Name__________________________
Training Lab – How To Test For Vitamin C Content In Beverages
Purpose:
In this laboratory, you will be determining the Vitamin C content in various
beverages. The beverages you will be testing are: orange juice, apple juice, V8 juice,
and grapefruit juice. Think about each of these beverages, and rank them from the lowest
vitamin C content to the highest.
______________
(lowest vitamin C)
_______________
_______________
_______________
(highest vitamin C)
Background:
You probably already know that vitamins are a necessary part of your diet and
help with many body functions. One of the vitamins your body needs is Vitamin C (also
known as ascorbic acid or citric acid) which helps in fighting off disease and healing
wounds. Vitamin C is an essential nutrient - meaning that we cannot make Vitamin C in
our bodies and must obtain it from our diet. A lack of sufficient Vitamin C in your diet
may lead to a disorder called scurvy – a dietary disease which causes weakness and tooth
loss.
It is recommended that each person take in 60 mg of Vitamin C everyday. A
good diet which includes fruits and vegetables will supply you with enough of this
vitamin to remain healthy. Some even argue that increasing the Vitamin C content well
above the recommended dosage will help fight off common illnesses. Many fruits, such
as oranges, grapefruit, and strawberries contain a great deal of Vitamin C.
In this lab, you will prepare a standard solution of Vitamin C using an over-thecounter vitamin tablet. Then you will be using a common quantitative laboratory
technique called a titration to determine the amount of Vitamin C contained in various
beverages.
Titrations: Ubiquitous in the Chemistry Lab
A common technique used in the chemistry lab is the titration. Titrations involve
the following three solutions: a standard solution, a test solution, and an indicator. A
standard solution is a solution whose concentration is accurately known. Commonly,
standard solutions are either made in the lab or bought commercially. The test solution
is, as the name implies, the solution that is to be tested. In this lab, the various beverages
are the test solutions. Finally, an indicator is a substance that changes color when the
reaction is complete. In this lab, the starch solution will be used as an indicator. The
indicator changes color when the endpoint is reached and the reaction between the test
solution and standard solution is complete.
The Chemistry Involved
Vitamin C will readily react with iodine (I2) in a very fast reaction to produce a
colorless product. Iodine will also react with starch to produce a very dark blue product.
Vitamin C + I2 → colorless product
I2 + Starch → dark blue product
When given a choice, the iodine will react with the Vitamin C instead of the
starch. So if iodine is added when both substances are present, the Vitamin C reacts and
the products are colorless. When the Vitamin C has been completely used up, the iodine
then reacts with the starch, and a dark blue product is formed. Hence, we will add iodine
dropwise, and we will know that all of the Vitamin C has reacted when the solution turns
dark blue, and we have reached the endpoint of the reaction.
Equipment and Chemicals:
Test Tube
Iodine
1000 mL beaker
Starch
(2) 50 mL Beaker
Vitamin C Tablets
(7) plastic pipets
Samples of Beverages
magnetic stirrer
stirring magnet
distilled water
Procedure:
Caution: Safety glasses must be worn throughout this lab!
Part 1: Preparation of a Standard Vitamin C Solution
Obtain 1 500-mg Vitamin C tablet and crush it to a powder with a mortar and
pestle. Without spilling any of the crushed powder, transfer the crushed tablet to the
1000 mL beaker. Rinse the mortar and pestle with distilled water, and transfer this rinse
water to the beaker. The goal is to get all of the crushed tablet into the beaker without
any loss. (Chemists would call this transferring quantitatively.)
Now accurately add 500.0 mL of distilled water. Measure the water with a
graduated cylinder – the markings on the side of the beaker are not always accurate. You
should include your rinse water as part of the 500.0 mL. Drop a Teflon-coated magnetic
stirring bar into the mixture, and place the beaker on top of a magnetic stirrer. Stir the
solution for several minutes to try to get all of the Vitamin C to dissolve. Take a glass
stir rod and crush any large pieces that are having trouble dissolving. Don’t worry if
there are small pieces of undissolved substance. Commercial vitamin tablets often have
substances other than vitamins that serve as a matrix or a binder to hold the tablet
together. If there are a lot of undissolved particles, you can filter a small portion of your
solution to a second container. We will only need a few mL of your standard solution to
perform the titration.
You have now created a standard solution of Vitamin C. A standard solution is a
solution whose concentration is known. In this case, you have dissolved 500. mg of
Vitamin C in 500.0 mL of water, so your standard solution has a concentration of 1.00
mg/mL
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Part 2: Using a Titration To Test For Vitamin C
1) Obtain seven (7) plastic pipets, and mark them with the following: I2, starch,
Standard, Orange, Apple, V8, Grapefruit.
2) Go over to the counter where the beverages are located, and draw up the correct
mixture into each of the pipets. You now have your own supply of the necessary
beverages, and you can go back to refill any of the pipets if you run out. Use the 50mL beaker to hold your pipets.
3) Place exactly 20 drops of the standard Vitamin C solution into a clean test tube. Then
add 3 drops of starch.
4) Now comes the test. You are going to be completing a titration, where you slowly
add a test chemical until you see a color change of some kind. It is important to keep
an exact count of the amount of test chemical needed to produce the color change.
5) Your test chemical will be a solution of iodine, which appears light brown when in
solution. Add one drop of the iodine to the test tube and gently flick the test tube to
mix. Look for a permanent color change. Continue slowly adding drops (counting
the number of drops) and swirling until the solution turns a dark blue for a second and
then turns back to clear. You are now getting close. Continue slowly dropping,
counting, and swirling until the solution turns blue and remains blue. You have just
reached what is called the endpoint (the point where the color change occurs). Record
the total number of drops of iodine needed to reach the endpoint in the data table
under Trial 1.
6) Rinse out the test tube with distilled water and get as much of the distilled water out
as you can. Then repeat steps 3-5 for the standard Vitamin C solution and record your
results under Trial 2.
7) Place 20 drops of orange juice into the test tube, add 3 drops of starch, and slowly
titrate with the iodine solution until a distinctive color change occurs. Record the
number of drops of iodine needed for that beverage.
8) Pour the contents of your test tube into the sink, and rinse the test tube with distilled
water.
9) Repeat steps 7 & 8 for the other beverages.
DATA
SAMPLE
Drops to
Endpoint
Vitamin C
Standard
Orange Juice
Apple Juice
Grapefruit Juice
V8 Juice
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Part 3: Calculating Vitamin C Concentration
To calculate the Vitamin C concentration in each beverage, we can just calculate a “drops
of iodine ratio” and use this ratio to determine the concentration in each of the samples
we tested:
Vitamin C Concentration =
drops of iodine to titrate sample
drops of iodine to titrate standard
X 1.00 mg/mL
Calculate the Vitamin C concentrations for the various beverages in the space below.
Just to be consistent, let’s give every answer three significant figures.
Vitamin C Concentration in Orange Juice
Vitamin C Concentration in Apple Juice
Vitamin C Concentration in Grapefruit Juice
Vitamin C Concentration in V8 Juice
Follow-up Questions:
1) Which of the beverages had the highest Vitamin C content? Which had the lowest?
2) Was your original guess of the order of Vitamin C content correct?
3) Suppose a student made the following mistakes when testing the orange juice. State
whether the results for the Vitamin C content would be too low, too high, or
unchanged, and explain why.
(a) Instead of using 20 drops of orange juice, 21 drops were used.
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(b) Instead of 3 drops of starch, 2 drops were used.
(c) Several drops of iodine were added well past the endpoint because the student
wasn’t paying close attention to the color change.
4) What measurement do you believe was least accurate in this experiment? Justify your
answer.
5) Since we only needed several mL of your standard solution, why did we make so
much? Why didn’t we just dissolve a few milligrams of Vitamin C into a few mL of
water?
6) Vitamin C has the following percent composition: 40.9 % carbon, 4.59 % hydrogen,
and 54.5 % oxygen. One molecule of Vitamin C has a mass of 2.92 X 10-22 g.
Calculate the empirical formula, molar mass and molecular formula of Vitamin C
using this information.
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Chemistry Honors
Vitamin C Pre-Lab
Name_________________________
1) Vitamin C is said to be an essential nutrient. What does it mean for a nutrient to be
“essential”?
2) What disease can result when a person doesn’t get enough Vitamin C? What are
some symptoms of this disease?
3) What are standard solutions and how do chemists obtain them?
4) What is the concentration of the standard solution you will make for this lab and how
will you make it?
5) Is it ok to use a beaker to measure a volume in this lab? Why or why not?
6) As you are adding the iodine dropwise to the test solutions, how will you know when
the endpoint is reached? What is the chemistry behind the color change?
7) When 20 drops of a standard Vitamin C solution with a concentration of 1.8 mg/mL is
titrated with iodine. It takes exactly 24 drops to reach the endpoint. When 20 drops of
pineapple juice is tested with the same iodine, it takes 36 drops to reach the endpoint.
What is the concentration of Vitamin C in the pineapple juice?
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