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Thread: Another relationship between p.H, K.H and G.H

  1. #1
    Nisai
    Join Date
    Nov 2008
    Posts
    84

    Another relationship between p.H, K.H and G.H

    Another chemistry question for you guys. Does running your G.H lower than your K.H. produce p.H stability or is it not that simple? It usually isn,t

  2. #2
    Sansai
    Join Date
    Oct 2011
    Posts
    158
    I'll probably get shot down in flames but here goes anyway.

    GH is Calcium and Magnesium positive ions. So you could have a pond made pH stable through sodium bicarbonate with a KH of say 3dh and a zero GH.

    When I've done this the pH is totally stable but the fish seem very unhappy with undetactable GH so I added a little, when they were then happy. Just my experience.

  3. #3
    Nisai
    Join Date
    Nov 2008
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    84
    Quote Originally Posted by pondlife View Post
    I'll probably get shot down in flames but here goes anyway.

    GH is Calcium and Magnesium positive ions. So you could have a pond made pH stable through sodium bicarbonate with a KH of say 3dh and a zero GH.

    When I've done this the pH is totaly stable but the fish seem very unhappy with undetactable GH so I added a little, when they were then happy. Just my experience.
    I am no expert but understand that if you ever get an algea bloom and a K.H in the form of bicarb only and with no G.H. you can end up with a very high p.H due to carbonates floating about which has a p.H. of 12. Not sure of the chemistry formulau or whether or not this is at all probable in a koi pond,(usually happens in a wild pond with plants) perhaps someone with more knowledge would care to have a go with explaining this .
    Best Regards Dave

  4. #4
    Sansai
    Join Date
    Jul 2009
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    pacific northwest, USA
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    177
    Quote Originally Posted by DaveB View Post
    Another chemistry question for you guys. Does running your G.H lower than your K.H. produce p.H stability or is it not that simple? It usually isn,t
    The pH of an aqueous solution does not directly correlate to the ratio of GH and KH values in a pond. Some sources of carbonate (which informs your KH and hence, pH stability) will introduce calcium, which will register in a GH test. Other sources of carbonate, such as sodium carbonate or sodium bicarbonate, will also increase your KH readings, while introducing no calcium, hence, no increase in GH readings.

    I think most people should just ignore GH. Outside of very extreme cases, there is little reason to try to move it off your source parameters, outside of the more esoteric pursuits of the 'perfect' water for sumi, shiroji, etc...

    -t

  5. #5
    Sansai
    Join Date
    Oct 2011
    Posts
    158
    Dave, seeing as no one else is answering I'll attempt an answer.
    I can see no effect of GH on KH buffering. They both have hardness in the title but have nothing in common.

    The buffering of bicarbonate resists changes in pH with the addition of acid through CO2 into the water. Obviously you have a finite amount of bicarb and it can be overwhelmed. But carbonate doesn't have a pH of 12 at pond type concentrations!

    I'm not sure where you have been told all this alkali comes from that can overwhelm a buffering system and raise the pH to 12? Remember that pH is logarithmic, so pH 7 to 8 is a 10 times change and pH 7 to 9 is one hundred times. So 7 to 12 is 100,000 more alkali.

    I tried a little experiment to test it out. Taking RO water with a TDS of zero I added lab grade sodium carbonate (not bicarb) to the water to get a TDS of 10dh. The pH was 11.2. Adding lots of further carbonate made it go up to 11.5 but very slowly. I didn't wish to waste an entire 1kg of lab grade reagent, but it's hard to imagine even a saturated solution going above 12.

    So in a pond with zero GH (which is what I had), even if all the bicarb was overwhelmed and converted to carbonate, the pH would still be 11 not 12 ie 10 times less.

    But basically these buffers are very effective and if you've got a KH of above 1.5dh then the plants aren't going to be able to overwhelm it (in the acid direction) unless you ignore it for days without measuring.

    Does this help? Still don't have any idea of what people think the chemistry is of GH protecting KH. One is anions and the other cations.

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