Modified Version of Exp 25, Part C. Properties of Buffers and Measurement of pH
NOTE: The following procedures will be done in place of Part C from the lab manual. It might be
helpful to copy Table I and Table II from the report form handout into your lab notebook
ahead of time. You will not be doing any other Part of this experiment from the lab manual.
Before beginning the experiment (see visual diagram on yellow sheet at back as well!):
1) Rinse and then fill your wash bottle with deionized (DI) water. Rinse out your 10- and 50-mL
graduated cylinders, one 50-mL Erlenmeyer flask, two 125-mL Erlenmeyer flasks, one 50-mL beaker,
and one 100-mL beaker with DI water. They need not be dry for any of the following procedures, just
rinsed (and shaken out to remove excess water). Also take out a 250- or 400- mL beaker (for rinse
waste) and a glass stirring rod (rinsed and wiped).
2) Label the 50-mL beaker “A’2”. Label the 50-mL Erlenmeyer “HC2H3O2”. Label one of the 125-mL
Erlenmeyer flasks NaC2H3O2, and the other 125-mL Erlenmeyer “H2O”.
3) Pour ~25 mL of 0.10 M HC2H3O2(aq) into the 50-mL Erlenmeyer, ~50 mL of 0.10 M NaC2H3O2(aq) into
the appropriate 125-mL Erlenmeyer flask, and fill the H2O-labelled flask with ~100 mL DI H2O.
4) Label one of your medium-sized test tubes “HCl” and fill it at least ¾ full with 0.1 M HCl(aq).
5) Label another of your medium-sized test tubes “NaOH” and fill it at least ¾ full with 0.1 M NaOH(aq).
Place a graduated disposable pipette in each of the test tubes, and place the tubes on a (small) test
tube rack. You may run out of both of these solutions during the experiment, so just add to the test
tubes a bit to finish the experiment, if needed.
6) Calibrate the pH meter as shown / instructed verbally (in short, you will remove the electrode from its
storage solution, rinse it with DI water, gently blot it to remove excess water, place it in the pH 7.0
buffer, swirl, hit “Cal”, wait until the beep [decimal point stops flashing], rinse and blot the electrode
again, place it in the pH 4.0 buffer, swirl, hit “Cal” again, and wait for the beep). To measure the pH of
a sample after this point, hit “Read” only (and wait for the beep). [Never hit “Cal” again all period! You
only calibrate the meter ONCE at the beginning.]
Important Notes to Remember During the Experiment:
● Once the pH meter is calibrated, do not hit the “Cal” button during the rest of the experiment. If you
accidentally hit the “Cal” button, you will need to redo the entire calibration sequence (step 5
above).
● Before measuring the pH of any sample, make sure it is well-mixed. Use a vigorous swirling
motion, or stir vigorously with a (clean) stirring rod. When the pH electrode is in the sample, swirl
several times as well after you hit the “Read” button. Data will be quite poor if you do not mix
solutions thoroughly!
● For the first couple of solutions, immediately after you measure the pH swirl the beaker a bit and hit
the “Read” button a second time to verify that the reading is consistent. If the value is essentially
the same as the first reading, record it in your notebook and go on. If not, that likely means that you
did not mix the solution well enough. If you continue to have this problem, inform your instructor.
● You may temporarily remove the electrode and leave it “in the air” briefly (for a minute or less)
when stirring a solution or changing a vessel, etc. But immerse it in the pH 7.0 solution when
preparing a new solution, etc.
The experiment: How do dilution and addition of a bit of strong acid or strong base each affect the pH
of buffer solutions relative to pure water?
Prepare the following solutions as described using (and reusing) a 100 mL beaker (mostly) and your 50.mL graduated cylinder. Record all pH measurements in your lab notebook.
Sample A.
 Add 15. mL of 0.1 M HC2H3O2(aq) to the (rinsed and shaken-out) (50-mL) graduated cylinder and
pour into the 100-mL beaker.
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Add 15. mL of 0.1 M NaC2H3O2(aq) to the (rinsed and shaken-out) (50-mL) graduated cylinder and
pour into the 100-mL beaker (along with the 15. mL of acid above).
Stir well, and (after rinsing and blotting the pH electrode) measure the pH (remember to swirl a bit
once the electrode is immersed). (Recheck it!) Leave the electrode in the solution.
After a quick rinse of the graduated cylinder with DI water, fill it to 30. mL with DI water, add that
to the beaker (i.e., dilute the solution two-fold), stir (remove electrode momentarily for this, as
noted above), and measure the pH again. (Remember to swirl, and to recheck pH!)
Remove the electrode and leave it in the air temporarily.
Using the unrinsed, but shaken-out graduated cylinder, transfer half (i.e., 30. mL) of the
solution in the beaker to the 50-mL beaker labeled A’2 (to be used a bit later), and keep (or
put) the remaining half (30. mL) of the solution (back) in(to) the 100-mL beaker for use in
the next step (this solution is now called “A’1”).
Sample A’1 (in the 100-mL beaker):
o Add 5 drops (about 0.3 mL) of 0.10 M HCl(aq) to the solution (in the 100-mL beaker!), stir
well, and place the (unrinsed) electrode back into the solution.
o Swirl well and measure the pH. (Recheck it only if prior measurements indicate it
necessary)
o Add an additional 3.0 milliliters of 0.10 M HCl(aq) [w/ graduated pipet], stir well (remove the
electrode and leave it “in the air” temporarily), and measure the pH. (Recheck it?).
o Leave the electrode in the solution for now.
Sample A’2 (in the 50-mL beaker):
o To the solution in the 50-mL beaker that was put aside earlier, add 5 drops of 0.10 M
NaOH(aq) and stir well.
o Remove the electrode from the 100-mL beaker, rinse and blot it, place it in the 50-mL
beaker, swirl the solution, and measure the pH of A’2. (Recheck it?)
o Add an additional 3.0 milliliters of 0.10 M NaOH(aq) [w/ graduated pipet], stir well (remove
the electrode and leave it “in the air” temporarily), and measure the pH. (Recheck it?).
o Rinse and blot the electrode, and place it back into the pH 7 buffer solution.
o Discard the solutions from the two beakers, rinse them with DI water, and shake out the
excess water.
o Wipe off the A’2 designation on the 50-mL beaker and relabel it B’2.
Sample B.
 Add 3.0 mL of 0.1 M HC2H3O2(aq) to the 100-mL beaker using the disposable pipet that you were
previously using for just water (make three, 1-mL additions). Do not use a graduated cylinder!
 Add 27. mL of 0.1 M NaC2H3O2(aq) to the (rinsed and shaken-out) 50-mL graduated cylinder and
pour into the 100-mL beaker (along with the 3.0 mL of acid from above).
 Stir well, and (after rinsing and blotting the pH electrode) measure the pH. (Recheck it?) Leave
the electrode in the solution.
 After a quick rinse of the graduated cylinder with DI water, fill it to 30. mL with DI water, add that
to the beaker (i.e., dilute the solution two-fold), stir (remove electrode momentarily for this, as
before), and measure the pH again. (Remember to swirl; recheck pH?)
 Remove the electrode and leave it in the air temporarily.
 Using the unrinsed, but shaken-out 50-mL graduated cylinder, transfer half (i.e., 30. mL) of the
solution in the beaker to the 50-mL beaker labeled B’2 (to be used a bit later), and keep (or
put) the remaining half (30. mL) of the solution (back) in(to) the 100-mL beaker for use in
the next step (this solution is now called “B’1”).
Sample B’1 (in the 100-mL beaker):
o Add 5 drops of 0.10 M HCl(aq) to the solution (in the 100-mL beaker!), stir well, and place
the (unrinsed) electrode back into the solution.
o Swirl well and measure the pH. (Recheck it?)
o Add an additional 3.0 milliliters of 0.10 M HCl(aq) [w/ graduated pipet], stir well (remove the
electrode and leave it “in the air” temporarily), and measure the pH. (Recheck it?).
o Leave the electrode in the solution for now.
2
Sample B’2 (in the 50-mL beaker):
o To the solution in the 50-mL beaker that was put aside earlier, add 5 drops of 0.10 M
NaOH(aq) and stir well.
o Remove the electrode from the 100-mL beaker, rinse and blot it, place it in the 50-mL
beaker, swirl the solution, and measure the pH of B’2. (Recheck it?)
o Add an additional 3.0 milliliters of 0.10 M NaOH(aq) [w/ graduated pipet], stir well (remove
the electrode and leave it “in the air” temporarily), and measure the pH. (Recheck it?).
o Rinse and blot the electrode, and place it back into the pH 7 buffer solution.
o Discard the solutions from the two beakers, rinse them with DI water, and shake out the
excess water.
o Wipe off the B’2 designation on the 50-mL beaker and relabel it C2.
Sample C.
 Pour 30. mL of deionized (DI) water in the 100-mL beaker and 30. mL of DI water into the 50-mL
beaker (labeled C2). Put aside C2 for now.
 After rinsing and blotting the pH electrode, attempt to measure the pH of the pure water in the
100-mL beaker.
(NOTE: the reading will likely fluctuate greatly because there is such a low concentration of ions. If the value is
between 5 and 8, just put an approximate value down [don’t wait for the meter to “beep”!]; if not, call me over).

Remove the electrode and leave it in the air temporarily.
Sample C1 (in the 100-mL beaker):
o Add 5 drops of 0.10 M HCl(aq) to the water (in the 100-mL beaker!), stir well, and place the
(unrinsed) electrode back into the solution.
o Swirl well and measure the pH of C1. (Recheck it?)
o Leave the electrode in the solution for now.
Sample C2 (in the 50-mL beaker):
o To the sample in the 50-mL beaker that was put aside earlier, add 5 drops of 0.10 M
NaOH(aq) and stir well.
o Remove the electrode from the 100-mL beaker, rinse and blot it, place it in the 50-mL
beaker, swirl the solution, and measure the pH of C2. (Recheck it?)
Finishing up:
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Rinse and blot the electrode, and replace the “plug” with the storage solution in it. Make sure the
electrode is submerged in the solution!!
Discard the solutions and clean up! (You made it!) 
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Experiment 25, Part C (Modified), Supplemental Sheet Preparations Before you Start the Experiment NaC2H3O2 No label ~50 mL (will be HC
H
O
2
3
2
A’2 reused ~25 mL (empty, throughout DI H2O put aside)
experiment)
DI H2O ~100 mL 10‐mL 50‐mL 100‐mL 50‐mL 50‐mL 125‐mL 125‐mL
Calibrating the pH meter
Waste 1) Remove electrode from storage solution.
2) Rinse electrode with DI water.
HCl 3) Gently blot electrode with Kimwipe.
NaOH 4) Place electrode in pH 7.0 (yellow) buffer.
5) Swirl, hit “Cal”, and wait for “beep” (decimal point
will stop flashing)
6) Rinse electrode with DI water and blot again.
250‐mL Small(er) (not large) test tubes, in small rack
Stirring rod 7) Place electrode in pH 4.0 (pink) buffer.
8) Swirl, hit “Cal”, and wait for “beep”.
**NEVER HIT “CAL” AGAIN!**
Only hit “Read” when you want to measure the pH!.
Visualization of Procedure Involving “Sample A” (this procedure will be analogous for Sample B and (almost) C) 2) Leave other ½ of sample + 5 drops of HCl
30 mL
A’ A A’1 + 30 mL H2O A’1 + 3 mL of HCl A’1 ~30 mL ~33 mL
Measure pH Measure pH
60 mL 1) Remove ½ of sample 30 mL Measure pH Measure pH A’2 30 mL
+ 5 drops of NaOH
A’2 + 3 mL of NaOH
A’2 ~30 mL ~33 mL
Measure pH Measure pH