Ch100: Fundamentals for Chemistry
Instructor: Tony Zable
1
Lab #10: Vitamin C in Fruit Juice
Objectives:
 Observe the oxidation of vitamin C in aqueous solution
 To determine the concentration (or molarity) of vitamin C in various fruit juices
 To measure and compare the vitamin C content in various kinds of fruit juices
Introduction:
You may have asked yourself “What is the best source of vitamin C and how much of it do I need
to eat or drink?” Orange juice is well-known for its vitamin C content but does the quantity of
vitamin C vary between different types of oranges? Many other fresh fruits and vegetables
provide substantial amounts of vitamin C. Most noteworthy are other citrus fruits like lemons,
tangerines and grapefruits. What about apples? These fruits grow in abundance the Pacific
Northwest. Are apples a good source of vitamin C? Is there variation in vitamin C content
between different types of apples?
In this lab, we will use an iodometric titration to measure the amount of vitamin C (ascorbic
acid) in the juice of various types of fruit. We will compare the vitamin C content of various types
of fresh-squeezed orange juice (or any other citrus fruit you care want to compare).
The iodometric titration method takes advantage of the fact that vitamin C is a water-soluble
organic compound that is easily oxidized and is therefore a good “reducing” agent. Iodine
oxidizes vitamin C according to the following chemical equation:
Ascorbic Acid + Iodine  Hydroiodic Acid + Dehydroascorbic Acid
or,
H2C6H6O6 (aq) + I2 (aq)  2HI (aq) + C6H6O6 (aq)
{Note: the chemical formula for ascorbic acid is more commonly written as C6H8O6}
What is the net ionic equation for this chemical reaction?
Because aqueous iodine solutions are unstable and inconvenient to work with, we will titrate the
fruit juice samples with potassium iodate, in the presence of an acidic iodide solution. The iodate
oxidizes the iodide to iodine, according the following net ionic equation:
IO3- + 5I- + 6H+  3I2 + 3H2O
The iodine formed in this reaction immediately oxidizes the vitamin C, according to the previous
reaction equation. Once all of the vitamin C is oxidized, there will be excess I2 that will react
with the starch to form the distinctive blue-black complex. This change serves as a good end
Ch100: Fundamentals for Chemistry
Instructor: Tony Zable
2
point. Note that for each reacted iodate ion, 3 molecules of diatomic iodine are produced. Since
each molecule of I2 reacts with one molecule of vitamin C, the molar ratio of vitamin C to iodate
in the combined chemical reaction is 3 to 1.
Procedure:
1. Obtain a piece(s) of fruit from the instructor. Weigh and record the mass for each piece of
fruit in Table 2.
2. Using a citrus press (or juicer), juice the whole piece of fruit and collect all of the available
juice. Measure and record the total mass of the extracted fruit juice.
3. Using a 10 mL pipet (instead of a graduated cylinder!) measure out a 10.00 mL volume of
juice.
4. Determine the density of the fruit juice by measuring the mass (in g) for the 10.00 mL
volume of juice. Divide the mass of the juice (in g) by the volume (in mL):
Density  D 
Mass

Volume
__________ g/mL
5. Using the density of the juice in step 4, determine the total volume of extracted juice in mL.
Divide the total mass of the juice (in g) by the density of the juice. Record total volume in
Table 2.
Total _ Volume  V 
Mass

Density
__________ mL
Table 2. Juice extraction from fresh fruit
Fruit Sample:
( & quantity)
Total Mass of
Fruit (g)
Total Mass of
Extracted Juice
(g)
Mass of 10 mL
juice sample
(g)
Density of
juice (g/mL)
Total Volume
of Extracted
Juice (mL)
4. Using the pipet, place 5.00 mL of juice from the 10 mL juice sample into a small beaker.
5. Determine the mass the 5.00 mL juice sample and record it in Table 3.
6. Add 10 drops of KI, 20 drops of HCl, and 5 drops of starch to the 5 mL juice sample.
7. Measure the mass of a 0.01M KIO3 ampule (before titrating).
8. Titrate the juice with 0.01M KIO3 to a jet black end point. If the end point is reached with a
single drop of KIO3, repeat steps 4-8 using 0.001M KIO3 instead of 0.01 M
Ch100: Fundamentals for Chemistry
Instructor: Tony Zable
3
9. Weigh the KIO3 ampule (after titrating).
10. Determine the mass loss of the KIO3. Record your results in Table 3.
11. Repeat for several other fruit/juice samples.
Table 3. The vitamin C in fruit juice
Fruit
Sample:
Volume of
Juice (mL)
Mass of Juice
(g)
Mass of KIO3
ampule (before)
Mass of KIO3
ampule (after)
Mass loss of
KIO3 (g)
Analysis:
The next step is to calculate the quantity of vitamin C in your sample and determine the molar
concentration. Perform this series of steps for each juice you tested.
A) Determine the volume of KIO3 reagent that was utilized in the chemical reaction. Divide the
mass loss of the KIO3 ampule by the density of the solution (1.000 g/mL).
Volume of KIO3 (in mL) = ______________
Volume of KIO3 (in L) = ______________
B) Determine the number of moles reacted KIO3. Multiply the molar concentration of the KIO3
by the volume of reacted KIO3. The “concentration” is the # of moles per unit volume ( i.e. 0.01
mol KIO3 per 1 L of solution). Be sure to use the volume of KIO3 in L for this calculation.
Number of moles of KIO3 = ______________
Ch100: Fundamentals for Chemistry
Instructor: Tony Zable
4
C) Determine the number of moles of reacted vitamin C. In this chemical reaction, the molar
ratio of vitamin C to KIO3 is 3 to 1. Multiply the number of moles of KIO3 by this ratio.
Number of moles of vitamin C = ______________
D) Determine the concentration of vitamin C in the fruit juice sample (in mol/mL). Divide the
number of moles of vitamin C by the volume of juice in the reaction.
Concentration of vitamin C (in mol/mL) = ______________
E) Determine the molar concentration of vitamin C in the fruit juice sample (in mol/L). Perform
a unit conversion to convert the mL units in (D) into L units.
Molar concentration of vitamin C (in mol/L) = ______________
F) Determine the mass concentration of vitamin C in the fruit juice sample, in grams per mL.
Multiply the molar concentration of vitamin C times the molecular mass of vitamin C (176
g/mol). Express your answer in mg/mL
Concentration of vitamin C (in g/L) = ______________
Concentration of vitamin C (in mg/mL) = ______________
G) Estimate the quantity of vitamin C in one piece of fruit (in mg). This is just a unit conversion
from mL of total fruit juice to mg of vitamin C. Multiply the volume of juice (in mL) in one piece
of fruit by the mass concentration of vitamin C (in mg/mL).
Quantity of vitamin C (mg) = ______________
H) Estimate the number of whole fruit that would need to be eaten to meet the RDA standard
recommendation for vitamin C. The RDA recommendation for vitamin C intake is 60 mg per day.
Divide the RDA value by the quantity of vitamin C per piece of fruit.
Quantity of fruit to meet RDA = ______________
Ch100: Fundamentals for Chemistry
Instructor: Tony Zable
5
Table 4. Summary of results
Sample 1:
Sample 2:
Sample 3:
Sample 4:
Volume of KIO3
(mL)
Number of moles of KIO3
Number of moles of
vitamin C
Concentration of vitamin C
(mol/mL)
Concentration of vitamin C
(mol/L)
Concentration of vitamin C
(g/L or mg/mL)
Quantity of vitamin C per
piece of fruit (mg)
Number of fruit to reach
RDA
Questions:
1) Which of the fruit tested has the highest concentration of vitamin C in its juice? Which has
the lowest?
2) Which fruit has the highest quantity of vitamin C per piece? Which has the lowest?
3) Which fruit has the highest quantity of vitamin C per gram (or ounce)? Which has the lowest?