TheBakwassingQueens#4(IO) - Page 37

Objective

The goal of this project is to determine the amount of acid in different types of vinegar using titration with a colored pH indicator to determine the endpoint.

Introduction

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 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's 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.

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 alkaline.

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, and
3. a means of indicating when the endpoint has been reached.

For the titrating solution, you'll 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 below).

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 the illustration below, I'd say that the fluid level is 14.58 mL.

Always read the fluid level in the buret from the bottom of the meniscus (G. Carboni, 2004).

In this experiment, you will use an indicator solution called pheonolphthalein. (I love to say that word: fee-nol-fthay-leen!) Phenolphthalein is colorless when the solution is acidic or neutral. 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 project.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Vinegar
• Acids and bases
  • Acetic acid (CH₃COOH)
  • Sodium hydroxide (NaOH)
• pH scale (see the Science Buddies Chemistry Resource, Acids, Bases, and the pH Scale)
• Indicator solutions (e.g., phenolphthalein)
• Stoichiometry
• Titration
• Meniscus
• Buret
• Titrant

Questions

• What value of pH is neutral?
• What range of pH values is acidic?
• What range of pH values is basic?
• How does a pH indicator work?
• At what pH does phenolphthalein change from colorless to pinkish?

Bibliography

• To get you started, here is a Wikipedia article on vinegar:
  Wikipedia contributors, 2007. "Vinegar," Wikipedia, The Free Encyclopedia [accessed July 24, 2006] http://en.wikipedia.org/w/index.php?title=Vinegar&oldid=146505883.
• Here is a good introduction to acid-base chemistry:
  
  • Carboni, G., 2004a. "Experiments with Acids and Bases," Fun Science Gallery [accessed July 24, 2007] http://www.funsci.com/fun3_en/acids/acids.htm.
  • This project is based on the vinegar titration experiment about 4/5 of the way down the page:
    Carboni, G., 2004b. "Experiments with Acids and Bases: How Acid Is That Vinegar?" Fun Science Gallery [accessed July 24, 2007] http://www.funsci.com/fun3_en/acids/acids.htm#11.
• These webpages have a quick review of exponents and logarithms:
  
  • U of MN, 2004a. "What Is an Exponent?" Math Review: Useful Math for Everyone, University of Minnesota, School of Public Health [accessed July 24, 2007] http://www.mclph.umn.edu/mathrefresh/exponents.html.
  • U of MN, 2004b. "What Is a Logarithm?" Math Review: Useful Math for Everyone, University of Minnesota, School of Public Health [accessed July 24, 2007] http://www.mclph.umn.edu/mathrefresh/logs.html.
• For more information about the pH scale, try these references:
  • Environment Canada, 2002. "Kids' Corner pH Scale," The Green Lane: Acid Rain, Environment Canada website [accessed July 24, 2006] http://www.ec.gc.ca/acidrain/kids.html.
  • Decelles, P., 2002. "The pH Scale," Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College [accessed July 24, 2006] http://staff.jccc.net/pdecell/chemistry/phscale.html.
• For more information on titration techniques see Chemistry Lab Techniques.
• CBSE Blog: http://cbse-sample-papers.blogspot.com

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Vinegar, at least three different types
  • Tip: it will be easier to see the indicator change color with lighter-colored vinegars.
• Distilled water
• Small funnel (do not use for food after using it for chemistry)
• The following items can be ordered from Science Kit & Boreal Laboratories:
  • Chemical safety goggles
  • Lab apron
  • Rubber (latex) gloves
  • 0.5% Phenolphthalein solution in alcohol (pH indicator solution)
  • 0.1 M sodium hydroxide solution
    • Sodium hydroxide is caustic, which means it will cause a chemical burn on bare skin.
    • You will have to order this chemical through your school.
    • This solution is fairly dilute and relatively safe to use with proper chemical safety precautions (chemical safety goggles, lab coat or apron, and rubber gloves).
    • Tip: 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.
  • 125 mL Ehrlenmeyer flask
  • 25 or 50 mL buret
  • 10 mL graduated cylinder
  • Ring stand
  • Buret clamp

Experimental Procedure

Performing the Titration

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. Since you will be working with dilute sodium hydroxide, you should take proper safety precautions:
   1. Wear safety goggles for chemistry, an apron (or lab coat), 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 ml of vinegar in an Ehrlenmeyer 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 Ehrlenmeyer 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 the illustration below, I'd say that the fluid level is 14.58 mL.
   

   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. The illustration below shows an example of the experimental setup at the beginning of the titration.
   

   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. The illustration below shows how the solution color changes at the endpoint of the titration.
    

    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 (M_s).
   2. The sample volume was 1.5 mL (V_s).
   3. You can use a proportion to determine the number of moles of acetic acid (M_x) in a standard volume (V_x = 1 L) of vinegar: M_s/V_s = M_x/V_x.
   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?

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).