Potentiometric Titration Curves 535x3r

Chemistry 153 Lab

ACID/BASE TITRATIONS & TITRATION CURVES Acid-base titration is a useful technique for determining the concentration of an acidic or basic solution. As you will see today, titration of a weak acid or base can also be used to determine the Ka or Kb for the acid/base. You have already been introduced to titration technique in previous labs. Today you will use a pH meter connected to a computer to generate a titration curve. A titration curve shows how the pH of a solution changes as acid or base is added. The shape of a titration curve is influenced by both the concentration and nature of the acid and base. You will see three different titration curves in today's lab. A typical strong acid-strong base titration curve is shown below. In previous titration experiments you have determined the endpoint (or equivalence point) of the titration by observing the color change of an indicator that is added to the solution. When using a pH meter to monitor the titration, the endpoint is determined by the inflection point of the curve. This provides a more accurate determination of the endpoint. The inflection point is the steepest part of the curve, and is also the point on the curve where the curvature changes. [In calculus , this is the point where the second derivative equals zero.]. Please review your text for a further discussion of titration curves, particularly the discussion of titrations involving weak acids and bases.

In this lab you will work with a partner to complete two titrations. First you will titrate a weak acid (KHP) with a ~0.1 M sodium hydroxide solution. From the results of this titration you will be able to calculate the exact concentration of the NaOH. (In other words, you will standardize the NaOH. Do you why this is necessary?) Next you will titrate an unknown polyprotic acid. From your titration data, you will be able to calculate the acid ionization constants and the molecular weight of the unknown acid. A pH meter will be used to measure the pH as each reaction proceeds. So that you can monitor each titration in “real time”, computers will be available so that data may be entered and plotted as the titration proceeds.

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Chemistry 153 Lab

General Directions The following are general directions for each titration. Specific details for each of the titrations follows. 1. Set up a titration apparatus as shown below. Position the pH electrode in the solution and adjust its position so that it is not struck by the stirring bar.

2. Open the graphing program Graphical Analysis from the Vernier programs menu. Label and scale the axes appropriately. 3. Use the buffer solutions to calibrate the pH probe as directed by the instructor. 4. Set up a buret with the NaOH solution provided. Fill to the 0.00 mL line. 5. For each titration, add the specified amount of acid to the beaker. If necessary add water to dissolve. Add a few drops of phenolphthalein to the acid. 6. Record the initial pH of the acid solution. This is the pH with no NaOH added. Record the initial volume (0.00 mL) of NaOH in the buret. Data should be recorded both in your lab notebook, and in the Graphical Analysis program. As you continue to enter data, the titration curve will be displayed on the screen. 7. Add NaOH a few drops at a time until you produce an approximate 0.2 pH unit change. Record the exact volume of NaOH added and the exact pH. that burets should be read to the hundredths place. Edmonds Community College Page 2 of 5

Chemistry 153 Lab

8. Continue adding increments of NaOH to produce increases of about 0.2 pH units. Do not worry if you “overshoot” a change of 0.2 pH units, but be sure to record the exact volumes and pH values. Note: When the pH begins to change rapidly you will need to add base in much smaller increments in order to produce a change of only 0.2 units. 9. Monitor the graph of pH vs. volume added. After the pH rises steeply and begins leveling off, you may end the titration. that polyprotic acids will produce multiple endpoints, so take care that you do not end the titration too early. 10. Add a column to your Graphical Analysis data table to calculate the second derivative of the data. Save the file to your ACS or to a removable drive. Do not save to the hard drive or your data will be lost.

Specific Directions for each titration: A. Titration of a known monoprotic weak acid (KHP) You will be given a vial containing KHP. Dissolve the entire sample in about 40 mL of water and determine the exact mass by weighing the vial before and after. Begin titrating with NaOH. Refer to your prelab for an estimate of the expected endpoint and add base in small increments as this point approaches. Continue titrating to beyond the endpoint. B. Titration of an unknown polyprotic acid You will be given a vial containing an unknown acid. Record the number of your unknown in your notebook. Dissolve the entire sample in about 40 mL of water and determine the exact mass by weighing the vial before and after. Titrate to beyond the final endpoint with NaOH, taking care to add base in small increments near each endpoint.

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Chemistry 153 Lab

Calculations Turn in your titration curves for each titration. 1. Determine the equivalence point for each titration and indicate the equivalence point(s) on each graph. 2. From the titration of KHP with NaOH, determine the exact concentration of the NaOH. 3. From the titration of KHP with NaOH, determine the Ka for KHP and compare with the accepted value. Do not expect exact agreement; look for order of magnitude agreement (same power of 10). 4. From the titration of NaOH with the unknown polyprotic acid, determine the molar mass of the acid. 5. From the titration of NaOH with the unknown polyprotic acid, calculate the ionization constants (Ka values) for the acid. 6. Consult the list of weak acids (posted on Blackboard) and by comparing your molar mass and Ka values, suggest an identity for the acid.

Questions 1. Why are the titration curves so steep near the endpoint?

2. Use the Ka value you calculated for your polyprotic acid to determine the pH of a 0.50 M solution of this acid.

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Chemistry 153 Lab

Name_______________________________

PRE-LAB: 1. Sketch the titration curve that should be obtained when a monoprotic weak acid is titrated with sodium hydroxide. Explain how and why this differs from a strong acidstrong base titration curve.

2. Estimate the expected endpoint of the titration when 0.30 g of KHP is titrated with 0.10 M NaOH.

3. Sketch the titration curve that should be obtained when a diprotic acid is titrated with sodium hydroxide.

4.

If the titration of 0.156 g of an unknown monoprotic acid requires 22.43 mL of 0.1027 M NaOH, what is the molecular weight of this unknown acid?

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