pH METER LABORATORY
Background:
pH meters measure the hydrogen ion (actually the activity of H+) concentration of a
solution by detecting the voltage between two electrodes in the solution. One of the
electrodes must be sensitive to the hydrogen ion content, e.g. the glass electrode. The
second (or reference) electrode must be insensitive to the hydrogen ion concentration,
e.g. the calomel electrode. If the hydrogen ion concentration (the pH) of the solution
changes, it selectively affects reactions at the pH-sensitive electrode. This changes the
voltage between the two electrodes. The change in voltage can be correlated with the
change in pH of the solution.
Laboratory:
This laboratory will demonstrate the correct use of the pH meters. ALWAYS set the pH
meter to the standby position before removing the electrodes from the solutions. Rinse
off the electrodes before immersing them into a new solution.
A. Calibration of the pH meter: The pH meters must be calibrated to ensure the pH
that is displayed is accurate, e.g. using a standard pH solution such as pH 7.0
buffer. Immerse the electrodes in 10-15mL of the pH 7.0 buffer; adjust the
pH to read 7.00, if necessary.
B. Unknowns: After calibrating the pH meter, measure the pH of each of the four
unknown solutions.
C. Buffering capacity measurements: Each group will test the effect of changing a
buffer’s pH molar concentration on the ability to resist change in pH.
1. Use the three 0.1M phosphate buffer solutions labeled pH 6.6, 7.6 and 8.6.
Measure out two equal volumes of 10-15 mL each into plastic beakers. Do this for
each of the three different pH solutions using the same volume uniformly. You will
have three pairs of beakers. Measure the pH of each beaker (it should be close to the
stated pH). After measuring the pH, add 200µL of 1.0N HCl to one beaker of each
pair. Add 200µL of 1.0N NaOH to the second beaker of the pairs. Mix all the
solutions in the beakers and then measure the pH.
2. To measure the effect of concentration on buffering capacity, each group will make
a 1:10 serial dilution of the stock 1.0M phosphate buffer (pH 7.6) with distilled
water. You will only need to make two dilutions so that you will have three
concentrations of 1.0M, 0.1M and 0.01M. Make sure to make enough (10-15mL of
each dilution) so the electrodes will be fully immersed. All three solutions must
have the same volume. Measure the pH of each solution (most should be close to
the original pH 7.6). Add 200µL of 1.0N HCl to each beaker and measure the pH
again.
(see Page 2 for problems to be solved)
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pH METER LABORATORY PROBLEMS
Problems:
1.
What is the pH of each of the unknown solutions? If each is a strong acid or base
what is the Molar concentration?
2.
From your buffering measurements, which solution best resisted changes in pH
when acid was added? How does the change in pH of the buffer affect its
buffering capacity?
3.
Calculate how much acid (acetic acid) and how much conjugate base (sodium
acetate) must be used to make 500mL of a 0.8M acetate buffer with pH 4.76.
Assume that the pKa for the dissociation of acetic acid 4.76. How much of each
must be added if the desired pH is 5.76?
4.
How many mL of 0.25M NaOH are required to neutralize 50mL of 0.4M H2SO4?
5.
Determine the pH of the following:
a.
5 x 10-4M HCl
b.
4 x 10-2M NaOH
c.
10-4M H3PO4
d.
2.5 x 10-3M NH4OH
6.
Define: oxidation, reduction, anode, cathode, electrochemical half-cell, and the
Standard Electrode Potential.
7.
You mix 500mL of 10-3M HCl with 500mL of 4 x 10-3M NaOH. What will be
the pH of this solution?
8.
If 50mL of 10-2M HCl is diluted to a final volume of 800mL with distilled water,
what will be the pH?
9.
You make a 1:5 serial dilution of 1.00M NaOH so that you have 10 tubes; what is
the pH of each?
10.
What is the pH of each of the following solutions?
a.
b.
c.
d.
10mL of 1.0M HCl diluted to 500mL with water
1mL of the above solution diluted to 100mL with water
10mL of 1.0M HCl diluted to 500mL with 0.1M NaOH
50mL of 0.1M HCl plus 50mL of 0.25M NaOH diluted to 250mL with
water.
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