KH <= GH

Discussion in 'Water Chemistry' started by Edward Shave, 19 Nov 2018.

  1. Edward Shave

    Edward Shave Member

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    Hi Marcel
    Sorry if my answer seemed a little kurt as it wasn't intended that way.
    The test kit manufacturers must share a lot of the blame for the confusion. It is relatively easy to measure Alkalinity (A) but not so easy to measure KH. Since A=KH so long as KH<=GH and since this is the case for the vast majority of us aquarists the easy answer is to label the Alkalinity test as KH. In fact my "KH" test kit label also contains the word alkalinity in smaller print.
    Your case is an example of why I think it's worth knowing the difference.
     
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  2. Edward Shave

    Edward Shave Member

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    No. At least that goes for the NT Labs test kit I use.
     
  3. zozo

    zozo Member

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    But still the alkalinity test result i get from that drop test is pretty spot on compaired next to the water company laboratory method in measuring mg/l HCO3
    Then i may expect that there is a accurate corelation in bot methods i can recalculate it back to same results.. Assuming the laboratory uses a different method than acid based tittation and convert it with a wet finger into mg/l HCO3 on their report.

    And reading this forum a lot i noticed the majority in the UK averagely has a higher Gh than kH, but where i'm from its other way around. That is actualy what i wanted to show.. That's all. I do not understand all chemistry behind it enough to go into it any further than basic knowledge and formulas. There surtainly will be exceptions not explained in the basics.

    All i have is a laboratory report and some simple home tests and both come up with pretty accurae summilar values if i recalculate it with what i know. Huh? always forget and can look back up i mean.. :)
     
  4. Edward Shave

    Edward Shave Member

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    Hi again Marcel. :)
    I have no argument with your figures which I'm sure are correct. I'm sure also that you are correct to say that KH (as measured by your test kit) is greater than GH in your area.:)

    addendum
    I just noticed your water company is reporting total carbonates not carbonate hardness. So again this figure refers to alkalinity.
     
    Last edited: 20 Nov 2018
  5. zozo

    zozo Member

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    Hi Edward, :)

    I believe your word for it.. Dunno what that all means..

    I thought to understand since 1° dKh equals 17.86 mg/l carbonate and bicarbonate which obviously has a coralation with alkilinity there aint much to argument.

    But obviously that seems not the whole story.

    Interesting. :)
     
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  6. dw1305

    dw1305 Expert

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    Hi all,
    I don't think it does, which bits are inaccurate? I've just looked back through it, and the science is right and it additionally mentions the ICP analysis that is in Marcel's water report etc.
    Alkalinity doesn't directly relate to either dGH or dKH, because other ions and compounds are also bases. Alkalinity is the ability of a solution to resist pH change when acids are added, so it is a measure of <"buffering">. Acids are "proton (H+ ion) donors" and bases (alkalis) are "proton acceptors".

    In water you usually have carbonate buffering, but you can have other buffered systems like <"sodium phosphate buffers">, where disodium phosphate (Na2HPO4) is a base, and sodium dihydrogen phosphate (NaH2PO4) the acid. You can tell which is which from the formulae, the base has two alkali metal atoms which will both become Na+ in solution and the acid has one H+ and one Na+. Sodium citrate/citric acid etc are other buffered systems.
    That is right, test kits use alkalinity as a proxy for hardness, because usually, in tank water, they are both derived from limestone and the alkalinity and hardness (both dKH and dGH) are linked if both derive from CaCO3, but they aren't if they originate from other salts.
    It can.

    <"Group I (alkali metal) carbonates are soluble">, the solubility of NaHCO3 is 96 g/L (20 °C) in H2O. When you add NaHCO3 you raise dKH, but not dGH, you haven't added any dGH (multivalent cations). If you add NaHCO3 (Na+, HCO3-) to a solution which is already saturated with Ca++ and HCO3- ions, CaCO3 will precipitate out, CaCO3 is less soluble than NaHCO3.

    The salts of all group I metals are soluble, many of the salts of group II metals (Ca, Mg etc.) are not, as an example the solubility of CaCO3 is 0.013 g/L (25 °C).

    cheers Darrel
     
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  7. Edward Shave

    Edward Shave Member

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    When I get a minute I'll go back to the original thread and try to point out the inaccuracies.

    My point exactly..!

    I think the main difference between us lies in how we define KH

    I think we both agree that GH is the sum of multivalent cations or total hardness
    These cations must be balanced by an equal amount (in terms of charge) of anions
    Historically it was found convenient to divide these cations into two groups, those balanced by carbonate anions KH and those balanced by other anions. So KH is that portion of GH balanced by carbonates.
    If there are enough carbonates present to balance all the hardness then KH=GH. Increasing the supply of carbonates beyond this point will have no effect on KH as all the cations have already been used up.
     
  8. dw1305

    dw1305 Expert

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    Hi all,
    They must.
    It is the amount of carbonate buffering, it doesn't matter which compound the HCO3- ions have come from. In solution an ion is an ion is an ion, they are all the same.
    Not exactly, it is back to the differing solubility of the (periodic table) group I "alkali metal" elements and the group II "alkaline earth" elements, and the <"common ion"> effect.

    You can have a lot of Na+ and HCO3 ions in solution, we know this because of the high solubility of sodium bicarbonate (or sodium carbonate (Na2CO3)) in water. You can also see why sodium carbonate is a stronger base than sodium bicarbonate (sodium carbonate has two sodium atoms (Na+ ions)).

    If you add Na+ and HCO3- ions to water, which is already saturated with Ca++ and HCO3- ions (and at 400 ppm CO2 a very small weight of CaCO3 dissolves), you will always get CaCO3 precipitated out, because it is the least soluble carbonate. You used up all the Ca++ ions, but you still have plenty of Na+, HCO3- ions and H2O molecules (the water).

    The H2O can surrender a H+ ion (to the CO3--), creating both a HCO3- and an OH- ion (H2O - H+ = OH-) and raising the pH.

    cheers Darrel
     
  9. Edward Shave

    Edward Shave Member

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    No, it is that portion of GH balanced by carbonates.

    This is just plain wrong. It's much simpler than you make it out to be.
    But if you wont take my word for it maybe a couple of citations will help...

    From...WATER CHEMISTRY
    An Introduction to the Chemistry of Natural
    and Engineered Aquatic Systems
    Patrick L. Brezonik and William A. Arnold
    Page 377
    "Hardness, the sum of multivalent cations in water, usually is dominated by Ca2+ and Mg2+. The carbonate hardness is that portion of the hardness that can be associated with the bicarbonate and carbonate present in the water.

    From... FRESHWATER AQUARIUM CHEMISTRY
    Kevin J Ruff PhD
    Page 38
    General hardness is comprised of two parts 1) "Carbonate Hardness" (KH), and 2) "Non Carbonate Hardness" (None KH or NKH). The K in KH comes from the German spelling of carbonate.) So.... GH=KH+NKH
     
  10. dw1305

    dw1305 Expert

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    Hi all,
    @Edward Shave, we are just going around in circles, and we will have to differ in our view.

    "Degrees of hardness" are strange terms, scientists don't tend to use them, they look at elements in solution (ions) in mols or ppm (mg/L) and alkalinity in milliequivalents/L, but the definition of dKH is:

    "One German degree of carbonate hardness (dKH) corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate (CaCO3) per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed "as CaCO3" – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions. Bicarbonate ions only contribute half as much carbonate hardness as carbonate ions, so bicarbonates that are present in the water are converted to an equivalent concentration of carbonates when determining KH."

    I'm pretty sure it doesn't matter where the carbonate/bicarbonate ions came from, we are expressing them in terms of CaCO3 (derived from CaO), but they don't have to originate from CaCO3.

    This is via <"Wikipedia"> "An aqueous solution containing 120 mg NaHCO3 (baking soda) per litre of water will contain 1.4285 mmol/l of bicarbonate, since the molar mass of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of (calcium) carbonate, or 71.485 mg/l of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO3, this solution has a KH of 4.0052 degrees.">

    cheers Darrel
     
  11. Edward Shave

    Edward Shave Member

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  12. Edward Shave

    Edward Shave Member

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    I don't disagree with any of that but what does it have to do with the definition of KH?

    I don't disagree with that either but I never suggested it matters where the carbonates came from.

    I would just like to say how much I appreciate your input even if we don't agree. :)
     
  13. Edward Shave

    Edward Shave Member

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    So is there no one who agrees with me?

    I find it fascinating that this misconception regarding KH is so ingrained in the hobby..!
     
  14. zozo

    zozo Member

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    Depends on where you put the focus, the hobby is still quite devided.... :) For most of the oldschool hobbyist still not dissolved into the the CO² hype actualy don't give a flyig figure about kH.
     
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  15. dw1305

    dw1305 Expert

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    Hi all,
    I agree it would make much more sense to actually stop using "dGH" & "dKH", and use non-ambiguous units like mg/L.

    If test kits referred to alkalinity, rather than carbonate hardness, it would also make things more transparent. In most UK waters the alkalinity, dGH and dKH all come just from dissolved CaCO3, and they are all clearly correlated with one another.

    This isn't true universally and a water body like <"Lake Tanganyika"> would have very different chemistry.

    cheers Darrel
     
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  16. Konsa

    Konsa Member

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    Hi
    I dont have the knowledge to get involved in the discussion but I find it fascinating too as I don't even bother testing my KH or GH or anything else (apart from PH for the CO2 profile when I was high tech)in my tanks for the last 6 years and my hobby is so much more enjoyable.I have what I have in my tap and the rest is just waterchanges, waterchanges and if I haven't mentioned it before more waterchanges.
    P.S. Please don't get me wrong as writen text is easily misunderstood. I dont mean to be rude and at the end of the day I may even learn sth new too.But overcomplicating things is not really my thing.
    Kind Regards
    Konsa
     
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  17. jcspotless

    jcspotless Newly Registered

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    Hi,
    I am new to this forum and finding it interesting this discussion on GH and KH. I am a analytical chemist by training so I think I know a little bit about water chemistry. My question is this: It seems to me that KH should be kept high if you want pH stability since this is a buffer. However low GH and KH seem to be what a lot of people are aiming for. I do not really get that. Why would you want to remove something which stabilises your pH?

    I can understand if want to change pH that you would want a low KH so you can change it. But when you get to your target pH, why would you not want to again have a high KH. I have not read anything concrete to suggest high KH per se is a bad thing.

    Am I missing something?
     
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  18. zozo

    zozo Member

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    No not realy and if you want to keep an aqaurium long term, something like a low energy Wallstad setup with a minimum of water changes than a good buffer might be more important.

    In the nowadays High Tech CO² era, metaphoricaly the Sports Car Setup among planted aqauriums, preferably metabolising at top speed, fuelled to the max with EI. Needs a lot more maintenance and water changes to flush out and delute build up organic waste products and excess ferts to maintain a form of stability and give it a weekly reset. Averagely at least 50% weekly is adviced with a proper substrate vacuum to get rid of decaying materials. That way you actualy prevent an accumulation of substances that possibly could influence pH negatively other than the CO² you're pumping in.

    I'm not 100% sure someone will correct if i'm wrong anyway :) But if i remember correctly that a Higher kH than ideal (> kH 6) can prevent certain fert ellements comming out of solution. For example Iron is such an ellement, than running an aqaurium at such high metabolic speed you might run into deficiencies. Something to do with Calcium staying in solution due to the CO² added counteracts Fe ions to come available for the plants. At least i believe it was something along that line..

    Than we actualy provide the needed stability with maintence and husbandry and don't need to rely on the waters kH buffer capacity regarding a buildup in organic acids.

    And if i rember correctly also for low energy setups the pH has an effect on micro elements comming out of solution and available to plants. For example i have kH 10 and a high alkalinity my average pH profile is pH 8 +.. Also not ideal and could run into Fe deficiencies if i grow plants that require more of this. Than a lower alkalinity gets easier to maintain a more ideal pH value.

    This higher alkalinity also prevents me ever from keeping fish that require a lower pH.. LIke some apistos.. It's very hard to get a lower required pH with a dkH 10 water
     
    Last edited: 1 Dec 2018
  19. Oldguy

    Oldguy Member

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    As you say its in the history, especially the determination of 'hardness' with a standard soap solution and scum/lather formation. Hate terms such as kH & gH, always have to look them up. Much prefer ppm and the substance of interest. Terminology also muddied by assuming all waters are natural and using Ca2+ as an industrial short hand. The water in our little glass boxes is anything but natural especially if you follow the EI path.
     
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  20. dw1305

    dw1305 Expert

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    Hi all,
    It would make a lot more sense.
    That is it. The carbonate hardness (which we use dKH as units for) stabilises the pH at ~pH 8 via the CO2/carbonate equilibrium (assuming ) 400ppm atmospheric CO2. The total amount of inorganic carbon (TIC/DIC) remains the same, it form just changes dependent upon pH. When you add CO2 you depress the pH, because you've added H2CO3, and that is a proton donor (the "spare" H+ ion).

    If you keep hard water fish (lake Tanganyika cichlids etc.) or hard water plants (Vallisneria etc.) they have evolved in carbonate rich water and have adaptations for this environment. Plants like Vallisneria etc. have the physiological pathways to use bicarbonate (HCO3-) as a carbon source in photosynthesis and to efficiently sequester nutrients like PO4 --- and Fe+++, which are of very limited in their availability in hard water. They also often don't have any mechanisms for avoiding iron toxicity in reducing or acidic conditions.

    The pH stability of hard water isn't as set in stone as many people suggest, mainly because <"in vegetated water there will always be diurnal swings in pH"> caused by the differing CO2:O2 ratio during (and after) photosynthesis. In carbonate buffered water the amplitude of the swing will be much reduced.

    Most commonly kept fish and plants don't have any particular water requirements, and will live and reproduce in a range of water types. In very hard water a lot of plant nutrients are less available and high levels of Ca++ ions may interfere with the uptake of Mg++, Fe+++ and K+ ions, and bio-calcification.

    A limited number of plants and fish come from water without any natural carbonate hardness, this includes many of the "black-water" fish from SE Asia (Parosphromenus spp.) and the central Amazon basin (Dicrossus filamentosus etc.), and plants like Syngonanthus & Tonina and Brasenia species. These plant and fish can only be maintained long term in soft, acid, nutrient poor water. The exact reason for this is not fully known, but may relate to their efficiency in obtaining scarce cations in their natural habitat, and problems with an excess of these elements when they are more available.

    Does any of this matter? <"Views on this forum vary">.

    cheers Darrel
     
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