by pH. Hence, your assumption that CO2 only plays with the inert gas laws is not accurate.

I think this is right when you don't add CO2, but I think it is different when you add CO2 .

Only a very small proportion of CO2 goes into solution as H2CO3, most of it remains as CO2 (gas), which is the "total carbon dioxide" figures on the left of the curve.

My understanding is that carbonate solubility (as HCO3-) is dependent upon the CO2 concentration, and the graphs are at equilibrium with atmospheric CO2 levels (~400 ppm).

If we continually add CO2, we can achieve much higher CO2 values, and we can <"estimate the level of dissolved CO2 from the pH drop for a known dKH value">.

cheers Darrel

 

[alto]

but I think it is different when you add CO2 .

When you alter the concentration of any component in an equilibrium, there will be an equilibrium shift

 

[fablau]

Depends on type of plants & how much ambient (sun) light tank receives .... you can often tell by looking at the plant leaves - are they already "day open" or still "night closed" (some plants are much more obvious about this than others).
If your tank is running well with your current settings, I'd just leave them as is 🙂

Plants are completely "open" at around 8am, and some of them really bend toward the near window, even though it is just indirect light.

My tank is ok, but not perfect. I still have some BBA around, and I 'd like to have pristine conditions. Maybe I should try to have more Co2 earlier in the day?

 

[rebel]

This is not true at all. There is no relationship between the KH or pH of a water sample and the solubility of CO2 in that water.
The solubility of CO2, or ANY gas is a function of pressure, temperature and to some extent, salinity. No other parameters or factors are relevant.

Words to live by.

Cheers,

Agree. If you want CO2 in your water. just add it. If not enough, (use plants to judge), then add more. If too much, (judge using fish), then reduce. Ignore LFS. Show them your glorious planted tank.

 

[alto]

Maybe I should try to have more Co2 earlier in the day?

Try this on days when you're able to monitor fish.
I tend to run low levels of CO2 24/7 (tap water is very soft with KH <1, GH<1 or occasionally 2), adjusting filter return to provide more surface agitation during dark/dim hours.

 

[ceg4048]

However, this is only true under the point of view that the gas concentrations of water are not affected by external factors.

The only external factor involved here is that we are increasing the partial pressure of CO2. The KH of the water cannot be considered an external factor because it is a fundamental property of the water. Increasing the carbonate concentration has the effect of buffering the water's pH against changes made by the liberated H+ as equation (2) moves to the right by virtue of the CO2 partial pressure increment introduced on the left by our injection.

The KH of the water has nothing to do with preventing CO2 solubility in that water. Given similar atmospheric conditions (pressure, temperature and salinity), around the world, CO2 dissolves equally well in all bodies of water, no matter their varying Carbonate content. The same amount of CO2 is dissolved in high carbonate Lake Tanganyika as in soft water Scottish streams because the atmosphere above the water is pushing the gasses into the water at similar rates (again, assuming temperatures are similar).

Where you are mistaken is that first, you have incorrectly assumed that 100% of the CO2 is involved in the Carbonic acid equilibrium equation, and that is simply not the case. It's exactly as Darrel has pointed out to you and it's as plain to see that they show that the percentage of CO2 that STAYS as CO2 is 99.85%. Only the 0.15% of the CO2 reacts to become Carbonic acid - and THAT is the percentage that becomes zero at pH 8 on your chart (and which is stated as 0.5% in the speciation discussion on your link).

The other issue that people who wave this chart around seem to conveniently forget, is that if you have water with a high natural pH then it can only be high pH because of high natural levels of carbonate/bicarbonate. So of course the percentage of Carbonic acid derived from the small percentage of CO2 reacting with the water compared to the highly buffered water will be very small.

Additionally, if this is an equilibrium equation, and if all the CO2 turned into something else, wouldn't the partial pressure of CO2 in the water then fall to zero? And wouldn't the atmosphere just above the water then push more CO2 into solution continuously? Also, if this is an equilibrium equation, and if the chemical conversion had indeed evacuated all the CO2 from the left side of the equation, wouldn't equilibrium then force the equation the other way, to the left. How can CO2 simply turn into something else and completely disappear if the equation is bi-directional?

The answer is: It doesn't disappear, no matter what the pH or KH because the solubility is a strict function of pressure and temperature and those parameters determine the equilibrium equation, they are not defeated by it, so CO2 will ALWAYS be present in the form of CO2 no matter what pH or KH.

This mechanism can limit the pCO2 in water, as in fact, the reason why CO2 is so soluble in water is because becomes in HCO3- in most of the total flux of CO2, not being a gas anymore, and then allowing more flux of CO2 to the water until the point of saturation of HCO3-, and then starts the carbonates formation.

Again, incorrect. CO2 must first dissolve into water before any of these reactions occur. CO2 is very soluble in water simply because it has a moderately high solubility coefficient. Hydrogen Sulfide (2 times more soluble), Chlorine (3 times more soluble), Sulfur Dioxide (65 times more soluble), Ammonia (250 times more soluble), all have higher solubility coefficient that CO2 and they do not involve Carbonic acid formation.

And also explains why so many aquatic plants have adaptations to use HOC3- in addition of CO2, like the mechanism you explained some time ago:

http://www.ukaps.org/forum/threads/what-form-of-carbon-do-water-plants-use.26887/

No this doesn't explain why plants use this Proton Pump strategy. Plant use the strategy because it works and the equilibrium equation is bi-directional. In many waters, the Carbonate content of the water is naturally due to the presence of dissolve carbonates while the CO2 availability is low due to many factors such as high temperature, poor flow, low gas diffusion rates, competition for CO2 by other plants and microbes and so forth.


Cheers,

 

[hard determinist]

CO2 will ALWAYS be present in the form of CO2 no matter what pH or KH

I'm afraid you forgot to take the TIC and degassing issue into account. It seems you make no distinction between a system with a constant TIC level vs. a system with artificially maintained (increased) TIC level. This is very important to distinguish when we speak about CO2 and its levels in water. When you add CO2 into water (from a cylinder), then you are increasing the TIC (= total inorganic carbon) level together with CO2 level, as more added CO2 adds to TIC also. So you have three main carbon "bodies" in water: [CO2] + [HCO3-] + [CO3--]. All these "bodies" make up the TIC content. So for example, under pH 6.5 and KH=4 the total inorganic carbon (TIC) level is 2.43 mM/L (this much carbon is there in the water). If you measure the HCO3- ions concentration and find out it's KH=4, it means that there is 1.43 mM/L HCO3- ions (or 87 ppm). Based on the equilibrium equation you can calculate the amount of the two other species: CO2 and CO3--. And you get 0.002 mM/L CO3-- (= 0 ppm), and 1.00 mM/L CO2 (= 45 ppm). Also, of this 1.00 mM/L CO2, a small fraction (about 0.2%) is there as H2CO3, of which small part dissociates into H+ ions increasing the total hydrogen ion concentration by 0.0003 mM/L H+. So under pH 6.5 you have 45 ppm [CO2] : 87 ppm [HCO3-] : 0 ppm [CO3--] in water. However, this state is not "natural". The water is oversaturated by CO2. Under normal (natural) conditions the CO2 level should be only about 0.4 to 0.6 ppm. So if you have such a tank (or river) with 45 ppm dissolved CO2, there must be some constant supply of CO2! Once you stop this supply, the CO2-oversaturated water will degas the excess CO2, and its level will balance out with its concentration in the air (400 ppm in the air = 0.4 ppm of dissolved CO2 in water). Due to this the TIC will drop, but the HCO3- concentration (= KH) will stay the same [unless we add some strong acid in there, or plants will deplete it during their photosynthesis]. So when you hypothesize about changing CO2 or HCO3- levels, don't forget to take into account the TIC also, and the rate of degassing. So there is (for sure) a strong correlation between the pH/KH and [CO2] in water! Because this correlation exists, you can calculate the total CO2 level in your tank based on the pH and KH values (when I'm speaking of "KH" here, I mean "HCO3-"). It's not true that only the small percentage (0.2%) becomes zero at pH 8. The chart shows the total [CO2], and not just the H2CO3 fraction! The total CO2 is however calculated based on this fraction (and its relationship or correlation to CO2/HCO3- and pH). So what happens when you lower the KH (HCO3- ions concentration), for example by adding a strong acid like HCl into water? The new H+ ions will reacts with HCO3- ions forming H2CO3 which quickly dissociates into CO2. So HCO3- ions are consumed up in this reaction (KH rapidly decreases), and CO2 molecules are created (total CO2 level is rapidly increasing). But under normal conditions, most of this newly created CO2 just quickly degas. So you won't be able to keep it in the water. How much HCO3- ions will be depleted and how much CO2 is created depends on the amount of added acid (H+ ions). If you have 1.43 mM/L (87 ppm) HCO3- ions in your water initially, and add 1.08 mM/L of HCl, you will end up with 0.35 mM/L (22 ppm) HCO3- and 1.08 mM/L H2CO3 (of which 0.25 mM/L = 11 ppm remains in water under pH 6.5 and continuous CO2 supply, and 0.83 mM/L = 37 ppm degas out of water). So if you had 2.43 mM/L TIC initially, you end up with just 0.6 mM/L TIC.

This matter is a really complicated one, and most confusion arises when someone who don't understand it well is playing on a guru.
No CO2 level above ~0.4 ppm will ever stay in water unless you continue to add more and more CO2 in there (thus overcoming the degassing rate)!
Once you increase the CO2 concentration in your water above this natural level, the H2CO3 concentration (and thus pH also) will adjust accordingly. And after you find out the HCO3- ions concentration in your water (aka "KH"), you can calculate the amount of total [CO2] in your water. This should be enough for most hobbyist to know.

 

[dw1305]

Hi all,
I'm not a CO2 user, but I think you are right, dissolved gases and gas exchange is generally a subject that aquarists find quite confusing and complicated.

This matter is a really complicated one.......No CO2 level above ~0.4 ppm will ever stay in water unless you continue to add more and more CO2 in there (thus overcoming the degassing rate)!

It seems you make no distinction between a system with a constant TIC level vs. a system with artificially maintained (increased) TIC level.

I think this is the important point, the drop checker etc is just for water with an "artificially maintained (increased) TIC level" (TIC = total inorganic carbon).

In this post I used the <"Bouncy Castle"> analogy to describe both the addition, and degassing, of CO2. I'm not sure if it helps.

cheers Darrel

 

[ceg4048]

Hello,
At no point have I stated that there is no relationship between various forms of TIC. I also ignore off-gassing because it is not germane. The degassing rate is a separate issue. It does occur, is significant and can be taken into account from a practical level, but if you think that 0.4ppm is the limit then it's clear you have never used CO2. You can annihilate the tank inhabitants in short order with CO2 - and they die from CO2 poisoning, not from Carbonic acid poisoning.

Also, I think you have missed the point of my argument. What I am saying is that the relationship between CO2 content and pH/KH is based on the 0.15% of CO2 that reacts with the water. As more CO2 is added the equations are driven by that 0.15%, whose absolute value increases as the CO2 partial pressure increases. The remaining 99.85% of the CO2 remains as dissolved CO2.

Cheers,

 

[hard determinist]

Only the 0.15% of the CO2 reacts to become Carbonic acid - and THAT is the percentage that becomes zero at pH 8 on your chart

This statement proves you guity of being mistaken in your understanding of CO2 behaviour. At pH >8 you have (nearly) no dissolved ΣCO2 in water. But it's OK. I know that some people here (and elsewhere) are unerring. I don't correct you because of you (because I know it is pointless); I do it for the ones who wants to listen.

 

[dw1305]

Hi all,

At pH >8 you have (nearly) no dissolved ΣCO2 in water.

We are just going around in circles here. When you add CO2 (so that the rate of addition exceeds the degassing rate) you push the HCO3- ~CO2 equilibrium towards CO2.

More CO2 in the water = more H2CO3 and the additional H+ ion cause the pH to fall. When you add 30ppm of CO2 you get a drop of about 1pH unit, it doesn't matter how much HCO3- you have (pH drop is independent of dKH).

Calcium carbonate is insoluble in water, but in water with carbonates present the small amount of CO2 (that goes into solution as H2CO3) is in equilibrium with the HCO3- to give a stable value of ~pH8 at atmospheric CO2 levels (400ppm CO2) and standard barometric pressure (1013mb). Carbonic acid (H2CO3) and bi-carbonate (HCO3-) are the weak acid and weak base pair in carbonate buffering.

When we add CO2 above atmospheric levels we drive the H2CO3 ~ HCO3- equilibrium towards H2CO3. We know that pH is a ratio, and that an acid is defined as a H+ ion donor and we've added extra H+ (from H2CO3), so the pH falls. How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH".

In a drop checker we have an air gap and use 4dKH solution with a narrow range pH indicator (bromothymol blue) to estimate the CO2 value from the dKH, pH, CO2 chart. The air gap means that the pH value (indicated by the change in colour of the pH indicator) is reliant on the CO2 degassing from the tank water, and then going back into solution as H2CO3 in the drop checker. The drop checker is isolated from both tank water (by the air gap) and atmosphere (it has a sealed top).

cheers Darrel