Published on Mar 02, 2020
To study and calculate the content of ascorbic acid in different citrus fruits.
Vitamin C or ascorbic acid in citrus fruits is a water soluble carbohydrate like substance involved in certain metabolic processes of animals. Although most of the animals can synthesise vitamin C, it is necessary in the diet of some including men and other primates. In order to prevent scurvy, disease characterized by haemorrhage especially in skin and mucous membranes Vitamin C was identified as a curative agent for survey in 1928. The name ascorbic acid is derived from the expression anaisearbatic vitamin, referring to vitamin’s ability to prevent and to curve scurvy. First isolated in 1928 by Hungarian biochemist and Nobel Prize winner Albert Seent Ceyorghi. Vitamin C has been the object of continued active laboratory research to determine the specific mechanism of action of cells.
Ascorbic acid (AA) is a simple sugar. It is the most active reducing agent known to occur naturally in living tissues, and is easily reversibly oxidized to Dehydroascorbic acid (DHA). Ascorbic acid is a white crystalline stable substance, when in dry light and air. It is easily soluble in water and easily oxidized especially in alkaline medium and exposure to heat and light, it reacts with metals, particularly copper. It is fairly soluble in cold acid solution. Though the first stage in its oxidation to DHA is reversible, oxidation to diketo-gulnic acid cannot be reversed.
Ascorbic acid, the formula of which C6H8O6, behaves as a vinologous carboxylic acid, wherein double bond (“vinyl”) transmits electron pairs between the hydroxyl and carbonyl. There are two resonance structures for the deprotonated form, differing in the position of double bond.
Another way to look at ascorbic acid is to consider it an enol. The deprotonated form is an enolate, which is usually strong basic. However, adjacent double bond stabilized the deprotonated form.
Ascorbic acid is rapidly in converts into two unstable diketone tautomers by proton transfer, although it is the most stable in the enol form. The proton of the enol is lost, reacquired by electrons in the form of double bond, to produce a diketone. It is an enol reaction there are two possible forms: 1,2-diketone and 1,3-diketone.
The concentration of the solution of ascorbic acid can be determined in many ways; the most common way involves titration with an oxidizing agent.
A commonly- used oxidizing agent is the dye 2,6-dichlorophenol – indophenols, or DCPIP for short. The blue dye is run into the ascorbic acid solution until a faint pink colour persists for 15 seconds.
Another method involving using iodine and a starch indicator, wherein iodine reacts with ascorbic acid, and when all the ascorbic acid had reacted, the iodine is excess, then forming a blue – black complex with starch indicator. This indicates the end point of the titration. As an alternative, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as indicator
IODATE AND IODINE
The above method involving iodine requires making up and standardizing the iodine solution. One way around this is to generate the iodine in presence of ascorbic acid by the reaction of iodate and iodide ion in acid solution.
N – BROMOSUCCINIMIDE
A much-less-common oxidizing agent is N bromosuccinimide, (NBS). In titration the NBS oxidizes the ascorbic acid 9 in presence of potassium iodide and starch). When the NBS in excess (i.e., the reaction is complete). The NBS liberates iodine from the potassium iodide, which then forms the blue/black complex with starch, indicating the end–point of the titration.
Ascorbic acid is easily oxidized and so is used a reductant in photographic developer solutions amongst ethers and as a preservative. Exposure to oxygen, metals, light, and heat destroys ascorbic acid, so it must be stored in dark, cold and not metal container. The L-enantiomer of ascorbic acid is known as vitamin C. The name “ascorbic” comes from its property of preventing and curing scurvy. Primates including humans and few other species of the same animal kingdom, notably the guinea pig, have lost the ability to synthesize ascorbic acid, and must obtain it in their food. Ascorbic and its sodium, potassium, calcium salts are commonly used as antioxidant food additives. These compounds are water-soluble and thus cannot protect fact from oxidation. For this purpose, the fat soluble esters of ascorbic acid with long chain fatty acid can be used as food antioxidants. 80% of World’s supply of ascorbic acid is produced in China
Citrus Maxima Juice
Weight 0.254g of solid iodine and pour in a dry beaker. Add 4g of solid potassium iodide. Then add distilled water then dissolve iodine and potassium in it. Transfer this solution to a clean 100ml volumetric flask and prepare required quantity of distilled water that was added to make 100ml of iodine solution. In this way another 100ml of iodine solution is prepared. This solution has a molarity of 0.01M.
Now a starch solution is prepared by adding a spatula of starch to 100ml of water and subsequently boiling it.
The fruit juices are extracted and filtered using muslin cloth.
The iodine solution of 0.01M is taken in a burette and 5ml of filtered juice is pippeted out in a conical flask. To the juice 1ml of starch solution is added.
The solution of juice is titrated against iodine solution. The process is stopped at the point the colour of solution in conical flask changes from fruit juice to violet colour.
Three concordant readings are taken.
The following are the percentage of ascorbic acid to various citrus fruits.
Lemon juice = 31.68%
Orange juice = 49.28%
Citrus Maxima = 70.4%
The percentage of ascorbic acid was found to be less in lemon juice and more in Citrus Maxima juice. Here starch solution was added to the fruit juices, before doing titration due to which some of the fruit content reacted starch and a permanent complex was formed which could not be oxidized
Sarawathi lab manual chemistry – XII
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