Formation of a precipitate indicates the presence of

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Unformatted text preview: 3 , Cu+2 , and Sn+4 (many qualitative analysis schemes also include mercury, arsenic, cadmium and antimony in this group). Because hydrogen sulfide gas is toxic, we will be using thioacetamide, which decomposes readily to produce hydrogen sulfide at lower, and therefore safer, concentrations. Separation of group II from groups III – V: Heat the decantate from group I to reduce the volume. When the volume is about ¼ to ½ the original volume, remove from the heat. Add concentrated HCl dropwise until the solution has a pH of less than 1 (use pHdrion paper to test). When acidified, add 5 drops of thioacetamide. Heat the solution to cause the decomposition of thioacetamide. Centrifuge the mixture. The solid contains group II ions. Dakota State University page 195 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual Check the pH of the decantate and be sure it is still acidic. Add two more drops of thioacetamide, and heat. If a precipitate forms, centrifuge and add the solid to the precipitate to that from the previous step. Repeat this step until addition of thioacetamide produces no more precipitate. Keep the liquid as it contains groups III through V ions. Separation of Sn+4 from Pb+2 , Bi+3 , Cu+2 : To the precipitate, add 1 mL of ammonium sulfide and agitate the solution. Tin should dissolve into the liquid, while other ions remain solid. Centrifuge and decant. Pb+2, Cu+2, Bi+3, Sn+4, groups III - V HCl/Thioacetamide PbS, CuS, Bi2S3, SnS4 (NH4)2S Sn+4 H2SO4 Sn(SO4)2 Groups III-V PbS, CuS, Bi2S3 Nitric Acid Pb+2 H2SO4 Cu+2 NH4OH Bi+3 NH4OH PbSO4 Cu(NH3)4+2 Bi(OH)3 Verification of Sn+4 : Add dilute HCl to the decantate from the previous step until it registers acidic to litmus paper. Formation of a precipitate indicates the presence of tin. Separation of Pb+2 Bi+3 , and Cu+2 from other impurities: To the precipitate from the separation of tin step, add 1 mL of dilute nitric acid. All ions should dissolve, leaving behind only solid impurities. Centrifuge and decant. Discard any remaining precipitate; separate the decantate into three portions. Verification of Pb+2 : Dakota State University page 196 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual Add dilute sulfuric acid to one portion the above decantate; the formation of a precipitate verifies the presence of lead. Notice that you should regard lead as present if it shows positive in EITHER group I or group II (or, naturally, in both). Verification of Bi+3 : Add ammonium hydroxide to the second portion of the above decantate until the solution is alkaline according to litmus paper. Formation of a precipitate is verification of the presence of bismuth. Verification of Cu+2 : Add concentrated ammonium hydroxide dropwise to the final portion. If a precipitate forms, continue adding ammonium hydroxide until it dissolves. A blue color of the solution is a verification of copper. Group III: Group III ions precipitate as sulfide ions, just as group II, except in an alkaline solution rather than acidic. These...
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This note was uploaded on 09/18/2012 for the course CHEMISTRY 1010 taught by Professor Kumar during the Fall '11 term at WPI.

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