VOLUMETRIC DETERMINATION OF CARBONATE AND BICARBONATE -...

21
VOLUMETRIC DETERMINATION OF CARBONATE AND
BICARBONATE
DETAILS DUE DATE
DURATION OF LAB: September 22-25, 2003 September 30, 2003
READING ASSIGNMENT: pp. 203-219, pp. 232-235
PROBLEM ASSIGNMENT: 11-11, 11-13, 11-19, 11-20, 11-
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September 23, 2003
ADDITIONAL ASSIGNMENTS:
*All page number in reference to 6
th
Ed. Quantitative Analysis, D.C. Harris
Background
Acid base titrations with weak acids and bases represent well the challenges and complexities of
many biological, medical, environmental, and industrial measurements. Further, mixtures
represent well the demands faced in modern analytical science. In the present laboratory, you will
receive a mixture containing sodium carbonate (a weak base), sodium bicarbonate (a weak acid
and base) and other un-reactive substances. Techniques used here will provide a sophisticated
analytical challenge with a demand to understand of weak acid-base equilibria and calculations
involving dissociation constants. The weak acid base character of these analytes and the
difficulties in resolution of composition using indicators will cause us to use some maneuvers not
commonly used in undergraduate laboratories.
The central challenge in this measurement is that we need to quantitatively determine both HCO
3
-
or CO
3
-
and then one of the components alone. The remaining constituent then can be calculated
by subtraction of the one component from total alkalinity determination (mindful of stoichiometry).
A titration of your sample directly with strong acid will lead to reactions with both carbonate and
bicarbonate in stoichiometric proportions (see reactions on last page of this laboratory). The sum
of all acid moles or equivalents used is a useful piece of information called total alkalinity in water
quality or environmental studies; but, total alkalinity alone does provide quantitative results for the
mixture of carbonate and bicarbonate.
But, how can we measure HCO
3
-
or CO
3
-
using volumetric methods? Sadly, end points to
distinguish individual constituents with mixtures such as NaHCO
3
and Na
2
CO
3
are difficult to
recognize using indicators or dyes. Perhaps a potentiometric titration might be possible using a
pH indicating electrode, but that is not our goal. The goal in this laboratory is to use the technique
of back-titration with analysis of a mixture (through a little help from precipitation chemistry).
One approach might be to titrate with strong base to determine HCO
3
-
however, the endpoint will
be difficult to determine. Thus, our measurement challenge would be simplified if we could
remove or isolate the bicarbonate from the carbonate. But this cannot be done easily or
conveniently. Both of these complications can be solved using the method of excess reagent and
back-titration with strong reagents. What is a back-titration? In a back-titration, an exact and
excess amount of strong base is added to solution to react with HCO
3
-
(there will be no reaction
between OH
-
and CO
3
-
) and some OH
-
will remain as residual base in solution. We may arrange a
clean back-titration of residual OH
-