How does aquasoil lower KH? - The Planted Tank Forum
 
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post #1 of 14 (permalink) Old 11-27-2015, 02:38 PM Thread Starter
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How does aquasoil lower KH?

So I've got a new AS setup, and it's widely known that it takes carbonate out of the water. My observations confirm this.

But how does it do it? The AS obviously has organic acids in it like tannins that would reduce pH, but I'm confused as to how it sucks KH away.

The substrate has a high affinity for positive ions like NH3 and K but where does the CO3-- and HCO3- go? It's not bound or adhered directly to the substrate is it?
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post #2 of 14 (permalink) Old 11-27-2015, 03:09 PM
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I think it is bound to the substrate.
A negatively charged material (the substrate) attracts positive particles, as you state. Then the outer surface is positively charged, which attracts negatively charged items (like the carbonates).
Some things are bound more strongly than others, so the carbonates happen to stick pretty well.
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post #3 of 14 (permalink) Old 11-27-2015, 03:37 PM
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As far as I understand.

CO2 + H2O <> H2CO3 <> HCO3 <> CO3.

These are the chemical species that CO3 can transform to, with the reaction capable of going in both directions. If you add 1 dKH of CO3 to the water, and all of this converts to HCO3 with the addition of H+ (acid), then the dKH would drop by half to 0.5 dKH, because HCO3 only contributes half to KH.

The reason why HCO3 only contributes half to KH, is because HCO3 is actually HCO3-, and CO3 is actually CO3--, meaning that CO3 can accept two H+ ions before being converted to an acid, whereas HCO3 can only accept one H+ ion before being converted to an acid.

When the transformation is complete to H2CO3, the acid has been formed, and thus concentration of CO3 in the water is no longer resistant to addition of acid, and no longer registers as KH.

https://en.wikipedia.org/wiki/Alkali...ddition_of_CO2
Alkalinity vs. KH

Feel free to edit.
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post #4 of 14 (permalink) Old 11-28-2015, 01:13 PM Thread Starter
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Quote:
Originally Posted by Diana View Post
I think it is bound to the substrate.
A negatively charged material (the substrate) attracts positive particles, as you state. Then the outer surface is positively charged, which attracts negatively charged items (like the carbonates).
Some things are bound more strongly than others, so the carbonates happen to stick pretty well.
That does make sense to me. I wonder if this layering effects extends past the addition of HCO3- ? Perhaps after th HCO3 is bound some K+ goes on top of it? Then HCO3- again and so on.

Quote:
Originally Posted by Audionut View Post
As far as I understand.

CO2 + H2O <> H2CO3 <> HCO3 <> CO3.

These are the chemical species that CO3 can transform to, with the reaction capable of going in both directions. If you add 1 dKH of CO3 to the water, and all of this converts to HCO3 with the addition of H+ (acid), then the dKH would drop by half to 0.5 dKH, because HCO3 only contributes half to KH.

The reason why HCO3 only contributes half to KH, is because HCO3 is actually HCO3-, and CO3 is actually CO3--, meaning that CO3 can accept two H+ ions before being converted to an acid, whereas HCO3 can only accept one H+ ion before being converted to an acid.

When the transformation is complete to H2CO3, the acid has been formed, and thus concentration of CO3 in the water is no longer resistant to addition of acid, and no longer registers as KH.

https://en.wikipedia.org/wiki/Alkali...ddition_of_CO2
Alkalinity vs. KH
So by this logic, as CO3-- becomes more hydrated (more H+), it gradually reduces it's buffering effect and becomes less strong at the same time. So if I add K2CO3 directly to the water, it would be able to accept two times the number of hydrogens than KHCO3. When it takes up a H+ and becomes HCO3- we are effectively lowering the pH, but the KH is now reduced by half.

I'm very confused about this. Because if we continue to add acid into the water, the pH would go down in proportion to the carbonate left. If no carbonate were present, the pH would go down according to the pKa value of the acid. However, he reaction is reversible, so it's possible for H2CO3 to become HCO3- again if the surrounding H+ ions lower in concentration.

What I'm trying to say is...no matter the amount of acid (tannins) you add to the water, the total carbonate content does not change, only the form that it is in changes. So with aquasoil, that adds alot of acid via tannins and such would lower the pH...yes I get that. But it lowers the alkalinity as well.

Alkalinity = total buffering capacity of ions in solution (CO3, PO4 ect ect)

KH = carbonate hardness, or carbonate buffering only.

In our aquariums usually the vast majority of the total buffering power comes from carbonate sources so KH works well for CO2 calculations. The problem is our test kits measure Alkalinity, or total buffering.

Anyway I'm still left wondering where the KH goes with AS. Even if it's as you say, Audio, and the CO3 becomes H2CO3, the KH titration test kit would still measure it. But it doesn't. I put in 2 dKH worth of carbonate hardness, and in 12 hours the KH test kit registers less than 1.

I'm still leaning towards Diana's answer as being the correct one, and only because KH is actually removed from the water, not exchanged with H2CO3 which would still be seen with test kits.
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post #5 of 14 (permalink) Old 11-28-2015, 01:53 PM
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Test kits that measure KH, work on the principle of adding acid to the sample, with the amount of acid added (number of drops) determining the concentration of things buffering against this added acid.

When CO3 has formed H2CO3, it can no longer accept anymore H+ ions, and thus, it cannot buffer pH against the addition of more H+ ions, and thus, those dinky KH test kits won't register the level of CO3.

Think of pure water that has no KH. pH only changes if you change the ratio of H+ to OH- ions. Just because the water doesn't have KH, doesn't mean the pH just randomly bounces all over the place, however, since there is nothing in the water for the addition of H+ to bond to, any H+ ions added to the water have a direct effect on pH. If you raise the KH of this pure water to 10 German degrees, and add a bunch of H+ ions (pH down for the win) to the water, the pH will drop with the addition of H+, and then rise to the original pH again. We all know the affect, pH down stuff gets asked every other week. The reduction in pH from pH down is the addition of free H+ ions to the water, and the subsequent rise in pH is the bonding of those free H+ ions to CO3 and/or HCO3, and thus reducing the concentration of free H+ ions affecting pH.

So again, since H2CO3 can not accept free H+ ions, it's affect on KH = 0.

The reaction is indeed reversible. However, one must not forget the final reaction, CO2 + H2O <> H2CO3. The atoms on the left hand side of the equation equal the atoms on the right hand side of the equation. One Carbon atom, two Hydrogen atoms and three Oxygen atoms.

As H2CO3 concentration is increased, some of this transforms into CO2 + H2O. Since the concentration of CO2 in water always wants to maintain equilibrium with the atmosphere, as the concentration of H2CO3 is increased, and hence the concentration of CO2 in the water is increased, some of this CO2 is lost to the atmosphere. In other words, one Carbon atom and two Oxygen atoms are lost to the atmosphere, quite a large chunk of CO3 don't you think. In fact, all that remains is two Hydrogen atoms and one Oxygen atom (H2O). In this situation, the addition of free H+ ions has had a direct effect on the chemical species (H2CO3, HCO3, CO3). Where we inject CO2 into the water, the existing HCO3 and CO3 species remain unaffected, as the increase in H2CO3 is thanks to CO2 + H2O, rather then the transformation of the existing HCO3 and CO3 species through the sole addition of H+.

edit: Also, I don't think KH test kits measure alkalinty, I think they do indeed only measure KH. Where they fail is when other sources are present in the water that affect pH. Since these test kits titrate the sample to a pH endpoint, any fluctuation in pH from sources other then KH will affect the reading. But it's just a theory at this stage.

Feel free to edit.

Last edited by Audionut; 11-28-2015 at 02:55 PM. Reason: CO2 is lost, leaving H2O.
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post #6 of 14 (permalink) Old 11-30-2015, 06:17 AM Thread Starter
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So you are saying the take-a-way from this is that directly adding H+ to the aquarium (in the form of weak acids from a multitude of sources) influences the KH reading i get with my test kit?

Bump: @Audionut
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post #7 of 14 (permalink) Old 11-30-2015, 07:16 AM
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That's correct. And a Bjerrum plot shows what the predominant species will be dependent on pH (providing no other ions are affecting pH).

Feel free to edit.
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post #8 of 14 (permalink) Old 11-30-2015, 08:42 AM
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As I understand it, the total alkalinity has two parts (see the picture):

pufry = buffers
kyseliny = acids
uhličitany = (bi)carbonates
hydroxokomplexy = hydroxo-complexes
fosforečnany = phosphates
humusové látky = humic substances

The total alkalinity consists of negatively charged anions and one positively charged cation (H+). While all the negatively charged anions increase alkalinity, the positively charged cation decrease it. So if you have these negatively charged guys in your water your alkalinity will increase. But if you have the positively charged guy in your water your alkalinity will decrease. This I see as a main reason why ADA Aqua Soil decreases the total alkalinity => it contains many H+ ions bound to the negatively charged carboxyl groups in the humus. Once you put the substrate into the water, these H+ ions unbound and decrease alkalinity + pH. On their place in the carboxyl groups other more "capable" cations go (mainly K+, Ca+2, Mg+2, and in a limited amount some NH4+ or heavy metals also). So once you flood the ADA substrate, a great number of H+ guys escape from the substrate into the water. The substrate becomes immediately hungry for other "+" guys, thus eating every K+, Ca+2 or Mg+2 that comes into contact with the substrate. The H+ guys in the water will be immediately neutralized by other guys called HCO3- (the number of CO3-- guys is usually very small in our tanks so we can ignore them). So the H+ guys marry the HCO3- guys creating a neutral H2CO3° guys. [In this reaction H+ guys and HCO3- guys are no longer in this form in your water which means that there is less and less of these guys in water. And because HCO3- guys are the main ingredient of alkalinity, it also means that your alkalinity is decreasing during this process.] Finally, these neutral H2CO3° guys divorce into H2O° (water) and CO2° (carbon dioxide). The H2O° (water) remains in our tank, while the CO2° escapes from our tank into the atmosphere. This process will continue to happen unless all the H+ guys in the substrate are not replaced by K+, Ca+2 or Mg+2 guys. In this process your pH and KH will drop significantly. If you want to speed it up, you can use hard water (with high content of Ca+2 and Mg+2 guys) in your tank. This way all the H+ guys will be replaced by Ca+2 or Mg+2 guys much faster. Also, you can rinse the substrate in hard water (this will make it quite neutral; although the organic part of the substrate will be always there, slowly decomposing and releasing other H+ guys ... but not that many).

What Diana said about "layering" anions on cations is highly unprobable here (or maybe in some very limited way). From what I know nearly all the bounding places on carboxyl groups are occupied by K+, Ca+2 or Mg+2 ions. That's also the reason why it's called CEC = Cation Exchange Capacity. You never see any AEC (Anion Exchange Capacity), as the anions (= negatively charged guys) have very limited ability to bind to organics (as most of the organics have negative charge, thus attract the positive charged guys only). There are some materials that can bind anions, but these are very rare.

Last edited by 58417; 11-30-2015 at 08:55 AM. Reason: [Less HCO3- guys = lower alkalinity]
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post #9 of 14 (permalink) Old 11-30-2015, 09:05 AM
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Quote:
Originally Posted by Audionut View Post
I don't think KH test kits measure alkalinty, I think they do indeed only measure KH.
Side note:
The hobby KH test kits contain usually just a diluted solution of HCl, and a methyl orange colouring. They do not measure carbonate alkalinity (= HCO3- and CO3-- ions content) only, but the total alkalinity (which may be affected by other ions also). Based on the results of these alkalinity tests, hobbyists then wrongly assume that the total alkalinity value is the same as the carbonate alkalinity value (which is needed for calculating the CO2 level from KH and pH).
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post #10 of 14 (permalink) Old 11-30-2015, 09:29 AM
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Thanks for the comments @Marcel G. I know the carbonate buffer system reasonably well, but haven't done any research on the other buffers. It never occurred to me that the level of other cations in the water determined the rate of release of H+, but of course, you never get anything without giving something in return.

Can you share your thoughts on why a solution bottle of KH2PO4 with a PO4 concentration > 13,000 ppm, doesn't register on a KH test kit?

Feel free to edit.
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post #11 of 14 (permalink) Old 11-30-2015, 09:51 AM
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Quote:
Originally Posted by Audionut View Post
Thanks for the comments @Marcel G. Can you share your thoughts on why a solution bottle of KH2PO4 with a PO4 concentration > 13,000 ppm, doesn't register on a KH test kit?
It seems strange, as H2PO4- should react with HCl in the KH test kit. One scenario that comes to my mind is if all your H2PO4- is already in a neutralized H3PO4° state. How exactly do you proceed with the test? If you add just one drop into your solution, and it immediately changes color, that would imply H3PO4 in the solution. If you add many drops into your solution, and nothing happens, then it may simply mean that your solution is too strong, so you need to add much more of the HCl to first neutralize all the H2PO4- before the solution change in color. Don't forget that your KH2PO4 is extremely concentrated while the HCl in KH test is diluted.

Last edited by 58417; 11-30-2015 at 09:58 AM. Reason: typo
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post #12 of 14 (permalink) Old 11-30-2015, 10:17 AM
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The sample goes immediately to the end point color (1 drop). I try diluting the solution 1ml to 29ml demineralised water with the same result. The solution was made with demineralised water (TDS 3 ppm), so very little contaminates.

I've run out of my previous supply of KH2PO4, and I will be using a different supplier in the future. I will try again with the new supply.

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post #13 of 14 (permalink) Old 11-30-2015, 04:38 PM Thread Starter
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From my observations AS buffers completely to an exact pH of 6.40 - 6.45 every time. I add KHCO3 enough to raise KH by 1. the pH raises by almost 0.2, and in less than 6 hours is back to 6.40, and my KH test kit reads 0.5. (use 10mL of water instead of 5mL. 0.5 KH increments).

So from this it would be logical to conclude that as long as there is an external source of acid (this being weak acids being dissociated from the AS), and I add HCO3- in any form, it will actually increase the amount of CO2 (yes CO2) in the aquarium?

H2CO3 + H20 <---> CO2 + H20

Basically as I put in more bicarbonate, the H+ "guys" find a HCO3- and become H2CO3. However, H2CO3 isn't that stable at pH 6.4 and will make CO2.

This would explain why my drop checker will not turn blue. After 12 hours of degas, it's still emerald green. Before i had the AS in there my drop checker would be blue in the morning.

Last edited by Positron; 11-30-2015 at 04:43 PM. Reason: typo
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post #14 of 14 (permalink) Old 12-07-2015, 07:48 PM
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Looking a bit more into phosphate affects on alkalinity readings with test kits.

Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog
Quote:
The precise endpoint of a total alkalinity titration isn't always the same pH, but rather depends a bit on the nature of the sample (both its ionic strength and its alkalinity). For normal seawater, this endpoint is about pH = 4.2. In freshwater it depends strongly on the alkalinity, with an endpoint of pH = 4.5 for an alkalinity of 2.2 meq/L, and pH = 5.2 for an alkalinity of 0.1 meq/L.
So there are some factors that control the endpoint pH of the titration, one of which in freshwater is the concentration of the alkalinity. Higher alkalinity means that the pH endpoint of the titration is lower, and vice-versa.

https://en.wikipedia.org/wiki/Phosph...cal_properties
Quote:
The phosphate ion carries a negative-three formal charge and is the conjugate base of the hydrogen phosphate ion, HPO42−, which is the conjugate base of H2PO4−, the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric acid.......................................Aqueous phosphate exists in four forms. In strongly basic conditions, the phosphate ion (PO43−) predominates, whereas in weakly basic conditions, the hydrogen phosphate ion (HPO42−) is prevalent. In weakly acid conditions, the dihydrogen phosphate ion (H2PO4−) is most common. In strongly acidic conditions, trihydrogen phosphate (H3PO4) is the main form.


It's not possible for the form of phosphate to be in the form of H3PO4, unless the pH is below 5.0. Looking at the above diagram, even at pH 4.0, only a very small fraction of total phosphate is in the form H3PO4. Since the KH test kits do not appear to titrate the sample below pH 4.5, the affects of phosphate on these test kits should be negligible.

Feel free to edit.

Last edited by Audionut; 12-07-2015 at 08:01 PM. Reason: speeling
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