Maximum solubility of O2 in water

[mr_wibble]

Greetings earth brewers and brewsters,

I was doing a bit of reading on under and over oxygenating wort. Obviously both are possible, but under oxygenating is the bigger problem. Fine. That's the baseline theory - you want the correct amount if at all possible.

But given it's really difficult to dissolve CO2 in room temperature beer at low pressure, I wondered if it was even possible to over oxygenate wort at all.

Wyeast says[1] generally, yeast want > 10ppm dissolved oxgyen. And using pure O2 for 60 seconds gives ~12ppm.

However assuming I'm reading it correctly, this graph:



Shows the solubility of O2 in (pure) water just under 10mg/litre (= 10 ppm) at 20oC [2]

Am I reading this incorrectly?
To me this says it's theoretically impossible to dissolve much more than 10ppm into pure water (at 1 bar / 20oC). Pure water being the best case.
Therefore, it is impossible to over oxygenate without further cooling, or higher pressure - right?

Obviously processes differ, but my water temperature is usually a bit under 20oC, and that's the best I can chill to. Sometimes I'll put it in the fridge before pitching, but usually it gets O2 + yeast at 20, then straight into the fridge (typically set to 18). Some over-estery Belgian-style beers led me to believe I was under-O2-ing, perhaps this is part of the reason.

[1] https://www.wyeastlab.com/oxygenation/ .
[2] http://www.engineeringtoolbox.com/oxygen-solubility-water-d_841.html .

 

[ianh]

I think that's basically correct. You may be able to get a slightly higher amount of oxygen in water when bubbling oxygen in. But part of that would not be dissolved and would be released to give you about 10 ppm (9.1) at 20 C and normal atmospheric pressure at equilibrium.

 

[MHB]

O2 solubility in wort is lower than it is in pure water, at Ale pitching temperatures I doubt its possible to over Oxygenate.
At Lager temps it is a possibility and its worth remembering that at high concentrations Oxygen is toxic to yeast. If you're worried just wait for 10 minutes between aerating and pitching, Oxygen comes out of solution faster than say CO2 does (not as fast as N2) but will reach the temperature pressure equilibrium pretty quickly.
Without a Dissolved O2 meter we are all taking an educated guess but fortunately most of the natural processes are pretty much working in our favor.
Mark

 

[technobabble66]

Always good to read more info on O2.
Thanks for that image, Mark. I definitely prefer to see that kind of published/research info. Bit more conclusive. [emoji4]
Also good to see that graph in the OP, Mr Wibble. Again, I prefer that kind of conclusive (& visual!) evidence.
Both save me much googling!

 

[mr_wibble]

What book/paper is that from Mark?

Basically this tells me, that at anything above 5oC and normal air-pressure, it's impossible to over oxygenate wort.
Perhaps it's a bit high in the very short term, but then anything excess should be soon out the airlock.

One less thing I have to be totally paranoid about.

 

[MHB]

Handbook of Basic Brewing Calculations, Steve (Brewman) gets them in from time to time, not cheap by home brewing standards but a bargain when talking about textbooks.
Mark

 

[Yob]

At Lager temps it is a possibility and its worth remembering that at high concentrations Oxygen is toxic to yeast. If you're worried just wait for 10 minutes between aerating and pitching,

sorta :icon_offtopic: but...

should yeast not be present during aeration? Ive not noticed any ill from doing so but if there is a rule of thumb,,,,, (the 10 minute thing?) I'd be happy to know why

 

[Brewman_]

Handbook of Basic Brewing Calculations, Steve (Brewman) gets them in from time to time, not cheap by home brewing standards but a bargain when talking about textbooks.
Mark

Yeah! Order them this week. Bumped me on this.
A few books still available. At cost.

 

[Lyrebird_Cycles]

The solubility of a gas in a liquid is governed by Henry's Law, which states that the equilibrium concentration is proportional to the partial pressure of the gas above the liquid. The constant of proportionality at a given temperature is the Henry's law coefficient, for oxygen in water at 20 oC this is about 770 l. atm / mol.

The coefficient changes with temperature according to the Van t'Hoff relationship K1 / K2 = e[C(1/t1-1/t2)]. The Van t'Hoff factor for oxygen in water is about 1700K.

Wort is generally 80-90% water, say 87% as an average value.

Unpacking this, for normal wort at atmospheric pressure at 20 oC the equilibrium oxygen concentration is 0.21 atm * .87 water / 770 = 0.24 mM = 7.6 mg/l (ppm),

Under pure oxygen, the equilibrium is much higher, for the same conditions it will be 1 atm * .87 / 770 = 1.1 M = 36 mg/l (ppm).

If the temperature changes the Van t”Hoff relationship kicks in, for 15 oC (typical pitching temp) the the new coefficient is given by K1 / K2 = e[1700 (1/ 288 – 1/293)] so the Henry’s Law coefficient at the new temperature becomes 770* e[1700 (1/ 293 – 1/288)] = 770 * 0.9 ~= 695.

Plugging this into the eqs given above the new equilibria are 0.2mM = 8.4 mg/l (ppm) and 1.25 mM = 40mg/l (ppm).

These figures are broadly in agreement with the figures Mark posted.



Take home lesson: it is very difficult to over aerate wort but quite easy to over oxygenate it.



Note that Henry's Law applies at infinite dilution with no reactions present, neither of these constraints is completely true of wort but it's close enough.

 

[MHB]

Yob - I think both options are open, I know commercial breweries who pitch before, after and during aeration.
I wouldn't want to leave an aerated wort sitting around too long for a couple of obvious reasons, but 10 minutes or so either way shouldn't make a great deal of difference.
Mark

 

[TSMill]

The graphs are showing equilibrium concentrations. When you oxygenate using pure oxygen, you are creating a saturated oxygen environment....that is, given enough time the oxygen will come out of soluton and equilibrate at a lower concentration. Sorry but nothing in the graphs to support your hypothesis.

 

[Yob]

If the temperature changes the Van t”Hoff relationship kicks in, the new coefficient is given by C1 / C2 = e[C(1/t1-1/t2)] so for 15 oC (typical pitching temp) the relationship is C1 / C2 = e[1700 (1/ 288 – 1/293)] so the Henry’s Law coefficient at the new temperature becomes 770* e[1700 (1/ 293 – 1/288)] = 770 * .9 ~= 695.

Thus it is indeed very difficult to over aerate wort but quite easy to over oxygenate it.

my eyes, in all honesty, blur when I look at **** like this.. it's like my brain is protecting itself, text like that is friction-less to my eyes..

fast forward to last line.. solid gold

Ive read it elsewhere, and was probably Mark again, but I dont worry to much about aeration for the shed Brewery