6-Lab6-iodometric detn of Cu - Chemistry 223 Experiment 6...

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Chemistry 223: Experiment 6 IODOMETRIC DETERMINATION OF COPPER IN BRASS References: 1. C, Chap. 10 (especially pp. 263-264 and 267-272) and pp.583-584. 2. H7, Chapter 16, especially section 16-7. 3. S & W, Chaps. 11, 12 (especially pp. 312-17, 320-23), pp.593-96. 4. SWH, Chaps. 13, 14 (especially pp. 339-344), 31 (pp. 781-784). This experimental procedure is mainly based on the lab. Iodometric determination of copper in brass. Chem. 223- UIUC I. Purpose Brass is an alloy consisting principally of copper, zinc, lead, and tin. In addition, several other elements, iron and nickel, for example, may be present in minor amounts. The iodometric method is convenient for estimating the copper content of such alloys. A solid brass sample is dissolved in acid. The Cu 2+ is reacted with I - to form CuI(s) and I 2 . The iodine generated is determined by titration with standardized thiosulfate, thus allowing calculation of the copper content of the brass. II. Introduction: In acid solution practically all oxidizing agents will oxidize iodide ion to iodine quantitatively. The iodine formed in the reaction can then be titrated by means of a standard sodium thiosulfate solution. This type of indirect titration is given the general name of iodometry. Iodometric methods of analysis have a wide applicability for the following reasons: 1. Potassium iodide, KI, is readily available in high purity. 2. A good indicator, starch, is available to signal the equivalence point in the reaction between iodine and thiosulfate. Starch turns blue-black in the presence of iodine. Therefore, when the blue-black color disappears, the iodine has been completely reduced to the iodide ion. 3. Iodometric reactions are rapid and quantitative. 4. A precise and stable reducing agent, sodium thiosulfate (Na 2 S 2 O 3 ), is available to react with the iodine. The amount of iodine liberated in the reaction between iodide ion and an oxidizing agent is a measure of the quantity of oxidizing agent originally present in the solution. The amount of standard sodium thiosulfate solution required to titrate the liberated iodine is then equivalent -- Converted from Word to PDF for free by Fast PDF -- www.fastpdf.com --
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to the amount of oxidizing agent. Iodometric methods can be used for the quantitative determination of strong oxidizing agents such as potassium dichromate, permanganate, hydrogen peroxide, cupric ion and oxygen. As has been mentioned above, the endpoint in a titration of iodine with thiosulfate is signaled by the color change of the starch indicator. When starch is heated in water, various decomposition products are formed, among which is beta-amylose which forms a deep blue- black complex with iodine. The sensitivity of the indicator is increased by the presence of iodide ion in solution. However, if the starch indicator solution is added in the presence of a high concentration of iodine, the disappearance of the blue-black color is very gradual. For use in indirect methods, the indicator is therefore added at a point when virtually all of the
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6-Lab6-iodometric detn of Cu - Chemistry 223 Experiment 6...

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