Test kit kh testing

Discussion in 'Water Chemistry' started by naz, 24 Jan 2020.

  1. naz

    naz Member

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    Hi forum

    I have been trying to measure my kh of my tank water for the last few weeks,I have 3 kh test kits that I have been measuring the tank water with, I no all test kits are not going to be accurate and will give different results.

    So I have tested all 3 test kits to a 4dkh solution that i use for my drop checker that I purchased, as a reference point thay were all very close in the results,then I measured them all on my tank water that is high tech with co2 in it and all 3 of them had very close results.

    Then I took a cup of my tank water and let' it degass of co2 on the side for 3 days just to make sure it was fully degassed of co2 and tested with the 3 test kits thinking it would be the same results as in the tank.

    But there was a really big difference, where as it's was taking 3 drops of test kit solution to turn from blue to yellow it was taking the degassed tank water 7 drops of test kit solution to turn from blue to yellow,these results happened on all three test kits

    Question
    Can anybody tell me why this is happening?
    What results should I take as my kh, the degassed tank water or water in tank ?
    Has anybody come across these results them selves ?

    Lee
     
  2. ian_m

    ian_m Global Moderator Staff Member

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  3. dw1305

    dw1305 Expert

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    Hi all,
    Yes, the result you need is for the degassed water.

    You've actually just described how a drop-checker works. There is a complete discussion in <"Drop checker color....">

    The important things are that the KH test kit doesn't actually measure carbonate hardness (dKH), it measures alkalinity, via an acid base titration. This wouldn't usually matter, because alkalinity and the carbonate hardness are fairly closely correlated, and carbonate hardness, pH and CO2 are linked via the carbonate ~ CO2 ~ pH equilibrium.

    Carbonates are insoluble in water, but soluble in weak acids. CO2 <"is very soluble in water"> and a small proportion of the 400 ppm of atmospheric CO2 goes into solution as carbonic acid H2CO3. This dissociates into a bicarbonate ion HCO3- and a proton, H+.

    Acids are defined as "proton donors", so you've added an acid and the pH falls, unless you have some carbonate buffering which supplies a base and maintains a steady pH. That pH level (~pH8) is as a result of the level of atmospheric CO2.

    When you initially did the test you just had more CO2 in the water. More CO2 means more H+ ions and that means you had to add less acid (via the KH test) to neutralise the alkalinity of the tank water.

    cheers Darrel
     
    Last edited: 25 Jan 2020
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  4. Hanuman

    Hanuman Member

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    Catching up here as this is interesting.

    So if I understand well, @Darell when one wants to keep plants that require a low KH one needs to take as reference the KH the water from the tank that has been taken out and that has been sitting several hours so that it degasses? Would taking water from the tank during the night when no co2 is being injected and immediately testing it yield the same result?

    Say for example I have some plants that cannot tolerate a dKH higher than 2 or 3. My reference point should be the KH from degassed tank water right?
     
  5. dw1305

    dw1305 Expert

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    Hi all,
    Possibly.
    Yes it should do. The additional CO2 (with a small proportion as H2CO3) will dissociate into HCO3- and H+ ions and those H+ ions will change the position of pH in the carbonate ~ pH ~ CO2 equilibrium. You have more <"Total Inorganic Carbon (TIC)"> when you are adding one form of it (added CO2).
    That is where the "possibly" comes in, I've never injected CO2 and I don't know whether it is pH, or base status, that is important.

    What you need are posters who have successfully (or not successfully) grown <"Tonina etc"> in harder water with CO2 injection.

    cheers Darrel
     
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  6. Hanuman

    Hanuman Member

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    Thinking this again. Wouldn't actually measuring KH without making the water rest be more realistic? If one uses a buffering substrate and if one removes the water for far too long then PH will rise to the point where KH testing will be irrelevant as the OP experienced. Am I wrong here?
     
  7. dw1305

    dw1305 Expert

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    Hi all,
    You've got two separate, but related, processes occurring there.

    CO2
    The pH drop, when you add CO2, is because you've changed the ratio of H+:OH- ions, by adding H+ ions. It is that ratio that we measure as pH. You have more ions present.

    Ion Exchange
    An <"active substrate"> works via ion exchange. It exchanges a lightly bound H+ ion, in the substrate, for a cation of higher valency in the water column. That ion is likely to be a calcium ion (Ca++), but ion exchange is dependent upon <"both the lyotropic series and the abundance of ions">. You have the same amount of ions present, but you've reduced the dGH. You may also have some <"anion exchange capacity">.
    That is why you need some-one who has tried Tonina etc. in harder water with CO2 injection.

    cheers Darrel
     
  8. Witcher

    Witcher Member

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    It will simply melt in hard water, CO2 doesn't seems to be that important (of course water must be soft and acidic anyway). Syngonanthus sp. will behave in the same way.
     
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  9. dw1305

    dw1305 Expert

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    Hi all,
    That was my guess.

    That is partially why I like conductivity as a measurement, rather than just pH. You can alter pH by adding acids, but they also add to the conductivity. I'm not convinced that low pH, high conductivity water offers much advantage over harder water.

    If you have very low conductivity water you know you don't have many ions of any description. Black-water plants and fish have evolved in water with really low conductivity values.

    cheers Darrel
     

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