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Unformatted text preview: 1 DETERMINATION OF ASCORBIC ACID IN VITAMIN C TABLETS BY REDOX TITRATION Background reading: Harris, 7th ed., Chap. 16. Skoog et al., 7th ed., Chap. 18; Introduction Iodine is a versatile redox reagent because its potential falls in the middle of the range of potentials observed in aqueous solutions. Thus in the presence of strong oxidants, such as dichromate, iodide is oxidized to iodine; in the presence of reducing agents, such as As (III), iodine is reduced to iodide. Solid I 2 is only slightly soluble in water, but in the presence of excess iodide it forms the soluble triiodide ion, I 3- , and it is in this form that it is used for redox titrations. Reducing agents are determined by direct titration with standard I 3- . For the determination of oxidizing agents it is not feasible to titrate directly with standard iodide, because a high concentration of iodide is needed to form the I 3- complex. Instead excess iodide is added to oxidizing agents, and the excess I 3- formed is titrated with a standard solution of a reducing agent, thiosulfate, S 2 O 3 2- . An advantage to all of these analyses is the ready availability of a specific indicator, starch. I 3- reacts with starch to form an intense blue color that is visible even at very low I 3- concentrations. In direct titrations with I 3- the endpoint is signaled by the appearance of the blue color when the first trace of I 3- is produced after the equivalence point. In titrations of triiodide with thiosulfate, the endpoint is signaled by the after the equivalence point....
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