Vitamin C

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 urine.

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