Hi Tom,
We're trying to get to the bottom of the issue of whether a higher CO2 bubble rate is required in higher Carbonate alkalinity water to achieve the same levels of dissolved CO2 compared to waters having lower carbonate alkalinity.

Most trusted sites seem to agree that solubility depends on the following factors:
1. Pressure
2. Temperature
3. pH
4. Presence of other substances

The pressure and temperature factors seem intuitive so no problem there. Page 11 of this online book mentions the effect of pH: Fundamentals of aquatic toxicology ... - Google Book Search

The second paragraph implies that CO2 solubility is directly related to pH so that at higher pH the Carbonic acid equilibrium equation is driven to the right.

The figure 13 chart shown on this page gives a similar explanation: http://openlearn.open.ac.uk/mod/reso....php?id=185884


Since higher carbonate/bicarbonate levels favor higher pH does it follow that higher KH waters, with higher alkalinity would have less dissolved CO2 and additional higher levels of carbonate/bicarbonate even if the injection rate is kept the same?

I think you've explained before that higher kH water has higher carbon content naturally but it's unclear whether factor number 4. presence of other solutes (carbonates/bicarbonates) act in concert with factor 3. pH to lower the dissolved CO2 content.

The implication would be that tanks running at higher kH would need higher injection rates to achieve the same CO2 concentration.

Could you clarify?

Cheers,

Here we go again........

Two glasses of water sitting on the table, one with a KH of say 1 and the other 10.

At equilibrium, do they have the same CO2 ppm?
At equilibrium, do they have the same Sum total DIC(Dissolved inorganic carbon)?

Think about CO2 gas.
If you need/want more CO2, what do you add?
It's self evident.

I think some folks believe they can monkey with KH to magically add super natural CO2 or some cockamaniy.
And that's where all the money business starts.
Then we add enriched levels of CO2 also, that causes even more confusion.

I can have a KH of 20 degrees and use CO2 to drop it to 7.4 pH.
This gives me 24 ppm of CO2.

Likewise, I can add CO2 and get the same CO2 ppm, at a KH of 8 by adding the exact same amount of gas(24 ppm) and have a pH of 7.0, or add the same amount again(24ppm) to get a pH of 6.4 with a KH of 2 degrees.

Pressure, yes, does not really apply to most aquarist living 2000 meters or less. maybe in you live in LaPaz Boliva............

Temp, we make that stable, so no issue there unless you deal with summer time heat, then higher temp= less gas. Adding to higher temps= faster MET rates and growth rates for many plant species. So more CO2 is needed during summer...........

Presence of other substances - likely pH alterning buffers other than Carbonates, like borates, PO4, tannins etc........

Now pH.

Adding more carbonate does what to pH?
Drives it up in general if you have soft water.
Adding CO2 does what?
Decreases pH.

No biggie there.

Now what about the total Carbon?
Look at the Sum total Carbon, see figure 5.7 there in your book reference.
Look at the Y axis, what does it say?
Sum total CO2.

The solubility of free CO2 increases when you have lower KH's.......but that's not really an issue for us if we enrich with CO2 as it's pretty soluble to begin with.
This affects the rate that CO2 comes in/out of solution, not the CO2 ppm at equilibrium.

The relative % changes, but the over all total is the same.
Now what happens if you lower pH with additions of CO2?
You get more CO2.........which is what you want.

So how much CO2 does RO water have at equilibrium(just a glass of water sitting there for 48 hours)? How about a Kh of 4? How about a glass with a KH of 10?

They will all have different pH's(thus the relative %), but the CO2 ppm's should all be the same since in all cases, the partial pressures are also the same with air.
the total Carbon is different in each case as well.

Hard water will have more total carbon as DIC, but at equilibrium, the ppm of CO2 should be the same for any water............since the partial pressures are the same everywhere.

Now can you make CO2 from HCO3, eg baking soda?
Can you do this by adding CO2? Will CO2 dissolve the HCO3 into more CO2?

"Thus, low-pH waters, with available carbon dioxide, are more favourable for photosynthesis."

Photosynthesis for what type of green thing?
Algae are not carbon limited, they have low Carbon demand vs plants.
The key in the above statement is really this: available CO2.

Hard or soft, does not matter much. As long as the rate added meets the demand.
That's why we add CO2 to begin with

Our systems rapidly deplete the CO2 at higher light.
You will find in many richly planted systems that the KH is low..........since after the plants are done with the CO2, they go after the HCO3 next.

Does not always mean they prefer anything, in many cases, harder water is where we find more plants because there is higher total carbon in the form of both CO2 and HCO3..........and if you do to Florida, you can find examples in both water types.

Figure 13 is the % relative to the other ions, not the actual amount of CO2.
Most of the total carbon at a pH of 11 is CO3-2. But at equilibrium with air etc, the water should have the same ppm's as a glass or RO water.
At a high pH, the amount of CO3 is huge, but the CO2 relative % is low.
At RO water?
The relative % is 100% CO2, no Bicarb or carb.

So think about it like this: low KH and high KH= same CO2 ppm's, but the higher KH has more total carbon.
That is what is meant.

Maybe that clears things up better.

Here's some background on water and gas:

http://en.wikipedia.org/wiki/Henry's_law
Partial pressure - Wikipedia, the free encyclopedia

Laws of Gas Transport

O2 is much less soluble than CO2 for example.

That's why we have 3ppm of CO2 or so and about 7ppm of O2, even though in the air we several orders of magnitude more O2.

Applied Aquatic Ecosystem Concepts - Google Book Search

This states that CO2 is mostly CO3 at pH's 12 and above.....
Which is another way to think about it.



Regards,
Tom Barr

Here is one detailed reference on that subject: www.co2-cato.nl/doc.php?lid=71

Another less detailed one: Saturation of CO2 in Water

Google has a lot of other references, from which I understand that the relationship of CO2 to water for the concentrations of interest to us is very complicated. That seems to be primarily because CO2 forms compounds with water, and the mix of those compounds depends on numerous things, including pH. But, it also seems that for water with the parameters of interest to us, the relationship if pretty simple. That my non-Chemist understanding. I will await Tom's much more informed comment with great interest.

And, Tom was commenting as I was reading my references. His answer is what I would intuitively expect.

Yes, yes...I remember this quiz question from a post 3 years ago...but I remember getting the answer wrong because I kept thinking it was a trick question. I remember thinking; Hey wait a minute, suppose the low KH glass was sitting on a table in Death Valley in Winter and the high KH glass was sitting on a table in La Paz in Summer?

The uncertainty comes about because your analogy uses static water glasses at equilibrium, but doesn't the fact that we are adding CO2 on one end and losing it on the other end due to evaporation and plant uptake mean that our systems are not at equilibrium? If so then it seems to me that the bicarbonate–carbonate equilibrium equation is constantly being messed about.


Well, gosh...this question points to the root of the uncertainty. If I look at the equation: CO2 + H20 -><- H + HCO3-><- 2H + CO3

If this were just the glass sitting on the table and baking soda was added, then the equation is driven to the right, no? That would be consistent with not being able to create more CO2 just by raising KH.

However, if more H is added due to injection and more baking soda is added which way is the equation driven? I 'm not really sure.


Again, this is the crux of the matter. Lets say instead of two glasses sitting on a table there are two otherwise identical tanks, planted as close to identical as possible, but one filled with RO water and the other with 20 KH tap water. Does the same CO2 injection rate create the same available dissolved CO2 in the same amount of time in both tanks?

Another scenario is one in which a tank has hardscape with a carbonate source. As the CO2 is injected the carbonic acid dissolves the material in the hardscape and the released carbonates buffer the acid reducing a pH drop. Is the available dissolved CO2 the same or is a higher injection rate required to obtain the same available dissolved CO2? As Hoppy says, intuitively it seems that no injection rate adjustment should be necessary but I can't rationalize it using the equation.

Apologies if this seems a bit dim witted.

Cheers,

Looks like Tom has answered the question


If CO2 solubility decreases with increasing KH then you get higher CO2 loss at water's surface and also slower dissolving of CO2 means more bubbles reach the surface and are lost to the atmosphere. What does this mean? You need a higher CO2 injection rate to obtain the same CO2 levels in a high KH tank than you do in a low KH tank.

Equilibrium levels of CO2 aren't being disputed, and are the same in different water samples having different KH's. What is being suggested is that at above equilibrium levels of CO2 there is a greater loss CO2 in water of high KH compared to one with a low KH.

Big question is how much of a difference does it really make within the KH ranges we see within our tanks? Might be worthy of an experiment

James

None near as I can tell since the solubility of CO2 is very high in both cases, even if it's higher in one vs the other. This might make subtle differences in ecological situations...............in nature..............but less in our tanks over time.
It's a rather prickly question and you can mess things up easily and many do....then repeat it and then we end up with even more confusion.

In all honesty, I'd just stay away from it.
You do not need to know any of this to do anything you might desire in this hobby.

Using lower KH vs higher KH for certain plant species and fish species is about all most need to fret over.

Much easier...........

Learn the things you need to, not those that you do not(unless you just are plain curious).

Otherwise once the brain is full, you cannot add anymore knowledge in there

Regards,
Tom Barr