Drop checker and tanks KH

Discussion in 'Carbon Dioxide (CO2)' started by wick, 10 Jun 2008.

  1. wick

    wick Member

    Messages:
    52
    Hi
    ive just ordered a drop checker and 4KH water.
    Ive been out of the planted tank hobby for a few years and things have changed a bit,one being the drop checker.
    Ive been used to the old kh kit and ph kit and charting the readings.
    My question is...does it matter what your tanks KH is.......would the drop checker give correct readings,if say your tanks KH was 2,3,4,5,6,7,......or does the tanks water have to be set to 4KH
    sorry if its a dumb question,but when you dont know the answers....
    many thanks for any replies,
     
  2. milla

    milla Member

    Messages:
    241
    Location:
    Leeds
  3. wick

    wick Member

    Messages:
    52
    Cool
    For many years ive always thought that the higher your KH the more co2 you needed to add.
    And i have read that on many sites!!

    just shows,that u learn something everyday
    thanks :oops:
     
  4. JamesC

    JamesC Member

    Messages:
    1,276
    Location:
    Bexley, Kent
    That is correct. The higher your KH the more CO2 is required to achieve the same CO2 concentration as in a lower KH water.

    James
     
  5. wick

    wick Member

    Messages:
    52
    sorry but im lost now
    reading what ceg4048 wrote...
    If hobbyist "B" suddenly added sodium bicarbonate to his water, his kH would rise. No CO2 would be lost. He would still have 30 ppm CO2 dissolved but the bicarbonate would bond to more acid in the water and he would immediately see a pH rise.


    ??
     
  6. aaronnorth

    aaronnorth Member

    Messages:
    3,955
    Location:
    worksop, nottinghamshire
    The CO2 will still be there, it wont take it out
     
  7. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Hi James,
    Sorry mate but could you clarify? If the article is in error then I need to adjust it, but I thought your statement statement should be "...The higher your KH the more CO2 is required to achieve the same [pH] as in a lower KH water..."

    The CO2 / KH / pH table and equation (3 * KH * 10**( 7-pH )) can only show the CO2 concentrations as a function kH/pH. While it clearly shows that the "delta pH" is the same regardless of kH it does not address how much injection rate is necessary to drive the same delta. Are you saying that a higher injection rate is necessary at higher kH to achieve the same delta pH as in a lower kH water? I don't see any evidence of that either on The Krib or on Chucks page.

    We may have discussed it before but I can't remember which thread. I can't see it in either of the three equations:
    1. CO2 + H20 <------> H2CO2 (Carbonic Acid)
    2. H2CO2 <------> H+ + HCO3- (Bicarbonate)
    3. HCO3- <------> H+ + CO32- (Carbonate)

    If for example a higher ambient carbonate content shifted equations 2 and 3 to the left, would it drive equation 1 to the left as well and send gas out of solution?

    I think Planter was having some issues with high kH so if what you say is true that might explain some of it. It may be more apparent at very high kH and not so obvious at medium values. Could you have a look?

    Cheers,
     
  8. JamesC

    JamesC Member

    Messages:
    1,276
    Location:
    Bexley, Kent
    This is a difficult one and I did somewhere write up an answer to it but I can't find it now. Done some searching around chemistry sites but can't find a nice easy answer to point to.

    From what I remember if you have a higher KH you also have a higher ph. This higher pH shifts the equilibrium to the right of the equation below so that there is more HCO3 and CO3 compared to CO2. With a high KH obtaining the same CO2 levels then becomes more difficult as it becomes more difficult to lower the pH due to the buffering. So to get the same pH drop as before with a lower KH you need to pump in more CO2.

    CO2 + H20 <------> H2CO2 <------> H+ + HCO3- <------> H+ + CO3--

    Not 100% clear I know but I am certain that the higher the KH the harder it becomes to dissolve CO2. This doesn't affect the pH/KH tables to work out the CO2 concentration as they are still valid as long as there are no other buffers present like PO4.

    Some of the marine forums are excellent and there are some very good chemistry boffins on them. If I find a decent write up then I'll post a link. Really should use my bookmarks more.

    James
     
  9. aaronnorth

    aaronnorth Member

    Messages:
    3,955
    Location:
    worksop, nottinghamshire
    This is why i like this site, you can get great quality answers :D
     

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