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Lab 9 - Titrations

Purpose

To determine the concentration of acetic acid in vinegar.

Goals

Introduction

Many laboratories analyze consumer products to determine accuracy in the labeling of the product. One very common and simple analytical technique known as titration will be demonstrated in this experiment. A titration is an analytical procedure in which a reaction is run under carefully controlled conditions. The stoichiometric volume of one reactant of known concentration, the titrant, that is required to react with another reactant of unknown concentration, the analyte, is measured. The concentration of the analyte is determined from the concentration and volume of titrant and the stoichiometry of the reaction between them. The experimental setup is shown in Figure 1. A buret, which contains the titrant, is calibrated so the volume of solution that it delivers can be determined with high accuracy and precision. Titrant is added to the analyte until the stoichiometric volume of titrant has been added. This is called the equivalence point, which, when reached, the volume of titrant delivered by the buret is read. Usually, the volume readings are estimated to the nearest 0.01 mL. The delivery of the titrant is adjusted with the stopcock on the buret. With practice, one can dispense fractions of a drop of titrant and control the procedure well enough that replicated titrations agree within 0.10 mL. For this first lab, you will need your titrations to agree to within 0.50 mL.
Figure 1

Figure 1: Titration Setup

The equivalence point can be determined by two methods. The pH can be monitored during the titration with a pH electrode and the equivalence point identified as the point of most rapid pH change. The equivalence point can also be determined visually with an indicator. The indicator, which is a substance that changes color near the equivalence point, is added to the analyte solution. Since the color change is near but not exactly at the equivalence point, the point at which the color change occurs is called the endpoint. Indicators are chosen so the endpoint is very close to the equivalence point. It is important to keep a titration well mixed, so the titrant and analyte can contact each other and react rapidly. Either manual swirling of the beaker or mechanical stirring can be used. You will use mechanical stirring in this experiment. The most common type of titration is the acid-base titration. In this experiment, you will determine the concentration of acetic acid, HC2H3O2 in commercial vinegar. Vinegar is a mixture of acetic acid and water. In this titration, aqueous NaOH is the titrant, and vinegar is the analyte. We assume that the strong base and the weak acid react completely according to the net equation:
( 1 )
HC2H3O2(aq) + OH(aq) → C2H3O2(aq) + H2O(l)
 
The balanced equation shows 1:1 stoichiometry, so we can write:
( 2 )
moles HC2H3O2 reacting = moles OH added
 
Or more generally:
( 3 )
moles of acid reacting = moles of base reacting
 
Moles of base can be calculated from concentration of base multiplied by the volume of base used (molesbase = Mbase × Vbase). As a reminder on concentration units: molarity is defined as the number of moles of solute in a liter of solution (M = mol/L). This is numerically equal to the number of millimoles of solute in a milliliter of solution (M = mmol/mL). It is often convenient to use this second definition of molarity in titrations and other work where small quantities are involved. There are 1000 mmol in 1 mol and 1000 mL in 1 liter. For example, 10.2 mL of 0.100 M NaOH solution contains 1.02 mmol of NaOH.
( 4 )
10.2 mL solution ×
0.100 mmol NaOH
1 mL solution
= 1.02 mmol NaOH
 
The molarity of the acetic acid in vinegar can be calculated from moles of acetic acid divided by volume of analyte
(Macid = molesacid / Vanalyte)
. Using the values above, if the titration requires 1.02 mmol of NaOH to reach the endpoint, the sample must also contain 1.02 mmol of acetic acid. If the volume of the vinegar used is 8.05 mL, the molarity of acetic acid is 1.02 mmol / 8.05 mL = 0.127 M.
In this experiment, a carefully measured volume of vinegar (Vanalyte) is placed into a beaker and the mass is determined. The sample of vinegar is then titrated with a NaOH solution of known concentration (Mbase), and the volume of NaOH solution required to reach the endpoint (Vbase) is determined. Vbase, Mbase, and Vanalyte are all known, so the concentration of the acid (Macid) can be determined as described above. In addition, the mass of acetic acid in the sample can be determined from the number of moles present and the molar mass of acetic acid (gacid = MWacid x molesacid). Finally, the mass percent of acetic acid in the vinegar can be determined from the mass of the acetic acid in the sample and mass of the vinegar (analayte) solution that was titrated.
( 5 )
Mass % =
mass of acetic acid in sample
mass of vinegar solution titrated
× 100
 
In the titration of acetic acid with aqueous NaOH, phenolphthalein is used as the indicator. Phenolphthalein is nearly colorless in acidic solution, but turns pink at a pH of about 8. This indicates that the base has neutralized all the acid. As you titrate the vinegar, you will observe that the pink color is more persistent as you add more base. This is a signal to slow the addition of base, and control it carefully. The endpoint has been reached when a faint pink color persists for at least 30 seconds. It is easy to overshoot the endpoint. If this happens, you will have a dark purple-pink solution, and you will have to repeat the titration, so be careful. Note the volume you have used; stop short of this volume in subsequent titrations, and add the last milliliter or so dropwise. Your instructor will show you how to control the stopcock of the buret to facilitate this. Note that the volume measurements in titrations are usually reported to four significant figures, so the concentrations are usually reported to four significant figures as well. Watch this in your work; when you calculate molar masses, make sure you have four significant figures.

Equipment

  • 1
    MicroLab Interface
  • 1
    MicroLab pH Measurement Instruction Sheet
  • 1
    pH electrode in pH 7.00 buffer
  • 1
    10.0 mL graduated cylinder
  • 1
    30 mL beaker
  • 1
    100 mL beaker
  • 2
    250 mL beakers
  • 1
    25 mL buret
  • 1
    ring stand
  • 1
    clamp
  • 1
    buret clamp
  • 1
    magnetic stir plate
  • 1
    magnetic stir bar
  • 1
    deionized water squirt bottle
  • 1
    box of Kimwipes

Reagents

  • ~50 mL 0.5 M sodium hydroxide (NaOH)
  • commercial vinegar (HC2H3O2)
  • pH 4.00 buffer
  • pH 7.00 buffer
  • pH 10.00 buffer
  • phenolphthalein solution
  • deionized water

Safety

NaOH is corrosive. It can attack the skin and cause permanent damage to the eyes. If NaOH solution splashes into your eyes, use the eyewash station immediately. Hold your eyes open and flush with water for at least 15 minutes. If contact with skin or clothing occurs, flush the affected area with water for at least 15 minutes. Have your lab partner notify your teaching assistant and the lab director about the spill and exposure. The chemicals used in this experiment are very dilute so gloves will not be available. Remember to wash your hands with soap and water when the experiment is completed.

Waste Disposal

All solutions can be discarded down the sink drain followed by flushing with plenty of water.

Prior to Class

Please read the following sections of the Introductory Material: Please review the following videos: Please complete your WebAssign prelab assignment. Check your WebAssign Account for due dates. Students who do not complete the WebAssign prelab assignment are required to bring and hand in the prelab worksheet.

PDF file

Lab Procedure

Please print the worksheet for this lab. You will need this sheet to record your data.

PDF file

In this experiment, you will be using pH electrodes connected to the MicroLab Interface. pH electrodes have a thin glass bulb at the tip. They break easily and are costly to replace. Be careful not to shove the electrode into the bottom of a beaker or drop the electrode. There is a protective guard around the tip, which should remain in place at all times. The guard will not protect against careless treatment. Please use extreme care when using this equipment. Best results in using the electrodes are obtained if:
  • Electrodes are kept in standard pH 7 buffer solution when not in use.
  • Immediately prior to use, the electrodes are rinsed with deionized water and gently blotted with a Kimwipe, then placed in the test solution.
  • The electrodes are rinsed and blotted again after the measurement and returned to the pH 7 buffer solution.

Part A: Calibrating the MicroLab pH Electrode

Part B: Titration of Vinegar Monitored by pH Probe and Indicator

Figure 2

Figure 2: Experimental Setup

Figure 3

Figure 3: Data Table B: Volume of Titrant Added to Vinegar vs pH

The following tables are located on the supplemental worksheet.
  • Data Table A1: Experimental Data
  • Data Table A2: Calculated Results
  • Data Table B: Volume of Titrant Added to Vinegar vs pH
Question 1: The titration curve of a weak acid like acetic acid with base has a distinctive appearance when the volume of titrant is plotted on the x-axis and the pH is plotted on the y-axis. Select the picture that most closely resembles this graph.
Figure 4

Figure 4

Question 2: What is the color of the solution at below pH 8? What is the color of the solution above pH 8? Find pH 8.00 on your titration graph. How close is the amount of titrant at pH 8.00 to the Equivalence Point Buret Reading? Within 0.50 mL? Within 1.00 mL?

Part C: Titration of Vinegar Monitored with Phenolphthalein Indicator

Question 3: Calculate the number of millimoles of NaOH required to reach the endpoint for each of the titrations. Show one calculation completely. What is the average? Record the values in Data Table A2.
Question 4: How many millimoles of acetic acid are in each vinegar sample? Show one calculation completely. What is the average? Record the values in Data Table A2.
Question 5: What is the mass of acetic acid in each vinegar sample? Show one calculation completely. What is the average? Record the values in Data Table A2.
Question 6: What is the molarity of acetic acid in each vinegar sample? Show one calculation completely. What is the average? Record the values in Data Table A2.
Question 7: What is the mass % of acetic acid in each vinegar sample? Show one calculation completely. What is the average? Record the values in Data Table A2.
Data Table A2: Calculated Results
Question 8: Do you prefer monitoring a titration with a pH probe or an indicator? Explain your choice.