Report 7 - Determination of Ascorbic Acid in Vitamin C...

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Determination of Ascorbic Acid in Vitamin C Tablets by Titration with Potassium Bromate (KBrO 3 ) Vincent Gallo CHM3120-903 March 21 st 2016
I. Introduction Ascorbic acid concentration in Vitamin C tablets can be determined using back titration methods. A back titration, also referred to as a indirect titration, is a two-step technique where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another second reagent. The second titration’s results show how much of the excess reagent was used in the first titration and the original analyte’s concentration can then be calculated. In this experiment, ascorbic acid can be oxidized by bromine, allowing for its concentration to be calculated given reaction 1 : + ¿ ¿ + 2 H ¿ Ascorbic Acid + Br 2 → Dehydroascorbicacid + 2 Br ¿ Adding potassium bromide to an acidified solution results in the production of a known amount of bromine by means of a titration involving standard KBrO 3 . The free bromate ions react with five moles of bromide and H + ions to produce three moles of bromine and 3 moles of water. The bromine ions react with ascorbic acid immediately after being produced, completely reacting all available ascorbic acid, resulting in excess bromine in solution and a faint yellow color. Reacting the bromine left over from the previous reaction with iodide results in the production of two moles of bromide ion and one mole of iodine. This procedure requires the production of iodine so that it can be reacted with thiosulfate in the back- titration portion of the experiment so that the number of moles of ascorbic acid can be calculated, and ultimately, the percent mass of ascorbic acid.
If the back-titration methods are completed correctly with reactants that aren’t contaminated, then an ascorbic acid percent mass will be found within four trials with a percent precision with respect to the mean value less than or equal to 0.5%. II. Experimental Procedure Prior to the beginning of the experiment, the preparation of starch indicator, sodium thiosulfate, and potassium bromate was completed by the teacher’s assistant. The pre-ground vitamin C powder was weighed out into four samples weighing .3 grams each. The samples were then transferred into their own 250 mL Erlenmeyer flask. Then, Two burets per trial were obtained and washed with distilled water. Buret one was rinsed

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