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I-6
B.
Determine the Effect of Acid and Base additions on the Buffer Solution.
1.
Dispense a fresh 25.0 mL sample of the original buffer solution (i.e. not the diluted
solution) into a clean 50 mL beaker. Re-measure and record the pH of this buffer solution.
(The pH should be the same as the initial measured in Step 3 above.)
To this sample, add
0.20 M HCl from a buret until the pH changes one full pH unit. Do this by adding the
acid slowly with constant stirring and monitoring the pH change continuously. Record the
final pH. Record the initial and final buret volumes to determine the amount of HCl
required to decrease the pH by one full pH unit.
2.
Continue to add HCl until a second full pH unit change
occurs. Record the initial and
final buret volumes for this second change as well as the final pH. (NOTE: This change
will require a much smaller volume of HCl, so add the HCl carefully.)
3.
Dispense a fresh 25.0 mL sample of the original buffer solution into a clean 50 mL beaker.
Re-measure and record the pH of this buffer solution. (The pH should be the same as the
initial measured in Step 3 above.) To this sample, add 0.20 M NaOH from a buret until the
pH changes one full pH unit. Again, add the base slowly with constant stirring, monitor
the pH change continuously, and record the final pH. Record the initial and final buret
readings to determine the volume of NaOH required to increase the pH by one full pH unit.
4.
Continue to add NaOH until a second full pH unit change
occurs. Record the initial and
final buret volumes for this second change as well as the final pH. (NOTE: This change
will require a much smaller volume of NaOH, so add the NaOH carefully.)
Part IV.
Study of the pH 7.00 Commercial Buffer Solution
1.
Repeat Parts A and B of Part III above substituting the pH 7.00 commercial buffer solution in
each step for the acetic acid–acetate buffer solution. Record all observations on your
Observations Sheet in the section provided for Part IV.
Before leaving the laboratory, check to make sure that all of your equipment is clean and back in your
drawer. It is important that the pH electrode has been washed and is stored in storage buffer of pH 7.
When you have finished all of your titrations, and have disposed of all of the wastes properly,
wash your burets with hot
tap water (four times) and rinse once with deionized water. Leave
them at your work station empty and upside down.
