Ascorbic acid has reacted excess br 2 is indicated by

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ascorbic acid has reacted. Excess Br 2 is indicated by a faint yellow color. The amount of excess Br 2 can then be determined by using the following reaction to stoichiometrically convert an excess of I - to I 2 : In the determination of ascorbic acid, the KBrO 3 method employs a back titration with a standard thiosulfate solution of the excess Br 2 generated by adding a precisely known amount of a primary standard KBrO 3 solution to an ascorbic acid solution. In this procedure, potassium iodide is added to react with Br 2 to form I 2 : Br 2 + 2I -  2Br - + I 2 ----- Eq. (3) The amount of I 2 produced is determined by the back-titration with standardized sodium thiosulfate solution according to the following reaction:
43 I 2 + 2S 2 O 3 -2  2I - + S 4 O 6 -2 ----- Eq. (4) II. Procedure: You may follow the procedure from “Step d”. a. Starch indicator (sufficient for about 100 titrations : Rub 1 g of soluble starch and 15 mL of water into a paste. Dilute to about 500 mL with boiling water, and heart until the mixture is clear. Cool, and store in a tightly stoppered bottle. For most titrations, 3 to 5 mL of the indicator is used. b. Preparation of the sodium thiosulfate solution: Boil about 1 L of distilled water for 10 to 15 min and cool to room temperature (an ice bath may be used). Weigh about 12-13 g of sodium thiosulfate (Na 2 S 2 O 3 5H 2 O, MW: 248.18 g/mol, approx. strength 0.048 ~ 0.052 M) * , and dissolve it in the cooled liter of water. Weigh out about 0.1 g of Na 2 CO 3 , and add to the thiosulfate solution. Stir until the solid has dissolved. c. Preparation of the potassium bromate solution: Dry about 1.5 g of KBrO 3 at 110 C for at least 1 h and cool in a desiccator. Weigh (to the nearest 0.0001 g) about 1.25 g into a 500-mL volumetric flask; use a powder funnel to ensure quantitative transfer of the solid. Rinse the funnel well with distilled water, dissolve the KBrO 3 in about 200 mL of distilled water, dilute to the mark, stopper and mix thoroughly (~ 0.015 M) * . * These are the approximate concentrations of sodium thiosulfate and potassium bromate solutions. TA will provide you the exact concentrations of sodium thiosulfate and potassium bromate solutions. d. Sample preparation: Grind and homogenize the 4 vitamin C tables in a mortar and transfer the powder to a dry weighing bottle. Weigh (to the nearest 0.0001 g) individual 0.40 g to 0.50 g samples (to the nearest 0.0001 g) into dry 250 mL Erlenmeyer flasks. Treat each sample individually beyond this step. e. Obtain two burettes (#1 and #2). Rinse both burettes with distilled water. Rinse the burette #1 with 5-10 mL portion of standard KBrO 3 solution, burette #2 with 5-10 mL of standard sodium thiosulfate solution, and fill both burettes with proper solutions. Read the initial volumes of each. f. Dissolve the sample in 50 mL of 1.5 M H 2 SO 4 ; then add about 5 g of KBr. Titrate immediately with standard KBrO 3 solution (in #1 burette) to the first faint yellow due to excess Br 2 . Record the volume of KBrO 3 solution used. Second, add 3 g of KI and 5 mL of starch indicator; back titrate with standard sodium thiosulfate solution (in #2 burette) to cloudy white. ** Record the volume of Na 2 S 2 O 3

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