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The Science Behind Api Kh Drop Test Question

Discussion in 'Test Kits' started by aniroc, Apr 17, 2019.

  1. anirocWell Known MemberMember

    I noticed that the KH in my CO2 injected tank is always low.
    I am wondering what exactly is the test measure: the amount of Carbonates/Bicarbonates or the remaining buffering capacity?
    The KH test solution itself is yellow. When added to the test vial (5 ml tank water) it turns blue. Few drops later, depending on the KH, will become yellow again.
    The color change looks familiar. Same in the drop checker.
    Lets say the KH test solution has Bromothymol blue as pH indicator in an acidic solution and testing is based on acid-base titration (add enough acid to neutralize the base).
    Since I inject CO2, I already have some Carbonic acid present. It will take less KH solution to neutralize the base (carbonates). If the acid in the KH solution is a stronger acid than Carbonic acid (almost all are stronger: acetic, citric, etc), the Carbonates will react with the acid and CO2 gas is formed. If that is true, KH test is measuring the amount of Carbonates as well.

    Any thoughts?
  2. nikm128Fishlore VIPMember

  3. SkavatarWell Known MemberMember

    wow, i did not realize how acidic the KH test is.

    pH (as supplied) 1.20-1.45

    here's an interesting diy titration test for KH
  4. toosieFishlore VIPMember

    Well, since carbonates and bicarbonates are what makes up the buffering capacity (alkalinity), I would say both. In the test, acid is used to neutralize the alkalinity, at which time it changes color.

    Any acid that is in the aquarium, whether it is tannic acid, nitrous acid, nitric acid, carbonic acid, etc., will have an impact on KH, and consume the buffering capacity (ie: carbonates and bicarbonates). Other processes also use KH, such as bacteria and plants. Plants when offered CO2 will use it as a source of carbon because it is easier than breaking down carbonate to obtain CO2. So while carbonic acid gets neutralized by the buffer, the weak acid doesn't use up the carbonates as readily as plants can when they are rapidly growing.

    If source water has plenty of KH, large water changes can replenish KH in the tank. If water changes aren't done often enough or large enough to keep up with demands, KH can have an unhappy ending. But too, large water changes also help replenish all sorts of minerals and trace elements that also get used up by various processes.

    As a side note:
    In the case of the GH test, which also changes color, the solution reacts with calcium and magnesium, and bonds to make a complex that makes it change color.
  5. anirocWell Known MemberMember

    Yes, i guess it's both. An acid that strong will first consume the buffer and next destroy the carbonate. However, I never noticed gas (CO2) bubbles in the test vial.
    Carbonic acid that is formed when CO2 is dissolved in water will never destroy the Carbonate. It will temporarily reduce it's buffering capacity. Once CO2 is used up by plants or it is outgassing, the KH returns to previous value.
  6. toosieFishlore VIPMember

    Well, carbonic acid dissociates into bicarbonate and hydrogen ions. But if I am understanding you correctly, you are saying that this doesn't impact carbonates?

    No, of course, you're right! Because carbonate is a salt of carbonic acid. It's a duuuh, moment. :) Thank you! But the acid in the test solution would.

    But, the Bromothymal blue (if it is part of the KH solution) would change color to yellow once pH of the test sample reached a pH of 6.0, correct? But, if we look at the link Skavatar provided, the pH needs to be brought down to 4.2. where all carbonate has been converted to carbonic acid. Let me provide that link again.
    So, it would seem unlikely Bromothymal blue is used. Would you agree with that?
    Last edited: Apr 17, 2019
  7. anirocWell Known MemberMember

    Yes. That is what I said: Carbonic acid does not make Carbonates, it does not increase KH.
    Carbonic acid (or CO2 in water) enters into an equilibrium with the alkalinity already present. The higher the KH, the more CO2 the water can hold without changing its pH.
    At the same time, Carbonic acid dissociates into CO2 (gas) and water. This is also an equilibrium reaction. The best way to describe it:

    CO2 +H2O <--> H2CO3 <--> H+ + HCO3- <--> CO3-- + 2H+

    When CO2 is injected, the entire chain of reactions shifts to the right and the water acidifies. Once injection is stopped, the equilibrium moves to the left, toward CO2 and water. CO2 is leaving the system, H+ is consumed and pH goes up. Back and forth every day pH changes but the KH does not.

    Same thing in the drop checker: CO2 enter and exit the solution every day but the Carbonic acid does not change the KH, it stays at 4dKH.

    As for the pH indicator used in API KH test drops, I still believe to be Bromothymol blue. If the test is based on acid-based titration, unless it is between a weak base and a weak acid (not the case), the titration curve shows that at the "equivalence point" the change in pH is drastic (4 to 5 units of pH) with no additional volume of acid (or base). In other words, a drop of KH test can change the pH in the test vial from 7.6 (blue) to way less than 6 (yellow), below 4.2- the point that breaks down the Carbonates into CO2 and water.
  8. toosieFishlore VIPMember

    No, you're right. I already agreed with you that it didn't decrease it, but I wasn't implying it increased it either. What I was saying is that carbonates and I'll include bicarbonates, are salts of carbonic acid. If carbonic acid loses hydrogen ions, it would become either a hydrogen bicarbonate or a hydrogen carbonate. So, if it loses one hydrogen ion, it would become a bicarbonate. If it lost 2 it would become a carbonate. So of course carbonic acid isn't going to decrease KH, and when broken down both carbonates and bicarbonates will release CO2 as part of the cycle. Which is why plants will use KH when another source of carbon such as CO2 isn't being provided.

    I'll concede on the pH indicator, it makes sense and I can't think of any other explanation for it turning color. I see your logic. :)

    Edit: Because CO2 in water doesn't all become carbonic acid, there is always dissolved CO2 present. If when the carbonates released the CO2 when doing the test, if it immediately dissolved in water, you wouldn't see the bubbles, any more than you see the dissolved CO2 that is already present.
    Last edited: Apr 18, 2019