Vitamin C, chemically known as L-ascorbic acid, is an important nutrient that provides support to body structures, contributes to wound healing and aids the immune system in preventing infection. In this activity, participants learn how to determine whether vitamin C is present in commercial drinks and, if so, how to measure the relative amount. The procedure is easy to perform, uses materials that can be found at a grocery store, and is suitable as either a demonstration or hands-on activity.
The basic experimental procedure combines 50 drops of the solution to be tested with 3 drops of starch solution. Iodine solution is then added dropwise until a blue-black color appears. The color is the result of the formation of the starch-iodine complex. If vitamin C is present, the brownish color of the iodine solution will become colorless — the vitamin C serves as a reducing agent and reduces iodine to iodide ions (colorless in solution). If there is no vitamin C (or very little), the blue-black coloration appears immediately. At the end, sodium thiosulfate can be added to the solutions with iodine to convert all the tri-iodide, iodine and remaining iodate into iodide — which then can be disposed of in the sink, if local regulations permit.
The vitamin C standard was prepared by crushing a vitamin C tablet and then dissolving it in 50 – 100 mL of water. Water was used as the other comparison standard. The audience then could compare the standard test results (with and without vitamin C) with the results produced from the sample beverages.
“C” the difference
Concepts
- Starch-iodine endpoint
- Titration
- Oxidation-reduction reaction
Materials
- Standard solutions — 50 drops of water and 50 drops of vitamin C solution (300 mg ground vitamin C tablet in 100 mL water)
Note: all water solutions were made with distilled water.
- Assortment of beverages (e.g., Gatorade®, apple juice, Hi-C®, Kool-Aid®
- Starch solution
- Iodine tincture
- Sodium thiosulfate solution
- Small test tubes with a test tube rack
- Disposable pipets
- Small dropper bottles to add the drops
Advance preparation
- Starch solution is prepared by heating water to boiling. Then using spray starch such as Niagara®, spray periodically into heated water until the solution is cloudy.
- Vitamin C solution can be prepared based on desired quantity for titration. Our solution was made by dissolving a 300-mg crushed vitamin C tablet in 100 mL of water. This solution required 10-15 drops of iodine to reach the endpoint.
Some of the insoluble binder in the tablet may remain at the bottom.
- Sodium thiosulfate solution can be found at photography stores still developing film. It can be used as is. If you have solid available, a 0.1 M solution works well — 2.48 g of sodium thiosulfate pentahydrate in 100 mL of water.
- If desired, test tubes can be prepared ahead of time by adding 50 drops of test solution to separate tubes and
3 drops of starch to each.
Procedure
- Add 50 drops of the vitamin C solution to one tube and 50 drops of water to another tube.
- Add 3 drops of starch solution to each.
- Add iodine tincture to each, drop by drop — recording the number of drops — until the iodine no longer reacts and a blue-black color appears. Approximately 1 drop for the water sample compared to 10-15 drops for sample with vitamin C.
- Then add 50 drops of selected beverage to a test tube and add 3 drops of starch. Repeat procedure for all beverages. These are your test solutions.
- To each test solution add iodine tincture, dropwise, mixing well after each drop until you see a permanent color change. Solution will generally be blue-black in color, sometimes brown. The color change indicates the endpoint of the titration.
- For each test solution, record, on a chart, the number of drops of iodine tincture needed to reach the endpoint.
- Have audience members using the chart determine which drinks contained no vitamin C, and which, the most.
- Add sodium thiosulfate, dropwise, to test tubes containing iodine; continue adding drops and mixing well until solution returns to its original color.
Safety and Disposal
- Wear protective gloves.
- Wear safety glasses/goggles.
- Some people are allergic to iodine. Be sure that they wear protective gloves.
- After demonstration, do not pour solutions containing iodine down the drain; add sodium thiosulfate solution to test tubes containing iodine. The thiosulfate will reduce iodine to iodide and is safe to pour down the drain. Check with local authorities before putting in the drain.
- Discard plastic pipets into the trash.
- Discard leftover beverages down the drain; do not drink.
Carolyn Rath, as part of the requirements for the chemistry outreach service learning course, selected this activity to perform in-class and to present at outreach events. The following are her reflections about learning and performing this activity.
I chose a presentation that was both informative and relevant to our everyday lives. I had stumbled upon the vitamin C testing experiment and was excited at the prospect of connecting chemical concepts to nutrition. What better way to engage my audience than to explore the nutritional value of some of their favorite beverages? Through preparing and performing this demonstration, I learned to establish goals for my presentations, center my talk on these goals, and always be myself.
The first time I practiced in front of Dr. Lyle, I was feeling confident in my preparation and ready for a larger audience. I had the sequence of the presentation down to a simple science: add 50 drops of solution, add 3 drops of starch, add the iodine, and voila — everyone would be able to “C the difference” in vitamin C content. This was going to be a breeze, right? Well, not exactly. As I launched into my talk and followed my carefully planned sequence of steps, Dr. Lyle stopped me with a simple question, “What did I want my audience to take away from this demonstration?” I stared at him blankly, wracking my brains for the answer. I knew that I wanted to show my audience how we can connect chemistry to the items we eat or drink. However, Dr. Lyle helped me to realize that my presentation could be much more effective if I first established a foundation of clear, simple goals. After discussions we decided on two main objectives for the presentation: (1) determine whether or not there is vitamin C in a beverage, and (2) determine the relative amount of vitamin C in various beverages. Once the goals were established, my approach became much more straightforward allowing me to ask my audience guided questions at different points to ensure their understanding.
In addition to planning the sequence of steps, I had also planned exactly what I wanted to say at each point in the demonstration. Not until I practiced my presentation for another person did I realize that rote memorization was not the route to take. I found myself becoming anxious and frustrated when I forgot the carefully crafted lines. Luckily, this is where Dr. Lyle’s suggestion to establish goals came in. If I focused on merely talking to my audience as we worked towards our two main objectives, my presentation became less of a monologue and more conversational. I appreciated this approach when I performed in front of my classmates. Several times, my peers interjected with questions or funny comments. Had I still been focused on reciting my lines, these interjections could have completely derailed my performance. Instead, I was able to use these moments to engage with audience members and reinforce concepts central to my main objectives.
Dr. Lyle’s final piece of advice turned out to be the most important, and I continue to rely on it in other outreach endeavors. He pointed out that, in my anxiety to say and do all the right things, I wasn’t showing my personality to my audience. The best way to connect was to express my love of chemistry in my own enthusiastic, bubbly and oftentimes goofy way. Thus, when I performed the demonstration I didn’t hold back my characteristic giggle or refrain from exclaiming with excitement when the blue-black starch-iodine complex formed. This strategy proved useful when an unexpected catastrophe arose — I had forgotten to add the starch to a test tube of Gatorade®, which stayed stubbornly yellow as I continued to add drops. However, I didn’t let this mishap deter me, but rather used the moment as an opportunity to laugh at my scatterbrained self and challenged my audience to help identify the missing step. By being myself, I was able to connect with audience members and incite their interest in science.
The most rewarding part of my “C the difference” experience came from adapting it for one of our premier outreach events — the Alumni Science Camp. I had the opportunity to hand over the test tubes and iodine droppers to alumni and their family members and guide them through the experiment. From the toddler who performed his first titration to the parents who asked excitedly if they could do this experiment at home, I felt privileged to be a part of each of their scientific discoveries. Despite early struggles, I was proud to see the demonstration’s progression to a hands-on activity with the potential to impact audience members of all ages.
We’ve all heard the phrase a million times: practice makes perfect. Although I have always believed this saying, my experience in developing the “C the difference” demonstration has altered my perspective. I now know that there is no such thing as a “perfect” presentation, and that sometimes, the imperfections are what can make a presentation really stand out. In the future, I hope to focus less on formulating flawless demonstrations and more on letting my goals and true personality guide me in my efforts to spark scientific interest.
*Carolyn Rath is a junior at Duke University majoring in Biology with minors in Chemistry and Global Health. She hopes to pursue a career in pediatric pharmacy after college. **Kenneth Lyle is a Lecturing Fellow at Duke University.
