9.1: Titration of Vinegar Lab Procedure

Learning Objectives

• To determine the molarity and percent by mass of acetic acid in vinegar

 

Background

Vinegar is essentially a solution of acetic acid (HC2H3O2) in water. The concentration of acetic acid in vinegar may be expressed as a molarity (in mol/L):

\[ \text{Molarity} = \frac{\text{Moles of Acetic Acid}}{\text{Volume of Vinegar (in L)} } \]

or as mass percent

\[ \text{Mass %} = \frac{\text{Mass of Acetic Acid}}{\text{Mass of Vinegar}} \times 100% \]

In this experiment, a technique known as a titration will be used to determine the concentration of acetic acid in vinegar. A titration involves performing a controlled reaction between a solution of known concentration (the titrant) and a solution of unknown concentration (the analyte). Here, the titrant is an aqueous solution of ~0.1 M sodium hydroxide (NaOH) and the analyte is vinegar. When mixed, a neutralization reaction occurs between sodium hydroxide and the acetic acid in vinegar:

\[ \ce{NaOH(aq) + HC2H3O2 (aq) -> NaC2H3O2(aq) + H2O(l)} \]

The sodium hydroxide will be gradually added to the vinegar in small amounts from a buret. A burette is a device that allows the precise delivery of a specific volume of a solution. The NaOH will be added to the vinegar sample until all the acetic acid in the vinegar has been exactly neutralized. At this point the reaction is completed, and no more NaOH is required. This is called the equivalence point of the titration.

In order to know when the equivalence point is reached, an indicator solution called phenolphthalein is added to the vinegar at the beginning of the titration. Phenolphthalein is a pH sensitive organic dye. Phenolphthalein is colorless in acidic solutions like vinegar, and deep pink in basic solutions like sodium hydroxide. At the equivalence point of the titration, just one drop of NaOH will cause the entire solution in the Erlenmeyer flask to change from colorless to a very pale pink.

As the titration is performed, the following data will be collected:

• The molarity of NaOH (aq) used
• The volume of NaOH (aq) used to neutralize the vinegar
• The volume of vinegar used. Using this data, the molarity and mass percent of acetic acid in vinegar can be determined.

 

Experimental Procedure

Materials and Equipment

50-mL burette*, 5-mL volumetric pipette*, pipette bulb*, ~ 0.1 M NaOH (aq), vinegar, phenolphthalein, burette stand, two 250-mL (or 125 mL) Erlenmeyer flasks, wash bottle with distilled water, funnel

 

Safety

Be especially careful when handling the sodium hydroxide base (NaOH), as it is corrosive and can cause chemical burns to the skin. If any NaOH spills on you, rinse immediately under running water for up to 15 minutes and report the accident to your instructor. Make certain to wear safety glasses when handling NaOH when doing the experiment!

 

Titration Procedure

Your instructor will demonstrate the correct use of the volumetric pipette and burette at the beginning of the lab session. Detailed instructions on how to use a pipette are also found on the last page of this handout. Note that three titrations must be performed.

1. Obtain a 50-mL burette from the drying rack, 5-mL volumetric pipette and a pipette bulb from the fume hood.
2. Setting up the buret and obtain the NaOH
3. Rinse the inside of the buret with distilled water. Allow the distilled water to drain out through the tip in order to ensure that the tip is also rinsed.
4. Now rinse the burette with a small amount of NaOH (aq). To do this, add about 5-mL of NaOH (aq) to the burette, then twirl the burette on its side (over the sink) to rinse its entire inner surface. Then allow the NaOH (aq) to drain out through the tip and collect in a small beaker.
5. Fill the buret with NaOH (aq) up to the top, between 0-mL and 5-mL. Use a funnel to do this carefully, below eye-level, and preferably over a large beaker. After this you will need to flush the tip of the burette – your instructor will show you how to do this. Now measure the volume at the level of the NaOH precisely, and record it as the “Initial Burette Reading” on your report. Also record the exact molarity of the NaOH (aq), which is labeled on the stock bottle.

Preparing the vinegar sample

1. The volumetric pipette used in this lab is designed to measure and transfer exactly 5.00 mL of solution. First, rinse the inside of the volumetric pipette with distilled water. Using the pipette bulb, draw the water into the pipette up above the 5-mL mark, then allow it to drain out through the tip. You may want to do this several times for practice. Then perform a final rinse, but this time use vinegar.
2. Now use the volumetric pipette to transfer 5.00-mL of vinegar into a clean 250-mL Erlenmeyer flask (see instructions on page 4). Record this volume of vinegar on your report. Then add about 20-mL of distilled water and 2-3 drops of phenolphthalein to this Erlenmeyer flask.

Performing the titration

1. Begin the titration by slowly adding NaOH (aq) from the burette to the vinegar in the Erlenmeyer flask. Swirl Erlenmeyer flask as you add the base in order to efficiently mix the chemicals. Some pinkness may appear briefly in the flask as the base is added, but it will quickly disappear as the flask is swirled.
2. As the equivalence point is approached, the pink color will become more pervasive and will take longer to disappear. When this occurs, start to add the NaOH (aq) drop by drop. Eventually the addition of just one drop of NaOH (aq) will turn the solution in the Erlenmeyer flask a pale pink color that does not disappear when swirled. This indicates that the equivalence point has been reached. Do not add any more NaOH (aq) at this point. Measure this volume of NaOH (aq) precisely, and record it as the “Final Burette Reading” on your report.
3. Refill your burette with NaOH (aq), and then repeat this procedure for a second sample of vinegar, and then a third sample of vinegar. You do not need to flush the tip of the burette again. Note that if you use less than 25-mL of NaOH (aq) for the second titration, you do not need to refill the burette for the third titration; also, that you will need to clean out and re-use one of your Erlenmeyer flasks for the third titration. You and your partner(s) will take turns performing these titrations.
4. When finished, dispose of your chemical waste as instructed.

Pipeting instructions

1. Get the appropriate amount of the solution you wish to pipet in a clean, dry beaker. Never pipet directly out of the stock bottles of solution. This creates a contamination risk.
2. Insert the tip of the pipet into the beaker of solution so that it is about a quarter inch from the bottom. Be sure not to press the tip against the bottom of the container.
3. If you are right handed, hold the pipet in your right hand, leaving your index finger free to place over the top of the pipet. With your left hand, squeeze the pipet bulb. Press it firmly over the top of the pipet, but DO NOT INSERT THE PIPET DEEP INTO THE BULB!
4. Release the pressure on the bulb and allow the solution to be drawn up into the pipet until it is above the volume mark. Do not allow the solution to be sucked into the bulb itself.
5. Quickly remove the bulb and place your index finger firmly over the top of the pipet. Then remove the pipette tip from the beaker of solution. 
6. Slowly roll your finger to one side and allow the liquid to drain until the bottom of the meniscus is aligned with the volume mark. With practice, you will be able to lower the liquid very, very slowly.
7. When the bottom of the meniscus is even with the volume mark, press your index finger firmly on the top of the pipet so no liquid leaks out. Touch the tip once to the side of the beaker to remove any hanging drops.
8. To transfer the solution, place the tip of the pipette against the wall of the receiving container at a slight angle. Then allow the liquid to drain from the pipet.
9. When the solution stops flowing, touch the pipet once to the side of the receiving container to remove any hanging drops. DO NOT blow out the remaining solution. The pipet has been calibrated to deliver the appropriate amount of solution with some remaining in the tip.

 

Calculations

Molarity of Acetic Acid in Vinegar

1. First, using the known molarity of the NaOH (aq) and the volume of NaOH (aq) required to reach the equivalence point, calculate the moles of NaOH used in the titration.
2. From this mole value (of NaOH), obtain the moles of HC2H3O2 in the vinegar sample, using the mole-to-mole ratio in the balanced equation.
3. Finally, calculate the molarity of acetic acid in vinegar from the moles of HC2H3O2 and the volume of the vinegar sample used.

Mass Percent of Acetic Acid in Vinegar

1. First, convert the moles of HC2H3O2 in the vinegar sample (previously calculated) to a mass of HC2H3O2, via its molar mass.
2. Then determine the total mass of the vinegar sample from the vinegar volume and the vinegar density. Assume that the vinegar density is 1.000 g/mL (= to the density of water).
3. Finally, calculate the mass percent of acetic acid in vinegar from the mass of HC2H3O2 and the mass of vinegar.