Consumer Chemistry: Vitamin C in Fruit Juices
The goal of this experiment is to determine the concentration of vitamin C in a range of different fruit juices (fresh and preserved) using titration and to rank these sources of vitamin C. Skills
At the end of the laboratory session you should be able to:
* use a pipette correctly,
* use a pipette filler safely,
* manipulate a burette and carry out a quantitative titration properly, * weigh a sample accurately,
* understand and utilise error analysis.
* You will extract vitamin C from a number of types of fresh fruit and bought juices. * You will present a conclusion concerning the ranking of different sources of vitamin C that includes clear reference to their interpretation of the error analysis. * You will explore the stability of vitamin C in solution and relate this to differences between fresh and preserved juices. Introduction
Vitamin C is a water-soluble compound that is essential for life. It is involved in many processes in the human body, including: the production of collagen in the connective tissue; the synthesis of dopamine, noradrenaline and adrenaline in the nervous system; and the synthesis of carnitine, which is important in the transfer of energy to the cell mitochondria. A deficiency in vitamin C causes scurvy, a disease that affected sailors in the 16th - 18Centuries. It was discovered that fresh fruit, e.g. limes and oranges, or sauerkraut (preserved cabbage) provided the sailors with protection from scurvy. In Australia and New Zealand, the recommended daily intake (RDI) of Vitamin C is 60 mg. The Nobel-prize winning scientist, Linus Pauling (1901 - 1994), believed in regular mega doses of vitamin C, but this is still regarded as unorthodox in conventional medicine. Vitamin C is often used as an antimicrobial and antioxidant in foodstuffs. It was first isolated in 1928 and in 1932 it was proved to be the agent which prevented scurvy (hence its scientific name of "ascorbic acid", which literally translates as "anti-scurvy acid"). Its structure was determined in 1933 and confirmed by total synthesis soon after. th
Enantiomers are isomers that are mirror images of each other, a concept dealt with in detail in E15. Vitamin C is the L-enantiomer of ascorbic acid, as shown in Figure 1. (See Skill 13 if you can't understand the stick structures used.) Ascorbic acid is a stable solid that does not react with air, however, it is rapidly oxidised on exposure to air and light when in aqueous solution. The product of this oxidation is dehydroascorbic acid, as shown below
You will come across the terms oxidation and reduction in the lecture course. These two processes are the opposite of each other and always accompany each other - as one species is oxidised, another is reduced - and the combined reaction is called a redox process. In organic chemistry, it is convenient to think of oxidation as a decrease in the number of H atoms in a molecule and reduction as an increase in the number of H atoms. Vitamin C the antioxidant
In biological systems, reactive oxidants are often produced from metabolic processes. They have the ability to react with other molecules (e.g. DNA), thus damaging the cell. The body protects its cells by utilising another group of molecules called antioxidants (to which vitamin C belongs) to reduce (and hence detoxify) the oxidants. This experiment uses this phenomenon in a reduction/oxidation (redox) titration, where vitamin C reduces the orange solution of iodine to the colourless iodide ion as shown in Volumetric analysis
Volumetric analysis is a technique that employs the measurement of volumes to determine quantitatively the amount of a substance in solution. In any reaction between two or more species, the reaction equation shows the stoichiometric ratio of reacting species. Take, for example, the reaction being investigated...