ABT: Acid-Base Titration

Purpose: Titration is an important quantitative analysis technique where a solution of accurately known concentration (the titrant) is added incrementally (usually by a buret) to another solution of unknown concentration (the analyte) until the chemical reaction between the two solutions is complete (i.e. no more analyte remains). This is known as the equivalence point. The titrant concentration and volume used to reach the equivalence point, along with the volume of the analyte titrated are used to calculate the concentration of the analyte. This technique is commonly used in biomedical diagnostic labs. Since there is often no visible change at eh equivalence point, an indicator must be added. Indicators may involve a change of color near the equivalence point, pH (if the reaction is an acid/base), or some other physical property. This lab will familiarize you with the following:

1. The titration procedure between an acid and a base. The reaction completion point will be determined through the use of both a color indicator and a pH meter.
2. Determining the concentration of an unknown solution by titration.
3. The use and care of a buret and pH meter.
4. Calibration of a pH meter.
5. The proper handling procedures for dilute acid and base solutions.
6. The use of a spreadsheet to graph a non-linear set of data.

Background reading:

Cooperative Chemistry Laboratory Manual

1. Page 45-46 Containers

2. Pages 47-48 Measuring Liquids by Volume

3. Pages 67-69 Titration

4. Page 91 pH Meter- Its Care and Use

Chemistry & Chemical Reactivity

5. Section 5.3 Acids and Bases (pp 185-190)

6. Sections 5.4 Reactions of Acids and Bases (pp191-193)

7. Sections 5.8 Measuring Concentration of Compounds in Solution (pp 205-208)

8. Section 5.9 pH, a Concentration Scale for Acids and Bases (pp 212-214)

9. Section 5.10 Stoichiometry of Reactions in Aqueous Solution (pp 214-217)

10. Section 1.8: Mathematics of Chemistry Section on Graphing only (pp 43-44)

Equipment and Materials:

You will work in-groups of two for this experiment. Each group will need the following:

1. One buret (obtain from the common equipment area)
2. One stand (common equipment cabinet next to _{your locker.)}
3. _{One buret clamp (common equipment area)}
4. _{One 5-mL volumetric pipet (common equipment area)}
5. _{20 mL of the unknown acid and 100 mL of the NaOH solution (common equipment area)}
6. _{A pH meter (The pH meters are already set up in the lab. Do not move the meters. Take your materials and equipment to the meter.)}
7. _{Two Erlenmeyer flasks and two beakers.}
8. _{Buffer solutions pH = 4.00, 7.00 and 10.00 (Use the buffer solutions in the jars that are provided. Do not pour the buffer solutions into another container.) (common equipment area)}

_{Notes on titrating and using burets:}

1. _{Burets are expensive devices and must be used carefully. Do not try to clean a buret by putting it under the faucet. To clean a buret, securely clamp it to a stand and, using a funnel, pour water into it from the top of the buret. NEVER TRY TO POUR LIQUIDS INTO AN OPENING ABOVE YOUR HEAD.}
2. _{When you are finished using your buret for the day, rinse it with distilled water and clamp it upside down to dry, with the stopcock open. If this procedure is not followed, each partner will be subjected to a 5-point penalty.}
3. _{The burets used in this experiment are 50-mL burets, accurate to 2 decimal places. Remember from your reading assignment that a buret measures the quantity of liquid dispensed, and} thus the zero mark is at the top of the buret.
4. Review how to read a meniscus from last week so that you will be able to properly read the liquid volume in a buret to calculate the volume delivered.
5. This is a quantitative experiment. Thus, you must be careful not to contaminate or change the concentration of any of the solutions you are using. Cleanliness will be extremely important. Therefore, if you are not absolutely sure that your glassware is clean and dry, it is best that you wash your glassware with a small amount of whatever solution you are planning to put in it. Example: You will be filling a buret with NaOH. Therefore, you will want to clean it first with a small amount of the NaOH solution.

Chemicals:

Dilute hydrochloric acid solution

Dilute sodium hydroxide solution

Phenolphthalein indicator

Distilled water

Buffer solution

Safety Precautions:

Sodium hydroxide and hydrochloric acid solutions are corrosive chemicals and skin-irritants. If you splash either on your skin, rinse the area with lots of running water. You must wear your safety goggles at all times. Remember: even though you may be finished in lab does not mean everyone else is finished and hence an accident could still happen. Any spills must be cleaned up immediately. Wash everything you used that contained sodium hydroxide and/or hydrochloric acid with water before putting it away. Remember, chemical residues may linger for weeks.

Overview:

The purpose of this lab will be to determine the concentration of a dilute hydrochloric acid solution. In order to do this, you will use an experimental technique called titration. A titration involves using two solutions: first the "titrant" which in this case will be a solution of sodium hydroxide of accurately known concentration, and the second solution is refereed to as the "unknown" which is a solution of hydrochloric acid of unknown concentration. The sodium hydroxide solution of accurately known concentration is added incrementally to the unknown solution until the chemical reaction between the two solutions is complete. The overall reaction is: NaOH + HCl ® NaCl + H₂O. Note that in this balanced reaction, one mole of NaOH reacts with one mole of HCl, and therefore the amounts of each reactant are related. Suppose we have an unknown number of moles of HCl. In the titration, we add a known number of moles of base (volume x concentration) to a known volume of the unknown acid solution until the reaction is complete. At the completion point, the moles of base added to the unknown acid is numerically equal to the number of moles of acid in the unknown acid solution (1 mol base = 1 mol acid). By knowing the volume of the unknown acid solution used in the titration, the concentration of the unknown acid solution may be calculated (M = moles acid/volume acid). Now, all we need is a method to determine when the reaction is complete. All reactants and products in the above reaction are clear solutions. Two techniques can be used to monitor the reaction. First, an acid/base indicator can by used to visually monitor the reaction. Phenolphthalein is an indicator that is colorless in acidic solutions and pink in basic solutions. Therefore, you will monitor the reaction between NaOH and HCl by monitoring the color change that takes place (as the reaction changes from an acidic solution to a basic solution). Secondly, a pH meter can be used to monitor the reaction. As you add the NaOH to the HCl, you will monitor the reaction for changes in pH. You will record the volume of titrant added and the pH of the unknown solution. Once sufficient data has been collected, you will use Excel to generate a titration curve and then you will be able to determine the concentration of the unknown acid.

Experimental Procedure:

Part A: Acid-Base Titration Using an Indicator.

1. Clean the appropriate glassware for the experiment.
2. Obtain 200 mL of the NaOH solution and 40 mL of the unknown acid solution and label the beakers.
3. Fill the buret with the NaOH solution to just below the 0.00-mL mark. (Did you clean the buret first?) Be sure the tip of the buret is filled with NaOH. Read the initial buret volume and record the volume in the appropriate space in the data sheet.
4. Using a clean 5-mL volumetric pipet, transfer 5-mL aliquots of the unknown acid to two Erlenmeyer flasks. Add about 75 mL of distilled water and 2 to 3 drops of the phenolphthalein indicator. The volume of water is not critical.
5. It is a good idea to do a rapid trial run with your first sample to get an idea of where the endpoint approximately is (refer to page 68 of your lab manual). Titrate the acid to the endpoint for one sample. Do not be concerned about getting the exact endpoint for this run. Remember to record the volume of NaOH that you added in the data table.
6. Now that you have an idea of how much of the NaOH solution will be needed, titrate the second sample. You can rapidly approach the endpoint, but slow down a few milliliters before the endpoint and add the base dropwise to the endpoint (the solution will be clear, and then 1 drop will cause the solution to change color). Be sure to mix thoroughly between drops. Be careful on recording volumes on the second run. This is your best data.

In this experiment, you will be using a pH meter. A pH meter must be calibrated before it can be used to collect data. Be sure your pH meter is calibrated correctly before collecting data. 

Calibration of a pH Meter.

1. Obtain the buffer solutions from the common equipment area.
2. Directions for using the pH meters are provided with each meter.
3. If you have problems using the pH meter, ask you instructor for help.
4. Calibrate the pH meter with buffers 4.00 & 7.00 and then use buffer 10.00 to check the calibration.
5. Each pH meter has an electrode that will need to be cleaned often with distilled water as outlined in the directions for the pH meters. Use a wash bottle with distilled water to rinse the electrode. Do not take the electrode to the sink to rinse.
6. When you are finished with it for the day, rinse and dry the electrode and return the electrode to the KCl solution for storage.

 Part C: Acid-Base Titration using a pH meter.

1. Using a clean calibrated 5-mL pipet, transfer 5-mL aliquots of the unknown acid to two beakers. Add about 75 ml of distilled water and measure the pH of this solution.
2. Titrate the acid, collecting sufficient data (volume of titrant added and pH of the solution) to plot a titration curve. (Refer to Figure 17.6, page 672 of your textbook) Be sure to record the total volume of NaOH that was added for each pH measurement.
3. Repeat the procedure for the second sample.
4. When you are finished with this step, it is a good idea to generate your titration curve before continuing.

Data Analysis:

A. Calculation of the Molarity of Unknown Acid

 Example:

 Five mL of HCl (aq) was titrated to the endpoint with 23.78 mL of a 0.1000-M solution of NaOH. Calculate the molarity of the HCl solution.

 Solution: Start with the balanced equation for our reaction:

HCl(aq) +	NaOH(aq) ®	NaCl(aq) +	H2O(l)
Unknown Concentration	Known Concentration
Known volume	Known volume

Since the acid was titrated to the endpoint (the point at which the unknown acid has been completely consumed) with the base and there is a one to one mole ratio between HCl and NaOH in the balanced equation, it follows that the moles of HCl that reacted equals the moles of NaOH added from the buret.

1. Calculate the number of moles of NaOH added to the HCl

2. Use the balanced equation to determine the number of moles of HCl that was titrated.

3. Calculate the concentration of HCl in moles/L.

1. (0.1000 mol NaOH/L)(0.02378 L NaOH) = 0.002378 mol NaOH

2. (0.02378 mol NaOH)(1 mol HCl/1 mol NaOH) = 0.002378 mol HCl

3. M_HCl = 0.002378 mol HCl / 0.00500 L HCl) = 0.4756 M

 *To summarize, if we need to find the concentration of an unknown acid, titrate a known volume of the unknown acid with a known concentration of base. We measure the volume of base required to get to the endpoint. We know when we are at the endpoint by using an indicator or a pH meter.

2. Creating a Titration Curve

Set up a spreadsheet with your data from Sample 2 only as follows:

	A	B
1	mL NaOH added	pH

1. Plot a titration curve with mL NaOH added as the x-axis and pH as the y-axis.

(The endpoint is volume of base added where the slope of the pH curve is greatest.)

2. Recalculate the concentration of the acid, as described above, using the pH curve endpoint. Generally, titration curves are more accurate in determining the endpoint, but require the use of a pH meter and generating a graph. Modern labs use computers and robot arms to generate titration curves.

Part A Data Sheet

Part C Data Sheet 1

 Part C Data Sheet 2

*No data sheet for Part B.