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Experiment 5 - Vitamin C Analysis (1/2 pages)

Introduction | Experimental

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Pre-lab Questions

1. Estimate how many lemons you would need to eat daily to consume 1 g of vitamin C (recall Pauling’s suggestion of 250 mg to 10 g daily).

2. Why is it easier to oxidize vitamin C under basic conditions than under acidic ones?

Introduction

The history of vitamin C is the history of the human disease scurvy, probably the first human illness to be recognized as a deficiency disease. Its symptoms include exhaustion, massive hemorrhaging of flesh and gums, general weakness and diarrhea. Resultant death was common. Scurvy is a disease unique to guinea pigs, various primates and humans. All other animal species have an enzyme that catalyzes the oxidation of L-gluconactone to L-ascorbic acid, allowing them to synthesize vitamin C in amounts adequate for metabolic needs.

As early as 1536, Jacques Cartier, a French explorer, reported the miraculous curative effects of infusions of pine bark and needles used by Native Americans. These items are now known to be good sources of ascorbic acid. However, some 400 years were to pass before vitamin C was isolated, characterized, and synthesized. In late 1700’s, the British Navy ordered the use of limes on ships to prevent scurvy. This practice was for many years considered to be quackery by the merchant marine, and the Navy sailors became known as ‘Limeys.’ At that time, scurvy aboard sailing vessels was a serious problem, with often up to 50% of the crew dying from scurvy on long voyages.

The Recommended Daily Allowance, or RDA, for vitamin C put forward by the Food and Nutrition Board of the National Research Counsel is 60 mg/day for adults. It is recommended that pregnant women consume an additional 20 mg/day. Lactating women are encouraged to take an additional 40 mg/day of ascorbic acid in order to assure an adequate supply of vitamin C in breast milk. Medical research shows that 10 mg/day of vitamin C will prevent scurvy in adults. There has been much controversy over speculation that vitamin C intake should be much higher than the RDA for the prevention of colds and flu. Linus Pauling, winner of both a Nobel Prize in Chemistry and the Nobel Peace Prize, has argued in his book, Vitamin C and the Common Cold, that humans should be consuming between 250 mg and 10 g of vitamin C depending on one’s physiology.

Vitamin C is a six-carbon chain, closely related chemically to glucose. It was first isolated in 1928 by the Hungarian-born scientist Szent-Gyorgi, and structurally characterized by Haworth in 1933. In 1934, Rechstein worked out a simple, inexpensive, four-step process for synthesizing ascorbic acid from glucose. This method has been used for commercial synthesis of vitamin C. Vitamin C occurs naturally primarily in fresh fruits and vegetables. A table of some typical vitamin C contents of foodstuffs is given below. Imagine how many lemons one would have to eat in order to ingest 1 g of vitamin C!

The redox method of analysis used in this lab takes advantage of the ease of oxidation of vitamin C. As shown below, vitamin C is readily oxidized in acidic conditions (the oxidation is more facile under basic conditions, why?) We will use iodine to oxidize the vitamin C.

When the ascorbic acid is completely gone, the I₂ will begin to build up and will react with the I^- already present to form the highly colored I₃^-/starch complex indicating the end of the titration. It is not practical to prepare an aqueous I₂ solution for titration. However, the water-soluble, colorless KIO₃ reacts under acidic conditions with I^- to form I₂. This reaction will quickly generate I₂ in the solution to be tested.

This reaction will quickly generate I₂ in the solution to be tested. Using this method we will be able to quantitatively measure the amount of vitamin C in a sample.