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Unformatted text preview: Fall 2007 Chemistry 300 Cornell University Lab 6 Determination of the Iodine Number of Fats (Part I) I. INTRODUCTION The goal of this three-week lab is to measure the degree of unsaturation of commercially available oils. The degree of unsaturation will be determined, following the U.S. Food and Drug Administration, by inferring the iodine number of the oil, defined as the grams of I 2 that would react with 100 g of oil. II. MATERIALS AND METHODS You will need First day Equipment * Platinum electrodes (1 paddle, 1 wire) * Current source * Multimeter, set to read current * Wires with banana and alligator clips * Ringstand * Hotplate and stirbar Materials * Glass tubes with Agar and KNO 3 solution Glassware * Buret: 50 mL * Plastic bottle: 1 L * Beakers: 400 mL and 600 mL * Volumetric pipet: 2 mL and 5 mL * Erlenmyer flask: 50 mL Chemicals * Potassium iodide * Cyclohexane * Sodium thiosulfate pentahydrate (Na 2 S 2 O 3 5 H 2 O) * Starch * Acetic acid, glacial * . 1 M ICl in acetic acid Second and Third day . You will need all the mate- rials from the first day, and in addition: Materials * Commercial oils (Day 2) * Unknown oil (Day 3) I 2 , or alternatively, ICl, reacts quantitatively with the double bonds in unsaturated oils. We will react oil with ICl and then, to determine the FDAs iodine number for the oil, back-calculate much I 2 the oil would have reacted with. To measure the degree of unsaturation of an oil, one must know only how much ICl it reacts with. Unfortu- nately, the oil cannot be titrated directly against ICl. An indirect approach to determining how much ICl reacted with the oil is required, as follows. A well known amount of ICl is added in excess to a determined mass of oil. Once the reaction has gone to completion, you will mea- sure the remaining ICl from which, by substraction, you can infer how much reacted. ICl is next converted to I- 3 and Cl- by the following displacement reaction with I- . The I- is provided by KI( aq ). ICl + 2 I-- I- 3 + Cl- The molar quantity of I- 3 will be exactly the same as that of the ICl before the displacement reaction. Starch in the solution acts as an indicator for iodine, giving a recognizeable bluish purple color; the I- 3 starch complex is blue. The iodine-starch solution is titrated with a thiosul- fate solution. The thiosulfate reduces iodine to iodide according to the following redox reaction: I- 3 + 2 S 2 O 2- 3- S 4 O 2- 6 + 3 I- At completion of the titration, I- 3 is used up and the solutions blue color disappears. From the end point you can compute how much iodine there is, and therefore how much ICl was left over. The endpoint of this titra- tion, however, is not as accurate as we would like. So a known amount of excess thiosulfate is added you will intentionally overtitrate. Then we can titrate coulombe- trically back to the endpoint very precisely. A steady current oxidizes iodide to iodine in the provided elecro- chemical cell:...
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- Fall '07
- Chemistry, ICL