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The ionization of an acid can be shown by the following equation:
HA(aq) « H+(aq) + A-(aq) (Eq. 1)
Since an equilibrium exists, an equilibrium constant, Ka, can be written:
Ka = [H+] [A-] (Eq. 2)
[HA]
The Ka value is an indication of acid strength. The larger the value of the Ka, the stronger the acid. This value is characteristic of the acid and can be used to help identify an unknown acid. A similar system exists for bases (Kb).
Two methods may be used to determine the Ka value. Both methods require the use of a pH meter. In the first method, a sample of acid is titrated with base. The pH values are plotted vs. the volume of base added. The equivalence point is determined from the graph. Next, the volume of base halfway to the equivalence point is found, and the pH at this volume is noted. The [H+] corresponding to this pH is equal to the Ka for the acid. At a point halfway to the equivalence point, [H+] = [HA] = [A-] for a monoprotic acid. Canceling out [A-] and [HA] in Equation 2 gives Ka = [H+].
The second method for determining Ka values involves a ?half volume? method. A solution of the acid is prepared and divided in half as accurately as possible. One portion is titrated to its endpoint with phenolphthalein. The two portions are then recombined, and the pH of the resulting solution is measured. Since half of the acid has been titrated, [H+] = [HA] = [A-]. Again, if [A-] and [HA] are canceled in Equation 2,
Ka = [H+]. The pH value of the combined solutions can be converted to [H+] to give a Ka value.
Purpose:
The purpose of this experiment is to determine the Ka value for a weak acid by two methods: potentiometric titration and ?half volume? methods.
Equipment/Materials:
pH meter acid sample
buret standard base solution (0.10 M NaOH)
buffer solutions beakers
distilled water phenolphthalein
Erlenmeyer flask graduated cylinder
Safety:
An apron and goggles should be worn at all times in the lab.
Procedure:
Part I: Potentiometric Titration
Note: The procedure for standardizing pH meters varies from model to model. For this reason, specific instructions for that step have not been included. Follow your instructor?s directions for this step or refer to the manual for the instrument.
Rinse a buret with tap water and then with distilled water. If the buret drains without leaving drops of water behind, it is clean. If not, wash the buret with a dilute detergent solution. Rinse the buret several times with tap water and then with distilled water.
Pour some of the sodium hydroxide solution into a beaker. Pour a small amount of this into the buret and let it drain through into a ?waste? beaker. Fill the buret with the sodium hydroxide solution. Make sure that the tip of the buret has been completely filled. Drain the sodium hydroxide solution to the 0.00 mL line or below.
Place half of the unknown acid in a beaker and add enough water so that the end of the pH electrode will be covered. Usually a titration is done in a flask. In this experiment, room for the pH electrode is needed.
Note the initial reading of the sodium hydroxide in the buret. Record the value.
Rinse the pH electrode and place it in the beaker with the acid. Adjust your set-up so that the buret containing the sodium hydroxide is over the beaker. If you are using a magnetic stirrer and a stirring bar, make sure that the pH electrode is above the stirring bar. The tips of the electrodes are fragile and break easily.
Begin adding small portions of the sodium hydroxide to the acid sample. After each addition, record the total volume of sodium hydroxide added and the pH of the solution.
When the pH begins to rise rapidly, add smaller portions of the sodium hydroxide. Rinse the sides of the beaker occasionally with distilled water from the wash bottle. Continue to add the sodium hydroxide until the pH is in the basic region AND has leveled off.
Discard the titrated acid sample in the beaker, and repeat the titration.
For each trial, plot pH (y-axis) as a function of volume of base added (x-axis). Determine the equivalence point on each graph. Find the volume of base that corresponds to the equivalence point. Divide this value by two to determine the amount of base needed to reach half way to the equivalence point. The pH at this volume of base is converted to [H+] to give the Ka of the acid.
Part II: ?Half Volume? Method
It is assumed that the pH meter and buret are ready for use. If not, refer to Steps 1 and 2 from Part I.
Obtain a sample of the unknown acid. Measure out 100 mL of distilled water with a graduated cylinder and pour into a 250 mL Erlenmeyer flask. Dissolve the sample of acid in the water, and stir to mix thoroughly.
Divide the solution you have prepared
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