Substrate and CO2 lower PH

[goodiehard]

Hi All,

Having some trouble with choosing the right substrate. Using kittylitter now in my first tank (here in the netherlands the "foetsie bah" brand is similar to the tesco stuff), but it's just too course to hold any plants. For my non-existing experience and skills anyway.

I've read about the baked clay substrates on the forum. But ADA is expensive and hard to come by in Holland. Akadama, the bonzai soil, looks a lot like the big grainsize of kittylitter.
Ebi gold is available, is a nice black color and a small grain in the powder variant. But the thing I'm worried about is that is says that it lowers the PH.
Because adding CO2 lowers the PH, if the substrate also lowers the PH (to about 6,5 according to spec), doesn't this limit the amount of CO2 I can add?
Tapwater PH in amsterdam is 8,0.

There a nice journal on this forum where the user stated that more co2 is better and to keep upping the bubbelrate untill the PH checker is almost yellow, I guess that's a PH of about 6,0. I think most fish dig 6,0 and maybe a little less, but not much below that, right?

Is this a real worry or can I add enough CO2 even if the substrate lowers it from 8,0 to 6,5?

Ciao
Alex

 

[dw1305]

Hi all,
Alex I'll need to deal with the 2 questions separately, but basically they are different processes and mutually exclusive, so you don't need to worry about the pH reduction from the added CO2.

if the substrate also lowers the PH

Both Akadama and cat litter are clay based, and clay minerals have the ability to hold on to and exchange ions. All salts (products of the neutralization reaction of an acid and a base), when they dissolve, go into solution as cations and anions, in the case of NaCl (sodium chloride or salt, hence "salts") this is Na+ the sodium ion a cation, and Cl-, the chloride ion, an anion.

The ability of any compound to hold on to and exchange cations is called its Cation Exchange Capacity or CEC (it will also have an Anion Exchange Capacity - AEC).

Sand has no CEC, but clay minerals have high CECs, this is why we like them for the planted tank, in the case of Akadama it is clay from an area which is naturally poor in "bases" or alkaline ions such as calcium and magnesium. The technical term is that it has a low "percent base saturation", and it means that most of the exchange sites naturally have an H+ ion. When you add Akadama to a solution containing K+, Ca2+, Mg2+ ions (your tank water), the H+ ion will be exchanged for one of the other cations, and the solution may become more acid. If you soak the Akadama in a solution of calcium chloride or magnesium sulphate, the H+ ions will be exchanges for Ca2+ or Mg2+ ions before it gets in the tank.

Because adding CO2 lowers the PH, if the substrate also lowers the PH (to about 6,5 according to spec), doesn't this limit the amount of CO2 I can add? Tapwater PH in amsterdam is 8,0.

The pH reduction caused by adding CO2 is different from the pH reduction caused by swapping cations for proton (H+ ions) that can occur in the substrate. In the case of CO2, adding CO2 alters the CO2 ~ HCO3 equilibrium, and a small proportion of the CO2 goes into solution as carbonic acid (H2CO3). This carbonic acid then depresses pH. When you stop adding CO2, the equilibrium returns to its initial state and the pH rises.

Equilibrium reaction details:
CO2 + H2O ~ H2CO3
CaCO3 ~ Ca2+ + CO3-–
HCO3– ~ H+ + CO32–
H2CO3 ~ H+ + HCO3–
H2O + CO2(dissolved) ~H2CO3

People who add CO2 usually buffer their water to 4dKH, this gives them a reserve of carbonate. Have a look here if that doesn't make sense: <http://www.theplantedtank.co.uk/co2.htm>.

cheers Darrel

 

[profile]

The Cation Exchange Capacity of clay and organic matter is a property that works pH dependent and is ion specific. There is also a saturation point for a certain ion, pH dependent. The Calcium and Potassium ions for example stay on the surface of clay and organic matter longer than others, like Magnesium, if the pH decreases. Organic matter for example binds readily Calcium and gets credited with higher values of CEC than clay(total CEC), though clay is useful and the larger part of a soil is composed of it.

There was also a misleading information saying that if there is CEC in a soil, it will also hold some AEC. Well, the common sense wins in saying that the clay with lower CEC has higher AEC.

In order for CEC to work, there is a need for a higher pH. So, the addition of CO2 creates the swing for this property of the clay to exist and to become useful. Around neutral pH/tap water of Amsterdam, the uptake of nutrients is poor and many ions are found on the binding sites of the substrate. When the CO2 gets there, the nutrients become more available and the uptake increases.

The are carbonates and bicarbonates and carbonic acid or water soluble carbon dioxide. Their ratio is pH dependent. The two former ones hold cations other than hydrogen (which is held by the latter one) and they hold the water hardness. Swapping cations and adding carbon dioxide work with the same stuff, though, one is messing with the Ca of CaCO3 for example and the other is messing with the amount of CO3.

 

[goodiehard]

@darrel and profile. thanks for your reactions. pretty scientific stuff, hope there won't be a test cause I don't think I'll be able to reproduce it...

The message is clear though, so thanks a lot for that.

@Darrel. Read the link to James' article. I think I've read it somewhere before. Anyway, I have the CO2 kit he describes and am using the techniques he explains.

 

[dw1305]

Hi all,
Profile, that is an interesting post, not all the images are showing unfortunately. Point taken about humus having a higher CEC than many clays, this is one of the reasons I like some humic compounds in my tank water, although many people try and remove them.

There was also a misleading information saying that if there is CEC in a soil, it will also hold some AEC

You mean because the clays that have low CEC tend to have some AEC?, I still think I'm right, compounds like clays have CEC/AEC, in a sesquioxide clay (laterite) you will have high AEC/low CEC (CEC about 3 meq, compared to 200meq for humus), but a silica based sand won't have any exchange sites at all. Additionally
AEC is a much weaker attraction than CEC, typically below 10meq.

We do have some other posts with more about the lyotropic series for both cations (Fe most strongly bound, Na+ least) and anions (probably in a post about nitrate ion exchange resins and why you can re-charge them with KCl).
You also have to take into account concentration of cations, some exchange sites will contain sodium (Na) ions if you have a solution with a high concentration of Na+ ions, even where other multivalent ions, with stronger bonding are present at lower concentrations.

Generally, in contrast to CEC, AEC generally will increase when pH drops and decrease when pH rises, but things are slightly different in aquarium substrates (as compared to within the soil), partially because we tend to add the salts (as fertilisers), to substrates, so that calcium and magnesium may not be associated with carbonates (HCO3- in solution) in the way they would be more normally in the soil.

The point about the availability of nutrients changing with pH is a valid one, and is one of the reasons we feed micro and macro-elements on different days. As an example if you add orthophosphates (PO4---) and unchelated iron (Fe+++) at the same time, insoluble iron phosphates are formed. The same process occurs with calcium at alkaline pH's, again insoluble calcium phosphate complexes are formed.

The are carbonates and bicarbonates and carbonic acid or water soluble carbon dioxide. Their ratio is pH dependent. The two former ones hold cations other than hydrogen (which is held by the latter one) and they hold the water hardness. Swapping cations and adding carbon dioxide work with the same stuff, though, one is messing with the Ca of CaCO3 for example and the other is messing with the amount of CO3.

I don't quite get what you are getting at here, when you stop adding CO2 (carbonic acid is an H+ ion donor) the pH will rise, because the ratio H+:OH- ions* will have changed (H+ ions are no longer being added, and as the CO2 out-gasses along it diffusion gradient, the amount of carbonic acid will fall).

This is the question the original poster was asking, adding CO2 is not the same as adding a base (H+ acceptor) and then neutralising it with an acid(H+ donor). You can prove this easily enough with a conductivity meter. Changes in pH when you add CO2 are exactly that, changes in pH, they aren't changes in alkalinity, and they don't effect the fish.

Personally I'm not a great fan of pH as a measurement for aquarists, unless you understand buffering, and why 2 similar pH values may not mean the same thing.

cheers Darrel

*I'm going to ignore hydronium - H3O+