Vacidmacid = Moles H+ = Moles OH = Vbasembase

Acids, Bases, and pH – 3 Percent Acetic Acid in Vinegar – Microscale Method

Introduction

Vinegar is composed mostly of acetic acid and water. In this experiment, you will find the amount of acetic acid in a vinegar sample by means of a titration (pronounced “tie-TRAY-shun”). In a titration, the concentration of an unknown substance can be found by comparing it to another substance of known concentration. The equation for monoprotic acids – acids with one acidic hydrogen, like acetic acid – reacting with bases that have a single hydroxide group is

+ – VacidMacid = moles H = moles OH = VbaseMbase where V is the volume (in any convenient units) and M is the molarity (moles of solute per liter of solution). In this titration, you will add a solution of sodium hydroxide to a vinegar solution to determine the percen- tage of acetic acid in the vinegar. The reaction taking place in the titration is

CH3COOH (aq) + NaOH (aq)  NaCH3COO (aq) + H2O (l) or, in words, “acetic acid and sodium hydroxide will react to form sodium acetate and water.” (What kind of reaction is this?) This titration works well in microscale – using small quantities of chemicals and apparatus that has been scaled down for the procedure. There are several good reasons for using microscale techniques to perform experi- ments in chemistry. First, the smaller quantities of chemicals used reduce costs and the risk of serious accidents. Second, the time required to complete a trial is shorter, meaning that many more trials can be completed in a single lab period. Third, the problem of waste disposal is reduced because less waste is generated. Finally, because everything is scaled down, the chemistry is the same as it would be if you used large amounts of chemicals. In this experiment, you will use micropipets in place of burets, and a well plate will take the place of beakers. This microscale method will allow you to complete many more trials than would be possible using traditional lab techniques.

Objectives

By doing this lab, students will be able to:

. define the term titration . determine the percent acetic acid in a vinegar solution using microscale techniques . list the advantages of using microscale techniques instead of traditional macroscale techniques

Materials and Equipment

24-well plate 0.1 M sodium hydroxide solution 3 Beral® pipets Vinegar (several samples) Plastic toothpicks Phenolphthalein indicator solution

Safety Considerations

. As always, you should wear safety glasses/goggles when working in the lab area. . Avoid getting the solutions on your skin and clothing. If you come in contact with them, wash thoroughly with soap and water. University of South Alabama Introductory Manual: Acids, Bases, and pH Science in Motion 2003 Percent Acetic Acid in Vinegar - Microscale Acids, Bases, and pH 3 – Percent Acetic Acid in Vinegar – Microscale – 2 . Sodium hydroxide can cause permanent, instantaneous eye damage; treat it with respect!

Procedure [NOTE: It’s a good idea to have one student actually do the procedure while the other records the data; this allows for uniformity in the procedure.]

1. Place your well sheet on a sheet of white paper (notebook paper will work well); this will allow you to better see the indicator change color.

2. Using a Beral pipet, place 5 drops of vinegar in each of six wells in the well plate; this will allow you to perform the titration six times. It’s important that the drops are of uniform size – to do this, make sure the same person uses the dropper throughout the experiment and holds the dropper at the same angle each time. Record the vinegar sample’s number or letter on the data sheet.

3. Add one drop of phenolphthalein indicator solution to each of the six wells containing vinegar. Gently swirl the plate to mix the vinegar and indicator, or stir the solutions with a clean toothpick.

4. Add sodium hydroxide solution to the first well, one drop at a time, and stir with a clean toothpick. Be sure to keep up with the number of drops of sodium hydroxide you add. Keep adding sodium hydroxide until the solution remains pink for at least 20 seconds after stirring. Record the number of drops of NaOH required to reach this endpoint.

5. Repeat Step 4 for each of the other five vinegar samples, using a fresh toothpick each time. Record the number of drops of sodium hydroxide used each time. If one trial varies greatly from the rest, disregard it and complete another trial.

6. If your instructor has made several brands of vinegar available and time permits, you may repeat the procedure for another vinegar brand.

Disposal and Cleanup

. When you’ve finished all your trials, call your instructor over so he or she can look at your well plate. . Take your well plate to the sink and turn the water on slowly. Gently put the well plate underneath the water stream and rinse it out thoroughly – phenolphthalein has an annoying tendency to linger in the wells and may contaminate someone’s future results. Take care to avoid splashing chemicals on yourself or your neighbors. . Place the well plate face down on a double thickness of paper toweling. . Discard the Beral pipets and toothpicks. . Wash your hands thoroughly with soap and water.

University of South Alabama Introductory Manual: Acids, Bases, and pH Science in Motion 2003 Percent Acetic Acid in Vinegar - Microscale