Potassium Carbonate (K2CO3) vs pH ?

[dw1305]

Hi all,

In terms of the query in laymans terms, if using RO water do we need to be adding carbonates? I know you use rainwater, but I've not seen you mention adding any carbonates salts directly?

Assuming I wasn't keeping black water fish I would probably add 1 or 2 dKH, for the reasons @MichaelJ alludes to. Because our rainwater has some carbonate buffering I never actually start from 0 dKH, in the way I would if I used DI water.

I've never tried DI water and 0 dKH, so I don't have any practical experience of it. I work with what I've got (rain-water) and try and find plants and fish that do well in it. If I had softer rain water, and I didn't have a hard tap supply, I would probably keep more obligate soft water organisms, and if I was obliged to use our tap water? I'd keep "hard water" fish and plants.

The other reason for having some dKH is that I like <"snails in the tank">, so I need some carbonate buffering for them. Now I just watch the red Ramshorn Snail shells and if all the <"snail shells are really pale">, and all the snails are small, I add a <"dash of tap water">, which is <"about 17dKH / 17 dGH">.

cheers Darrel

 

[dw1305]

Hi all,

Please notice that the table describes K2O as a basic oxide , not "the potassium ion" K+ .

I maybe missing something, but isn't that why the table uses the oxides? The oxygen doesn't change, but the oxides are either basic, amphoteric or acidic. Group one and group two oxides are basic, because these are <"alkali"> and <"alkaline earth"> metals respectively, but the point is that they are the alkaline metals.

As an example of an acidic oxide, one that people will be familiar with <"because it is how a drop checker works">, carbon dioxide (CO2) (in solution) lowers pH and this is because a small proportion of the gas becomes carbonic acid (H2CO3) and that then disassociates into H+ and HCO3- ions.

A proton is H+, we've added a proton and "proton donors" are defined as acids. If we want to know how much acid we've added? We can use a <"modified titrimetric method with a base of known strength and a narrow range pH indicator">.

It may be worth highlighting that when we add KHCO3 or K2CO3 to our aquariums at no point are we adding metallic potassium (K). We are adding the potassium ion K+. Here is a video showing how K metal looks and reacts with water

Yes, agreed, you are never going to find potassium (K) as a metal, it is much too reactive. It oxidises in air and water, so you have to store in oil to keep it as potassium metal.

In nature potassium is always going to be in a mineral or solid compound or as an ion (K+), in solution (or bound to a clay, etc) and plants can only take up mineral nutrients as ions from solution.

cheers Darrel

 

[Soilwork]

Hi All,

Just a warning about inverts when using potassium bicarbonate. All my shrimp acted very strange and erratic after using it to raise pH. All MTS closed up in their shells and all crypts melted. May have added way to much to my soft water. Potential ammonia issues becoming toxic as pH rose that was never confirmed but it unlikely. I suspect the sudden TDS change was the issue given the melting of the crypts. I repeated this to confirm it was after adding potassium bicarbonate so not a fluke. Only other thread I could find was the one below. Never used it since.

Potassium (bi)carbonate and inverts

www.plantedtank.net

Regards

CJ

 

[MichaelJ]

Hi All,

Just a warning about inverts when using potassium bicarbonate. All my shrimp acted very strange and erratic after using it to raise pH. All MTS closed up in their shells and all crypts melted. May have added way to much to my soft water. Potential ammonia issues becoming toxic as pH rose that was never confirmed but it unlikely. I suspect the sudden TDS change was the issue given the melting of the crypts. I repeated this to confirm it was after adding potassium bicarbonate so not a fluke. Only other thread I could find was the one below. Never used it since.

Potassium (bi)carbonate and inverts

www.plantedtank.net

Regards

CJ

Hi @Soilwork Thanks. It essentially brings it back to my OP, because when I realized that potassium carbonates is a relatively strong base I wanted to understand the possible implication on pH without just having to wing it by measuring it with my pH test kit and possibly take comfort in a crude / erroneous low reading. With very soft water (in CO3 terms), It is very likely that an abrupt change to pH freaked out your shrimps - and regardless of KH, a large abrupt change to TDS definitely will - and might also have affected the crypts. It's the pH bit that worries me though. I keep my TDS change fairly consistent and never introduce more than a 10% net change with my 40% WCs.

Cheers,
Michael

 

[MichaelJ]

Ok, as a test I tried targeting ~1.3 KH with 330 mg of K2CO3 in 10 liters of RO water. That gave me a pH around 7.5, which is reasonably consistent with the math @dw1305 pointed to above, so for tomorrows WC's I think I will just target that and should give me 18 ppm of K as well 🙂 ... this should work out.

EDIT: Just to test: In the 10 liter test bucket above I added another 170 mg. to reach 2 KH... and it immediately spiked to pH ~9... Note to self: dosing K2CO3 is definitely not something you want to do with a cavalier attitude!

Cheers,
Michael

 

[dw1305]

Hi all,

Just to test: In the 10 liter test bucket above I added another 170 mg. to reach 2 KH... and it immediately spiked to pH ~9... Note to self: dosing K2CO3 is definitely not something you want to do with a cavalier attitude!

If you have calcium (Ca++) (or magnesium (Mg++)) ions in solution they will always moderate the pH to ~pH8. This isn't dependent on how much potassium carbonate (K2CO3) you add (until you run out of divalent cations).

This happens because of the <"common ion effect"> and the <"limited solubility of calcium and magnesium carbonate">.

cheers Darrel

 

[Nick potts]

Hi All,

Just a warning about inverts when using potassium bicarbonate. All my shrimp acted very strange and erratic after using it to raise pH. All MTS closed up in their shells and all crypts melted. May have added way to much to my soft water. Potential ammonia issues becoming toxic as pH rose that was never confirmed but it unlikely. I suspect the sudden TDS change was the issue given the melting of the crypts. I repeated this to confirm it was after adding potassium bicarbonate so not a fluke. Only other thread I could find was the one below. Never used it since.

Potassium (bi)carbonate and inverts

www.plantedtank.net

Regards

CJ

I use KHCO3 to raise my KH to 1 in my RO water, I have only been using it a few weeks but have so far not noticed any issues with my shrimp colonies. Is this an issue only if using large amounts?

I also haven't checked the PH of my freshly made water, but the tank runs at 7.3 before CO2 injection and 6.2 after.

 

[MichaelJ]

Hi all,

If you have calcium (Ca++) (or magnesium (Mg++)) ions in solution they will always moderate the pH to ~pH8. This isn't dependent on how much potassium carbonate (K2CO3) you add (until you run out of divalent cations).

So what your saying is, if I would start out adding my typical target of CaSO4.2H2O (~30ppm) and then add the K2CO3 I wouldn't see the spike, as the Ca would moderate/stabilize the pH around the equilibrium? If so, that makes me less worried about the situation.

Cheers,
Michael

 

[dw1305]

Hi all,

I wouldn't see the spike, as the Ca would moderate/stabilize the pH around the equilibrium?

Yes.

cheers Darrel

 

[MichaelJ]

Hi all,

Yes.

cheers Darrel

Awesome Darrel, thanks for clarifying... yeah, I suspected my experiment was a bit construed given that I was using pure RO water. I will run an experiment where I start adding all my minerals and salts (MgSO4,CaSO4,Mg(NO3)2,KH2PO4) and afterward the K2CO3 and see what the effect is.

Cheers,
Michael

 

[Happi]

Your absolutely right @Wookii, My answer would be that I want that little bit of buffering that 1-2 KH would give in terms of stabilizing my pH... But I am definitely not 100% sure if I really, really need it. If I would think enough about it, I would ask myself why would my pH not be stable ? what could possibly cause "severe" pH swings in a healthy well-maintained low-tech tank, and would said swings even be an issue for my plants or livestock?



Yes, your probably right... but still, I am not 100% sure zero KH is the way to go.... But I would love to hear what the experts have to say on the matter.

Cheers,
Michael

kept the 0 KH for decade now and am confident enough to say that you are more likely to kill your stock with overdose of CO2 rather than the 0 KH. you can expect the PH to stay between 5-6 and I have never seen it crash

here is a good discussion about the KH and PH: KH and plants

 

[arcturus]

Several commercial remineralization salts claim that they do not change the pH while raising the GH and KH. Given the discussion in this thread, how is this possible? Are these remineralizers using additional substances to neutralize the alkalinity increase? Or are they using other salts to increase GH and KH that have a minor effect on pH? (sorry for the dumb question, but I am not a chemist).

 

[dw1305]

Hi all,

they do not change the pH while raising the GH and KH

You can add dGH without raising the pH, using a <"neutral salt"> like calcium chloride dihydrate (CaCl2.2H2O). The only way you could add dKH without raising the pH is if the pH was already at the CO2 ~ HCO3 ~ pH equilibrium point of ~pH 8. Once you get more than about 2dKH the pH will always return to ~pH 7.8 ish, because the solubility of calcium carbonate (CaCO3) is dependent upon the amount of atmospheric CO2.

Are these remineralizers using additional substances to neutralize the alkalinity increase

If you added an acid as well, it <"would counteract the addition of dKH">, but by turning the added <"DIC / TIC from HCO3- to CO2">, which would then out-gas, so the net effect would be that you haven't actually added any dKH.

cheers Darrel

 

[MichaelJ]

kept the 0 KH for decade now and am confident enough to say that you are more likely to kill your stock with overdose of CO2 rather than the 0 KH. you can expect the PH to stay between 5-6 and I have never seen it crash

Thanks @Happi Since I don't inject, I don't have to worry about that part. Speaking about crashes - its a very vague concept IMO. What actually constitute a crash? On a daily basis the CO2 injectors are forcing a 1 - 1.5 pH drop (thats a 10-50 times change in acidity level!) over the course of 6-8 hours, so that seems to counter the general idea of the importance of pH stability... (you see a similar change in some natural habitats with low alkalinity), but I suppose the relative drop vs. the timeframe within the drop occurs may play the biggest role here. So along that line of reasoning, crashes happening out of the blue, without introducing any compounds that would drastically alter the water chemistry, in a well kept tank seem like a very unlikely event to occur. What I am worried about is the general pH level being too low. The 5-6 range would be too low for my shrimps. So having that little bit of buffering in the 1-2 KH range is likely a good idea in my case if that helps maintain the equilibrium towards the 6-7 range.

On a sidenote: I went ahead and made the WC yesterday using the K2CO3 and the rest of my minerals mixed in. When I tested in the evening my pH was 6.5-6.7 which is very close to what it was before the WC and my KH was about ~1 KH. My TDS is now down at ~180ppm. The next couple of 40% WC's should bring this particular tank down to the mid 150 ppm range which is probably the lowest level I can expect with the chemical markup of my WC water. I will try and see if I can replace at least some of the CaSO4 with Ca Gluconate, to cut back on the SO4, without the Ca Gluconate turning my water into skim milk 🙂

Cheers,
Michael

 

[MichaelJ]

I will run an experiment where I start adding all my minerals and salts (MgSO4,CaSO4,Mg(NO3)2,KH2PO4) and afterward the K2CO3 and see what the effect is.

Cheers,
Michael

Hi @dw1305 I ran this experiment today. Adding all the minerals and salts and letting it dissolve before adding the K2CO3 made the pH for the 2.0 KH target somewhere between 7.5-8... definitely nowhere near 9 pH or above as in my original experiment with pure RO and K2CO3 only. So for my purpose, given my low ~1.5 KH target, the use of K2CO3 seems to fit the bill for now.

Cheers,
Michael

 

[Cédric]

Once you get more than about 2dKH the pH will always return to ~pH 7.8 ish, because the solubility of calcium carbonate (CaCO3) is dependent upon the amount of atmospheric CO2.

Hi Darrel,
I don't get it. May you please explain the "mechanism" behind this process ?

 

[dw1305]

Hi all,

Hi Darrel,
I don't get it. May you please explain the "mechanism" behind this process ?

Limestone (calcium carbonate)(CaCO3) <"isn't soluble in water">, but it is soluble in weak acid.

Rain water becomes a weak acid as it falls, because a very small amount of CO2 dissolves as carbonic acid (H2CO3), which disassociates into a proton H+ (acids are proton donors) and a bicarbonate ion HCO3-.

When the protonated water <"encounters some CaCO3">, it is neutralised and the <"Dissolved Inorganic Carbon"> is <"converted from CO2"> into HCO3-. <"This is a buffered system"> with <"CO2 and (H)CO3 as weak acid and weak base">.

You've got to get the hydrogen ion (H+) from somewhere and it comes from the H2O, leaving behind hydroxide ion (OH-), <"bases are "proton acceptors"> and you can get back to H2O by adding a proton to the OH- ion.

Because calcium (Ca) is a divalent ion for every Ca++ion that goes into solution you need 2HCO3- ions to balance. The calcium ions (Ca++) in solution is what we describe as dGH, the bicarbonate ions (HCO3-) the dKH.

The amount of H2CO3 that dissolves is <"dependent upon the amount of CO2 in the atmosphere">. At <"420 ppm CO2"> the equilibrium pH value is pH ~8. You don't need much dKH to buffer the small addition of H+ ions from the dissolved CO2, and in practice 2dKH is plenty.

As you add more CO2 that equilibrium pH value will fall. This is how a <"drop checker works">.

Cheers Darrel

 

[Cédric]

Because calcium (Ca) is a divalent ion for every Ca++ion that goes into solution you need 2HCO3- ions to balance.

Is this related to electrical neutrality in any way ?

 

[dw1305]

Hi all,

Is this related to electrical neutrality in any way ?

Yes, solutions are electrically neutral.

cheers Darrel

 

[Cédric]

Yes, solutions are electrically neutral.

Hi Darrel,
Thank you very much. I hope you are not repeating yourself too much because of my questions. But i guess your answers are helpful not only for me (trying to ease my conscience). I am ignorant about water chemistry but UKAPS made me want to know more. Just let me know when you've had enough of these questions.