VIRTUAL EXPERIMENT 1. Determination of Vitamin C in Supplements
- Read this experiment thoroughly and watch the associated lab videos on MyLS, including: buret set-up tutorial, how to use a volumetric pipette, and how to use a volumetric flask
- Complete the online MyLS open book quiz prior to Wednesday November 11th at 11:59pm.
The objective of today’s experiment is to use volumetric analysis to determine the amount of Vitamin C (ascorbic acid) in three types of individual tablet supplements – Kirkland ®, Jamieson ®, and Wampole ®. Then, by compiling research, qualitative and class data, determine which variety contains the most accurate amount of vitamin C as compared to the advertised quantity of 500mg.
Course Learning Outcomes
- Analyze and solve problems in a consistent and organized manner
- Solve mass number-molar mass type problems
- Perform common unit conversions
- Balance chemical reactions
- Calculate amount of product from the amounts of the reactants and a balanced chemical reaction
- Draw hydrocarbons using the IUPAC system
- Complete WHMIS and Academic Integrity Training and upload the completed certificate to the CH110 MyLearningSpace (MyLS) drop box. (Courses available from “Self-Registration” tab on MyLS home page).
- Complete the Health and Safety Questionnaire.
- Read the experiment.
- Watch all of the mandatory videos in order to get access to the quiz.
- Complete the open book prelab quiz.
Vitamins are organic constituents of food that are essential for life. Not only do they prevent what are known as ‘vitamin deficiency diseases’ such as scurvy, pellagra and beriberi; they perform specific tasks in metabolic processes that are vital to growth regulation, tissue replacement and general cellular activity.
There are two categories of vitamins: fat soluble and water soluble. The fat soluble vitamins (A, D, E and K) are stored in the body and are not readily lost in cooking water or destroyed at cooking temperatures. Since these vitamins are stored in the body, they can be taken in one relatively large dose to be used over a long period of time. However, overdosing on fat soluble vitamins should be avoided because the accumulation of large quantities of these vitamins can cause harm. Water soluble vitamins (C and the B complex) are not stored in the body, are easily leached into cooking water, and are destroyed by light and heat. Since they are not stored in the body, these vitamins must be ingested in small quantities at frequent intervals. For example, the Reference Daily Intake (RDI) of vitamin C (ascorbic acid) for adults and children over the age of 4 is 60 mg/day. This value, however, is only enough to reduce the risk of scurvy. Taking large doses quickly, say greater than 500 mg, increases the urinary excretion since vitamin C is water soluble, so the doses should be taken in small amounts throughout the day. The side effects of vitamin C overdosing can include diarrhea and increased flatulence.
One way to obtain the RDI of vitamins is through supplements. Supplements can either be extracted from natural sources, such as citrus fruits, or chemically synthesized. It is very cost efficient to chemically synthesize Vitamin C. In addition, it contains very high potency that works effectively and can be produced in a small tablet making it more user friendly. In the case of this experiment the chemically synthesized supplements from tablet form, Ener-C ® and Emergen-C ® each claim to contain 1000mg of vitamin C per packet or tablet.
One method of synthesizing ascorbic acid that is most commonly used involves the fermentation of corn sugar to eventually form a ketoacid, which can then be converted into LAscorbic acid. Various additives are included in the manufacturing of these supplements to enable the human body to properly process the supplement. For tasting purposes, artificial flavours and aromas are normally added as well.
In this experiment, the vitamin C is quantified by titration of the pure ascorbic acid with an iodine, I2(aq), solution. The titrant, the iodine solution of known concentration, is reduced by the vitamin C while the vitamin C is oxidized. The indicator is a 1% starch solution. When the ascorbic acid (vitamin C), C6H8O6 in the supplements is oxidized by the iodine three colourless products are formed: dehydroascorbic acid (C6H6O6), hydrogen ions, and iodide ions (reaction 1).
When all the ascorbic acid in the supplement has been oxidized, the next drop of iodine is free to react with starch and a blue-black starch-I2 complex is formed. The colour change signals the endpoint of the titration.
This type of reaction, when ascorbic acid reacts with iodine, is known as a reductionoxidation or redox reaction. Electrons are transferred in this reaction where the element or compound losing electrons is oxidized and the element or compound gaining electrons is reduced. An easy mnemonic tool to remember this is “LEO the lion says GER”. LEO stands for “Losing Electrons – Oxidation” and GER represents “Gaining Electrons – Reduction”.
The loss and gain of electrons can be shown easily by breaking up the overall reaction into two half reactions. For example, zinc metal displaces hydrogen ions from aqueous solutions of acids according to equation 2:
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g) (2)
In this case, zinc loses electrons in the oxidation half reaction 3
Zn(s) → Zn2+(aq) + 2e– (3)
while hydrogen gains electrons in the half reaction 4.
2H+(aq) + 2e– → H2(g) (4)
The symbol e– represents a single electron with one negative charge. In any redox reaction, the number of electrons transferred must be the same between each half reaction and each individual half reaction must be balanced both with number of atoms and total charge.
An oxidizing agent is a substance that takes electrons from another substance. A reducing agent is a substance that gives electrons to another substance. In the example above, the Zinc is the reducing agent and the hydrogen ions are the oxidizing agent.
One of the ways to identify if a redox reaction occurs is to determine the oxidation numbers or states of the substances involved. There are a number of rules to follow when determining oxidation number that can be found in your text (for some substances). For the reaction (6) example listed below, the oxygen and hydrogen are labeled with oxidation numbers of -2 and +1 respectively.
Consider the following reaction 5 used in the Breathalyzer test to determine the amount of ethanol on the breath of people who are suspected of driving while under the influence of alcohol.
3 CH3CH2OH (g) + 2 Cr2O72-(aq) + 16H+(aq) → 3 CH3CO2H (aq) + 4 Cr3+(aq) + 11 H2O (l) (5)
The unbalanced oxidation half-reaction is CH3CH2OH (g) → CH3CO2H (aq). Since each carbon is not identical, the oxidation numbers must be determined separately as follows in 6:
Ethanol Acetic Acid (6) The carbon atom in the –CH3 group in ethanol is assigned an oxidation state of -3 so that it can balance the oxidation states of the three H atoms it is bonded to. In the –CH2OH group, the oxygen and three hydrogens are labeled first giving a total of (-2) + 3(-1) = +1. In order to balance this part of the molecule, the carbon must have an oxidation state of -1.
The carbon in the –CH3 group in the acetic acid formed in this reaction has the same oxidation state as it did in the starting material: -3. There is a change in the oxidation number of +3 to balance the 2(-2) x 1(+1) = -3 total provided by the two oxygen’s and the hydrogen. Therefore, the oxidation half-reaction must have a loss of four electrons by one of the carbon atoms. The chromate ion (Cr2O72-) is involved in the reduction half reaction.
Note: Each lab bench will be assigned only one kind of vitamin C supplement – a 500mg tablet of Jamieson, Kirkland, or Wampole. Data for all types will be shared with the class for comparison purposes via the CH110 MyLearningSpace (MyLS).
- Vitamin C Stock Samples
Note: Some steps may be altered or changed due to Covid protocol. Read the additional instructions provided before starting.
- Record observations of your supplement before and after preparation, minimum of three observations.
- Using a mortar and pestle, thoroughly crush a 500mg Vitamin C tablet. Place the entire tablet to be used in a clean 250 mL Volumetric flask using a dry funnel. Rinse the mortar and pestle, placing the rinse in the volumetric flask.
- Add deionized water to roughly half way full in the volumetric flask. Refer to Figure 1.
- Cover the flask with Parafilm® and swirl until the sample is completely dissolved. Remove the Parafilm and then fill the flask with deionized water such that the bottom of the meniscus is exactly at the etched line. DO NOT go past the etched line. It will be helpful to use a disposable pipette looking at eye level to fill to the line.
Figure 1: Proper procedure to prepare a solution in a Volumetric flask from a solid sample.
- Place Parafilm tightly on top. Mix by inverting 17 times. This will be your stock solution. Record qualitative observations.
- Titration of Vitamin C Sample
The titrations are completed with only one of the three possible supplements.
1. Clean the burette with tap water and a very small amount of soap. Remove all traces of soap with tap water and then rinse with deionized water. Obtain ~5 mL of the 0.0037M iodine solution and rinse the burette with it. Place the 5 mL rinse in a small “waste” beaker. Set up the burette at your lab bench using a burette
clamp and retort stand
Carefully fill the burette with the
solution to approximately the 0.00 mL mark.
Remove all bubbles/air p
ockets from the tip of the
by draining out the solution quickly
stopcock fully opened
Ensure the funnel is out of the burette.
Record the actual initial volume in the burette to two
decimal places on you
r data sheet. Refer to figure
low for an example in proper burette readings.
Get your burette reading checked by your lab
assistant (IA/TA) or lab coordinator. Their initials
are required on your data sheet before continuing.
Figure 2: The bottom of the concave meniscus must be read precisely. To make the reading easier, a “meniscus reader” containing a black horizontal stripe can be placed behind the burette. The solution surface then appears to be black and is easier to see as in the diagram on the right hand side. The correct volume reading for the burette is 1.42 mL.
- Practice pipetting with DI water in a beaker first. Using a volumetric pipette as shown in figure 3, transfer 10.0 mL of the stock solution into a clean 250 mL Erlenmeyer flask. Refer to the tips at the beginning of the lab manual to review correct volumetric pipette usage.
- Fill the Erlenmeyer flask to approximately the 100 mL mark with deionized water.
Etched line. Do
- Add 10 drops of starch indicator and swirl the flask. not go past!
- Place the Erlenmeyer flask under the burette on a meniscus must white piece of paper to better detect the endpoint. touch line.
- Titrate by adding the iodine solution from the burette as you gently swirl the flask.. Flashes of blue/purple color will begin to appear in the flask. Continue until the endpoint is reached, as indicated by the persisting faint blueish colour.
- Record the endpoint burette volume to TWO decimal places on your data sheet.
- Repeat steps 7-12 two more times for a total of three titrations. You must refill the burette before beginning the second and/or third titration.
- Dispose of solutions in hazardous waste containers in the lab.
Figure 3: 10.0mL volumetric pipette with etched line (which must be read at eye level)
- Clean all glassware and equipment and return to original location at your station.
- Calculate milligrams of vitamin C for each of the three trials. Remember to account for the preparation of the stock solution.
- Check with the IA/TA or lab coordinator that your value is reasonable and then record your average value in milligrams on the board.
- Record your locker number station and IA name and email at the front of your lab manual. You will use the same bench for the term.
- Once your station is fully clean, obtain your IA or TA or lab coordinator signature on your data sheet.
- Obtain class set of results from MyLS
LAB REPORT QUESTIONS
Include all components of a lab report as stated in the “Lab Report Outline” posted on MyLS. In particular, ensure your discussion includes comments on the results of your own and class qualitative and quantitative data.
1. Calculate the milligrams of Vitamin C present in your supplement AND the stock sample. Show the complete calculations including ALL units for one trial only. Ensure you have the correct number of significant figures in the final answer.
(Remember to set up your calculations according to the description in the “lab report outline”, listed here again for your reference:
- Begin each new calculation under a descriptive title and indicate where the data comes from.
e.g. Sample Calculation 1: Calculation of the molarity of KMnO4 from trial 1, Part A If this title is missing or is not descriptive you will lose one mark per question.
- Show all formulas and carry the units with your numerical values right through the calculation.
- Marks will be deducted if the number of significant digits in the final answer is incorrect. Please refer to significant figures tips in the manual for proper use of significant figures.
- Calculations may be submitted in pen (never pencil) instead of in typed format. However, typed format is the preferred method as this facilitates easier upload to the drop box.
- Calculations must be inserted into the question when asked and not as an appendix at the end of the experiment. Any text as part of a calculation (summary statement, question,) must be typed. )
- Calculate percentage error between your average sample and the reported value. Use the formula given below:
|𝐴𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑣𝑎𝑙𝑢𝑒 − 𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒|
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑒𝑟𝑟𝑜𝑟 = 𝑥 100%
Accepted value is also called “true” value or “theoretical” value.
- a. Calculate the mean class value and standard deviation for the amount of ascorbic acid found in each of the three supplements. Sample calculation not required for this part (a) only. You must use Excel (or similar) to complete this calculation (Refer to Excel Assignment for help!)
Reminder: Comment on what the standard deviations represent for your data in your discussion section.
- Calculate the percentage error between mean class data and the reported mg of Vitamin C for each supplement. (Note: Assume 4 significant digits in the supplements mass). One sample calculation required.
- Create a labelled table to summarize your data. Include all class data as an appendix or in results section.
- Identify the reducing agent in the experimental redox reaction. Explain how you determined this using a chemical reaction.
Insert your own academic integrity statement in this question to earn 2.5 marks in the
“Other category”. As a reminder here are the instructions:
In EVERY lab report, you will make an academic integrity declaration. This statement will ensure your submission is your own work and that you did not share your work with others. Example statement:
• “I declare that this submission is my own original work. I did not use file sharing websites or other student assignments or Chegg to complete the lab report. I did not take pictures of my work or file share my own document with anyone. I am familiar with the academic integrity policies and penalties at WLU” • You must create and write your OWN statement
- The polymer starch, used an indicator in our reaction, can be oxidized by NaOCl, according to the reaction below. Draw (only one repeating unit required) and identify (by circling) exactly the carbon atom(s) on starch which are being oxidized. State the oxidation number of the carbon atom(s) before and after the reaction.
Breakdown of marks for Experiment 1.
Lab Report Section
Mark Earned (for your reference)
Discussion (including source of
Remember: Handing in a partial lab report, even late, is better than handing in nothing.
We are here to help! Ask questions!!
Copyright Infringement Notice
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TIPS for UPLOADING DOCUMENTS to MyLS
- Use a simple, short file name with your initials in the name. (eg. Vitamin C sv.doc)
- Do not use any symbols in the file name. (Avoid *,&,$ etc.)
- Check the file size – it must be less than 20MB.
- Do not insert images and, if necessary, use pictures of the smallest file size.
- Submit ONE document in .doc or .pdf format. Note: .pages is not an acceptable format.
- If you are uploading multiple files, the LAST file to be uploaded will be marked unless otherwise noted.
- If your last file uploaded is AFTER the deadline, be sure improvements are worth more than 10 marks!
- Double check that you upload YOUR FILE. If you “accidentally” upload someone else’s, this is ACADEMIC MISCONDUCT as you shouldn’t have their file at all.
- Try again at a later time (as long as it’s still before your due date and time) as sometimes MyLS system is updating and cannot accept files.
Double check it’s the right dropbox! Your marker only has access to your actual dropbox!