Lower KH By Raising Temperature - The Planted Tank Forum
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post #1 of 17 (permalink) Old 09-25-2015, 02:25 PM Thread Starter
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Lower KH By Raising Temperature

I just experimented and found that you can lower KH significantly by raising temperature to 90 degrees for about 24 hours. Of course you have to remove fish. My KH went from 10dKH to 5dKH. pH also went from 7.6 to 7.2 in process. Interesting. Apparently the ionic bond is reversible.
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post #2 of 17 (permalink) Old 09-25-2015, 04:26 PM
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I'm a little surprised it happens at 90.. but boiling water to drive down KH is a long-standing practice...

There's a reason KH is sometimes called "temporary hardness" by water treatment folks...

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post #3 of 17 (permalink) Old 09-25-2015, 04:32 PM
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I wonder if some of this is due to increased plant consumption i.e. raise the temp, which raises metabolism etc.

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post #4 of 17 (permalink) Old 09-25-2015, 04:41 PM
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what type of test kits were used to get these results?


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post #5 of 17 (permalink) Old 09-25-2015, 05:05 PM
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KH is just carbonates/bicarbonates which decomposes to co2 and O when heated.
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post #6 of 17 (permalink) Old 09-25-2015, 05:17 PM
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Quote:
Originally Posted by Buu View Post
KH is just carbonates/bicarbonates which decomposes to co2 and O when heated.
Yes, but this generally happens at quite high temperatures, ie:at boiling for some compounds in water, but needs to be over 850C for dry calcium carbonate.


Does thermal decomposition of carbonates dissolved in water really happen noticeably at temperatures as low as 90F? That's pretty close to the 75F we normally keep tanks at, and pretty far from the 212F normally used to precipitate hardness from water...

I tend to agree that the modest difference in temperature suggests some other mechanism at work.. most plants and beneficial bacteria can consume carbonates, and the increased temperatures are going to reduce dissolved CO2 levels, making carbonate usage more necessary for plants.

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post #7 of 17 (permalink) Old 09-25-2015, 06:17 PM Thread Starter
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Quote:
Originally Posted by burr740 View Post
what type of test kits were used to get these results?
API drop, dKH and pH separately

Bump: I have this:
In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms H2CO3 (carbonic acid), which is a weak acid since its ionization in water is incomplete.

https://en.wikipedia.org/wiki/Carbon_dioxide#Structure_and_bonding
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post #8 of 17 (permalink) Old 09-25-2015, 06:42 PM
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I don't think 90F vs 75-ish is going to change CO2 solubility enough to change you by 1 dKH, much less 5.

If you are going with the idea the carbonate in the carbonic acid will raise KH, and lack of it will lower KH:
----------------------------------------------------------------------------------------------------------------

It will, but at normal tank temperatures, CO2 will reach air equilibrium at around 3ppm. An actual tank could be higher or lower due to bacterial/fish respiration, injection or plant consumption. That said, you are very unlikely get over 10ppm without injection.

a 5dKH shift would take a lot more than 10ppm of CO2's worth of carbonic acid.

5 dKH It suggests the same alkalinity as would be provided by 89.24 ppm of CaCO3. 12% of that is pure carbon, so you'd need 10.708 ppm of carbon (as carbonate) to be removed for this much shift

10ppm of CO2 is only 2.79ppm of carbon... and most of that is not in carbonate form.

It would take more than 38ppm of CO2 to make enough carbonate to have a 5 dKH shift. Again, assuming all of the carbon in the CO2 became carbonate (which it won't).

The realistic impacts of CO2, without resorting to pressurized carbonation of water, on KH are very modest, and less than 1 degree. If you manage to dissolve enough CO2 to make soda water, you might be able to measure the KH shift..

If you are going with the idea the acidity of carbonic acid will lower KH:
----------------------------------------------------------------------------

It won't. Carbonic acid can react with carbonates, but you're basically playing "pass the carbon". A CO2 becomes carbonic acid, and the acid's free hydrogen reacts with an existing carbonate, producing CO2... Carbonic acid will never lower KH, because for every free hydrogen ion produced, a carbonate ion is also produced..

Also, higher temperatures would lower CO2 solubility, thus lowering concentration, so this mechanism would actually result in higher KH as temperature rises.



edit:

I also realize this whole post is making some mistakes/simplificaitons, in assuming that our KH test kits test KH... Pretty much all of them, and for sure the API test, actually measure total alkalinity, and express it in CaCO3-equivalent german degrees.

That said, the general argument of all of these being small effects unless you have a LOT of CO2 involved are still valid.

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Last edited by mattinmd; 09-25-2015 at 06:53 PM. Reason: KH vs alkalinity.
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post #9 of 17 (permalink) Old 09-25-2015, 07:26 PM
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interesting discussion.
without much calculations, 5KH drop sounds unrealistic, for that temperatures.

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post #10 of 17 (permalink) Old 09-25-2015, 09:30 PM Thread Starter
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If your kit is measuring in meq/L, it is measuring total alkalinity. If it is measuring in dKH, it is most likely measuring for carbonates. dKH is what my drop tests measure. If you assume that all of your alkalinity is made up of carbonates, (no substrate, rocks, sand, other materials) then the correlation to co2 is applicable. KH, refers to your carbonate hardness (carbonates/bicarbonates) and GH (general hardness) is actually referring to the calcium and magnesium content of your water. These two (KH/GH) are totally separate entities. In a fish tank, whether salt or fresh water, these readings are affected by decoration in the tank. I have bare bottom tanks, nothing other than plants and fish. So my readings for dKH should be quite realistic. I also fertilize my plants only outside of the tank, in a bin, then wash them off and replace them into the fish tank. So I am not adding any nutrient other than what is given off by plants, fish, and tap water. If API dKH drop tests are not accurate for dKH, then I'm sorry for the thread. But if it is accurate, and the molecular bonds are very weak, then I can see where raising the temp by 15 degrees could work. I read a thread somewhere, maybe here in PT, where the poster did just this, and that is where I got the idea. I kick myself for not bookmarking the thread. I'll find it one day I hope.
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post #11 of 17 (permalink) Old 09-25-2015, 10:46 PM
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Quote:
Originally Posted by AWolf View Post
If your kit is measuring in meq/L, it is measuring total alkalinity. If it is measuring in dKH, it is most likely measuring for carbonates.
While you should be right here in a book sense... in the aquarium trade you are not.

KH is a standardized lie in aquarium trade/hobby.

The API KH test kit "measures" in dKH, and says carbonate hardness on the label.. But really it is an acid-titration test measuring total alkalinity.

This is completely expected in the aquarium trade.. Nobody really cares about KH in most contexts, as we really only care about resistance to pH change. That is measured by total alkalinity from all buffers, not carbonate hardness from carbonates only.

So when we say "this fish needs 5 dKH" we really mean "this fish needs water with sufficient buffering to have the same total alkalinity as a solution containing 5 dKH worth of calcium carbonate"..

edit:

To be specific about the API test, if you look at the MSDS for it, they don't reveal any ingredients. However, they do reveal that the solution is pH 1.20-1.45, a strong acid.

Gee, I wonder why the solution is acidic.. oh, that's right.. it's an acid titration.. the rest of the solution is an indicator that changes color with pH... going from blue at alkaline/neutral pH and becoming yellow/orange in acid.. probably bromophenol blue or the like.....

So with each drop you are adding more pH sensitive indicator, and acid. When the acid overcomes the alkalinity of the solution under test, the indicator changes color and you have your reading based on the number of drops..

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Last edited by mattinmd; 09-26-2015 at 12:15 AM. Reason: added notes about API test in specifc...
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post #12 of 17 (permalink) Old 09-26-2015, 12:15 AM Thread Starter
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So if the alkalinity went down 5 points, and the pH down 4 points, what caused it? It was staying there until I raised the temp 15 degrees. I guess the only way to find out what is going on is to see how many days it takes for the 'KH' to lower the same amount without raising the temp. I'll give it a try over the next week and see. Wow, KH tests that don't measure KH. I'm pissed. So how do people measure their CO2 if not with a KH test? All this time I have been using this test to measure my CO2. Bummed and confused.
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post #13 of 17 (permalink) Old 09-26-2015, 12:52 AM
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Quote:
Originally Posted by AWolf View Post
All this time I have been using this test to measure my CO2. Bummed and confused.
https://www.plantedtank.net/forums/8129177-post12.html

I would highly recommend you reset your testing procedure, and start the test again. Lets rule out user error (of which I have lots of experience).

Feel free to edit.

Last edited by Audionut; 09-26-2015 at 01:20 AM. Reason: add experience
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post #14 of 17 (permalink) Old 09-26-2015, 10:42 AM Thread Starter
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Originally Posted by Audionut View Post
https://www.plantedtank.net/forums/8129177-post12.html

I would highly recommend you reset your testing procedure, and start the test again. Lets rule out user error (of which I have lots of experience).
I found this: http://www.advancedaquarist.com/2002/2/chemistry

"...As the pH drops from 8.2 to about 7.5, the most important thing happening is that the carbonate is converted into bicarbonate (equation 1). In figures 1 and 2 this part of the titration can be seen to take about 0.6 meq/L in my tank, and represents about 17% of the total alkalinity, in line with expectations for a tank that starts at a relatively high pH (8.45). All of the other minor contributors also get protonated at this point, and we see a shift to: ..."

http://www.advancedaquarist.com/2002/2/chemistry

Last edited by AWolf; 09-26-2015 at 11:18 AM. Reason: Clarification
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post #15 of 17 (permalink) Old 09-26-2015, 03:58 PM Thread Starter
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It may be the KH tests are considered inaccurate because most aquariums have substrate and decoration that mineralize the water in such a way that KH is confounded. From all I have read so far, including the article cited above, a bare bottom tank without decoration other than fish and live plants should show accurate KH readings. If you have the typical planted tank with substrate, your KH readings would be more of a 'industry lie' as it were. So how do most people with these tanks test for CO2? Are people really able to test accurately since KH tests are false due to substrate, or has it always been a guess? If you can't test for CO2, you really are running blind. Just measuring the CO2 going into a tank doesn't mean much, if you ask me. I would want to know how much dissolved CO2. It seems crazy to me that there is not a definitive way to test for CO2 in tanks with substrate and other decoration.
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