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by: Edwardo Fadel I


Edwardo Fadel I
GPA 3.66

Delana Nivens

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Delana Nivens
Class Notes
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This 2 page Class Notes was uploaded by Edwardo Fadel I on Saturday October 3, 2015. The Class Notes belongs to CHEM 2300 at Armstrong Atlantic State University taught by Delana Nivens in Fall. Since its upload, it has received 36 views. For similar materials see /class/217880/chem-2300-armstrong-atlantic-state-university in Chemistry at Armstrong Atlantic State University.




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Date Created: 10/03/15
Determination of Ascorbic Acid in Vitamin C tablets by Titration with Potassinm Bromate Back Titration Review the relevant sections in your textbook regarding the method of back titration and redox titrations paying particular attention to the discussion of the standardization of iodine Follow the guidelines for an informal report Remember signi cant gures accuracy and precision gure into your grade You will work in pairs and use 2 burets Be sure to get your partners buret correction values Reagents Vitamin C tablets 15 M H2804 you have to make this 250 mL KBr KI KBr03 00150M 500mL prepared from primary standard grade Starch Indicator 005 M NazSzO35HzO must be standardized before use 250 mL mortar and pestle Procedure Standardization of Sodium T hiosnlfate l Pipet 2500mL aliquots of KBr03 solution into 3 Erlenmeyer asks 2 Add 23 g excess of K1 3 Add 10 mL of 15M H2804 The reaction between 1amp2 produces 12 BrO39 611 6Haq gtBr7aq 3 3 09 mg 3HZO a I is not very soluble or in solution but the presence of excess 139 converts I to 1339 triiodide ion by the following redox reaction 12 aq 2e cgt 21 aq 12 0611 GCI G 137061 4 Immediately Titrate with Na8203 until the solution becomes pale yellow The 1 generated in a reacts with thiosulfate For calculation simplicity assume that all iodine is in the form 1339 and is reacted with thiosulfate according to the following reaction I 2SZOZ GSAOZ 311 309 3 m1 5 09 b 5 Add 05 mL of starch indicator indicator is added at a late stage to prevent decomposition and to minimize insolubility Starch interacts with various forms of Iodine and Iodide in solution to form a blueblack color 6 Titrate the solution to the disappearance of the blue color stoichiometric consumption of 1339 by thiosulfate Even if you think that you have titrated it all wait a few minutes to be sure Intermediates of thiosulfate and iodide can form that will redissociate to free 1339 and reform the blueblack color 7 Repeat a total of3 times 8 Calculate the concentration of the thiosulfate This should be straightforward Titration of the Vitamin C Tablet to determine Ascorbic Acid Concentration l Weigh 4 Vitamin C tablets 2 Pulverize them thoroughly in a mortar 3 Weigh individual 040 g samples 3x into dry asks Remember to weigh to the nearest 01 mg 4 Treat each sample individually beyond this point 5 Add 50 mL 15 M H2804 to each ask 6 The binder in the tablet may not dissolve 7 Add approximately 5 g of KBr 8 Titrate immediately with standard KBr03 The reaction between steps 7 and 8 is BrOgm SBrqu 6Haq gt 3Br2 3HZO c The Bromine produced in c immediately oxidizes the ascorbic acid in the Vitamin C tablet to dehydroascorbic acid C6H806 W Brz W a 0614606 M d 9 Titrate until you see the first faint yellow color indicating excess Brz 10 Record the volume of KBrO3 used 11 Add N3 g of KI and starch indicator The KI reacts with any excess Brz B 311 GZBVTW 13 Hg 6 12 Backtitrate the triiodide with the standardized sodium thiosulfate This is a redox reaction You should be able to track the movement of the electrons and assure that the reaction is balanced 1 220 cape 31 3 m1 3 m1 5 m1 Repeat a total of 3x 14 Calculate the average of milligrams of ascorbic acid in each sample analyzed your standard deviation relative standard deviation and 95 confidence 15 Back calculate to the average of milligrams PER TABLET Again calculate your standard deviation relative standard deviation and 95 confidence 16 Compare your answer to the quoted mg of Vitamin C ascorbic acid in each tablet assume it is the true value What is your accuracy relative and absolute Be sure to answer the following questions in your report 09 2Haq ZBrqu rung f


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