Buffering carbonates when dosing CO2

[Mitchel]

I often read that you should start adding CO2 two hours before the lighting comes on.
This has something to do with the carbonates that must first be saturated.

my comment/questions:
A: You can simply open the CO2 tap a little further. Then you will reach the desired ppm faster, right?
B: if you use RO water you don't need 2 hours. After all, there are no carbonates to bond to?
C: If you use a soil (i.e. pH-lowering) you will reach the desired ppm CO2 much sooner, right?

Why then is the 2 hour rule adhered to?

regards, Mitch

 

[simon_the_plant_nerd]

I often read that you should start adding CO2 two hours before the lighting comes on.
This has something to do with the carbonates that must first be saturated.

my comment/questions:
A: You can simply open the CO2 tap a little further. Then you will reach the desired ppm faster, right?
B: if you use RO water you don't need 2 hours. After all, there are no carbonates to bond to?
C: If you use a soil (i.e. pH-lowering) you will reach the desired ppm CO2 much sooner, right?

Why then is the 2 hour rule adhered to?

regards, Mitch

Two hours is roughly the time it takes for a drop checker to accurately represent the amount of dissolved CO2 in the water (assuming the CO2 level stays consistent throughout that 2 hours). Once your lights turn on, you can see if you have reached the correct amount of CO2. I think this has been determined experimentally as a rule of thumb. Your water hardness will affect PH drop as CO2 is injected so the drop checker, which is independent of your water, is the best way to see how much is dissolved.

If you just crank up the CO2 you risk asphyxiating your fish and inverts. You should set the correct value and leave it there.

You shouldn’t be using RO water as it has no nutrients in it for plants or invertebrates. You need to remineralise the water first.

CO2 isn’t substantially more soluble in lower PH solutions. As far as I’m aware, it’s more soluble in basic solutions but I’m not a chemist. The substrate will make no difference (based on the PH).

Plants don’t require CO2 when the lights are off, they put CO2 into the water and absorb oxygen.

I don’t know what the carbonate thing is you’re talking about but someone else may.

EDIT: Added more information and better explained myself (I hope).

 

[simon_the_plant_nerd]

Here you go:

Co2: 2 hours before lights turns on. Why?

When I'm doing research everywhere I go it's the same advice: two hours before the lights turn on you start dosing the CO2. So when the light turn on there is the right amount of CO2 in the tank. But why two hours? Isn't it depending on the size of the tank and the amount of ppm? An tank of 50...

www.ukaps.org

 

[dw1305]

Hi all,

if you use RO water you don't need 2 hours. After all, there are no carbonates to bond to?

It isn't the alkalinity of the water in the tank that's relevant.

Two hours is roughly the time it takes for a drop checker to accurately represent the amount of dissolved CO2 in the water (assuming the CO2 level stays consistent throughout that 2 hours). Once your lights turn on, you can see if you have reached the correct amount of CO2. I think this has been determined experimentally as a rule of thumb. Your water hardness will affect PH drop as CO2 is injected so the drop checker, which is independent of your water, is the best way to see how much is dissolved.

That one. It is only the carbonate hardness of the <"4 dKH solution in the drop checker"> that matters. The base in solution (sodium bicarbonate (NaHCO3) etc.) is the <"proton acceptor">.

Because the drop checker has an air gap, the colour change of the bromothymol blue, narrow range, pH indicator is entirely dependent upon CO2 that diffuses across that air gap, and the small proportion of that CO2 that becomes carbonic acid (H2CO3). That is why you have the 2 hour lag period, it gives time for the drop checker to "catch up". I still like the <"bouncy castle analogy">.

The carbonic acid then disassociates (into HCO3- and H+ ions) and it is the "extra" proton (H+) that causes <"the pH indicator to change"> from blue (deprotonated) to yellow (protonated). When you have a mixture of protonated and deprotonated bromothymol blue? It is green, simply because you've mixed together yellow and blue.

cheers Darrel

 

[LMuhlen]

I often read that you should start adding CO2 two hours before the lighting comes on.
This has something to do with the carbonates that must first be saturated.

People who are adding CO2 before lights on are aiming at having a constant CO2 concentration in the water throughout the entire photoperiod, which is when the plants feel the demand for CO2. Many people believe that a constant CO2 level during lights on is important for keeping plants happy, especially the more delicate ones, and as a consequence for keeping algae under control.

It is not directly related to carbonates. It is just concentration of dissolved CO2 in water. It is a balance between the rate at which CO2 dissolves into the water (dependent on characteristics of the reactor/diffuser and the rate of CO2 being pushed through it); the rate of CO2 that escapes back to the atmosphere (which depends on the tank's surface area and the degree of surface agitation); and the total volume of water in the tank. Both rates of CO2 dissolution and CO2 escaping to the atmosphere change depending on how much CO2 is already dissolved. When you are first starting and there is very little CO2 dissolved, the CO2 dissolves very easily and little CO2 is lost. When the concentration rises, this changes and it gets harder to dissolve more CO2 and the rate of CO2 losses increase. During this period, the CO2 concentration in water changes a lot and people don't want the plants to notice this, so this is done while the lights are out.

At some point, the rate at which you add CO2 to the water is the same as the CO2 losses and the concentration becomes constant. At this point, you are free to turn on the lights and supposedly appease your delicate plants with a mostly constant CO2 concentration in water. The time it takes for this equilibrium to be reached depends on all the parameters mentioned above, but it can go from 30 minutes to 3 hours. When in doubt, the 2 hours suggestion is a good starting point.

It is said that keeping a higher degree of surface agitation and a higher rate of CO2 injection will allow you to reach equilibrium faster while keeping CO2 concentration in the desired range. On the other hand, you will consume more CO2.

A: You can simply open the CO2 tap a little further. Then you will reach the desired ppm faster, right?

If you open the CO2 tap, as long as it remains within the capacity of your dissolving equipment (reactor or diffuser), you will dissolve CO2 faster. On the other hand, you will reach equilibrium at a higher concentration. If you could open the tap before lights on and then reduce the flow to the desired working flow after that, you would optimize this, but our standard equipment doesn't do that and it is a bad idea to set your CO2 flow manually twice a day. Automatic systems using pH probes try to do this by turning the CO2 valve on and off, but they still can't open more or less.

B: if you use RO water you don't need 2 hours. After all, there are no carbonates to bond to?
C: If you use a soil (i.e. pH-lowering) you will reach the desired ppm CO2 much sooner, right?

As far as I can tell, neither has a relevant influence on CO2 dissolution rate. They will affect the water pH at CO2 equilibrium, but not the concentration equilibrium itself (or the rate).

 

[Mitchel]

Thanks all for the responses so far.

I have also read that when using hard water you need to add more CO2 to reach the desired pH (or ppm/ph drop).
Is this not correct?

 

[LMuhlen]

Thanks all for the responses so far.

I have also read that when using hard water you need to add more CO2 to reach the desired pH (or ppm/ph drop).
Is this not correct?

I see this being brought up occasionally. I'm not sure, but I assume that it is false that the hard water makes a difference in CO2 dissolution. Of course, I would love to be reassured on this matter by someone with a more solid understanding of the chemical process.

On the other hand, it is very possible that a higher KH water will have a different pH drop between 2 different CO2 concentrations, when compared to a lower KH water. The 1.0 pH drop rule of thumb may be relevant for a fixed range of KH values. So if you measure your CO2 concentration by means of a pH drop, it is possible to have different results when different KHs are used in the tank. That shouldn't mean that the CO2 concentration or how easy it is to reach it changes, only that the effect on pH changes.

I'm not sure if the pH drop is higher or lower for a higher KH water, though...

 

[simon_the_plant_nerd]

Thanks all for the responses so far.

I have also read that when using hard water you need to add more CO2 to reach the desired pH (or ppm/ph drop).
Is this not correct?

This is incorrect (sort of). The ppm of dissolved CO2 and pH drop of water are not the same thing.

CO2 will dissolve in the water, however the hardness of the water has a buffering affect on the pH meaning more CO2 is required to achieve the same pH drop than in softer water.

The pH drop is NOT equivalent to PPM unless you know exactly what is in the water and can very accurately use a look up table (which I don’t think most people would do). This is why I said a drop checker is better. It is independent of your water hardness and is therefore a better indicator of how much CO2 is dissolved in the water.

If you aim for a specific pH drop without knowing the exact hardness of your water you risk putting too much CO2 into the water and asphyxiating your fish.

@dw1305 explained the science a couple of posts back up.

 

[dw1305]

Hi all,

This is incorrect (sort of). The ppm of dissolved CO2 and pH drop of water are not the same thing.

You need @ian_m, @Jose, @hax47 or @Andy Pierce for a proper comment, but this should link in to a thread <"with an explanation"> - <"Quick question around co2">.

If any-one really wants the inorganic chemistry? We have threads <"Gas-exchange experiments"> & <"CO2 concentration versus time. The maths...">.

You might like to get comfortable and <"pour yourself a cold one"> before you start.

So Fick's first law of diffusion:
It describes the flux of molecules (also gases) and can be applied both within a solution and between air and solution (water). If someone reads the Wikipedia site, it can be overwhelming, but there are a few simplified forms/derivatives:

J = D * A * (Δp / Δx)

where
J is the amount of gas transferred per unit of time
D is the diffusion coefficient specific to the gas in question; it is different for CO2 and O2
A is the surface area for gas exchange
Δp is the partial pressure difference of gas between two compartments
Δx is the distance in which the molecule diffuses

So the bigger the surface, the shorter the distance, the bigger the partial pressure difference (diffusion driving force) and the diffusion coefficient, and more molecules will diffuse per unit time.

It is not easy to calculate with the surface in the case of a complex system like an aquarium, especially if it is not a still surface, and the distance is also ambiguous. But we can merge Δx, A, and D into a single coefficient DC, the diffusion capacity or conductance of the aquarium. You can find this form (with concentration instead of partial pressure) of the law on Wikipedia under the "Biological perspective" part:

J = -DC * (p2 - p1)

cheers Darrel

 

[Mitchel]

CO2 will dissolve in the water, however the hardness of the water has a buffering affect on the pH meaning more CO2 is required to achieve the same pH drop than in softer water.

this is what I ment; the buffering effect

The pH drop is NOT equivalent to PPM unless you know exactly what is in the water and can very accurately use a look up table (which I don’t think most people would do). This is why I said a drop checker is better. It is independent of your water hardness and is therefore a better indicator of how much CO2 is dissolved in the water.

strange . I Always assumed that with a pH drop of 1 point, to obtain (approx) 30 ppm CO2, the KH is not relevant.

 

[dw1305]

Hi all,

I Always assumed that with a pH drop of 1 point, to obtain (approx) 30 ppm CO2, the KH is not relevant.

I think you are right, but it because of the log10 nature of the pH scale.

Assuming you still have a reserve of buffering then the fall in pH may appear to be linear (exponential data plotted as log10 values will form a straight line)

This what @Jose said <"Question about pressurised CO2 and water disturbance"> and <"my reply">

Jose I'll ask a colleague who is a chemist. But I think I've got it.

Assuming that the "pH drop of 1" for ~30ppm CO2 is right, it is because it is a buffered system, and pH is a log10 scale. The drop in pH is determined only by the concentration of CO2 in the water. In the same way a reserve of HCO3- (dKH) will maintain pH at ~pH8 at atmospheric CO2 levels, as you increase the CO2 levels you drive the equilibrium towards H2CO3 and the pH falls.

Assuming you still have a reserve of buffering then the fall in pH may appear to be linear (exponential data plotted as log10 values will form a straight line).

I think everything starts from the assumption that water in equilibrium with atmospheric CO2 levels contains ~3ppm CO2. A drop of 1 pH unit is actually an increase of 10 in the ratio of H+:O-H ions, when you have a pH drop of 1 unit you have:

3ppm x 10 (1 as log10) = 30ppm CO2.

cheers Darrel

 

[simon_the_plant_nerd]

this is what I ment; the buffering effect

strange . I Always assumed that with a pH drop of 1 point, to obtain (approx) 30 ppm CO2, the KH is not relevant.

Sorry, I don’t think I’m explaining myself well.

Let’s say you have pure RO water and you add a known amount of KH to the water to reach a specific hardness. You record the pH. If you then inject CO2 into the water until a pH drop of 1 is achieved, the concentration of CO2 in the water will be 30ppm give or take (assuming we are not at extremes of pH).

But this doesn’t take into account other things that may buffer the pH such as phosphate which is present in tap water and the fertilisers we add to our water. Also fish poo.

As soon as you go away from perfect conditions, the 30ppm/1pH drop stops working.

Also pressure, temperature and salinity will have an effect but these can be ignored for most people.

 

[Andy Pierce]

As soon as you go away from perfect conditions, the 30ppm/1pH drop stops working.

It doesn't actually work this way. In the steady-state situation (after all the moving pieces become stable and stop moving, for example 2 hours after you've been injecting CO2 gas) the amount of CO2 dissolved in the water (what we care about) is independent of the pH of the water and also independent of all the other various carbonate species (e.g. bicarbonate levels) as well - you can see that as the green horizontal line in this dissolved organic carbon (DIC) species breakdown vs. pH graph. That means it doesn't matter what your starting KH was.

The pH drop piece relates to the equilibrium between CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

pH is the measurement of the H+ concentration on a log10 scale subtracted from 14. That means if you increase the H+ concentration 10x you have decreased the pH by 1 unit. Because of the above equilibrium, if you have increased the H+ concentration by 10x, that means you have also increased the CO2 concentration 10x because these move together. Where the 30 ppm comes in is in the assumption that in the absence of CO2 gas injection the concentration of CO2 dissolved in water is 3 ppm, where then a 10x increase gets you from 3 ppm to 30 ppm. The assumption piece is critical here. If the starting CO2 concentration was actually 5 ppm then injecting CO2 gas to get a 1 unit pH drop will make the CO2 concentration 50 ppm. Similarly if the starting CO2 concentration was actually 1 ppm then injecting CO2 gas to get a 1 unit pH drop will make the CO2 concentration be 10 ppm. If you use a drop checker you don't have to worry about any of that because the colour of the drop checker solution (which relates to the pH of the drop checker solution) indicates the absolute concentration of CO2 dissolved in water instead of a change in concentration.

Interestingly (maybe) in the same way that you can add CO2 to increase the H+ levels and thereby decrease the pH, you can also do the reverse. You can add acid which increases the levels of H+ (thereby decreasing the pH) which will convert existing bicarbonate into dissolved CO2 gas. The trick is that while you can bubble in CO2 from a CO2 tank until the tank runs out, you can't play the same game with adding acid from an acid source because the bicarbonate in the aquarium will run out first (being converted to CO2) and then you crash your pH.

 

[ceg4048]

Thanks all for the responses so far.

I have also read that when using hard water you need to add more CO2 to reach the desired pH (or ppm/ph drop).
Is this not correct?

Yes, this is not correct. As mentioned, CO2 saturation in water has nothing to do with carbonate concentration in the water. The effect that carbonate has in the water column in it's interaction with CO2 is purely that of change in pH.
CO2 dissolves in high carbonate content water in exactly the same way and in exactly the same amount of time as in low carbonate content water. The difference in the two will strictly be limited to the pH.
CO2, as with other gases dissolves in water as a function of atmospheric pressure and water temperature. The effect of carbonate content in the water column is miniscule.
Since the pH difference can be confusing, the dropchecker is filled with distilled water adjusted to a known carbonate content (typically 4 dKH).
The CO2 that dissolves in the tank water "evaporates" into the open mouth of the dropchecker and in so doing it re-dissolves into the dropchecker's water, thus acidifying that water. Gases do not move very easily in water so it takes a while for the gas in the tank water to move across the boundary of the tank water to the dropchecker air bubble and then from the bubble into the dropchecker water. As you can imagine this path for a gas to trave from tank water-to-air bubble-to dropchecker water takes a long time, so it's not really easy to interpret the actual tank water's pH due to this lag time. Only when there is steady state conditions and when the tank's gas concentration is stable can the dropchecker indication be considered "accurate". Before that stabilization you will always be reading what the tank waters gas concentration was some time ago.

As mentioned by other posters, it is important that the gas concentration is adequate and stable by the time the lights go on.

Therefore, as a rule of thumb, it's a standard practice to run the gas at a constant rate for a while in order to stabilize the gas concentration.
Again, 2 hours is a simple rule of thumb "assuming" typical gas injection rates. If the injection rate is higher then that normally means the gas pressure is higher and, as mentioned at higher pressures the dissolution of the gas is quicker.

There is no way to be exact because every tank has different variables, such as temperature , evaporation rate, filter flow and so on, thus, the two hour rule of thumb. Feel free however, to experiment with these variables to find what works best for fish and fauna.

Cheers,

 

[Mitchel]

Great comments from everyone. A lot has already become clear to me.

Still one more remark:
Where does the comment come from, for example, that Seiryu stones release lime and thus influence the entire CO2 process? Lime is also a carbonate, right?
and that you therefore have an increasing of KH in the tank. If the KH has no influence on the CO2 why this is spread in the hobby? Is it a mythe?

 

[dw1305]

Hi all,

Where the 30 ppm comes in is in the assumption that in the absence of CO2 gas injection the concentration of CO2 dissolved in water is 3 ppm

We have a thread about where that assumption <"may have originated">. One issue is:

...... there are plenty of papers on natural CO2 levels in freshwater, but they nearly all rely on the carbonate equilibrium (and temperature) to estimate the CO2 level - <"A 30-year dataset of CO2 in flowing freshwaters in the United States - Scientific Data">, rather than actual CO2 measurements. This really just reflects the difficulty of <"measuring dissolved CO2 directly">.

So we don't really know that "30 ppm CO2" is actually 30 ppm CO2, as @Andy Pierce <"says"> above .

Where does the comment come from, for example, that Seiryu stones release lime and thus influence the entire CO2 process? Lime is also a carbonate, right? and that you therefore have an increasing of KH in the tank.

Your limestone (CaCO3) rocks will <"dissolve more quickly"> when <"you add CO2">. You will have raised dGH (the Ca++ ions) and dKH (the 2HCO3- ions) in your tank water. @Happi mentions this in <"Am i missing anything)?">

If the KH has no influence on the CO2 why this is spread in the hobby? Is it a mythe?

The CO2 level remains the same, it is dependent on Henry's law. You have more DIC at higher pH levels, but most of it as carbonate or bicarbonate ions. @Andy Pierce writes about this in <"My first adventure into aquascaping - Aquael 125L Walstad">

Higher dKH can make some nutrients less plant available, have a look at @Roland 's <"Soft water tank">.

Last one, but if your water is already saturated with bicarbonate ions (at atmospheric CO2 levels, so ~18 dKH) the least soluble carbonate will precipitate out as the level of dissolved CO2 falls back to atmospheric levels and, <"for us">, that "least soluble carbonate" is calcium carbonate (CaCO3).

cheers Darrel

 

[Andy Pierce]

the KH has no influence on the CO2 why this is spread in the hobby? Is it a mythe?

The confusion arises because in a closed system not open to atmosphere KH does influence CO2. The closed system is where most of the chemistry discussions take place so this is a quite natural mistake to make. We however are nearly always dealing with an open system system where KH does not influence CO2 levels.