rosse

Hello everyone,

In few sites (for example: http://www.chelonia.org/Articles/waterchemistry.htm) i've read that in order to lower KH, you can do so by injecting CO2 in your aquarium water. In others it says that injecting CO2 doesn't affect KH (for example: http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm)

But this is how i understand this: KH is concentration of carbonate and bicarbonate ions, and when you add CO2 into the water, some of it combines with water to form carbonic acid:

CO2 + H20 <-> H2CO3

Reaction above progresses from left to right faster because of our constant addition of CO2 into system.

So, H2CO3 then dissolves into H+ + HCO3-, or 2H+ + CO3--. In both cases, we get more H+ ions (a drop in pH), but more carbonate or bicarbonate ions, too, which should INCREASE KH.

I understand that addition of acids consumes carbonate ions. For example if we added HCl into water, it would dissolve to H+ and Cl+, and H+ would then form H2CO3 (and eventually H20 + CO2?) with carbonate ion in water, lowering KH..

But I see carbonic acid as an exception, becaue adding CO2 actually produces two extra H+ ions and one extra CO3-- ion.

So why some say that KH drops with CO2, some say that KH levels don't change with CO2 injection, and I think (for the reasons listed above) that KH should rise in conjunction with pH. Could anyone explain this to me? Where am I wrong? Thanks!
Simas

JamesC

I believe you are right. This is exactly how I see it as well in that adding CO2 increases KH.

James

fabry

Hi Simas,
it is impossible to answer your question in a concise yet exaustive way.
Nevertheless I try to do my best. (Anyhow I see from your reasoning that you are quite close to solution )

First of all let me point out that what you are calling KH is not really KH but Alkalinity (the two are different things even if we are used to refer to Akalinity by calling it KH).

So let me continue here to call it Alkalinity (because you are referring to that) and not KH.

Alkalinity is not just "concentration of carbonate and bicarbonate ions” but it is related to buffer capacity of the water defined as capability to “neutralize” H+ ions.

So, any chemical species able to neutralize H+ ions contributes positively (increases Alkalinity) and H+ ions themselves contribute negatively (decrease Alkalinity).

Several ions in this way can raise Alkalinity.

For example negatively charged ions of weak acids like acetate or bicarbonate.

These weak acids react with H+ ions by incorporating them and since they are weak acids they do not dissociate entirely to give back all the H+ they reacted with.

In this way they can lower the H+ content of water and thus are contributing to Alkalinity by raising it.

For example with the acetate ion you can write:

CH3COO- + H+ => CH3COOH

You can see from this reaction that acetate captures H+ ions to form Acetic acid.
Acetic acid is only slightly dissociated so once it captured the H+ ions it doesn’t give them back to water.

So if you have water with acetate ion in it you could add a certain amount of H+ before seeing a change in the pH (as acetate captures H+ ions as you add them).

In this case Acetate ion is buffering the water and is a part of its buffer system that is called Alkalinity and that we can measure with our aquarium testers called “KH test” (which in the reality are Alkalinity testers).

So a general expression for Total Alkalinity, keeping into account all the possible contributors could be written more or less as:

Total Alkalinity (meq./l) = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [SiO(OH)3-] + [MgOH+] + [H2PO4-] + 2[HPO4- -] + 3[PO4- - -] + [CH3COO-] - [H+]


In this equation you will see several ions contributing positively and the ion H+ contributing negatively to total alkalinity.


In the waters of our interests however most of these terms can be disregarded for a variety of reasons and our formula becomes simply:

Total Alkalinity (meq/l) = [HCO3-] + [OH-] - [H+]

And if we are close to neutrality (pH=7):

Total Alkalinity (meq/l) = [HCO3-]

So in the waters of our interest Total Alkalinity is mainly dependent on its Bicarbonate content (as we already know)


This said let’s come back to your question.

From what we said and looking at the formula above we can say that:

If you dose bicarbonate into the water you increase total Alkalinity.

If you dose some H+ ions you decrease total Alkalinity


If you dose CO2 into water that will form Bicarbonate ions and H+ ions according to the following reactions:

1) CO2 + H2O <=> H2CO3
2) H2CO3 <=> H+ + HCO3-

Then you see that supplying CO2 you are introducing equal amounts of bicarbonates (HCO3-) and H+

According to the equation:

Total Alkalinity (meq/l) = [HCO3-] + [OH-] - [H+]

you will see that:

The equal amounts of HCO3- and H+ added can be cancelled and then:

Total Alkalinity remains unchanged

That’s why when we introduce bicarbonate ions (as Sodium Bicarbonate for example) we raise Alkalinity, but when we supply CO2, Alkalinity remains unchanged

Hope I have been clear enough.

My regards,

Fabrizio.

freydo

fabry,

that is an awesome explanation... very easy to understand by the non scientific crowd.

fabry

Hi Freydo,
Thanks.

The big difficulty with this matter infact is to keep it simple.

Alkalinity in particular is still so controversial under some aspects (how to best define and measure it for example) that scientists still debate it and organize from now and then international symposiums to revise the argument.

Regards,

Fabrizio

rosse

hello, fabry!

Thank you for your time and explainations! It was really helpful, and now that I'm doing some research and learning more about the basic chemistry in aquarium water it's making more sense!

So, if I understood it correctly, I was right that injecting CO2 raises KH by adding bicarbonate ions to water, but since equal amounts of H+ ions are being added, total alkalinity doesn't change.

The main confusion about this probably came from terms KH and Alkalinity being used interchangeably in aquarium keeping community.

Thanks again!

fabry

Yes and no.

It is not correct to say that injecting CO2 raises KH.

KH is carbonatic hardness and, being a hardness, it has to do with Calcium and Magnesium as much as (and even more than) with Bicarbonates.

I try to explain.

Total hardness (GH) is the total amount of Calcium and Magnesium in water.

It can be divided in two parts:

Carbonatic hardness (KH)= the amount of Calcium and Magnesium that we can think balanced (in eq/l) by bicarbonates

Permanent hardness = the amount of Calcium and Magnesium that we can think balanced (in eq/l) by all the other anions (Cl-, SO4--, NO3- etc.)

Carbonatic hardness can be brought to zero just boiling the water; since thermal energy breaks down Bicarbonates into Carbonates and CO2 (that evaporates).
Carbonates instead precipitate with Calcium and Magnesium (as Calcium and Magnesium Carbonates are almost insoluble in water) and can be eliminated (filtered out for example).

For this reason Carbonatic Hardness (KH) is often referred to as "Temporary Hardness"

What remains after the boiling is the residual amount of Calcium and Magnesium that can be considered balanced by other anions.

This residual amount cannot be removed with such simple tricks and is therefore defined as "Permanent Hardness".


In this way it is always: Total Hardness (GH) = Temporary Hardness (KH) + Permanent Hardness.


So now I hope is clear the concept of KH.

Practically there is no easy and straightforward way to measure KH; nor we are particularly interested in measuring this parameter.


What we call KH is instead another parameter (much more important for us) which is Alkalinity (also called Buffer Capacity of water).

The liquid colorimetric tests we use and call KH tests are instead Alkalinity tests.


So, now that is clear (I hope) what KH is, is easy to understand that there is no way we can increase KH by adding just CO2 or just Bicarbonates into water.

For increasing KH we should add instead Calcium and/or Magnesium Bicarbonates.


Thus, adding just Bicarbonates we raise instead Alkalinity!

In partcular:

1) If we add Sodium or Potassium Bicarbonates we raise just Alkalinity (besides obviously Sodium or
Potassium concentration)

2) If we add Calcium or Magnesum Bicarbonates we raise at once Alkalinity, KH and GH (besides
obviously Calcium or Magnesium concentration)


It has to be pointed out that we could rise Alkalinity also by adding some other weak acids like acetic, fulvic, citric, gluconic etc.

All of these will be red by our colorimetric Alkalinity (KH) tests.

That's why in waters with a high content of such acids (from peat extract for example) we cannot rely on the reading of our Alcalinity (KH) tests to calculate CO2 concentration (from equations or tables and in conjunction with pH value).

CO2-Bicarbonates-pH relationship always holds true;
but in these cases we cannot know Bicarbonates concentration because our tests give us Alkalinity (which in these cases will be an overextimation of just Bicarbonates).



So,...Yes; you are right regarding the CO2, Bicarbonates and Alkalinity relationship.
Adding CO2 raises Bicarbonates, but not Alkalinity nor KH.

Most of the confusion comes indeed from using the terms "KH" (or carbonatic hardness) and "KH test" instead of the correct ones "Alkalinity" and "Alkalinity test"


Hope I helped.
Cheers,

Fabrizio

intothenew

Awesome write-up, and not in your native tongue.

JamesC

Thank you fabry for such an excellent explanation. I can now see where I was going wrong as I presumed that as bicarbonates were produced by adding CO2, this meant that the KH increased.

James

autorhino

Hi Thanks for the explanation !!
But can you tell me if this make sense with your explanation ..
When testing my water i add -- 1 dKH and 5,6 dGH with PH 7,1 no CO2 injection
Started CO2 injection and after 3 days i have 3dKH and 7,8 dGH with PH 6,8
Is the shift in KH and GH test are from CO2 injection or they came from something else in my thank ?
From what i understand even if the total Alkalinity haven't change the KH and GH are reporting the increase of available bicarbonate and carbonate ?
Am i making sense ?
thanks
Alain