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Contents:
Excess CaCO3 ("A.J deLange")
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Date: Fri, 1 May 2009 13:55:32 -0400
From: "A.J deLange" <ajdel at cox.net>
Subject: Excess CaCO3
Matt posted yesterday about storing brett over an excess of CaCO3 and
wondering what the pH of the solution might be. This is a problem that
can be handled by the Nearly Universal Brewing Water Spreadsheet (you
knew I was going to say that, didn't you?). What may be surprising to
most is that the independent variable here and the thing that sets the
pH is the partial pressure of CO2 over the container. I'm going to
describe briefly how the answers to be given were obtained from NUWBS
(obtainable at www.wetnewf.org). If you want to try this the remarks
should suffice to get you started. Otherwise just look at the answers.
Start by setting up the NUBWS for deionized source water (carbo mode
"C", value 0, pH 7.0, temperature 20 C) and zero all inputs to the
Synthesis section. Now ask the Solver to set proton deficit to 0 while
varying added CaCO3, CO2 and pH subject to the constraints that the
pressure of CO2 is 0.0003 atmospheres and that the saturation pH
equals the actual pH. Solver will find pH 8.33. This is the pH DI
water over chalk will come to if it is allowed to stand in air at a
partial pressure of 0.0003 but it may take days for this to be
reached. IOW we can calculate the equilibrium pH from thermodynamics
but can't say how long it may take to reach it. NUBWS will also
calculate that 55 mg/L of CaCO3 will be dissolved.
Should Al Gore be right and the partial pressure of CO2 double to
0.0006 atm the equilibrium pH would drop to 8.13 and believers worry
about, for example, what the effects on marine animals might be
(generate more shell material and send it to the bottom or die - shell
material is more soluble - and really louse up the system).
Now when we add the brett and they start to chug away and produce acid
the situation isn't quite so simple. Evolved CO2 will accumulate in
the container so that the partial pressure of that gas over the water
will be higher. Lets assume that you do this in an open beaker with a
fan blowing over it so that evolved CO2 is swept away and the partial
pressure of CO2 is again 0.0003 atm. Reset the CO2 pressure constraint
to 0.0003 atmospheres in NUBWS, choose citric as the 'other' acid and
enter 10 mg/L of it to simulate the acid production of the yeast. Now
have the Solver do its thing again. It should calculate a pH of 8.31
and show 61 mg/L chalk dissolved. For 20 mg/L citric produced the pH
will drop to 8.29 and 67 mg/L chalk will dissolve. Thus it appears
that the extra acid produced by the yeast dissolves a bit more chalk
than the CO2 from the air does by itself.
Now suppose that you had the yeast and chalk in a flask with an
airlock. Eventually the CO2 would fill the flask to the point where
the partial pressure of CO2 over the liquid was 1 atm. Put that in for
the CO2 pressure constraint ad the Solver will find a pH of 5.83 for
20 mg/L citrate with 572 mg/L chalk dissolved. With 20 mg/L citrate the
20 mg citrate the pH changes by less than 0.01 unit (i.e. still 5.83
to 2 decimal places) and the dissolved chalk is up to 576 mg/L.
Thus you can conclude that the acid produced by the yeast has
relatively little effect on the pH relative to the exposure of the
broth to CO2 with whether or not you allow the evolved CO2 to
accumulate or not having a major effect.
Remember again that these calculations are based on thermodynamics and
may not reflect what you observe unless you take steps to see to it
that equilibrium conditions are being approached. Shaking the flask to
liberate dissolved CO2 would be an example of one action that will
move the system towards equilibrium. Sweeping away evolved CO2 with a
fan as mentioned above would be another.
A.J.
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End of HOMEBREW Digest #5544, 05/01/09
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