Quantitative Analysis of Vitamin C by Acid-Base Titration
Vitamin C is a vital part of the human diet. A deficiency of vitamin C causes a wasting diseased called scurvy,
which until the 18th century plagued sailors and others whose regular diets provided few fresh fruits and
vegetables. In fact, the chemical name for vitamin C, ascorbic acid, is derived from scorobutus, the Latin name
for scurvy.
In 1753, British Royal Navy surgeon James Lind published a book, A Treatse on the Scurvy, which concluded
that scurvy could be prevented by including citrus fruits in the diet, and the active cases of scurvy could be
treated almost miraculously simply by having the patient consume citrus fruits or juices. The Royal Navy began
adding lime juice to the regular tots of rum provided to all sailors, accidentally creating the world’s first rum
punch. Scurvy disappeared , and British sailors became known as Limeys.
Scurvy is almost unheard of in the developed world, because our regular diets include sufficient vitamin C to
prevent scurvy. But many people take vitamin supplements that provide much higher dosages of vitamin C than
the recommended daily minimum.
In this lab session, we’ll use acid-base titration with standardized 0.1000 M sodium hydroxide titrant to
determine the actual amount of ascorbic acid (C6H8O6) present in a 500 mg vitamin C tablet. One mole of
ascorbic acid reacts with one mole of sodium hydroxide to yield one mole of sodium ascorbate and one mole of
water, according to the following balanced equation:
C6H8O6 + NaOH  C6H7O6Na + H2O
Using phenolphthalein indicator provides a sharp endpoint for the reaction. The solution remains colorless
while ascorbic acid is in excess. When sodium hydroxide is event slightly in excess, the pH of the solution
rapidly rises to a point above the color change range of phenolphthalein, an the solution quickly turns bright
pink.
Pre – Lab
1. Write the complete chemical equation for the reaction of a solution of sodium hydroxide (NaOH) with hydrochloric acid
(HCl).
2. How many mL of 0.1 M HCl are required to react completely with 5 mL of 0.1 M NaOH? NOTE – There are 1000 mL in 1 L
and this can be used to convert mL to L.
3. If equal molar amounts of NaOH and HCl are mixed, when the reaction is complete what will be the chemical species in the
resulting solution?
4. What will be the pH of the mixture in question 3, acidic, neutral, or basic? Explain.
5. Write the complete chemical equation for the reaction of a 0.1 M solution of acetic acid (CH3COOH) with a 0.1 M solution of
NaOH.
6. How many mL of the 0.1 M NaOH solution will required to react completely with 5 mL of a 0.1 M acetic acid (CH3COOH)
solution? Explain. NOTE – 1 L = 1000 mL.
7. How is it possible to determine when an acid-base reaction is complete when the concentration of one of the reactants is
unknown?
8. Using the table below, explain how indicators are chosen and used during titrations.
Indicator
Methyl orange
Methyl red
Bromthymol blue
Phenolphthalein
pH range
3.1–4.4
4.2–6.2
6.0–7.6
8.3–10.0
Color Change
Orange to yellow
Red to yellow
Yellow to green-blue
Colorless to pink
Thymol blue
9. Watch the following narrated animation:
1.2–2.8
Red to yellow
http://auth.mhhe.com/physsci/chemistry/animations/chang_7e_esp/crm3s5_5.swf
Take notes on the following:
 How are titrations are performed?
 Why there is only a minor change in pH until the equivalence point?
 Draw a titration curve for the reaction that was performed in the video.
 Draw a picture of the molecules in solution at (1) the beginning before any base is added, (2) after 10 mL of
base has been added, (3) at the equivalence point, and (4) at the end of the titration.
10. Explore the following simulated titration animation showing the determination of the molarity of an acid using a strong
base:
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/stoichiometry/acid_base.html
Be sure to note the following as you walk through the simulated titration:
 Where is the solution of known concentration placed?
 Where is the solution of unknown concentration placed?
 When is the molarity of the unknown solution canceled?
 Why do you believe you need to go drop by drop as you approach the equivalence point.
11. Finally go to the following link and click on “Animation 8 – Titration of Weak Acid, HA, with Strong
Base”:
http://www.chembio.uoguelph.ca/educmat/chm19104/chemtoons/chemtoons.htm
Be sure to note the following as you walk through the simulated titration:
 What is the difference between a strong acid/strong base titration and a strong base/weak acid titration?
 What extra component is left in solution?
 How does the titration curve differ from the one that you drew previously?
12. You may want to also check out the following resources in order to help develop your final procedure:
A virtual acid-base titration experiment in which the student may select the acid, base, and indicator, and perform the
experiment, including calculations:
■■
http://lrs.ed.uiuc.edu/students/mihyewon/chemlab_introduction.html
■■
A particulate view of a neutralization reaction between HCl and NaOH:
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/animations/HClandNaOHtgV8.html
Materials Available
Materials
Balance
Hot Plate
Erlenmeyer Flask, 250 mL
Graduated Cylinder, 100 mL
Buret, 50 mL
Ring Stand, rings and clamps
Vitamin C Tablet
Sodium Hydroxide, 0.1000 M
Phenolphtalein Indicator
Quantity
1
1
1
1
1
1
500 mg
50 mL
2 – 3 drops
Procedure:
1. You will be designing and running a lab based on the information that you have researched. Based on the
introduction and pre-lab material, formulate a research question that you relates to the amount of acid contained
within a 500 mg Vitamin C tablet.
Our Research Question: ________________________________________
___________________________________________________________.
2. Based on the group that you selected and the information that you have researched, formulate a hypothesis on the amount of
ascorbic acid
Our Hypothesis: _____________________________________________
___________________________________________________________.
3. Using the research from the pre – lab, outline a procedure below. Be sure to clearly identify which pieces of
equipment (listed in the Materials section) you plan to use during your investigation.
4. Make a simple sketch showing how you plan to set up your equipment, labeling the following: analyte, titrant,
buret, Erlenmeyer flask, volumetric pipette. Show where the indicator solution will be added, and comment on your
choice of indicator: Why did you choose this one and not another?
5. Create data tables for what you will record data on and where you will put this data. Put this is the data collection
section of this handout.
6. Consult with your instructor for approval before beginning your experiment.
7. As you conduct your experiment, keep detailed written records. Be sure to list all steps taken as you perform your
experiment, and all measurements and observations made during the experiment.
Data Collection
Data Tables:
Calculations
1. Using the molarity of NaOH that was on the bottle you obtained, calculate the moles of NaOH that you
added. Record answer in your Data Table.
2. Since the moles of base = moles of acid when the pink color lasts for 30 seconds, the answer to step 1 is also
the moles of ascorbic acid. Using this knowledge, calculate the mass (grams) of ascorbic acid knowing that the
molar mass of the ascorbic acid is 176.14 g/mol. Record final mass in your Data Table.
Discussion
1. Does your data indicate that the initial hypothesis should be accepted or rejected? Cite specific data to support your
acceptance or rejection!
2. What were the inherent structural errors that were introduced as a result of your experimental design? How did these impact
your final data? How can the lab be fixed in order to improve your results should you repeat the lab?
3. What next steps could you take in order to further investigate this topic? Cite specific areas that you believe you could study
and how your data allows you to better study these areas.
4. It is likely that the calculated value of Vitamin C contained within the pill and the advertised about did not match up. Do you
think it is safe for companies to advertise the wrong about of vitamins contained within each pill? Research into federal laws
that regulate the labeling of vitamin supplements and propose a way in which to better monitor that the amount advertised
aligns with the amount that is actually in a supplement that is taken.