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Calculating Carbonic Acid Content
Name:Lachlan
Status: student
Grade: 9-12
Location: N/A
Country: USA
Date: November 2007
Question:
My group has been doing a term
assignment that is assessing the solubility of
Carbonic Acid in soda water at different
temperatures. For this test we heat the soda
water to successive temperatures (30 deg C, 40
deg C, etc.) and once at the temperature we
titrate using 0.01M NaOH with Phenolphthalein.
We have all our results but the problem now
is... we cannot find the method to calculate the
actual Carbonic Acid content (anywhere on the
Internet). Carbonic Acid has 2 dissociations:
H2CO3 HCO3 + H+ Ka1 = 2.5×10 4 mol/L; pKa1 =
3.60 at 25 °C. HCO3 CO3(2 ) + H+ Ka2 = 5.61×10
11 mol/L; pKa2 = 10.25 at 25 °C. We used 20mL of
soda water for the titrations. The closest
result for example at 25 deg C is 31.9mL of
titrant (0.01M NaOH) used.
Replies:
Lachy,
The titration of the carbonic acid is independent of its
dissociation constant. Imagine if you were titrating the
H(+) in solution, as you consume the H(+) (by reacting it
with the base), by the Le Chatelier Principle, the
equilibrium will shift to the production of more H(+).
This will happen until all the H(+) from the source
(carbonic acid) is reacted with the base. Therefore,
the standard stoichiometric techniques of calculating
molarity will still work.
Greg (Roberto Gregorius)
This is NOT an easy experiment -- for several reasons:
1. Phenolphthalein changes color over a range of pH values between 8 and 10.
That is a factor of 100 in terms of concentration of hydrogen ion, so the
amount of indicator solution and a sharp eye is required to get reproducible
results.
2. There is a significant concentration of carbon dioxide in the atmosphere
that is readily absorbed by alkaline solutions, so your sample is acting as
an absorber of atmospheric CO2, which will distort the results.
3. You indicate that you are titrating with NaOH. Sodium hydroxide reacts
with atmospheric CO2. All carbonate titrations should be carried out with
KOH instead. The latter is much less sensitive to atmospheric CO2. Any one
or combination of these complications will cause your results to be inaccurate.
While I do not have a procedure in hand, I think a more reliable approach
might be to allow each solution to equilibrate at the various temperatures
and then add a substantial excess of an insoluble salt, for example, CaCl2,
rapidly and filter or centrifuge the insoluble carbonate.
But even before undertaking this alternative, use the Ka's to make sure
that you will be able to distinguish changes in the solubility of the
carbonic acid over the temperature range you intend to use. Remember that
you may also have to take Henry's Law for CO2 into consideration.
Good luck on a tough experiment.
Vince Calder
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Update: February 2012
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