[kramerog]

kamerog mentions bicarbonate (HCO3-). the relation between H2CO3 and HCO3- is determined by pH of the solution and at beer pH (~4.3) the vast majority of the carbo species will be H2CO3. So we are stuck here.

I wrote a longer explanation but I timed out so here is the gist.

Anyway, if my calcs are correct [HCO3-]= 4 [H2CO3] because Ka1=2 x 10-4.

Kai, perhaps you used the commonly given value of Ka1 is 4.45x10-7 which is based on the concentration of aqueous CO2 and H2CO3 combined?

[Kaiser]

Kai, perhaps you used the commonly given value of Ka1 is 4.45x10-7 which is based on the concentration of aqueous CO2 and H2CO3 combined?

Yes, I was working with  Ka1 = 4.45x10-7 by looking at commonly used distribution of carbo species over pH:



We can't use this simplification here.

So the pKa1 for HCO3- + H+ <-> H2CO3 is around 3.7 then. At beer pH this should give you about 10% HCO3- and 90% H2CO3. I'm eyeballing this from the above chart assuming the shift of pKa1.

That would allow us to create 10 times more (H2CO3 +HCO3-) from the dissolved CO2. But that reduces the dissolved CO2 by 2% instead of 0.2% which may not be enough to explain  this effect.

Further more, in a draft system the head pressure is constant and thus the CO2 concentration has to remain constant. And this effect has been noted both for daft and bottled beer.

This is a good discussion and I'd like to keep it going. Please feel free to correct any incorrect assumptions that I'm making.

Kai

[kramerog]

That would allow us to create 10 times more (H2CO3 +HCO3-) from the dissolved CO2. But that reduces the dissolved CO2 by 2% instead of 0.2% which may not be enough to explain  this effect.

Kai

I agree that if only 2% of the dissolved CO2 is in a carbonate form then it would be hard for me to imagine that fine carbonation could be explained solely by the slow dehydration of carbonic acid.  I believe that with [HCO3-] being the dominant form of carbonate in beer that carbonates could reversibly react with metals, sugars, protein, alcohol etc. as alluded to by Brungard.

[Kaiser]

I believe that with [HCO3-] being the dominant form of carbonate in beer that carbonates could reversibly react with metals, sugars, protein, alcohol etc. as alluded to by Brungard.

That might be possible, but we are still far from a good explanation though.

On the time scale of things, how long does it take for the CO2+H2O -> H2CO3 to be 50% towards equilibrium? I see the kintetic constants, but don't know how to use them since I have not dabbled in reaction kinetics yet since there was no need so far.

Are we talking seconds, minutes days or weeks here?

Kai

[kramerog]

On the time scale of things, how long does it take for the CO2+H2O -> H2CO3 to be 50% towards equilibrium? I see the kintetic constants, but don't know how to use them since I have not dabbled in reaction kinetics yet since there was no need so far.

Are we talking seconds, minutes days or weeks here?

Kai

I was a practicing chemical engineer so I should be able to handle the reaction kinetics after I review my kinetics textbook.  Where did you find the kinetic constants?

Roger (Kramer Original Gravity)

[Kaiser]

Where did you find the kinetic constants?

Wkipidia

The rate constants are 0.039 L mol−1 s−1 for the forward reaction (CO2 + H2O → H2CO3) and 23 s−1 for the reverse reaction (H2CO3 → CO2 + H2O).

Kai

[kramerog]

On the time scale of things, how long does it take for the CO2+H2O -> H2CO3 to be 50% towards equilibrium? I see the kintetic constants, but don't know how to use them since I have not dabbled in reaction kinetics yet since there was no need so far.

Are we talking seconds, minutes days or weeks here?

Kai

After an extended trip down memory lane and looking for books I no longer had, I constructed a scenario where I assumed that during carbonation it was possible to reach equilibrium for [CO2 aq] rapidly while the conversion to H2CO3 was negligible because that conversion to carbonic acid is "slow."  The equation for carbonic aid is then according to my very rusty differential equations is then
[H2CO3] = k1/k2 [CO2 aq] x (1-exp(-k2t))

I'm not quite sure if I did everything correctly, but the form of the equation should be correct as the equation correctly predicts the equilibrium concentration of H2CO3 = k1/k2 [CO2 aq], with k1/k2 = the equilibrium constant and the form looks right from hazy recollection of reaction kinetics.

According to this equation, 50% of the equilibrium concentration of H2CO3 is achieved within 0.03 seconds.

I think if I did the derivation correctly and Wikipedia is correct that this pretty much blows my theory of the slow conversion of aqueous CO2 to carbonic acid as an explanation for achieving fine carbonation a few days after achieving gross carbonation away.

But I'm going to stick to my guns anyway.  Some kind of acidification reaction takes a long time to occur because I've noticed in carbonating tap water that the water doesn't develop an acidic bite as quickly as gross carbonation is achieved.

Any ideas, Kai, mabrungard or others?

[Kaiser]

Thanks.

I think what's needed now are controlled experiments. They can be on a small scale. One experiment I was thinking about is this:

Take clear beer (there are theories that tie the smoothing carbonation to particulate that settled and we need to eliminate this) and fill it into 2 small soda bottles. Attach carbonator caps (ball lock quick connects) and keep venting the CO2. It makes sense to create foam initially and allow the foam to come out of the quick connect to purge any O2 that may change the taste of the beer.

Allow the beer to decarbonate to a head space pressure of 100 kPa (atmospheric pressure). By regularly venting. Now carbonate one beer with CO2 and let it sit for 2 weeks. Then carbonate the other and let it sit for 1 day. Now compare carbonation quality of both beers.

This is easy enough to do. I only need a 2nd carbonator cap.

Kai

[tomsawyer]

Ever do the experiment Kai?

Doesn't carbonation drop pH significantly?  Isn't this a big part of why beer is more acidic than wort?  I'd suppose that tying up carbonic acid and bicarbonate as salts with organic and/or inorganic cations would allow the hydration reaction to proceed to a greater extent than the hydration constant would predict.

Still, I don't think the "finer bubbles" are coming from reduced dissolved CO2 levels per se, but they could be due to changes in beer proteins as pH lowers further.  Maybe more proteins drop out at their isoelectric point and and there are more and smaller nucleation sites for the gas to form back into bubbles.

Sorry to resurrect an old thread, someone asked me a question about this the other day and it got me searching the web for answers.

[dean_palmer]

I usually do a rapid force carb following a fairly controlled method, then leave to stabilize for about a day connected at serving pressure. Here's my info http://thebeerjournals.com/carbonation.html

Either set and forget, or follow a structure. If you have variables you will eventually have problems and be the one posting about foamy flat beer problems :-)