Summary

In this lesson, students will learn about a career in the skilled technical workforce, develop skills utilized in a quality control lab, and obtain data that may not have a clear “right answer.” For example, though many over-the-counter medications and vitamins state the amount of active ingredient, any individual tablet may have between 97 to 103% of the stated label claim. In addition, any products past the expiry date may have less due to potential decomposition. Students practice scientific communication by reporting their findings in a professional manner.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information

Objectives

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

• Explain the role of chemical technicians in a quality control lab.
• Prepare solutions and conduct titrations.
• Analyze titration data and communicate results and quality control recommendations in a technical memo.

Chemistry Topics

This lesson supports students’ understanding of:

• Titrations
• Chemical reactions
• Oxidation-reduction reactions
• Data analysis and significant figures

Time

Teacher Preparation:

• 30-45 minutes to prepare solutions
• 30 minutes to standardize iodine solution

Lesson:

• 30 minutes for the introductory activity on Chemistry and Quality Control (can be assigned as homework as a pre-lab assignment)
• 60-90 minutes for lab activity (depends on whether students complete 2 or more titrations per analysis)

Materials

• Glassware and equipment per lab group:
  • 100 mL volumetric flask
  • 100-mL & 10-mL graduated cylinders
  • two 125-mL Erlenmeyer flasks
  • 50-mL burette, ring stand and burette clamp
  • 1-mL volumetric pipette and pipette bulb
  • Spatula
• Deionized or distilled water
• Vitamin C tablets (1 per group)
  • 1000 mg Vitamin C tablets are recommended, see teacher notes for more detail; do not use multivitamins as the other ingredients may interfere with the titration.
• 1% starch solution (3 mL per lab group, see teacher notes for preparation details)
• 2% Iodine-Potassium Iodide Solution (referred to throughout this lesson as “iodine solution;” see teacher notes for preparation and standardization details)
• Ascorbic acid stock solution, about 1 mg/mL (20-30 mL total for teacher standardization, or 10-20 mL per lab group if students standardize iodine solution; see teacher notes)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Iodine solutions may stain, so gloves and lab coat or apron may be worn.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Do not consume lab solutions, even if they are otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• The National Science Board estimates that there are over 16 million jobs in the Skilled Technical Workforce (STW) that don’t require a 4-year college degree, but require STEM knowledge such as the use of chemicals, application of arithmetic and algebra, and the knowledge of quality control and other techniques for manufacturing goods. This lesson was developed as part of a content writing team to support the ACS Strategic Initiative on Fostering a Skilled Technical Workforce, with the goal of increasing awareness of and appreciation for STW opportunities in the chemistry enterprise at the high school level.
• This lesson plan introduces students to quality control (QC) in pharmaceuticals and nutritional supplements. Students practice using some of the skills needed by chemical technicians, including preparing solutions, conducting chemical analyses, and preparing a report to summarize results.
• The first question in the introductory activity has students respond to a short video on quality control/chemical technician careers. Below are a few other video options – you may choose to substitute one of these if it would be more appropriate/interesting for your class.
  • Lilly Quality Control Laboratories Help Ensure High Quality Medicines from Eli Lilly and Company
  • Chemical Technician Career Video from Career One Stop
  • Lab Technician | What I do & how much I make | Part 1 from Khan Academy
• For the lab portion of this lesson, 1000 mg vitamin C tablets are recommended. Lower dose vitamins could be used and would take less time and titrant to analyze, but care should be taken to ensure that amounts are not too small for students to measure or have too few significant figures for meaningful data analysis. Be sure it is not a multivitamin, as other ingredients may interfere with titration data.
• In this lab, ascorbic acid (vitamin C) is directly titrated with iodine (iodimetry). The titrant iodine is reduced and the ascorbic acid analyte is oxidized as shown below. A starch solution is used as an indicator, as it forms a dark blue-black complex with I₂ once all the ascorbic acid is consumed. This color change marks the endpoint of the reaction.

• Generally, titrations should be run in triplicate, but time may limit students to fewer. It is helpful to have students share data (for example, via a shared Google Sheets) to allow more meaningful (and realistic) error analysis.
• For the final report, students can research the recommended daily amounts and upper limits for Vitamin C themselves, or you can provide the following information. While many students may know the problem with too little Vitamin C (scurvy), they may not know that too much Vitamin C can cause diarrhea or stomach cramps, and increase the risk of kidney stones. If time permits, a discussion about the importance of quality control as it pertains to these recommendations could help students further appreciate the relevance and real-world impact of QC careers.

• At the end of the final report, students are asked to submit raw data and full calculations as appendices. You could have them just submit the lab handout for this section, or, to make it more formal, you could have them neatly write or type up their data tables and calculations.
• The scenario can be modified to test fruit juices (see Ballentine, 1941 in “Further reading about techniques” section below). Light-colored juices must be used in order to see the endpoint.
• To increase the challenge for a more advanced class, you could remove the SOP section from the student handout and have the students research methods to measure ascorbic acid and write their own procedure. There are two methods of using iodine to oxidize ascorbic acid (iodometry and iodimetry) that differ only in how the reagents are combined.
• Another option to increase the level of difficulty of this lesson is to remove the guided calculations and have students independently determine how to analyze the results.

Standardization of Iodine Solution with Ascorbic Acid

• You can use 2% Tincture of Iodine Solution from a drug store (which contains 1.8 to 2.2 grams of I₂/100 mL solution with an approximate molarity of 0.08 M I₂), or Lugol solution (1% iodine solution which contains 1 g I₂/100 mL of solution with an approximate molarity of 0.04 M I₂). In either case, it must be standardized relative to ascorbic acid (see below).
• The procedures in the student document assume that time is limited, and that the teacher standardizes the iodine solution in advance and provides the conversion factor to the students. Alternatively, provide the standardization procedures (below) to the class, and each lab group can perform one titration (or more, if time allows) and contribute a data point for a group calibration. This standardization step would be a valuable addition to help students understand another example of the roles and responsibilities of skilled technical workers if time allows.
• Ascorbic acid oxidizes quickly in the presence of oxygen. The standard solution should preferably be prepared on the day of the lab but may be prepared the day before if stored in a brown bottle or a bottle wrapped in aluminum foil and refrigerated. If the teacher is completing the standardization and providing students with a conversion factor, the entire standardization procedure can occur the day before/when the ascorbic acid solution is fresh.
• The 1% starch solution is the indicator that will identify the endpoint of the titration when excess iodine reacts with starch and turns the solution blue-black. The exact concentration is not important, and the solution can be prepared by dissolving 1 g of soluble starch in 100 mL of distilled or deionized water (water should be heated for easier dissolving), or by spraying ironing starch into distilled or deionized water. More detailed instructions are available here.
• Procedure to standardize the iodine solution:
1. Using a 100-mL volumetric flask, dissolve 0.10 g of ascorbic acid in distilled or deionized water to prepare a stock solution with a concentration of ~1 mg/mL of solution. Record the exact mass of ascorbic acid used and label the concentration of the stock solution $\ce{{$\frac{mg~asorbic~acid} {100~mL~solution} $}}$. (It should be approximately 100 mg ascorbic acid/100 mL solution.)
2. Using a volumetric pipette, transfer 10 mL of the ascorbic acid solution to an Erlenmeyer flask.
3. Using the 10-mL graduated cylinder, add 1 mL of 1% starch solution to the ascorbic acid solution in the flask.
4. Fill the burette with the iodine solution and record the initial volume to the nearest 0.01 mL (or as appropriate for the particular burettes used).
5. Add the iodine solution by drops to the flask, gently swirling to mix, until the dark blue color of the iodine-starch complex persists.
6. Record the final volume and determine conversion factor $\ce{{$\frac{mg~asorbic~acid} {100~mL~solution} $}}$ as shown below.

Calculations and Sample Data for Standardization

• Rather than molarity, a gravimetric conversion factor relates the volume of iodine solution to the mass of ascorbic acid. This is the method used in the references cited and provides a direct route to the mass of ascorbic acid present, which matches the units you generally see on vitamin labels (mg) and makes it easier to directly compare the experimental results to the label.
  • Use the most precise balances available for best results.
• Mass of ascorbic acid: 0.103 g ascorbic acid = 103 mg ascorbic acid
• Concentration of ascorbic acid stock solution from step 1: $\ce{{$\frac{103~mg~asorbic~acid} {100~mL~solution} $}}$

• Determine the mass, in mg, of ascorbic acid in 10 mL solution:

Trial 1 & 2:

$\ce{10.0~mL~of~ascorbic~acid~solution * {$\frac{103~mg~ascorbic~acid}{100~mL~solution}$} = 10.3~mg~of~ascorbic~acid}$

• For each trial, determine how many mL of iodine solution needed to react with the mass of ascorbic acid in 10 mL solution.

Trial 1: 3.36 mL reacted with 10.3 mg of ascorbic acid

Trial 2: 3.42 mL reacted with 10.3 mg of ascorbic acid

• Calculate a conversion factor that relates 1 mL of iodine solution to mass of ascorbic acid.

Trial 1:

$\ce{{$\frac{10.3~mg~ascorbic~acid}{3.36~mL~solution}$} = {$\frac{3.07~mg~ascorbic~acid}{1.00~mL~solution}$}}$

Trial 2:

$\ce{{$\frac{10.3~mg~ascorbic~acid}{3.36~mL~solution}$} = {$\frac{3.01~mg~ascorbic~acid}{1.00~mL~solution}$}}$

Average Conversion Factor: 1 mL iodine solution = 3.04 mg ascorbic acid (provide this value to the students)

• If desired, you can have the students calculate molarities of the solutions instead of mg/mL, but this gravimetric conversion factor is more direct and is more likely to be used in a QC laboratory.

Further reading about techniques

• The use of iodine to titrate vitamin C is a common analytical technique that has been used for over 80 years. Some references are below:
  • Ballentine, R. (1941). Determination of Ascorbic Acid in Citrus Fruit Juices. Industrial and Engineering Chemistry, Analytical Edition, 13(2), 89. doi: doi.org/10.1021/i560090a011
  • Moore, C. E. (1948). The determination of vitamin C as a means of teaching iodimetry. Journal of Chemical Education, 25(12), 671. doi: doi.org/10.1021/ed025p671
  • Cesar R. Silva, J. A. (1999). Ascorbic Acid as a Standard for Iodometric Titrations. Journal of Chemical Education, 76(10), 1421-1422. doi: doi.org/10.1021/ed076p1421
• This resource provides a very useful reference for use and care of burettes.