# KH, PH, and CO2



## aquabillpers (Apr 13, 2006)

Is it true that the higher the KH, the less dissolved CO2 will be in the water column, all other things being equal? (This excludes aquariums with injected CO2, of course.)

If it is true, it would seem to follow that the PH would be higher in a tank with high KH. Then, the PH could be lowered by lowering the KH.

Right?

Bill


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## BryceM (Nov 6, 2005)

This is an old chemistry question that keeps coming up. In a nutshell the three are related, but knowing one doesn't mean you can figure out the other two. A high KH is a high KH, period. Water with a high KH tends to have a high pH, all other things being equal. CO2 levels are independent of KH. You can dissolve CO2 just as easily in water with a KH of 1 as a KH of 30.

In pure water with only carbonate and CO2 (which does not represent an actual situation) a high KH, and low pH would imply lots of CO2.


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## flagg (Nov 29, 2004)

aquabillpers said:


> Is it true that the higher the KH, the less dissolved CO2 will be in the water column, all other things being equal? (This excludes aquariums with injected CO2, of course.)
> 
> If it is true, it would seem to follow that the PH would be higher in a tank with high KH. Then, the PH could be lowered by lowering the KH.
> 
> ...


Lowering the KH should lower the pH too. If I remember an article from thekrib.com correctly, as the acids that are the by-product of nitrification break down they release more H+ into the water which lowers the pH more quickly since there are less carbonate and bicarbonate ions in the water for the free H+ ions to bind with. Of course, if the KH is too low then those carbonate ions would be used up more quickly until the pH crashes.

Question remains, just how much can the KH be lowered to reduce pH before you end up with a pH crash? Also, how much will the pH be lowered by a lowering of KH?

-ricardo

PS Another question came to mind, how would one go about lowering the KH? I guess you could peat filter, but that's ineffective, messy and time consuming as the peat needs to be replaced every couple of days. I guess you could dilute with RO water too....ds


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## Red_Rose (Mar 18, 2007)

The KH from my tap water is 15dKH and my pH is 7.8 yet in my tank, before I had added peat to the filter, my pH was at 8.2 and my KH was over 20! Once the peat was added to the filter, my KH had lowered down to 13dKH but my pH had still remained at 8.2. I don't know why my pH won't budge from there.

Flagg, what is messy about adding peat moss to a filter? I've never had a mess from it before.


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## david f (Mar 24, 2006)

Hi there ,

I am also working on trying to understand this CO2 + H2O <> H2CO3 <> H+ HCO3 <> CO32- + 2 H+ chemistry of the alkalinity's pH buffering system (pg 92 of Diana Walstad's book),would like to learn more about this.It sounds like there must a balance ,of certain relative proportions Of CO2 ,bicarbonates (baking soda KH ,alkilinity)And carbonates .get the balance of all these and minerals etc, to get the right water chemistry just right.


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## david f (Mar 24, 2006)

your GH may be holding the pH up


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## BryceM (Nov 6, 2005)

GH does not affect pH at all.


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## Squawkbert (Jan 3, 2007)

Elevated KH means that it takes _more _CO2 to get the pH to move down, because the KH is buffering the system. How ever much CO2 you have in thewater column is just that, and it is independent of KH. The CO2 concentration will alter your pH, more if you have low buffering capacity (partly a function of KH), less if you have lots of buffering capacity (higher KH).

Altering your pH will not alter your CO2 concentration.


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## aquabillpers (Apr 13, 2006)

Thanks to all of you.

I now finally understand that the amount of dissolved CO2 in a non-CO2-injected aquarium is not a function of the KH. The amount of CO2 in such a tank is constant.

So my next question will be about the effect of pH on plants, in a new thread.

Bill


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## Squawkbert (Jan 3, 2007)

aquabillpers said:


> The amount of CO2 in such a tank is constant.


Provided you have some surface agitation or an airstone, it will be a constant 3ppm (give or take). Without some agitation to encourage gas exchange at the surface, it can actually drop below that value in a planted tank.


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## Diana K (Dec 20, 2007)

I have Soil Master Select as a substrate in several tanks. This stuff will remove KH, and the pH drops. 

Tap water: KH 5 deg. pH 7.5-9 (Yes, it varies. Water company adds something. KH is quite stable, 4-5 degrees year round)

Tank a week after a water change: KH 0 deg. pH 6.0

Tank, after adding 1 tablespoon of baking soda. Tank is 29 (American) gallons, with enough substrate, rocks, wood etc. that there might really be about 25 gallons or so. 
KH 2 deg. pH 6.4
Within a day or so the SMS has removed the carbonates, and the tank is back to 0 deg. KH and pH 6.0

After about a year or so the SMS quit removing the KH quite so fast. 
Readings from last week: KH 4, pH 7.2 (tap water was KH 5 deg, pH 7.5)

I also have peat moss in the filter of this tank, but I have other tanks with and without peat and SMS. The results above are due to the SMS a lot more than because of the peat moss.

Peat moss, no SMS: KH 4, pH 6.5

GH is not really test-able, I add Epsom salt for magnesium, and Equilibrium to some tanks, and these keep the GH pretty close to 5 degrees or higher.


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## BryceM (Nov 6, 2005)

Knowing the GH of your aquarium is probably less important than knowing the KH, but it's actually quite easy to test for. There are several good kits on the market.

If you like spending money, Lamotte even makes one that will tell you the toal GH and the proportions GH arising from Ca & Mg.


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## Red_Rose (Mar 18, 2007)

What would the KH normally be for a pH of 7.8?

I had contacted our city's water company to find out more about our water and they don't make any changes to the water so aside from adding things like chlorine, etc., the pH, GH and KH are all untouched. The pH in our town varies in different areas from 7.0-7.8 but both the GH and KH are very high because the water source comes from an area where there's a lot of limestone. For a pH of 7.8, I just find it odd that the KH would be so high.


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## david f (Mar 24, 2006)

I wonder if that is why calcium carbonate is soluble at a very slow rate, (3ppm co2)


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## Marcel G (May 29, 2012)

It's a little old discussion, but in case someone is interested in the logic of how CO2, pH and alkalinity all relate, you can have a look at the following page, where I tried to do some interactive CO2:HCO3:CO3 calculator with some comments:

http://www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2

I hope it helps to understand what happens when acid is added to our tank (H2CO3, HCl ...), and why alkalinity is not changing with CO2 addition. I work on the translation of the first part (Chemistry #1) also, where I'll explain in deep what's the water hardness, alkalinity, pH, and how CO2 behave in the water.

Marcel


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