In honor of National Chemistry Week 2014, let’s do some redox reactions using something sweet.
Experimental Title: Lab 20: Reduction-Oxidation Reactions
Date of laboratory: October 21, 2014
Purpose: The purpose of this laboratory is to evaluate redox reactions.
Part 1: Permanganate Changes Color with Oxidation State
Sometimes it is possible to determine the oxidation state of a substance by looking at its color.
In a basic solution with small amounts of sugar added, permanganate ion is reduced to manganate ion:
MnO4– + e → MnO42–
and the solution changes from deep purple to deep green in color.
When there is excess sugar, the manganate is reduced to MnO2.
MnO42– + 2H2O + 2e → MnO2 + 4OH–
MnO2 is yellowish brown at the low concentrations, which is the final color change. This reaction is commonly called the “chameleon reaction” because many intermediate colors are often seen as the reaction progresses.
Here is a video that shows the reaction (you might want to turn down your speakers before playing it).
Part 2. Redox titration to find the relative amount of Vitamin C (ascorbic acid) in different foods
In this reaction, iodine oxidizes ascorbic acid to dehydroascorbic acid as it is reduced to iodide ions.
ascorbic acid + I2 → 2 I− + dehydroascorbic acid
Once all the ascorbic acid has been oxidized, the excess iodine is free to react with the starch indicator, forming the blue-black starch-iodine complex.
Special safety concerns for Lab 20:
- Please use gloves and eye protection when handling hazardous materials.
- If an acid spills, cover it with baking soda as we discussed. If a base such as NaOH spills, neutralize it with vinegar. If either spills on your skin, immediately wash with water.
- If anything else spills, please clean it up immediately with a paper towel and let your instructor know.
- If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
- Be sure to wash your hands when you are finished with this lab.
- Distilled water
- Transfer pipettes
- Sodium hydroxide
- Sugar (granulated)
- Glass beaker
- Potassium permanganate, 0.1 M
- Plastic cup
- Starch indicator solution
- Orange juice
- Cranberry juice
- Red pepper juice
- Lemon juice
- Strawberry juice
Part 1. Permanganate Changes Color with Oxidation State
- Fill a glass beaker with 100 mL distilled water.
- Add 1 small spatula of granulated sugar and mix with stirring rod.
- Using the transfer pipette, add 0.5 mL sodium hydroxide.
- Using a clean pipette, add 0.5 mL of potassium permanganate.
Note: If the reaction does not proceed completely to yellowish-brown after about 10 minutes, try adding another spatula full of sugar.
Part 2. Perform a REDox Titration to Determine the Relative Amounts of Vitamin C (Ascorbic Acid) In Different Foods
1. Add 5 mL of distilled water to a plastic cup. Add 0.5 mL of starch indicator and stir. Draw up iodine in a transfer pipette and add one drop to the solution (Set the filled pipette in a clean cup). Swirl the solution. It should turn blue-black, indicating the presence of iodine. This shows you the color you should achieve at the endpoint of titration. Now add a few mL of orange juice to see what happens.
2. Set up empty plastic cups for each type of juice. Predict which juices will contain the most Vitamin C (ascorbic acid) and thus require the most drops of iodine to reach the endpoint (turn blue-black).
Most vitamin C (ascorbic acid): _________________
Least vitamin C: _____________
3. Add 5 mL of orange juice to a clean, empty cup. Add 0.5 mL of starch indicator. Draw up iodine into a pipette or use the pipette you filled previously. Carefully add iodine a drop at a time, counting the drops. Swirl after adding each drop and check to see if the solution has reached the titration endpoint (turned blue-black). Once the solution remains blue-black, record the number of drops of iodine you added.
Number of drops:
4. Repeat step 3 for the remaining types of juice.
Number drops for:
- Cranberry juice:
- Red pepper juice:
- Lemon juice:
- Strawberry juice:
Did your results match your predictions?
Once you have completed the two parts, sit down and write a sentence or two to explain the results of each part.
Record any thoughts you have about the experiments, including:
- Possible improvements to the procedures or how to tweak techniques
- How the results differed from your expectations
- Suggestions for other experiments
- What key concepts you learned about redox reactions
Please leave a comment or send an e-mail if you have any questions before our meeting.