Vita C

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Determination of Ascorbic Acid in Vitamin C Tablets Done by: lora ID #: Date:12/3/2011 Objectives: The aim of this experiment is to determine the weight of ascorbic acid in three vitamin C tablets by titrating it directly with KBrO3, then back titration by Na2S2O3. Introduction: Vitamin C is a weak acid and a water soluble substance and it is often called ascorbic acid. It is useful for many biological activities and main functions such as: it is used to provide reducing equivalents for biochemical reactions and it works like a cofactor for reactions that need reduced metal ions. Moreover, it is like a protective antioxidant. Ascorbic acid also helps in formation of collagen, intercellular material, bond and teeth. We can get ascorbic acid from different sources like fruits and vegetables. Figure 1: Vitamin C In this experiment we need to determine the wt% of the ascorbic acid in three samples by using direct and back titration. A back titration, in which a known excess of one standard reagent is added to analyte. Then, the second standard reagent is used to titrate the excess of the first reagent. For this experiment, an unmeasured excess of KBrO3 will be added to an acidified solution of the standard potassium bromate till the faint yellow color appears. This excess of bromine can be determined iodometrically with standard thiosulfate solution. The ascorbic acid is oxidize by air so the titration should be performed quickly. + Br2 → + 2Br- + 2H+ The bromine or iodine used in titration is generated by adding an excess amount of KBr or KI to an acidified solution of the sample according to the following reaction: BrO3- + 5Br- + 6H+ 3Br2 + 3 H2O IO3- + 5I- + 6H+ 3I2 + 3 H2O Methods: Two vitamin C tablets were weighed and pulverized thoroughly in a mortar to get a homogenous powder. A 0.4028 g, 0.4081 g and 0.4042 g of samples were weighed into three separated 250 ml conical flasks. Then each sample was dissolved in 50 ml of 0.5 M H2SO4 and then about 5 g of KBr...
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