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Measuring the Amount of Acid in Vinegar by Titration with an Indicator Solution

Difficulty
Time Required Short (2-5 days)
Prerequisites None
Material Availability Specialty items. Note: this project requires the use of a sodium hydroxide solution, which is caustic. You will need to order this chemical through your school.
Cost High ($100 - $150)
Safety Adult supervision required. Follow appropriate chemical safety procedures.

Abstract

There are many different types of vinegar that you can buy to use around the kitchen for cooking and pickling. The chemical compound that gives vinegar its tart taste and pungent smell is acetic acid. Do different vinegars have different amounts of acetic acid? How much variation is there between the different types? Find out for yourself with this science project.

Objective

Determine the amount of acid in different types of vinegar using titration with a colored pH indicator to determine the endpoint.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on the vinegar titration experiment described here:

Cite This Page

MLA Style

Science Buddies Staff. "Measuring the Amount of Acid in Vinegar by Titration with an Indicator Solution" Science Buddies. Science Buddies, 26 Feb. 2014. Web. 27 Aug. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p045.shtml>

APA Style

Science Buddies Staff. (2014, February 26). Measuring the Amount of Acid in Vinegar by Titration with an Indicator Solution. Retrieved August 27, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p045.shtml

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Last edit date: 2014-02-26

Introduction

You may be familiar with vinegar's tart taste and pungent smell, but do you know how vinegar is made? Vinegar is a solution made from the fermentation of ethanol (CH3CH2OH), which in turn was previously fermented from sugar. The fermentation of ethanol results in the production of acetic acid (CH3COOH). There are many different types of vinegar, each starting from a different original sugar source (e.g., rice, wine, malt, etc.). The amount of acetic acid in vinegar can vary, typically between 4 to 6% for table vinegar, but up to three times higher (18%) for pickling vinegar (Wikipedia contributors, 2007).

In this science project, you will determine the amount of acid in different vinegars using titration, a common technique in chemistry. Titration is a way to measure the unknown amount of a chemical in a solution (the titrant) by adding a measured amount of a chemical with a known concentration (the titrating solution). The titrating solution reacts with the titrant, and the endpoint of the reaction is monitored in some way. The concentration of the titrant can now be calculated from the amount of titrating solution added, and the ratio of the two chemicals in the chemical equation for the reaction. Let us go through the process with a specific example: the titration of acetic acid. But before we go over titration, here is a quick review of the chemistry of acids and bases.

It all has to do with hydrogen ions (abbreviated with the chemical symbol H+). In water (H2O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxyl ions (OH). The "lost" hydrogen ions join up with water molecules to form hydronium ions (H3O+). By convention (and for simplicity in writing chemical equations), hydronium ions are referred to as hydrogen ions H+. In pure water, there are an equal number of hydrogen ions and hydroxyl ions. The solution is neither acidic or basic.

Acids and bases are defined by whether they donate, or accept, hydrogen ions. An acid, like acetic acid, is a substance that donates hydrogen ions. When acetic acid is dissolved in water, the balance between hydrogen ions and hydroxyl ions is shifted. Now there are more hydrogen ions than hydroxyl ions in the solution. This kind of solution is acidic. A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxyl ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxyl ions than hydrogen ions. This kind of solution is basic, or alkaline. The pH of a solution is a measure of how acidic or basic it is. A neutral pH is 7, such as distilled water. An acidic pH is below 7, such as acetic acid, lemon juice, battery acid. A basic pH is above 7, such as baking soda or bleach. For a refresher, see the Science Buddies page on Acids, Bases, & the pH scale.

To measure the acidity of a vinegar solution, you can add enough hydroxyl ions to balance out the added hydrogen ions from the acid. The hydroxyl ions will react with the hydrogen ions to produce water. In order for a titration to work, you need three things:

  1. A titration solution (contains hydroxyl ions with a precisely known concentration)
  2. A method for delivering a precisely measured volume of the titrating solution
  3. A means of indicating when the endpoint has been reached

For the titrating solution, you will use a dilute solution of sodium hydroxide (NaOH). Sodium hydroxide is a strong base, which means that it dissociates almost completely in water. So for every NaOH molecule that you add to the solution, you can expect to produce a hydroxyl ion.

To dispense an accurately measured volume of the titrating solution, you will use a buret. A buret is a long tube with a valve at the bottom and graduated markings on the outside to measure the volume contained in the buret. The buret is mounted on a ring stand, directly above the titrant solution, as shown in Figure 1, below.

buret set up for an acid-base titration
Figure 1. The illustration shows a buret (filled with titration solution) mounted on a ring stand above a beaker (containing the titrant solution) (G. Carboni, 2004).

Solutions in the buret tend to creep up the sides of the glass at the surface of the liquid. This is due to the surface tension of water. The surface of the liquid thus forms a curve, called a meniscus. To measure the volume of the liquid in the buret, always read from the bottom of the meniscus. In Figure 2, below, by reading the bottom of the meniscus you can see that the fluid level is about 14.58 milliliters (mL).

buret with sodium hydroxide titrating solution
Figure 2. Always read the fluid level in the buret from the bottom of the meniscus (G. Carboni, 2004).

Lastly, in this experiment, you will use an indicator solution called pheonolphthalein. (I love to say that word: fee-nol-fthay-leen!) Phenolphthalein is a pH indicator solution that is colorless when the solution is acidic or neutral, but when the solution becomes slightly basic, phenolphthalein turns pinkish, and then light purple as the solution becomes more basic. So when your vinegar solution starts to turn pink, you know that the titration is complete.

Which type of vinegar do you think will have the most acetic acid? Find out for yourself with this science project.

Terms and Concepts

  • Vinegar
  • Fermentation
  • Acetic acid (CH3COOH)
  • Titration
  • Titrant
  • Acid
  • Base
  • pH
  • Sodium hydroxide (NaOH)
  • Buret
  • Meniscus
  • pH indicator solution, e.g., phenolphthalein

Questions

  • How can you calculate the amount of a chemical in a solution using titration?
  • What range of pH values do you expect different types of vinegar to have?
  • How does a pH indicator solution work?
  • At what pH does phenolphthalein change from colorless to pinkish?

Bibliography

Materials and Equipment

These items can be purchased from Carolina Biological Supply Company, a Science Buddies Approved Supplier:
  • Small funnel, used only for chemistry and not for food
  • Chemical splash safety goggles
  • 0.5% Phenolphthalein solution in alcohol, used as the pH indicator solution
    • Note: If you are ordering this chemical through Carolina Biological Supply Company, the chemical must be ordered by a teacher and shipped to a school or business address, so plan accordingly.
  • 0.1 M sodium hydroxide solution
    • Caution: Sodium hydroxide is caustic, which means it will cause a chemical burn on bare skin. It is also flammable. However, the 0.1 M solution is fairly dilute and relatively safe to use with proper chemical safety precautions (chemical safety goggles, lab coat or apron, and rubber gloves).
    • Note: Since each molecule of sodium hydroxide (NaOH) can produce one hydroxide ion (OH), 0.1 N sodium hydroxide is the same as 0.1 M.
    • Note: If you are ordering this chemical through Carolina Biological Supply Company, the chemical must be ordered by a teacher and shipped to a school or business address, so plan accordingly.
  • 125 mL Erlenmeyer flask. Alternatively, you can use a beaker and a stirring rod.
  • 25 or 50 mL buret and ring stand with buret clamp
  • 10 mL graduated cylinder
You will also need to gather these items:
  • Vinegar, at least three different types
    • Tip: It will be easier to see the indicator change color with lighter-colored vinegars.
  • Distilled water. This can be purchased from a grocery store.
  • Lab coat or lab apron. Can be purchased at scientific supply companies, or through an online supplier like Carolina Biological Supply Company. Alternatively, old, protective clothing may be used instead.
  • Disposable gloves. Can be purchased at a local drug store or pharmacy, or through an online supplier like Carolina Biological Supply Company. If you are allergic to latex, use vinyl or polyethylene gloves.

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies does participate in affiliate programs with Amazon.comsciencebuddies, Carolina Biological, and AquaPhoenix Education. Proceeds from the affiliate programs help support Science Buddies, a 501( c ) 3 public charity. If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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Experimental Procedure

Note: this project requires the use of a sodium hydroxide solution, which is caustic. You will have to order this chemical through your school. Proper safety precautions should be used when working with this solution, including:

  • Chemical splash safety goggles
  • Lab coat/apron
  • Gloves

Performing the Titration

  1. Do your background research so that you are knowledgeable about the terms, concepts, and questions in the Background section. For information about doing a titration, visit the Science Buddies webpage Titration Tutorial: Tips & Tricks for Titrating.
  2. Since you will be working with dilute sodium hydroxide, you should take proper safety precautions:
    1. Wear chemical splash safety goggles, a lab coat (or apron), and a pair of rubber gloves.
    2. If you spill sodium hydroxide on your skin, wash it off quickly with lots of running water.
  3. Pour 1.5 milliliters (ml) of vinegar in an Erlenmeyer flask.
    1. These flasks are designed so that you can swirl the solution inside without spilling it.
    2. Tip: you can also use a regular beaker and a stirring rod to keep the solution mixed as you titrate.
  4. Dilute the vinegar with about 50 ml of distilled water.
  5. Add 3 drops of 0.5% phenolphthalein solution.
    1. Phenolphthalein solution is colorless at acidic pH, and turns light purple at about pH 8.3.
    2. The vinegar solution is acidic, so it should remain colorless.
  6. Use the buret clamp to attach the buret to the ring stand. The opening at the bottom of the buret should be just above the height of the Erlenmeyer flask you use for the vinegar/water/phenolphthalein solution.
  7. Use a funnel to fill the buret with a 0.1 M solution of sodium hydroxide.
  8. Note the starting level of the sodium hydroxide solution in the buret. Remember to read from the bottom of the meniscus. In Figure 3, below, for example the fluid level is about 14.58 mL.

    buret with sodium hydroxide titrating solution
    Figure 3. Always read the fluid level in the buret from the bottom of the meniscus. (G. Carboni, 2004)

  9. Put the vinegar solution to be titrated under the buret. Figure 4, below, shows an example of the experimental setup at the beginning of the titration.

    start of vinegar titration, phenolphthalein is colorless
    Figure 4. At the start of the vinegar titration, the phenolphthalein is colorless. (G. Carboni, 2004)

  10. Slowly drip the solution of sodium hydroxide into the vinegar solution. Swirl the flask gently to mix the solution, while keeping the opening underneath the buret. (Alternative is to use a beaker and stirring rod—but be careful not to hit the buret with the stirring rod.)
  11. At some point you will see a pink color in the vinegar solution when the sodium hydroxide is added, but the color will quickly disappear as the solution is mixed. When this happens, slow the buret to drop-by-drop addition.
  12. When the vinegar solution turns pink and remains that color even with mixing, the titration is complete. Close the tap (or pinch valve) of the buret. Figure 5, below, shows how the solution color changes at the endpoint of the titration.

    end of vinegar titration, phenolphthalein makes the solution pinkish
    Figure 5. The endpoint of the titration is reached when the phenolphthalein in solution turns pinkish. (G. Carboni, 2004)

  13. Note the remaining level of the sodium hydroxide solution in the buret. Remember to read from the bottom of the meniscus.
  14. Subtract the initial level from the remaining level to figure out how much titrating solution you have used.
  15. For each vinegar that you test, repeat the titration at least three times. If you are careful with all of your volume measurements, the results of your three repeated trials should agree within 0.1 mL.

Analyzing Your Results

Here's how to figure out how much acetic acid was in each sample.

  1. Determine the number of moles of sodium hydroxide used to titrate the vinegar.
    1. Multiply the volume of added sodium hydroxide (in liters) by the concentration (in moles/liter).
    2. For example, if you added 12.5 mL of sodium hydroxide, the number of moles would be 0.0125 L × 0.1 moles/L = 0.00125 moles.
  2. Determine the concentration of acetic acid in the vinegar.
    1. The number of moles of sodium hydroxide equals the number of moles of acetic acid in your vinegar sample (Ms).
    2. The sample volume was 1.5 mL (Vs).
    3. You can use a proportion to determine the number of moles of acetic acid (Mx) in a standard volume (Vx = 1 L) of vinegar: Ms/Vs = Mx/Vx.
    4. Continuing with the previous example, the number of moles of acetic acid would be 0.00125. Dividing by 0.0015 L gives 0.833 moles of acetic acid per liter, or a concentration of 0.833 M.
    5. You can also calculate the concentration in terms of grams of acetic acid per liter. To do this, multiply the molar concentration of acetic acid by the molecular mass of acetic acid, which is 60. In the case of our example, the concentration would be 0.833 × 60 = 50 g/L, or 5.0%.
  3. Repeat the calculations for each type of vinegar you tested. Which vinegar had the highest concentration of acetic acid?

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Variations

  • Measure the acidity of solutions such as beer or wine.
  • Measure the acidity of other beverages, such as: different fruit juices, soda, sport drinks, coffee, or teas.
  • Measure the acidity of fermenting apple cider over time.
  • Measure the acidity of uncorked wine over time (check once a day over the course of a week).

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I did this project I Did This Project! Please log in and let us know how things went.

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