Summary

In this lesson, students will use a color-changing indicator called indophenol and a simplified titration method to determine if vitamin C is present in a variety of store-bought juices. The indicator solution will turn from dark blue to colorless once all the indophenol has reacted with vitamin C in the juices. Students will count how many drops of juice it takes to produce this color change in a 5-mL sample of indicator solution. The greater number of drops it takes to cause the color change, the less vitamin C is present in each drop. They will use their data to compare the relative amounts of vitamin C in the juices to a solution prepared from a vitamin C tablet.

Grade Level

Middle School, High School

NGSS Alignment

This lesson will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lesson, students should be able to:

• Use a simplified titration procedure to determine if vitamin C is present in different commercially available juices and compare relative amounts.
• Understand the basic principles of titration.

Chemistry Topics

This lesson supports students’ understanding of:

• Concentration
• Chemical reactions
• Indicators
• Titration

Time

Teacher Preparation: 30 minutes
Lesson: 45 minutes

Materials

• 2,6 dichloroindophenol sodium salt, 0.025% w/v solution in water (see teacher notes for prep)
• Graduated cylinders (10 mL) or disposable pipette (5 mL) to dispense indicator solution
• Clear plastic cups (8-ounces)
  • 1 cup per student lab station plus 1 cup for vitamin C solution and each juice sample
• Disposable plastic pipettes for vitamin C solution and juice samples, 1 for each sample
• Vitamin C tablet (500 mg), ground and dissolved in 10-mL distilled water
• Orange juice (prepared from pulp-free frozen concentrate, e.g. Minute Maid)
• Selection of juices – for example: Capri Sun, Arizona green tea, apple juice, pineapple juice, lemonade, Gatorade, Sunny D, Vitamin Water, etc.
• Plastic rinse bottles with water, 1 per student lab station
• “Dump” buckets (~500 mL plastic container), 1 per student lab station
• Plastic trays or paper plates for reagent and lab stations (to control spills)
• Student handouts

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them on how to clean up their materials and properly dispose of chemicals.
• Do not consume lab solutions, even if they’re otherwise edible products.

Teacher Notes

• This lab is appropriate for middle school students or general level high school students. Prior to completing this lab, students should have a basic understanding of concentration (though they do not need to know specific units like molarity). They will be introduced to the fundamentals of titration and indicators in an approachable way that is appropriate for younger and less experienced students, as they will compare relative amounts of vitamin C in various juices rather than having them calculate how much vitamin C is present by mass. (See the extensions listed at the end of the teacher notes for ways to introduce a more quantitative analysis.)
• The background reading may be a bit advanced for middle school students or less confident readers – this could be a good opportunity to help them develop their confidence and reading skills! There are “check in” questions after each paragraph to help break up the reading and give students a chance to make sure they have processed and understood what they have read, but it might also be useful to read through that information and answer the questions as a whole class. The teacher or student volunteers could model active reading techniques, such as circling unfamiliar words and looking them up, underlining important ideas, writing questions in the margins, etc. The Chemistry Close Read activity in the AACT resource library and the related Chemistry Solutions article provide good guidance on modeling and encouraging active reading.
• Lab groups should include 2-4 students, depending on class size.
• Preparation of 0.025% (w/v) Indophenol Solution: Weigh 125 mg (0.125 g) of the 2,6-dichloroindophenol sodium salt (can be purchased from such sites as Flinn Scientific and Home Science Tools) and dissolve in distilled water to a final volume of 500 mL. This will be enough for 10 lab groups to test 8 or 9 different juice samples with a 5-mL sample of indicator solution (80 or 90 tests total), with a little left over to account for pre-lab testing by the teacher, spills, etc. Amounts can be scaled up or down to accommodate the number of 5-mL samples and lab groups anticipated for the activity.
  • Be sure to prepare enough solution for all lab groups in a single batch (if possible) so that the concentration of the indicator solution is uniform to allow for a valid comparison of data between groups.
  • Indophenol solution is not very shelf stable, so prepare it as close to the lab day as possible (preferably on lab day) and store refrigerated in a dark or foil-covered bottle.
  • It is recommended that the instructor test out several juice samples on their own to make sure the indicator concentration works for the variety of juices being tested. An “ideal” standard would be 5 drops of orange juice (prepared from frozen concentrate) turning 5 mL indicator solution from blue to colorless. You can adjust the concentration slightly higher or lower if needed. Testing other juices will give the instructor a preview of what to expect from students’ determinations.
  • Place indophenol solution in a capped bottle on a plastic tray to contain spills. It is recommended that the indophenol solution is staged away from the juice samples. Pour about 100 mL into a beaker with a dedicated 10-mL graduated cylinder or a 5-mL plastic disposable pipette for students to measure out their samples. Refill beaker as needed. Since each test requires a new indophenol sample, several beakers of indophenol can be placed around the room so that all students aren’t waiting in line for just one beaker.
  • For indophenol spills or stains, simply treat with vitamin C solution. The blue stain will disappear.
• Prepare the vitamin C solution by crushing one 500 mg vitamin C tablet and mixing with 10 mL distilled water. (Vitamin C tablets should be available in most grocery stores or drug stores, such as Walgreens, CVS, etc.) The vitamin is soluble in water, but some tablet bulking agents may not be. In this case, simply let solid materials settle and decant solution to a clean sample cup. This volume is enough for at least two full classes, and the solution must be made fresh each day. The pure vitamin C sample should be the last sample tested by each group.
• Set each juice sample on a tray to control spills. Pour approximately ¼ cup of each juice into a labeled 8-ounce cup and set on a long table, counter, or on any stable flat surface around the classroom. To control foot traffic, have lab groups send one student to retrieve one juice sample at a time to test at their lab station. Each juice sample must have a dedicated plastic pipette.
• A brief tutorial on the proper use of a graduated cylinder is recommended. This AACT simulation could help students practice with significant figures and uncertainty when reading graduated cylinders.
• You may want to give students some pointers on proper pipette technique:
  • Remind them to hold the pipette perpendicular to the lab table (or at least holding it the same way every time they add a drop) to make sure drop size is as consistent as possible.
  • Encourage them to pay close attention to which dropper goes with which juice to avoid cross-contamination, and remind them not to touch the dropper to any other solutions.
  • Inconsistencies in dropper technique may be cited as a source of variation or error in data analysis.
• The data table and graph in the student document can be customized to fit the number of juice samples you would like students to test.
• Depending on the grade level, time allotted for the activity, availability of supplies, etc. the number of juice samples tested can be limited. For example, perhaps 10 different juice options are offered but each group only picks 5 to test.
  • To promote student ownership of the learning activity, students could be given the option to bring in a juice of their own to test.
• Following the lab activity, allow students to compare results with other groups, discuss patterns and potential sources of variation in data within and between lab groups.
• Extension opportunities:
  • Compare the vitamin C content of bottled or processed juice to that of freshly squeezed juice (ex: store bought bottle of orange juice vs. orange juice from concentrate vs. freshly squeezed oranges, or bottled lemon juice vs. lemonade vs. freshly squeezed lemons)
  • Provide samples of juice that have been subjected to a variety of lighting and temperature conditions to study the effect of light and heat exposure on vitamin C content of a favorite bottled juice. A student can make connections between recommended storage in a refrigerator, packaging, and expiration dates. This study is appropriate for a Science Fair presentation.
  • For more advanced students, particularly at the high school level, reagent grade vitamin C (L-ascorbic acid) could be used in place of the drugstore tablet to make a vitamin C solution with high levels of precision (ex: 0.100 g per 100 mL). A solution whose concentration is known very precisely would serve as a standard that students could use to determine quantitatively the mass of vitamin C per a given volume of juice. See Flinn Scientific’s “Vitamin C Testing” resource for sample calculations, chemical equations, molecular structures, etc..