Exercises
Introduction to pH, Titration Curves, and Buffers:
Answers to Questions
- Do the values for the pKa you
found in the two methods agree? Which value is better for this
mathematical model? Which would be more useful experimentally?
Why?
- They should be close, but not exact (unless
by chance).
- The spreadsheet answer would be better for
the mathematical model, since the mathematical model represents
an exactly known system with defined values etc.
- The graphical method is better for experimental
data since no data point is known with certainty and the best
fit line represents an average of many points. Hopefully
in this situation the best fit line will provide a value in which
the errors have canceled to give a more accurate and precise
estimate of reality. Remember, in the real world, we don't know
the exact function followed by the data. Our models may be close
without being "real." (For example, Newtonian physics
gives marvelously accurate and precise predictions for everyday
phenomena, but is known to be a "false" picture
of the world.)
- Do the values for the equivalence point
you found in the two methods agree? Which value is better for
this mathematical model? Which would be more useful experimentally?
Why?
- Again, they should be close, but not exact
(unless by chance).
- As above, the spreadsheet answer would be
better for the mathematical model, since the mathematical model
represents an exactly known system with defined values etc.
- See 1.3 above, the graphical method is better
for experimental data since no data point is known with certainty
and the best fit line represents an average of many points.
- Does the buffer region you found graphically
correspond to the range found on the sheet (pKa±1)?
- Here the correspondence depends on how you
perceive the "linear" region. If one looks for a very
close fit to a straight line, then a narrower region than ±1
pH unit will be seen, as shown in the figure below, where I would
guess the "buffer region" goes from about pH 4 to about
pH 5.5 which is a range more like ± 0.75 pH units:
- Any definition
of the buffer region will be somewhat arbitrary, we are really
trying to make a pragmatic judgment on the behavior of the system
in maintaining pH within a range needed for a particular experimental,
biological, etc. situation.
- What is a buffer?
For pH, a substance which when added to a solution resists changes
in pH (hydrogen ion concentration). A pH buffer is a mixture
of an acid and its conjugate base (or vice-versa), with significant
fractions of each present.
- Why does the pH level off at high pH values?
Is this another titration and buffer region? (Hint: What's the
pH of 0.1 molar base? 1.0 molar?) The
highest pH achievable in a titration with a base will be the
pH of the pure base (e.g. for 0.1M NaOH the pH = 13, for 1.0M
pH = 14 in aqueous solutions). Thus as base is added a limiting
value will be approached, causing the curve to level off. This
is thus not another buffer region.
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- Last modified 22 August 2001
- © R. Paselk 1999
Laboratory
