dkH drops with time?

Discussion in 'Water Chemistry' started by davideyre, 13 Mar 2008.

  1. davideyre

    davideyre Member

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    looking on the tropica website:

    www.tropica.com/article_fullscreen.asp?type=news&id=683

    it shows a graph at the bottom of the page showing that in a planted tank dkH drops with time. why does this happen? my local water has a dGH of 15, does this mean i can expect that to fall with time in my planted tank? by how much? why does this happen?

    thanks.
     
  2. JamesC

    JamesC Member

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    Without knowing what substrates were used it's hard to say. What they are trying to say is that their substrate does not affect the water's KH. Some substrates will raise KH.

    James
     
  3. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    James, correct me if I'm wrong but I believe there is a low tech, non-injected scenario in which plants are able to fix carbon by extracting it from the Carbonates and Bicarbonates in the water column. This would lower the buffering capability and lower the kH slowly over time.

    In any case, 15 GH is only indirectly related to the dKH of the water as it is possible to have a high general hardness while having only a mildly high alkalinity. GH is essentially a measure of the Calcium and Magnesium content, while KH is more related to the Carbonate/Bicarbonate content.

    davideyre, I mention this because there seems to be an inconsistency as your question is regarding kH, yet you give the value of your tap water's GH not it's kH.

    Cheers,
     
  4. JamesC

    JamesC Member

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    Very true. When CO2 levels are very low some plants can extract carbon from bicarbonates in the water which would have the effect of lowering KH.

    James
     
  5. Ed Seeley

    Ed Seeley Member

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    If water changes aren't very frequent the KH can also drop due to the products of nitrogen metabolism and the processing from the bacteria.
     
  6. Moss Man

    Moss Man Member

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    Yes, biogenic decalcification I believe, Vallis is a typical plant species which use this.
     
  7. davideyre

    davideyre Member

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    thanks for the replies,

    so in the end vallis lowers both the kh and gh by removing calcium carbonate/bicarbonate from the water! thanks though for clarifying my original post which was not entirely clear. had the gh of the water to hand, but not the kh, which was why it ended up getting included.

    thanks.
     
  8. beeky

    beeky Member

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    Would the production of acids in the water lower KH?
     
  9. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    Hi beeky,
    Acidity is essentially the number of H+ ions contained in the water. kH is the number of carbonates (CO3--) and bicaronates (HCO3-) contained in the water. Adding more acid will cause bonding with the carbonates/bicarbonates but the number of carbonate/bicarbonate molecules are still there so the kH won't change.

    Cheers,
     
  10. JamesC

    JamesC Member

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    This isn't quite correct. Adding an acid to the water WILL lower the KH.

    In water there is an equilibrium between acid and carbonates that can be shown like this:
    CO2 + H2O <---> H2CO3 <---> H + HCO3

    If you add more acid (H+) then the equilibrium is changed and so more H2CO3 is produced which in turn produces more CO2 and H2O. As more acid is added more H2CO3 is produced at the expense of HCO3 which reduces.

    This is the same behaviour that is seen with tanks that have no water changes done when the pH and KH drop. The reason being that nitrate (nitric acid) builds up which uses up the HCO3 (bicarbonate) and so KH and pH drop.

    Hopefully this is correct as it's done off the top of my head.
    James
     
  11. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    OK, but this H+ addition happens every day when the CO2 goes on and we have 4 Kh water in the drop checker no? As the carbonic acid is added to the sample the ph drops and we are "detaining" or occupying CO3/HCO3. But when the gas goes off H+ disappears and the CO3/HCO3 liberated again, right? These ions haven't gone anywhere, just captured. Seems like the bonding to H+ is just as transient as the appearance of H+. I guess at some point you can add enough acid to completely occupy all CO3/HCO3 but the kH measurement intrinsically describes the ability of the water to resist pH drop, which it does until overwhelmed by acid addition. Are we saying that the drop checker water has a different kH when it turns green/yellow than when it's blue? I'm a bit unclear on this. :wideyed:

    Cheers,
     
  12. JamesC

    JamesC Member

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    Another way of showing the equation is as follows:
    CO2 + H2O <---> H+ + HCO3-

    If we add more H+ the equation becomes unbalanced and so to become balaced again more CO2 and H2O has to be produced. This has the effect of lowering the HCO3-. The only way to get the HCO3- to increase is to either add more HCO3- or to add CO2 which has the effect of moving the equilibrium in the opposite direction as before. Most people don't realise that adding CO2 to water actually increases KH.

    We don't actually add carbonic acid to the tank but CO2. Dissolved CO2 is in equilibrium with carbonic acid which is equilibrium with H+ and bicarbonate which in turn is equilibrium with carbonate.

    Not sure if that makes any sense to anyone, but if it doesn't there are some good articles all about KH on the Krib website.

    EDIT: Just done some searching to see if there was anywhere that could explain it a lot better than me. Found this article on the Krib which covers it a bit - http://www.thekrib.com/Plants/CO2/khgh.html.

    James
     
  13. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

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    OK, got it. Found the set of three equations you referenced:

    1. CO2 + H20 <------> H2CO2 (Carbonic Acid)
    2. H2CO2 <------> H+ + HCO3- (Bicarbonate)
    3. HCO3- <------> H+ + CO32- (Carbonate)

    Adding CO2 drives equation 1. to the right which drives 2. and 3 to the right producing more carbonates and bicarbonates. This yields a higher kH. This makes sens in that if I were to measure the KH during CO2 saturation the kH would have to be higher in order for the ph/kH/CO2 table to work.

    I couldn't find a similar set for the nitric acid production due to nitrogen cycle but found this extract from The Skeptical Aquarist:

    The nitrification cycle, in which ammonia is eventually metabolized to nitrate, has an additional side effect that generates more acid: the bacterially-oxidized molecule of ammonium finally produces--— in addition to nitrate--— a molecule of water and two protons (H+). Those dissociated H+ ions released into the water additionally lower the pH. Since the nitrifying communities also contribute CO2, they are an essential part of bio-acidification...

    Nitrous and nitric acids are also produced when cellulose is decomposed by a consortium of fungi and bacteria in the biofilm. As soon as the nitro groups are detached from the cellulose polymer during decomposition, they combine with water to form nitrous acid (HNO2), which is rapidly metabolized by nitrifying bacteria to nitric acid.


    It seems the difference between carbonic acid addition and nitric acid addition is that the nitric acid production contributes only H+ lowering pH and occupying whatever free carbonates are still available so the kH also drops, whereas the CO2 addition yields extra free carbonate/bicarbonate which raises the kH. Does this sound right?

    Cheers,
     
  14. davideyre

    davideyre Member

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    sounds good to me. if NH3 ammonia is oxidised to NO3 nitrate, then 3 H+ ions are potentially produced, which shift the equilibrium in the CO2 + H20 <=> H2CO3 <=> H+ + HCO3- equation towards the left and thus lower the dKH.

    so answering my original question, dkH drops with time in a closed system:
    1. due to the nitrogen cycle
    2. due to plants with ability to extract carbon from bicarbonate/carbonate

    thanks everyone.
     
  15. beeky

    beeky Member

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    Presumably there are other acids produced in a closed system due to the breakdown of plant material, otherwise peat wouldn't be so acidic. I'm assuming tannic acid and such like.
     
  16. JamesC

    JamesC Member

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    Sounds good to me.

    The nitrogen cycle produces H+ and NO3-, so putting these two together gives you HNO3 or more commonly know as Nitric Acid.

    James
     

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