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Thread: Relationship between CO2, KH, and pH

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    Default Relationship between CO2, KH, and pH

    This was written in response to a question over in Water Works regarding the effect of CO2 supplimentation on KH. This afternoon I realized it would be a good idea to put it here too.

    Enjoy!





    To answer the question "Is KH affected by adding CO2 to an aquarium?"

    _no_

    KH is the measurement of Carbonate (CO3 2-) in the water and that measurement isn't going to be affected by the amount of CO2 in the water. Your KH remains stable as long as the CO3 molecules aren't broken down, such as happens during Biogenic Decalcification, which will actually _raise_ the KH.

    Carbonate (CO3 2-)alone cannot be used for Biogenic Decalcification as it contains no Calcium, only Carbon and Oxygen. Biogenic Decalcification only occurs in environments high in Calcium Carbonate (CaCO3) and severly deficient in Carbon (organic or inorganic). The plants will absorb the CaCO3 and break the molecule down to get at the Carbon atom. The result is Calcium deposits on the leaves and Oxygen. In a system where Carbon is available readily (CO2 gas or organic carbon in the environment) the plants will use that instead. Biogenic Decalcification is much much more energy costly than breaking the O-C bond found in CO2.

    Statement 2:
    Yes, your wrong! c02 has a big impact on the kh and ph.

    Not quite. CO2 along with KH have an impact on pH. More on that later.

    If your c02 unit is pumping to much c02 into your tank it is causes a reaction kh and the kh drops!

    Read below.

    Aquarium with very low kh are subject to rapid ph shifts, and this is what kills fish.

    Yes and no. Stress caused by large pH shifts can lead to disease which can kill fish. The drop in pH is caused by increased amounts of Carbonic Acid in the water. That Carbonic Acid is dissolved CO2. Increasing the concentration of an acid in solution lowers the pH. Therefore, as the pH lowers, the concentration of dissolved CO2 increases which will suffocate your fish.

    On a kh test kit you will see on the chart its 1-16 dkh and from 17.9-286.4 ppm kh.

    KH and dKH are two totally different things. KH is Carbonate Hardness and is commonly referred to as the Alkalinity (opposite of Acidity) of the water. dKH is what most Europeans use to refer to what we in North America call General or Total Hardness (GH). GH is the measurement of dissolved Calcium and Magnesium in an aquarium. It's very important to not get those two mixed up.

    Check this link out for more information:
    http://www.fishdoc.co.uk/water/hardness.htm


    Here's a discussion of water chemistry regarding the relationships between CO2, CO3, pH, and KH. This is the chemistry of an aquarium where CaCO3 and CO2 are supplimented and are in excess for purposes of chemical reactivity.

    Don't forget the states:
    (g)=gas, (s)=solid, (l)=liquid, (aq)=aequeous solution
    -->/<-- denotes chemical equilibrium


    CO2 is added to water creating Carbonic Acid:
    CO2(g) + H2O(l) --> H2CO3(aq)

    Being an acid H2CO3 will dissociate and release a Proton(H+, a Hydrogen atom without the electron) into the solution forming Bicarbonate (HCO3-)
    Here's the dissociation equation:

    H2CO3(aq) --> HCO3-(aq) + H+(aq)

    CaCO3 is added:
    CaCO3(s) + H2O(l) --> Ca+2(aq) + CO3 2-(aq)

    We now have in solution an acidic Proton (H+), Carbonic Acid (H2CO3), Calcium (Ca), and Carbonate (CO3 2-). These four substances will react to form and aqueous solution containing Hydronium Ions, Calcium ions, Calcium Bicarbonate, and Carbonate.

    H2CO3(aq) + CaCO3(aq) -->/<-- H+(aq) +Ca 2+ (aq) + Ca(HCO3)2(aq) + CO3 2-(aq)

    This actually results in the increase in the concentration of Carbonate in solution for a moment. In this system Carbonate is the conjugate base of Carbonic Acid. Those free acidic protons are quick to react with the free Carbonate to form Bicarbonate and Carbonic Acid again.

    H+(aq) + CO3 2-(aq) --> HCO3-(aq)

    or

    2H+(aq) + CO3 2-(aq) --> H2CO3(aq)

    Therefore, we have a buffered solution, which is why we use CO3 to buffer our pH. Keep in mind, this is nearly instanteous so there will be no change in KH measurable by our hobby test kits.

    What we have here is a weak acid-weak base titration which will continue as long as we keep adding our acid. As we add more CO2 to our artificial environments the concentration of Carbonic Acid increases until the concentration of CO3 2- in solution (KH) is no longer able to compensate and the pH goes down.

    The relationship is lograthmic, here's a link to a great visual of the correlations.

    http://www.sfbaaps.com/reference/table_01.shtml

    I hope that's answered some questions.


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    Default Re:Relationship between CO2, KH, and pH

    Excuse me, Phil; but kh measures alcalinity; therefore Kh indicates CO3=, HCO3-, (OH)-, .... anions in the water.(No only CO3=).

    If you dissolves (by example) Ca(oh)2 in a destilated water cup then kh goes up. (there are´t CO3= anions in solution).
    If you put amoniac in a glass of water Kh goes up (because NH4(OH) is formed)

    A second thing: Carbonate calcium dissolves in water very bad; it goes in solution by first changing in calcium bicarbonate in the presence of carbonic acid. This soluble bicarbonate rises KH (no CO3=(aq))

    In typical aquarium water the measurement of KH is produced by bicarbonates in solution.

    Please,excuse me my english.

    Regards, Juan Carlos.

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    Default Re:Relationship between CO2, KH, and pH

    Juan Carlos,

    "Excuse me, Phil; but kh measures alcalinity; therefore Kh indicates CO3=, HCO3-, (OH)-, .... anions in the water.(No only CO3=)."

    Actually, KH is a measure of Carbonate in the water. Unfortunately in the hobby the term "Alkalinity" has been used to describe both pH above 7 _and_ KH. The true meaning of the term Alkalinity refers to a pH above 7.0. However, since CO3- is a base it tends to increase the pH in the absence of acids which is why people mistakenly use Alkalinity in conjunction with KH.

    HCO3- is actually amphoteric and can act as both a base and an acid. Since it dissociates so easily it's not usually around long enough, or in enough quantity to be measurable.

    Similarly, OH- is such a strong base that it quickly bonds with positive molecules and protons and is neutralized in short order. Therefore our test kits don't pick it up.


    "If you dissolves (by example) Ca(oh)2 in a destilated water cup then kh goes up. (there are´t CO3= anions in solution)."

    Actually, the pH goes up, not the KH. The GH (also known as dKH) will increase due to the increased amount of Calcium in solution.

    "If you put amoniac in a glass of water Kh goes up (because NH4(OH) is formed)"

    Actually, in this reaction you'll get both NH4(OH) and NH3 and H2O. Ammonium is a weak acid so 99.999% of the time it will donate it's proton to OH-, which is a strong base, and will become Ammonia (NH3). There will be some very slight amount of Ammonium Hydroxide in solution, but it is so minute that it's considered negligable for the purposes of the chemistry of the system.


    "A second thing: Carbonate calcium dissolves in water very bad; it goes in solution by first changing in calcium bicarbonate in the presence of carbonic acid. This soluble bicarbonate rises KH (no CO3=(aq))"

    You're right, it's a slower reaction than most of the other salts of the Alkaline Earth Metals but it still completely dissociates. Because of the charges associated with the CaCO3 molecule it's not possible to get Calcium Bicarbonate. The 2+ charge on Ca and the 2- charge on Carbonate balance eachother out. It won't react with other atoms/molecules until it's dissociated and that's when you get Bicarbonate. With the increase of (Bi)Carbonate the KH increases.


    "In typical aquarium water the measurement of KH is produced by bicarbonates in solution."

    Yes, you're right. Bicarbonate is more common in our aquariums than Carbonate because of all the free chemicals floating around in the solution. However, KH is still a measure of Carbonate. Our titration based test kits isolate the Carbonate molecule and measure the amount of it in the testing solution.

    "Please,excuse me my english."

    Tus Ingles es mas bien que mis Espanol.

    Regards,
    Phil

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    Default Re: Relationship between CO2, KH, and pH

    good little write up. However I never heard anyone calling GH = dKH. Most people I talked to (european and UK sionce I am in UK) called KH dKH and gh dGH - its literally just a degrees of measurement instead of a ppm measurement.

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    Default Re: Relationship between CO2, KH, and pH

    the relationship is interesting but agree with kingkano, in asia as well, never heard of GH=dKH.

    dKH is simplified as measurement of temporary hardness[bicarbonates mostly and carbonates] as kH numbers.

    dGH numbers as measurement of permanent hardness[mostly dissolved magnesium and calcium salts]
    GH numbers for simplified layman people like me.
    Stan Chung

    simple but not easy

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    Default Re: Relationship between CO2, KH, and pH

    Usually when we want to raise kH more directly, we can add sodium bicarbonate.
    kH will be increased, even in distilled water, no?
    My Sera test kit says my dkH is so high after adding a chunk of sodium bicarb to a half cup RO water that I stopped at 10 dkH.
    And I should think Sera is German.
    Isn't this because the tests measure not the calcium carbonate, but the equivalent of so much calcium carbonate?

    In this set of equations given, there seems to be no answer indicated as to why pH should continue to drop as time goes on, as people are concerned with when they buffer.
    Dave
    Last edited by raglanroad; 01-24-2006 at 10:53 PM.

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    Default Re: Relationship between CO2, KH, and pH

    Quote Originally Posted by Juan C. Nieto
    Excuse me, Phil; but kh measures alcalinity; therefore Kh indicates CO3=, HCO3-, (OH)-, .... anions in the water.(No only CO3=).

    If you dissolves (by example) Ca(oh)2 in a destilated water cup then kh goes up. (there are´t CO3= anions in solution).
    If you put amoniac in a glass of water Kh goes up (because NH4(OH) is formed)

    A second thing: Carbonate calcium dissolves in water very bad; it goes in solution by first changing in calcium bicarbonate in the presence of carbonic acid. This soluble bicarbonate rises KH (no CO3=(aq))

    In typical aquarium water the measurement of KH is produced by bicarbonates in solution.

    Please,excuse me my english.


    Regards, Juan Carlos.
    Actually, KH is a measure of Carbonate in the water. Unfortunately in the hobby the term "Alkalinity" has been used to describe both pH above 7 _and_ KH. The true meaning of the term Alkalinity refers to a pH above 7.0. However, since CO3- is a base it tends to increase the pH in the absence of acids which is why people mistakenly use Alkalinity in conjunction with KH.
    Regards,
    Phil

    Phil,
    I think your article was mostly excellent. However in this regard Jaun Carlos is correct and it is you that have it wrong. Alkalinity is correctly used to describe carbonate hardness not high pH. High pH is usually caused by high carbonates and is why there is confusion in this regard. As an example this statement is true "It is possible to have high alkalinity and low pH."
    Also Europeans use KH and GH exactly as we North Americans do (actually both KH and GH were invented by Germans). dGH in it's truest sense actually means Deutsch General Hartten basically German General Hardness and we use the K in KH because in German carbonates are spelled Karbonates, and as I'm sure you've guessed, KH means Karbonate Hartten.
    Again I think your article was excellent and I apollogize for the corrections.

    Larry

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    Default Re: Relationship between CO2, KH, and pH

    CO2, pH and alkalinity from carbonates

    CO2, ph and carbonates are all relatedby the following 3 equations.
    1. CO2+H2O<----->H2CO2 (cabonic Acid)
    2. H2CO2<----->H+HCO3- (bicarbonate)
    3. HCO3<----->H+CO3 2-(carbonate)
    Example of how increasing cabonates will either raise the pH or need counterbalancing CO2 added to maintain the pH level.
    If NaHCO2 is added to aquarium water the additional carbonate ions will cause a shift to the left side of the equation (2) This will form more carbonic acid extracting a H+ ion and thus raising the pH. The additional carbonic acid will drive equation (1) to the left creating CO2 which will dissapate out of solution bringing equalibrium at a higher pH. In order to maintain equalibrium at the original pH (1) and (2) must be shifted back to the right. This can be accomplished with the addition of more CO2 into the aquarium
    The higher the amounts of carbonates/bicarbonates in the aquarium the more CO2 is needed to maintain a specific pH.

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    Default Re: Relationship between CO2, KH, and pH

    Hey,
    I'm trying to better understand the co2 injection systems.
    For a planted tank you need a co2 reading of 20-30ppm
    Now looking at the co2/ph/kh chart in order to have that you need
    ph 6.8-7.0 and kh 5-6dh (89-107 ppm)
    to my simple understanding, higher kh means more co2 needed to lower the ph there fore more co2 in your water, correct?
    So if my water is 1.5kh (27ppm) i would need my ph at 6.4 hich is way to low for me.
    So is there a way to increase kh? like um, something you can put in your filter to increase it?
    I do what I must do, to do what I wish to do!
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    Default Re: Relationship between CO2, KH, and pH

    Baking soda (sodium bicarbonate) will increase KH, also the relationship between PH, KH and the amount of dissolved Co2 is iffy at best. Any amount of Phosphates for example will alter the results. The best way to know how much co2 you have in your tank is to use a drop checker with a stable KH solution. I like to use a 4dKH solution for example. Also Co2 does not change your KH it only changes the PH of the water.

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    Default Re: Relationship between CO2, KH, and pH

    Dissolved CO2 should be more like 15 ppm for a planted tank so, for a pH range of 6.8 to 7.0, you'd be targeting dKH ~ 3 to 5 (lethal range for fish starts at about 40 ppm so I'd think 30 ppm is starting to get uncomfortable). Also, warmer water has a diminished capacity to hold dissolved gasses so the more CO2 you do manage to get into solution, the less O will be available.
    I know the human being and fish can coexist peacefully.

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    Default Re: Relationship between CO2, KH, and pH

    Quote Originally Posted by kush View Post
    Dissolved CO2 should be more like 15 ppm for a planted tank so, for a pH range of 6.8 to 7.0, you'd be targeting dKH ~ 3 to 5 (lethal range for fish starts at about 40 ppm so I'd think 30 ppm is starting to get uncomfortable). Also, warmer water has a diminished capacity to hold dissolved gasses so the more CO2 you do manage to get into solution, the less O will be available.
    There's no relationship between Co2 and O2. Having more or less co2 does not decrease or increase the amount of O2 dissolved in the water. What causes problems with fish at high levels of Co2 is their gills aren't able to expel co2 into the water as easily with high levels of co2 already dissolved in the water. Also 15ppm Co2 is not going to cut it in most high light planted tanks, specially ones with 3wpg or more. At 15ppm co2 your going to end up with lots of algae problems. I keep my co2 between 24-30ppm with a water temp of 85-86 with no problems. None of my fish show any signs of stress. Discus can handle up to 45ppm co2 even at high temps. Of course anything above 30ppm co2 is really unnecessary as the plants won't even use it.

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    Default Re: Relationship between CO2, KH, and pH

    is there another way to increase your kH besides baking soda?
    I've tried it before and it works, but for a short time. Isn't there a solid material i can place in filter to increase it? Crushed coral? that's to increase dH am i right? Will it effect kh?
    I do what I must do, to do what I wish to do!
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    Default Re: Relationship between CO2, KH, and pH

    Any material containing Carbonate or Bicarbonate will increase the alkalinity of your water over time. Dolomite and Calcite rock are good choices for long term use. You can buy limestone (calcite) and try that, but I'd recommend getting a small piece of reef rock without any algae to avoid impurities. Seachem's Gray Coast Calcite is good as well.
    I'm not sure what Im looking at, but its huge and I think its going to be cool!

    Aquatic Gardeners Association
    www.aquatic-gardeners.org

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    Default Re: Relationship between CO2, KH, and pH

    Quote Originally Posted by ejhart View Post
    There's no relationship between Co2 and O2. Having more or less co2 does not decrease or increase the amount of O2 dissolved in the water...
    There is no direct relationship. The point is, that there is a limit to the amount of gas that can be held in solution, and that that limit decreases as water temperature increases.
    I know the human being and fish can coexist peacefully.

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