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KH is an Analog of PH?

Discussion in 'General Plant Topics' started by aquabillpers, Jan 10, 2009.

  1. aquabillpers

    aquabillpers Lifetime Charter Member
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    After measuring both KH and pH for years (not often, BTW) it came to me that when the KH is high, the pH also is high, and vice versa.

    My question is, ignoring CO2 levels, decomposition, plant and fish respiration, and similar variables, is there a direct correlation between KH and pH in an aquarium?

    Stated another way, in a non-CO2 tank, is KH control a practical way to control pH? Is there a reason to check both KH and pH, if one is into testing?

    Thanks.

    Bill
     
  2. SuperColey1

    SuperColey1 Guru Class Expert

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    Not being scientifically minded but I know from this subject regularly being brought up on the many forums that 2 aren't relate although harder water is normally alkaline it isn't necessarily the case.

    Can't explain in any detail as like I said I don't know the facts on this. lol

    Someone will though I'm sure

    AC
     
  3. aquabillpers

    aquabillpers Lifetime Charter Member
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    I think some people say that because when CO2 is injected it lowers pH regardless of KH, although I think it takes more CO2 to do that with higher KH rather than lower KH.

    Bill
     
  4. kid creole

    kid creole Prolific Poster

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    pH is the concentrations of H+ to the solution. Water disassociates into H and OH. The formula for pH is pH=-log(concentration of H+ to solution).

    In plain old water, the concentration of H+ is 0.0000001 to water. The -log of this is 7.

    KH is stuff that causes less H+ to show up, so you might have .000000000001 H+ to water, which would be a pH of 11.

    Chemistry was a long time ago. Forgive me any small transgressions. :)
     
  5. kid creole

    kid creole Prolific Poster

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    CO2 should in theory make roughly the same change to pH no matter what your starting KH is.
     
  6. aquabillpers

    aquabillpers Lifetime Charter Member
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    Are you sure? Two bubble per second of CO2 would produce the same pH in KH 2 water as in KH 10 water????

    bill
     
  7. SuperColey1

    SuperColey1 Guru Class Expert

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    No it wouldn't which is why we use 4dKH water in our solutions.

    When we use 4dKH then our Ph turns green at 25-30ppm
    When we use 2dKH then our Ph turns green at 12-15ppm.

    This is oversimplifying slightly as I am talking just the KH of our drop checkers. this is why we use a known reference in our DCs. Using the tank water would mean that we could be seeing the perfect green but in reality it would not mean 30ppm. It could be very low or incredibly high depending on our tank water.

    From doing a little research since I last posted I can see that in general KH and PH do tend to move together but I know I have read many discussions saying that this is not necessarily the case and that although they will often move together that they are not linked.

    I think this is one reason why the KH/Ph tables are so inaccurate because they assume a direct link but again not entirely sure on this.

    All I know is my water board say the water is very hard and the tap reads 7.2-7.4Ph when I test it.

    AC
     
  8. kid creole

    kid creole Prolific Poster

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    It would make the same change, not the same result.
     
  9. SuperColey1

    SuperColey1 Guru Class Expert

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    For simplicity's sake If we take 2 bps as being known to be 30ppm then in a tank of Ph7.2 KH10 then Ph should go down by less than in a tank of Ph7.2 KH6.

    Therefore the change would be different. The result of the CO2ppm would still be 30ppm but a different Ph would signal the 30ppm.

    This is why we use a reference known to be 4dKH as we know that green means 25-30ppm.

    People using the PPS Pro method who aim for 15ppm would use 2dKH knowing green was right for them.

    AC
     
  10. kid creole

    kid creole Prolific Poster

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    I agree with that.
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    Unless your home has some really heavy breathers :p and is well sealed from the outside air, you should be able to say with confidence that the ppm of CO2 in your tanks is constant. That reduces the equation relating CO2 to KH and pH to one relating KH to pH. So, yes, you could measure just the easiest one to measure and use that as an analog for the other. And, you could adjust the pH by adding baking soda.
     
  12. Tom Barr

    Tom Barr Founder
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    It should work fairly well as long as the tank has mainly bicarb as the alk component and not tannins, PO4's, borates, etc influencing things intermittently here or there etc, add peat every 3 months etc and the effects wear off etc, or lots of NO3 production which can lead to some HNO3 which can destory up KH(not much IMO, some say yes, I have little evidence to suggest it is a factor).

    And within the range of bicarb's pH(see here:)
    Carbon model worksheet 3 - J_Models

    Regards,
    Tom Barr
     

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