Potassium Carbonate (K2CO3) vs pH ?

[MichaelJ]

Hello, anyone of you chemistry wizards know the guiding math for how much a certain amount of K2CO3 will raise the pH?

2.5 grams of K2CO3 dissolved in 100 liter of water will raise the KH by 1 degree and add ~14 ppm of K and ~11 ppm of CO3 - but since K2CO3 is a relatively strong base how much will it affect pH?

Haven't been able to find any good guiding references relative to pH.

Cheers,
Michael

 

[JoshP12]

Hey @MichaelJ!

Could use pH/KH chart and just look when the CO2 is around 3 for the new pH (based on your new projected KH).

Example:


If your KH was zero, and you added enough to go to .5, then your pH would probably be around 6.6/6.7 and it was probably at something like 6.2/6.3? before. If you remember < this post > that guy used pure RO and had low KH around 0 --- pH around 5.7 ... probably some of that "extra" from 6.2ish would have been just decay, acids, or even error in test kit (below 6 is harder to see with bromothymol, unless he has a probe - even then calibrating is not fun).

Suppose you were at 1.5, added enough for a 1 degree increase, call it 2.5 now -- pH around 7.4. Call it a bit lower since we never quite hit the 3ppm in tank, then mask it slightly based on age + decomposition ... 7.2?

Hope that helps.

The actual shift in carbonate equilibrium can be calculated - would have to sit down and do it- but I think the above approximation works.

Josh

 

[MichaelJ]

Hi @JoshP12 Thanks much appreciated. I am still trying to figure this one out..

I did some rudimentary testing earlier today. I made two buckets of 10 liter of RO water @ TDS 7. Amounts below measured out on a fairly accurate microgram scale and tested using the fairly inaccurate API pH and KH test kits. However the TDS checks out almost spot on using my Hanna TDS meter vs the amounts of Potassium and Carbon trioxide in the K2CO3 that I added.

Bucket 1: To increase the KH by 1 I added 0.25 gram of K2CO3 and measure a pH at about 7 (could be anything between 6.5 and 7.5) and measure TDS at 32

Bucket 2: To increase the KH by 2 I added 0.50 gram of K2CO3 and measure a pH at about 9 (maxes out the Wide pH API test) and measure TDS at 54 (should be 57).

I can draw a line between these two points, but I am sure there is a more rigid mathematical correlation here. ? Even though I studied up quite a bit on basic chemistry lately, I realize I am still just mostly a math and computer science guy 🙂

Cheers,
Michael

 

[hypnogogia]

Ph scale isn’t linear, it’s logarithmic.

 

[JoshP12]

Hi @JoshP12 Thanks much appreciated. I am still trying to figure this one out..

I did some rudimentary testing earlier today. I made two buckets of 10 liter of RO water @ TDS 7. Amounts below measured out on a fairly accurate microgram scale and tested using the fairly inaccurate API pH and KH test kits. However the TDS checks out almost spot on using my Hanna TDS meter vs the amounts of Potassium and Carbon trioxide in the K2CO3 that I added.

Bucket 1: To increase the KH by 1 I added 0.25 gram of K2CO3 and measure a pH at about 7 (could be anything between 6.5 and 7.5) and measure TDS at 32

Bucket 2: To increase the KH by 2 I added 0.50 gram of K2CO3 and measure a pH at about 9 (maxes out the Wide pH API test) and measure TDS at 54 (should be 57).

I can draw a line between these two points, but I am sure there is a more rigid mathematical correlation here. ? Even though I studied up quite a bit on basic chemistry lately, I realize I am still just mostly a math and computer science guy 🙂

Cheers,
Michael

There are maths - this might give a lens into it:

https://lawr.ucdavis.edu/classes/ssc102/Section5.pdf

You will need to fix co2 with a bubbler or something in the bucket.

Also, I cross referenced Rotala butterfly and it gave me 492 mg - might sound silly but 8mg in 10 L will make a difference if you compound that error throughout.

 

[X3NiTH]

The Potassium Ion is a strong base so in the first instance there is enough acid neutralisation of the water to balance out at a lower pH adding more pushes the equilibrium further, the pH addition from the carbonate has its own rise but the potassium is busy forming hydroxides which have a much greater effect on pH hence the sky high reading.

🙂

 

[Ria95]

given the complex chemistry of the aquarium water the easiest way is to test on a small sample, but as you found out the initial pH will increase a lot. It will decrease over time as the CO3 gets converted to HCO3 by CO2 (atmospheric or injected). This is the reason most prefer to prepare their water a few days before and aerate heavily. Alternatively you can use KHCO3.

 

[Ria95]

A Brønsted-Lowry base is a proton acceptor. The potassium ion is a cation (fewer electrons than protons, positive charge), so it will not want to accept more protons but rather donate them, so not a base. K+ is not considered to have much in the way of acidity or basicity One can easily test how little K affects the pH by adding KCl at an equivalent K concentration. Then adding Na2CO3, MgCO3 or CaCO3 at an equivalent CO3 cocentration and see the major increase.

 

[MichaelJ]

given the complex chemistry of the aquarium water the easiest way is to test on a small sample, but as you found out the initial pH will increase a lot. It will decrease over time as the CO3 gets converted to HCO3 by CO2 (atmospheric or injected). This is the reason most prefer to prepare their water a few days before and aerate heavily. Alternatively you can use KHCO3.

Hi @Ria95 Yes, seems like Potassium bicarbonate (KHCO3) might be a better choice if I can avoid the huge and "unpredictable" spike in pH - I just need to raise the KH in my RO water to about 1.5 and at the same time provide a fair amount of potassium.

Cheers,
Michael

 

[arcturus]

Hi @Ria95 Yes, seems like Potassium bicarbonate (KHCO3) might be a better choice if I can avoid the huge and "unpredictable" spike in pH - I just need to raise the KH in my RO water to about 1.5 and at the same time provide a fair amount of potassium.

The issue with using KHCO3 to remineralize the RO is that we cannot keep the remineralizer mix and the fertilization mix independent due to K. Using KHCO3 to raise 1.5 dGH adds ~20 ppm K in the process.

The fertilizer mix uses monopotassium phosphate (KH2PO4) for PO4, which will further increase K. This means we probably need to find another source for NO3 because KNO3 will further increase K. Magnesium nitrate Mg(NO3)2 might be an option.

But this raises a question about the fertilization regime. With this mix, we would be adding almost 100% of the weekly K at water change. But NO3 and PO4 would then be added incrementally during the week... or should they also be added at the same time as K, during the WC? Has anyone experience dosing at near EI levels (with CO2 injection) just once a week, at WC?

 

[X3NiTH]

KCl is charge balanced in water whereas K2CO3 isn’t so hydroxide has to form to balance the charge.

🙂

 

[dw1305]

Hi all,

One can easily test how little K affects the pH by adding KCl at an equivalent K concentration.

KCl is charge balanced in water whereas K2CO3 isn’t so hydroxide has to form to balance the charge.

I was just going to write that, anyway now I'll agree.



@X3NiTH is right, potassium chloride (KCl) is a <"neutral salt"> because is the product of the reaction between a strong base (KOH) and a strong acid (HCl). Potassium (K) is a <"group one alkali metal">. Group 1 metals are highly reactive and only occur as compounds (salts), <"those salts are all soluble">.

If you had any meaningful amount of K2CO3 the pH will rise, if you wanted to work out theoretically how much you would need, <"the calculations are here">.

cheers Darrel

 

[MichaelJ]

If you had any meaningful amount of K2CO3 the pH will rise, if you wanted to work out theoretically how much you would need, <"the calculations are here">.

Thanks Darrel, thats exactly what I was looking for 🙂

Cheers,
Michael

 

[MichaelJ]

The issue with using KHCO3 to remineralize the RO is that we cannot keep the remineralizer mix and the fertilization mix independent due to K. Using KHCO3 to raise 1.5 dGH adds ~20 ppm K in the process.

That is fine with me... I was intending to use the K2CO3 if I can get it to work, or the KHCO3 as a replacement for the Tap water I am mixing in with my RO water... (My tap water runs thought a KCl resin based water softener so I have a huge amount of K and plenty of CO3 in my tap water but no Ca or Mg so I essentially source all my CO3 and Potassium from my tap water, but I also get unknown quantities of other compounds...) The idea was to lower my TDS a bit by not adding as much K (and potentially Cl) But looking at the numbers for KHCO3 I am starting to think this is going to be a wash vs. my overall TDS as KHCO3 will add an additional 17ppm to my TDS compared to K2CO3 for the KH 1.5 and K (20 ppm). Then I might as well just stick to what I am doing now with my Tap water mix.

The fertilizer mix uses monopotassium phosphate (KH2PO4) for PO4, which will further increase K.

Yes, but only fairly marginally - if I target 5 ppm of PO4 I only get 2 ppm of K...

This means we probably need to find another source for NO3 because KNO3 will further increase K. Magnesium nitrate Mg(NO3)2 might be an option.

Yes, I am using Mg(NO3)2 which I think works well.

But this raises a question about the fertilization regime. With this mix, we would be adding almost 100% of the weekly K at water change. But NO3 and PO4 would then be added incrementally during the week... or should they also be added at the same time as K, during the WC? Has anyone experience dosing at near EI levels (with CO2 injection) just once a week, at WC?

I am not injecting CO2 but I do my whole dose of EI level dosing once a week. I usually mix it into my WC water.

Cheers,
Michael

 

[Wookii]

The real question we have to ask @MichaelJ is why we are bothering to add carbonates and increase the KH at all (and I’m including myself in that ‘we’ because I add K2CO3 to remineralise my RO also).

I have always added it as something of a safety net to counter the assumed pH reducing effects of CO2 injection. However I’m acutely aware I’m doing this blindly without any real understanding of why. @dw1305 or @X3NiTH might be able to explain why it isn’t really necessary.

For practical examples, @Conort2 (if you read his journal here) injects CO2 whilst maintaining a zero KH with no ill effects.

Edit: sorry, I’ve just read you don’t inject CO2 anyway, so the case for not adding anything to boost the KH is even stronger.

 

[MichaelJ]

The real question we have to ask @MichaelJ is why we are bothering to add carbonates and increase the KH at all (and I’m including myself in that ‘we’ because I add K2CO3 to remineralise my RO also).

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?

I have always added it as something of a safety net to counter the assumed pH reducing effects of CO2 injection. However I’m acutely aware I’m doing this blindly without any real understanding of why. @dw1305 or @X3NiTH might be able to explain why it isn’t really necessary.

For practical examples, @Conort2 (if you read his journal here) injects CO2 whilst maintaining a zero KH with no ill effects.

Edit: sorry, I’ve just read you don’t inject CO2 anyway, so the case for not adding anything to boost the KH is even stronger.

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

 

[Ria95]

It really depends on the lifestock and the bioload of the tank. A little KH can go a long way if you don't have a lot of acid producing reactions in the tank. Other components like phosphates can buffer the pH and prevent wild swings.

The Potassium Ion is a strong base

Please see in the image posted by Darrel which component of K2CO3 goes on to act as a proton acceptor and is labeled a base. That is the point I am making.

View attachment 179890

 

[dw1305]

Hi all,

which component of K2CO3 goes on to act as a proton acceptor and is labeled a base

A Brønsted-Lowry base is a proton acceptor

I like the <"Bronsted-Lowry"> definition of acids and bases, I think it <"makes things easier">. I'm not a chemist and I really struggled with alkalinity and buffering before it was explained to me in terms I could understand.

....... OK, this is because KH2PO4 is an acid (you have two hydrogen atoms), and the tests for dKH actually measure alkalinity. You've added an acid ("H+ ion donor"), so alkalinity has declined.

The conjugate base for KH2PO4 is K2HPO4 (which is a base or "H+ ion acceptor", because you have two potassium atoms) and together they are phosphate buffers which you can combine together, in differing proportions, to make <"buffered solutions of a known pH">.

Carbonate hardness (dKH) is also a measure of a buffered system, but in this case the acid and base are CO3-- (HCO3-) and H2CO3 (from CO2). If you want to add carbonate hardness you can use potassium bicarbonate (KHCO3) (or potassium carbonate (K2CO3)), have a look at <"Questions regarding....">, there are amounts to add at <"James' Planted Tank">

<"Potassium (K) is basic">. If you look at the periodic table of the oxides of elements, bases are to the left and the acids to the right (excluding the "noble gases").

Periodic table showing basic (blue), amphoteric (green) and acidic (red) oxides. The metal-nonmetal boundary is indicated by the grey staircase line.

Water is an amphoteric compounds. A substance that can act either as an acid and as a base.

H2O molecules may either donate a hydrogen ion or accept one. This property makes water an amphoteric solvent. In the situation where an acid dissociates in solution, water is acting as a base. Conversely, water acts as an acid when bases dissociate. ..........

cheers Darrel

 

[Wookii]

... But I am definitely not 100% sure if I really, really need it.

Yes, your probably right... but still, I am not 100% sure zero KH is the way to go....

I'm not sure either to be honest, it would be good to get a definitive answer. Obviously in natural bodies of water like blackwater habitats you frequently get very low TDS, and presumably zero carbonates - but I am cautious in comparing a natural body of water with a glass box.

Other components like phosphates can buffer the pH and prevent wild swings.

Do you know what levels phosphates would need to be in the water column in order to successfully achieve that. For example would the typical dosing ranges of 1-3ppm be sufficient?

Hi all,


I like the <"Bronsted-Lowry"> definition of acids and bases, I think it <"makes things easier">. I'm not a chemist and I really struggled with alkalinity and buffering before it was explained to me in terms I could understand.

<"Potassium (K) is basic">. If you look at the periodic table of the oxides of elements, bases are to the left and the acids to the right (excluding the "noble gases").

cheers Darrel

The chemistry is well above my paygrade Darrel.

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?

If we don't any carbonates what are the risks if any, and how does that picture change if we inject CO2?

 

[Ria95]

Nice find on the table. Please notice that the table describes K2O as a basic oxide , not "the potassium ion" K+ . There is a difference. In a similar manner one can say KOH is a base, NaOH is a base, -OH is a base. K+ , Na+ is not a base . A Brønsted-Lowry base is a proton acceptor. If we have a proton H+ is it more likely to be accepted by OH-, CO3 (2-) or by K+ ? It's really that simple in this case.

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 .

Do you know what levels phosphates would need to be in the water column in order to successfully achieve that. For example would the typical dosing ranges of 1-3ppm be sufficient?

It will buffer at any concentration. If it's enough to prevent swings depends on how much acid is produced, other buffers in the system... At 3ppm it's a small player compared to 1KH from HCO3