Potassium Carbonate (K2CO3) vs pH ?

[dw1305]

Hi all,

I hope you are not repeating yourself too much because of my questions.

No, you are good. We have people browsing the forum all the time, and if people find content of interest they may become members etc.

I am ignorant about water chemistry but UKAPS made me want to know more.

I'll be honest, I didn't <"really understand"> a lot of water chemistry before I joined UKAPS.

cheers Darrel

 

[Cédric]

Hi all,

No, you are good. We have people browsing the forum all the time, and if people find content of interest they may become members etc.

I'll be honest, I didn't <"really understand"> a lot of water chemistry before I joined UKAPS.

cheers Darrel

It gives me hope. Can you tell me if these statements are true ? :

• If my water is pH 7 , KH 5 and GH 10, adding moderate amount acid or base will not move the pH (or only a little) ;
• If my water is pH, KH 5 and GH 10, adding KH will raise the pH to the equilibrium point (~pH 8) ;
• If KH is only KH3 the pH is likely to fluctuate (depending on CO2:O2 ratio for example) but its upper limit will be the equilibrium point pH 8.

 

[dw1305]

Hi all,

Can you tell me if these statements are true ? :

Sort of yes and no.

For your question we can ignore the dGH, although dGH and dKH are <"linked if they both come from the dissolution of CaCO3">, you can add dGH without adding any carbonates via a <"neutral salt">, like CaCl2.nH2O or MgSO4.7H2O <"Some handy facts about water">.

If my water is pH 7 , KH 5 and GH 10,

If you have pH7 you won't have KH5, because of the pH dependent nature of the form of the <"Dissolved Inorganic Carbon">. You could theoretically maintain the pH at pH7 by adding another <"weak acid / weak base buffer pair">.

If my water is pH, KH 5 and GH 10, adding KH will raise the pH to the equilibrium point (~pH 8) ;

If you don't have another buffer present the pH will rise to pH~8

If my water is pH 7 , KH 5 and GH 10, adding moderate amount acid or base will not move the pH (or only a little) ;

When you add an acid it will convert the bicarbonate (HCO3-) to CO2, some will be outgassed and the dissolved portion will lower the pH. You can lower the pH because you no longer have 5 dKH, some of that has been converted into CO2. If you have a reserve of carbonates (from limestone (CaCO3) etc) that will go into solution. If you add a base the pH will rise, unless pH is being artificially maintained by another (non-carbonate) buffer system.

If KH is only KH3 the pH is likely to fluctuate (depending on CO2:O2 ratio for example) but its upper limit will be the equilibrium point pH 8.

Yes, exactly that.

In exactly the same way if you have a very heavily carbonate buffered system (like sea water) it needs a lot of added acid to lower the pH. You can see this from the <"pH ~ dKH ~ CO2 chart">.

The whole concept of <"pH stability"> is what allows companies to <"sell pH buffers"> etc. There is more explanation in <"Corydoras in hard water...">

cheers Darrel

 

[Cédric]

If you have pH7 you won't have KH5, because of the pH dependent nature of the form of the <"Dissolved Inorganic Carbon">.

Does it means KH would be much higher at pH7 because of the high proportion of HCO3- ?

 

[John q]

It gives me hope. Can you tell me if these statements are true ? :
[LIST
[*]If my water is pH 7 , KH 5 and GH 10, adding moderate amount acid or base will not move the pH (or only a little) ;]
Probably won't have a 7ph reading in kh 5 water....
[*]If my water is pH, KH 5 and GH 10, adding KH will raise the pH to the equilibrium point (~pH 8) ;
Yes raising the kh will likely raise the ph.

[*]If KH is only KH3 the pH is likely to fluctuate (depending on CO2:O2 ratio for example) but its upper limit will be the equilibrium point pH 8.
[/LIST]

Don't know 😕

 

[dw1305]

Hi all,

Does it means KH would be much higher at pH7 because of the high proportion of HCO3- ?

Other way around, the pH would be higher (pH ~8) at 5 dKH.

The only way it could be <"maintained at pH 7">, would be by adding another buffer pair, either <"sodium citrate (HOC(COONa)(CH2COONa)2 · 2H2O) & citric acid" (HOC(CO2H)(CH2CO2H)2)> or "<phosphate buffers"> (Na2HPO4.7H2O & NaH2PO4.H2O) etc.

You can get buffer pairs (a conjugated weak acid and base pair) that maintain any <"particular pH level">, useful in human physiology, but absolutely pointless in aquariums.

cheers Darrel

 

[Cédric]

Hi all,

Other way around, the pH would be higher (pH ~8) at 5 dKH.

Hmmmm...3 statements and only 1 OK. My note is 3,33/10 and make me a bad student. Sad ! Lol. In low-tech aquarium (no CO2 injection) it happens that pH do no reach the equilibrium point (~pH8) with KH 2 or higher (tap water with CaCO3). It can be 7,4 or 7,5 for example. Does it means there are only 2 options in that case : 1/ test kit is not accurate enough 2/ Another buffer maintain pH under equilibrium point. No other option at all ?

 

[dw1305]

Hi all,

it happens that pH do no reach the equilibrium point (~pH8) with KH 2 or higher (tap water with CaCO3). It can be 7,4 or 7,5 for example. Does it means there are only 2 options in that case : 1/ test kit is not accurate enough 2/ Another buffer maintain pH under equilibrium point. No other option at all ?

Yes, one or the other. Because of the <"log10 nature of the pH scale"> when <"you are around pH 7"> it is any <"minor difference in ratio of proton donor : acceptor"> that changes the pH, so I'd guess that it is possibly the test kit.

I always look on pH measurement as a bit like photocopying colours, fine with red and blue, but much more difficult with muted tones.

One of the problems with pH measurement is that it is easy in salt rich, carbonate buffered water (the ocean etc.), but you already know that it is pH 8. Same with RO or rainwater, when you add plenty of humic substances (or a couple of drops of an acid) and you will have pH4 or similar.

cheers Darrel