QUANTITATIVE ANALYSIS
Vitamin C, or ascorbic acid, plays an important role in body metabolism,
especially in the repair of damaged tissue. Ascorbic acid is chemically similar
to the sugar molecule glucose, and most animals synthesize vitamin C in their bodies from
glucose. The primates, including humans, lack an essential enzyme for this conversion. We
therefore cannot synthesize our own ascorbic acid, which is why we need it as a vitamin
obtained from foods such as fruits and vegetables. Oddly enough, humans share this dubious
honor with apes and the guinea pig!
Deficiency in vitamin C leads to various troubles, including the disease scurvy, and the prospect
of ultimate death. British sailors, who for centuries suffered from scurvy, finally discovered that
drinking lime juice protected them from the dread of scurvy (hence the nickname "limeys"). At
present, the official Recommended Daily Allowance (RDA) for vitamin C is 60 mg.
USING EQUIVALENT RATIOS
Consider the following problem: A person “burned” about 150 Calories while skateboarding for
30 minutes. How many Calories would the person “burn” while skateboarding for 60 minutes?
Setting up an equivalent ratio is a useful method for solving these types of problems. For
example:
150 Calories
30 minutes
=
X Calories
Known Ratio
=
Unknown Ratio
60 minutes
In this case, the known ratio is set equal to an unknown ratio, and the unknown value is
obtained through cross-multiplication. Throughout this laboratory you will use this method of
establishing a known ratio to determine the value of an unknown quantity of a chemical
ingredient.
ANALYSIS OF FRUIT JUICES
In order to accurately measure the quantity of vitamin C in an unknown sample, a solution with
a known volume of a known concentration is reacted with the vitamin C. When the reaction is
complete, the amount of known solution is equal to the amount of unknown vitamin C. In this
experiment, iodine reacts with vitamin C in a 1:1 ratio. That is, for every molecule of vitamin C
present, one molecule of iodine reacts with it. By knowing how much iodine was added to the
sample the exact amount of vitamin C present can be determined. When excess iodine is
added to the vitamin C solution, it can no longer react with the vitamin C. To determine the
presence of excess iodine, a starch solution is added. When the starch reacts with the excess
iodine a distinctive purple color appears.
Materials: Mortar and pestle, vitamin C tablet, 50 mL, 150 mL and 250 mL beakers, stirring rod,
10 mL syringe, iodine solution, starch indicator solution, hydrochloric acid solution, fruit juices.
Quantitative Analysis
1
PROCEDURE
Testing a KNOWN Amount of Vitamin C
Using a mortar and pestle, crush a 250 mg (0.25 g) vitamin C tablet, add it to a 250 mL
beaker and dissolve in 100 mL of distilled water. Mix it thoroughly. Some of the tablet may
not completely dissolve.
Calculate the mass of vitamin C in 1 mL of this solution and record the mass below. You
will use this solution for both trials.
Mass of vitamin C per 1 mL of solution: 250 mg Vit C / 100 mL H2O = 2.5 mg Vit C / 1 mL H2O
With a 10 mL syringe, remove 5.0 mL of this solution and place it into a 150 mL beaker with
45.0 mL of distilled water. To this solution add 2 drops of 1.0 M HCl solution and 20 drops of
starch indicator solution.
Mass of vitamin C in solution: 2.5 mg Vit C / 1 mL H2O = 12.5 mg Vit C / 5 mL H2O
Fill the 10 mL syringe with iodine solution and add the iodine slowly until the solution turns
a pale purple and remains purple for 30 seconds while stirring (place a white paper towel
under the beaker to observe the endpoint). This may take more than one full syringe of
iodine. Record the volume of iodine added. (Results vary.)
Trial 1
mL Iodine 15.8 mL Iodine
Trial 2
mL Iodine 15.4 mL Iodine
After the 2nd trial, dispose of the vitamin C solution and flush it down the drain. Rinse and dry
the 250 mL beaker for use in the following experiment.
Testing Fruit Juices: An UNKNOWN Amount of Vitamin C
Measure 20 mL of juice and place it into a 250 mL beaker and dilute the solution with distilled
water to a total volume of 100 mL. Add 2 drops of 1.0 M HCl and 20 drops of starch indicator
solution. With the 10 mL syringe, add the iodine until the purple endpoint is reached. Record
the volumes of iodine added until the pale purple endpoint is reached.
Juice Sample Apple Juice
Juice Sample Orange Juice
Trial 1 mL Iodine 10.2 mL Iodine
Trial 1
mL Iodine 12.4 mL Iodine
Trial 2 mL Iodine 11.0 mL Iodine
Trial 2
mL Iodine 12.6 mL Iodine
Quantitative Analysis
2
ANALYSIS
Apple Juice
Orange Juice
1. Using equivalent ratios with the known and unknown amounts of vitamin C and quantity of
Iodine, calculate the amount (mg) of vitamin C in each unkown sample of juice tested.
12.5 mg Vit C
8.50 mg Vit C
12.5 mg Vit C
=
15.6 mL Iodine
10.0 mg Vit C
`=
10.6 mL Iodine
15.6 mL Iodine
12.5 mL Iodine
2. Examine the juice containers. According to your data, calculate the amount of vitamin C in
one serving of juice.
8.5 mg Vit C
85.0 mg Vit C
10.0 mg Vit C
=
20.0 mL Juice
100. mg Vit C
=
200 mL Juice
20.0 mL Juice
200 mL Juice
3. Examine the juice containers. How many milligrams of vitamin C does the manufacturer
guarantee in each serving? (Hint: What is the Recommended Daily Allowance?)
100% RDA
60 mg Vitamin C per serving
100% RDA
60 mg Vitamin C per serving
4. Determine the experimental error for each juice.
85.0 mg Vit C - 60.0 mg Vit C
100. mg Vit C - 60.0 mg Vit C
X 100 = 42 %
X 100 = 67 %
60 mg Vit C
60 mg Vit C
5. If you were to consume the recommended daily allowance of vitamin C (60 mg), how many
milliliters would you need to drink for each of the juices tested?
85.0 mg Vit C
60 mg Vit C
100. mg Vit C
141 mL Juice
200. mL Juice
=
200. mL Juice
60 mg Vit C
=
120 mL Juice
6. If you were to drink a large glass of juice (500 mL), how much vitamin C would you
consume?
85.0 mg Vit C
213 mg Vit C
100. mg Vit C
500. mL Juice
200. mL Juice
=
200. mL Juice
250 mg Vit C
=
500. mL Juice
7. What percentage of the RDA would you consume?
60 mg Vit C
213 mg Vit C
60 mg Vit C
355 % RDA
100% RDA
=
100% RDA
250 mg Vit C
=
417 % RDA
Quantitative Analysis
3