Sodium Hydroxide in Planted Tank? - The Planted Tank Forum
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post #1 of 50 (permalink) Old 02-11-2019, 08:31 PM Thread Starter
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Sodium Hydroxide in Planted Tank?

@mbkemp Pointed out to me that Sodium Hydroxide can be used in water treatment to soften water. It has the effect of creating water with high pH and low KH. This describes my tap water. It may contain NaOH or have been created using it.

Some digging found this:

http://www.esc.nsw.gov.au/council-se..._Chemicals.pdf

If they are using Sodium Hydroxide as a water softener that raises pH, then apparently it binds with CO2...

Not a chemist, but what I'm reading has me very concerned. If excess CO2 is converted with NaOH, does that mean my CO2 injection is for naught?

Looks like CO2 binds with NaOH and precipitates Calcium. This would be a very bad thing to have in planted tank water, imo. It might explain what appears as a Ca deficiency.


Would someone care to guess if/what effect Sodium Hydroxide would have on a planted tank and fertilizers?

@Greggz Provided a table with the pH/KH relationship of popular forum tanks, and none of them showed parameters like mine.

It would be great if there was a forum directive that could explain what to do when water like mine is found. Special dosing? Do nothing? Buy RO system? I am hoping that someone can explain the chemistry of this water so I can make educated decisions on dosing.
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post #2 of 50 (permalink) Old 02-11-2019, 09:09 PM
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post #3 of 50 (permalink) Old 02-11-2019, 09:17 PM
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post #4 of 50 (permalink) Old 02-11-2019, 09:25 PM Thread Starter
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Check your link.

File not found error at my end
http://www.esc.nsw.gov.au/council-se..._Chemicals.pdf


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post #5 of 50 (permalink) Old 02-11-2019, 10:18 PM
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I believe NaOH will actually increase KH but not GH. The equilibrium with atmospheric CO2 that determines KH depends only on the number of cations that are not balanced by anions like Cl-, SO4--, and so on that do not participate in CO2 equilibrium. NaOH adds unbalanced cations since OH- (and CO3-- and HCO3-) are involved in the CO2 equilibrium and so NaOH raises KH. However, Na+ is not Ca++ or Mg++ which are the chief ions determining total hardness, so NaOH leaves GH unaffected. (Sodium in your tank is another issue; plants don't like it much.)

Put another way, NaOH exposed to air gradually absorbs CO2 to become Na2CO3, sodium carbonate, which obviously adds to carbonate hardness. If you add NaOH directly to your tank, it just does its carbon dioxide absorption there to give the same final result.
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post #6 of 50 (permalink) Old 02-11-2019, 10:42 PM
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Looking again at your original post: The usual way to soften water is either through reverse osmosis, which lowers all dissolved solids, or by ion exchange that replaces Ca and Mg with Na. Adding NaOH directly to the water to soften it is a new one to me, but it would have the unusual effect of making KH greater than GH. Water softened with sodium is going to be very hard on plants.

The usual economical way to get rid of it is reverse osmosis. A novel alternative might be to reverse the ion exchange, using ion exchange medium that has been saturated with calcium chloride.
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post #7 of 50 (permalink) Old 02-11-2019, 10:54 PM Thread Starter
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Originally Posted by kgbudge View Post
I believe NaOH will actually increase KH but not GH. The equilibrium with atmospheric CO2 that determines KH depends only on the number of cations that are not balanced by anions like Cl-, SO4--, and so on that do not participate in CO2 equilibrium. NaOH adds unbalanced cations since OH- (and CO3-- and HCO3-) are involved in the CO2 equilibrium and so NaOH raises KH. However, Na+ is not Ca++ or Mg++ which are the chief ions determining total hardness, so NaOH leaves GH unaffected. (Sodium in your tank is another issue; plants don't like it much.)

Put another way, NaOH exposed to air gradually absorbs CO2 to become Na2CO3, sodium carbonate, which obviously adds to carbonate hardness. If you add NaOH directly to your tank, it just does its carbon dioxide absorption there to give the same final result.
My tap water KH = 2, GH = 4, pH = 8.2.

I did two 50% WC yesterday and did not add my small token baking soda, so I was expecting KH to be in the 2-3 range.

I really cranked up the CO2 to see if that was the problem. I tested KH an hour ago and it was 6!!

There is only small amount of Equilibrium (Mg,Ca), EI fertz and DTPA Fe. Inert PFS substrate.

Really having a hard time understanding how increasing CO2 would raise KH!?! Or what in the tap water is interacting with my fertz to do this.

Dosing EI by the book. Plants are pearling tons but show chlorosis. Always have.
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post #8 of 50 (permalink) Old 02-12-2019, 12:50 AM
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The way I understand the explanation is sodium hydroxide will convert to sodium carbonate when co2 is added. So, that would explain why your kh went up to 6.


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post #9 of 50 (permalink) Old 02-12-2019, 12:57 AM Thread Starter
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Originally Posted by mbkemp View Post
The way I understand the explanation is sodium hydroxide will convert to sodium carbonate when co2 is added. So, that would explain why your kh went up to 6.


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Is it that simple? Wonder why I haven't noticed KH creep with lower CO2?

And when this process happens, is it taking anything important from the water, like CO2 or Calcium? IOW, the CO2 that converted into sodium carbonate, is that not available to the plants? Is it taking anything else hostage?

Will it convert back sometime after CO2 has been turned off?

And does this mean there is a bunch of Sodium in my tank?..


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post #10 of 50 (permalink) Old 02-12-2019, 03:49 PM
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Quote:
Originally Posted by mbkemp View Post
The way I understand the explanation is sodium hydroxide will convert to sodium carbonate when co2 is added. So, that would explain why your kh went up to 6.

It depends a lot on how your carbonate hardness test works. Strictly speaking, carbonate hardness is [HCO3-] + [CO3--]. In water that is in equilibrium with atmospheric CO2, this will depend in turn strongly on the balance of other major cations and anions in your water, typically 2[Ca++] + 2[Mg++] + [Na+] + [K+] - ([Cl-] + 2[SO4--] + 3[PO4---]). I haven't found a good explanation online for how most carbonate hardness tests work, but I have a vague notion that common, inexpensive test kits do not directly measure [HCO3-] + [CO3--]. They measure the cation/anion balance instead and assume equilibrium with atmospheric CO2.


If your test does actually directly measure [HCO3-] + [CO3--], and if the water has an excess of cations over noncarbonate/nonhydroxide anions, and if it has somehow been prevented from absorbing atmospheric CO2 before coming out your tap, the water will have a high pH in spite of a rather low carbonate hardness, 2 dKH. And, yes, as it absorbs CO2, the carbonate hardness will go up. It might be interesting to take a sample of your tap water and do nothing with it except bubble air through it with an airstone. If the carbonate hardness goes up, then bingo. Alkali is being added to your tap water just before it comes out the tap, giving it no time to absorb atmospheric CO2, and your test is sophisticated enough that it actually measures [HCO3-]+[CO3--] directly, yielding the otherwise unusual pH 8.4, dKH 2.
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post #11 of 50 (permalink) Old 02-12-2019, 04:08 PM Thread Starter
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Originally Posted by kgbudge View Post
It depends a lot on how your carbonate hardness test works. Strictly speaking, carbonate hardness is [HCO3-] + [CO3--]. In water that is in equilibrium with atmospheric CO2, this will depend in turn strongly on the balance of other major cations and anions in your water, typically 2[Ca++] + 2[Mg++] + [Na+] + [K+] - ([Cl-] + 2[SO4--] + 3[PO4---]). I haven't found a good explanation online for how most carbonate hardness tests work, but I have a vague notion that common, inexpensive test kits do not directly measure [HCO3-] + [CO3--]. They measure the cation/anion balance instead and assume equilibrium with atmospheric CO2.


If your test does actually directly measure [HCO3-] + [CO3--], and if the water has an excess of cations over noncarbonate/nonhydroxide anions, and if it has somehow been prevented from absorbing atmospheric CO2 before coming out your tap, the water will have a high pH in spite of a rather low carbonate hardness, 2 dKH. And, yes, as it absorbs CO2, the carbonate hardness will go up. It might be interesting to take a sample of your tap water and do nothing with it except bubble air through it with an airstone. If the carbonate hardness goes up, then bingo. Alkali is being added to your tap water just before it comes out the tap, giving it no time to absorb atmospheric CO2, and your test is sophisticated enough that it actually measures [HCO3-]+[CO3--] directly, yielding the otherwise unusual pH 8.4, dKH 2.
If this is not what's happening, can you think of any other conditions that would create high pH, low KH water?


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post #12 of 50 (permalink) Old 02-12-2019, 04:09 PM
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To take a crack at your other questions:



Quote:
Originally Posted by ChrisX View Post
Is it that simple? Wonder why I haven't noticed KH creep with lower CO2?

I would expect to see this, too, unless at the lower CO2 injection rate, it is being extracted by plants as fast as it is added. Actively photosynthesizing plants lower carbonate hardness.


Quote:
And when this process happens, is it taking anything important from the water, like CO2 or Calcium? IOW, the CO2 that converted into sodium carbonate, is that not available to the plants? Is it taking anything else hostage?

The carbon dioxide being absorbed is being converted to bicarbonate and carbonate by reacting with the hydroxide in the water. Bicarbonate is bioavailable and so this not a problem. Carbonate will not be too important if your pH is below 8 or so. At 8.4 it is somewhat important, and I haven't found a good online article on its bioavailability, but I suspect this is also fairly good. So that shouldn't be an issue.


On the other hand, if the carbonate gets high enough, it can start precipitating out calcium as calcium carbonate -- the reverse process to adding crushed coral to a tank to increase carbonate hardness. Keep the pH below 8 and that should not be a problem. What is your target pH in the tank?

[/quote]


Will it convert back sometime after CO2 has been turned off?[/quote]


Some will. Not all at that tap pH; you've got enough excess of cations over "permanent" anions to keep some of the CO2 in the water. It would be useful to try the experiment, though, just to see if anything else weird is going on. Take a sample of your tank water and aerate it with another air stone, and see if the carbonate hardness drops any.



Quote:

And does this mean there is a bunch of Sodium in my tank?..

It certainly seems like there is some kind of alkali being added to your water, and the only other likely candidate is lime, CaOH. Which would increase general hardness, which seems unlikely given your tap dGH.


Sodium is a touch tricky to test for, but there are two possibilities. You could boil down a liter of your water and see how much solid residue is left. If you got access to a laboratory scale, you can even weigh it and immediately have total dissolved solids. (No, I don't have access to a laboratory scale, either, unless I cash in a lot of "good boy" chips with my wife the biochemist.) You can also taste-test the residue to see if it's noticeably salty; lime does not taste salty.


Or you could try a flame test. If you can fix up some kind of metal loop -- something inert like aluminum is best, if you don't have nichrome handy (see above: good boy chips) -- you can dip this in your tank water and stick it in a blue flame; a Bunsen burner or even just a gas cooking flame will work. If it turns yellow, sodium is present. You can try boiling down the sample of water first to concentrate the sodium if it's present and make the test more sensitive. Alas, neither the taste test nor flame test is a quantitative test, and sodium is ubiquitous.

Bump:
Quote:
Originally Posted by ChrisX View Post
If this is not what's happening, can you think of any other conditions that would create high pH, low KH water?

You'd need a chemistry in which [H+] is low and [HCO3-] + [CO3--] are also low. That's actually hard to do, since the relationship

[H+][HCO3-]/[H2CO3] = k


has to be satisfied at equilibrium. For a given carbonic acid content, raising pH and lowering HCO3- simultaneously is just not possible. (Remember that high pH = low [H+]). The carbonic acid content must be lowered; and since that depends almost entirely on temperature and on rate of CO2 injection, there just aren't any other knobs to twiddle. (I'm ignoring CO3-- because the same ideas apply and it's not important below pH 8.)
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post #13 of 50 (permalink) Old 02-12-2019, 04:22 PM
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One other thought: At pH 8.4, you'll precipitate out a lot of minor nutrients, particularly iron, if they aren't chelated. Since almost everyone chelates their trace elements nowadays, that's not too likely to be a problem, but should be considered if there are any signs of chlorosis.
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post #14 of 50 (permalink) Old 02-12-2019, 04:38 PM Thread Starter
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Just to level set, I will set out some tap water overnight to test pH, KH, GH.

If the water indeed has a high pH, low KH, then NaOH may be the only feasible explanation.

Edit: Just realize I had tap water sitting in unused 10g tank for past two weeks. This water has NOT been aerated.

The API high range test kit is hard to pinpoint, but it appears to be 7.8 pH, 2 KH.

Does that still suggest NaOH? Why would the water come out of the tap at 8.2 pH?


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Last edited by ChrisX; 02-12-2019 at 04:51 PM. Reason: asdf
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post #15 of 50 (permalink) Old 02-12-2019, 06:09 PM
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Quote:
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The API high range test kit is hard to pinpoint, but it appears to be 7.8 pH, 2 KH.

Does that still suggest NaOH? Why would the water come out of the tap at 8.2 pH?

The pH dropping is consistent with the water absorbing CO2, but [HCO3-]+[CO3--] should be going up if that is taking place.



The other possibility is that something other than CO2 is lowering the pH. Adding a non-carbonate acid to water will lower both carbonate hardness and pH. If there is anything in the unused tank that could introduce non-carbonate acidity, such as a piece of rotting wood, peat, or decaying organic matter releasing tannic acid, that could lower pH without raising KH.
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