Biochemistry I Fall Term, 2001

pH Titration: Answer Sheet Example

Example Problem:

You are given 100 ml of a "Good" buffer of unknown concentration.
You have a 1.0 M NaOH solution to use in a titration experiment.
a) Determine the pKa and concentration of the buffer.
b) Submit your graph of pH vs. equivalents of NaOH.
c) Identify the "Good" buffer from the list in Campbell, Table 2.7.
What we know from the problem:
1) Volume of the buffer = 100 ml.
2) [NaOH] = 1.0 M.
What we found in the experiment:
Vol. NaOH (ml)     pH mequiv.* 
    0
   0.5
   1.0
   2.0
   3.0
   4.0
   5.0
   6.0
   7.0
   8.0
   9.0
   9.5
   9.9
  10.0
 
 4.274
 6.31
 6.61
 6.95
 7.19
 7.39
 7.56
 7.75
 7.93
 8.16
 8.46
 8.81
 9.59
"too large"
 
 0.0
 0.5
 1.0
 2.0
 3.0
 4.0
 5.0
 6.0
 7.0
 8.0
 9.0
 9.5
 9.9
10.0
*Equivalents of NaOH = Volume ¥ concentration.
mequiv. = ml ¥ 1.0 M (i.e. mequiv. = 10-3 ¥ equivalents.) 
 
Graph of the calculated results:

The buffer concentration = 10 mequiv/100 ml = 100 mM or 0.10 M.
The pKa = 7.5-7.6.   (The pH at 5.0 mequiv.)
From Table 2.7 of Campbell, the "Good" buffer is either HEPES or TES.

HEPES.xls is the Excel spreadsheet used to calculate and graph these results; it can be copied to your disk.

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8.20.01