Early seamen on long voyages often developed bleeding gums, hemorrhaging, and general
muscular weakness, symptoms of the deficiency disease that became known as scurvy. In the
middle 1700s, the British discovered that fresh fruit and vegetables in the diet would
cure and prevent scurvy. Since fresh vegetables could not be properly stored for long
voyages, barrels of lemons and limes became staples on British ships, and British sailors
soon became known as "limeys." In the 1930s, the water soluble vitamin,
L-ascorbic acid (vitamin C), was determined to be the substance that prevented scurvy.
For humans and other primates, vitamin C is an essential vitamin, that is, it is an
essential part of our diet since we are unable to synthesize the vitamin in our bodies.
Citrus fruits have long been recognized as an excellent source of vitamin C, though many
fresh vegetables such as cabbage, tomatoes, lettuce, strawberries and broccoli contain
more vitamin C per gram than freshly squeezed orange juice (see Table 1). Unfortunately,
much of the vitamin C in foods is destroyed as food is processed and cooked. To compensate
for this loss, fresh or frozen fruit juices, beverages fortified with vitamin C or fresh
fruits and vegetables should be included in our daily diet.
The complete role of vitamin C in the body is not clearly understood. It does seem to
be involved in the transfer of hydrogen atoms (2e- + 2H+) during
cellular respiration. It is also involved in the formation of connective tissue and its
use by the body increases during periods of infection.
Dr. Linus Pauling, winner of Nobel Prizes in Chemistry (1954) and in Peace (1962) and a
graduate of OSU , announced in the early 1970s that ingesting large amounts of vitamin C
could prevent or lessen the effects of the common cold. Clinical evidence to support this
claim is controversial. One side effect accompanying the use of massive amounts of this
vitamin is diarrhea. Vitamin C present in digested food or from commercial tablets is
absorbed from the digestive tract into the bloodstream and carried to the tissues. The
current recommended daily allowance (RDA) for adults is 60 mg. The normal level of vitamin
C is about 1.2 mg per 100 mL of plasma. If this level is exceeded, it is excreted in the
Pure vitamin C is a white, crystalline solid that is soluble in water and ethyl
alcohol. Its chemical name is ascorbic acid. Because the vitamin is an acid, it is
reasonably stable in acidic solutions, but in neutral or basic solutions it is easily and
rapidly oxidized by dissolved oxygen. Because vitamin C is so readily oxidized, it is
easier to analyze vitamin C using an oxidation-reduction titration rather than an
acid-base titration. Also, because many fruits and vegetables contain acidic compounds
other than vitamin C, the oxidation-reduction procedure can be more accurate.
The oxidation of vitamin C, ascorbic acid, is brought about by the loss of two hydrogen
atoms as the vitamin is converted to dehydroascorbic acid. Examine the two large
structures shown below to see where the changes occur.
In this experiment, vitamin C will be oxidized by 2,6-dichloroindophenol, an organic
dye, simply referred to as "DCP." To increase the solubility of the DCP, its
sodium salt is used. It is convenient in this experiment to describe oxidation as
either the gain of oxygen (+ O) or the loss of hydrogen (- H) by a species. Reduction is
just the opposite, the loss of oxygen (- O) or the gain of hydrogen (+ H) by a species.
Oxidation and reduction always occur simultaneously. As vitamin C is oxidized, the DCP is
reduced and gains the two hydrogen atoms lost by the vitamin. As the DCP is reduced, it
changes from red to colorless. Since the reaction between the DCP and vitamin C is very
fast, this color change can be used to indicate the point in a titration at which all of
the DCP has just been consumed by the vitamin. This visual observation in a titration is
called the end point of the procedure. An equation showing the reaction of vitamin C with
the DCP is shown below.
During the analysis for vitamin C, the solution to be analyzed (juice, beverage or
tablet) is added from a micro buret to multiple wells of a well tray. Next the solution in
each well will be acidified (the acidity is used to stabilize the vitamin C solutions that
are being analyzed.. Then a measured volume of DCP solution will be added in a serial
titration scheme: to the first well containing acidified vitamin C, one drop of DCP is
added; to the next well, two drops of DCP are added; to the third well, three drops of
DCP; etc. As obtained from the laboratory supply, the DCP solution is blue, but when it is
added to the acidic solution, you will notice its color changes. If there is more vitamin
C present than DCP, the vitamin C will consume the added DCP, reducing it to the colorless
form. After the end point has been passed, the solution will remain red, since the DCP is
now present in greater quantity than the vitamin C. This red color tells you that all of
the vitamin C has been consumed by the DCP that is added to the solution.
You will first titrate a solution with a known number of milligrams of vitamin
C. This will enable you to determine the potency of the DCP solution. Then you can titrate
other unknown solutions of vitamin C using the standardized DCP solution, and you will be
able to calculate the amount of vitamin C in the unknown solutions. You can quickly and
accurately determine the vitamin C content in foods by this method.
The amount of vitamin C in different foods (stated as mg vitamin C per gram of food)
varies considerably, as shown in Table I.
Table I Vitamin C Content of Several Foods
|Foodstuff||Vitamin C (mg/g)|
|fresh orange juice (changes with variety and season)||0.27 - 0.61|
|reconstituted frozen orange juice||0.40 - 0.45|
|canned orange juice||0.40 - 0.60|
|fresh grapefruit juice||0.16 - 0.19|
|canned grapefruit juice (unsweetened)||0.14 - 0.34|
|fresh lemon juice||0.38 - 0.46|
|fresh tomato juice||0.10 - 0.23|
|fresh cabbage (uncooked)||0.38 - 0.42|
|frozen brussel sprouts (uncooked)||0.50 - 0.87|
|fresh green bell peppers (uncooked)||0.95- 1.05|
|fresh spinach (uncooked)||0.48 - 0.51|
|fresh turnip greens (uncooked)||1.30- 1.40|
|kale leaves (uncooked)||1.70- 1.90|
|mustard greens (uncooked)||0.88- 1.00|
|potato (uncooked)||0.10 - 0.20|
|broccoli (uncooked)||1.00- 1.20|
|Tang® (varies with concentration-will age)||0.50 - 4.00|
|Twist® (prepared as directed-will age)||0.50 - 0.70|
|Eggs, milk, carrots, beets, cooked meat||< 0.10|