Spectrophotometric Determination of an Indicator's pK
Version of 9/4/10
From 1990 to 2004, a variant of this lab was part of Chem 223 and its predecessors.
problems making the lab work reliably, it was dropped.
With several changes, we re-introduced the
experiment in 2009.
It worked better, but it still wasn't all that we hoped.
The changes included
measuring the pH of buffers, not just hoping they were as marked.
It also included improvements to the
way we dealt with activity coefficients.
Now, for 2010, we include two more improvements.
First, the way
that data are analyzed is being improved.
Second, we include instructions on doing adequate signal
averaging with the spectrophotometers to improve data precision.
Will the lab finally work?
If it fails, will
it fail in a way that indicates how to change it for the better (as it did in 2009)?
If it works, everyone will
get a nice, neat result.
If it fails, your insight can perhaps point the direction to future success.
this as a graded research project.
R. W. Ramette,
Chemical Equilibria and Analysis
, Addison-Wesley, Reading, MA, 1981, Chap. 13.
H. A. Laitinen and W. E. Harris,
ed., McGraw-Hill, New York, 1975, pp. 48-51.
D. J. Harris,
Quantitative Chemical Analysis
ed., W. H. Freeman, New York, 1998, Chap. 12. (in
edition, relevant material is in chapters 8-11).
In this experiment, the Stellarnet Diode Array Spectrophotometers will be used to evaluate the pK
In the process, fundamental characteristics of indicators and acid-base equilibria in
general should become apparent.
As seen in a previous lab, titration is a common method of quantitative analysis.
Titrations depend upon
accurate determination of the equivalence point for a given stoichiometric reaction.
(In fact, the "end
point" rather than the "equivalence point" is measured.)
Indicators make this quantitative analysis
Harris defines an indicator as "a compound having a physical property that changes abruptly
near the equivalence point of a chemical reaction."
For the acid-base indicators used in this lab, the
"physical property" is a color that is dependent on the chemical state of the indicator.
Thymol blue (thymolsulfonephthalein) is one of a member of a large class of acid-base indicators that are
known as the "sulfone-phthaleins."
The possible forms of the indicator and their colors are shown in the
figure on the next page.
The pH at which an indicator undergoes a color transition determines if it is
acidic or basic, and corresponds to that indicator's pK
i.e. an acid indicator has a transition range at an
Most indicators have only one transition range, but some indicators are polyprotic and can
have multiple transitions as seen above for thymol blue.