(Heat added)CH3CSNH2 + H20 --> CH3COO- (aq) + NH4+(aq) + H2S(aq)
A. Precipitation of the Cations for Analysis:
1. Oxidation of Tin and pH Adjustgment:
Place 2mL of test solution from Qual l, part B.2 or a solution containing the four test cations in an evaporating dish and add 15 drops 6M HNO3. Heat gently until a moist residue remains, then cool and add 2mL deionized water. Now add drops of 6M NH3 until the solution is basic to red litmus. Finally, add drops of 6M HCl until the solution is acidic to litmus. Once this is so, add 2 more drops of 6M HCl. This should adjust the pH of the solution to about 0.5. Transfer the solution to a centrifugable test tube.
2. Precipitation of Cations as Sulfide:
Add 10 - 15 drops of CH3CSNH2 (thioacetamide) to the solution. Heat in a hot water bath for 5 minutes, then cool and centrifuge. Repeat this process once more to ensure complete precipitation. Decant the supernatant and save it for Qual III.
B. Test for Tin Ion:|
1.Wash the precipitate from part A.2(above) with 1 mL 0.1M HCl. Stir, centrifuge, and discard the supernatant. Repeat washing once.
2. To the washed precipitate, add 2 mL 6M KOH and 3 drops CH3CSNH2. Heat the solution in a hot water bath for 8 minutes. While the solution is still warm, centrifuge and decant the supernatant into another test tube. Save the precipitate for the next test in part C.
3.To the supernatant, add concentrated HCl drop by drop until the solution is just acid to blue litmus. Heat the solution in a hot water bath for 5 minutes. Centrifuge and discard the supernatant.
4.To the precipitate, add 2mL 6M HCl, stir, and heat in a hot water bath for 5 minutes. This serves to boil off all excess sulfide as H2S. You may notice a foul odor, but that is normal. After heating for 5 minutes, add 2mL deionized water and 2 drops 6M HCl to the solution. Place a piece of polished aluminum approximately .25" square into the solution. Continue heating until the aluminum dissolves completely. Add 1 drop 6M HCl and heat for 3 more minutes.
5.Add 5 drops 0.2M HgCl2 to the solution. A gray or black precipitate confirms the presence of tin.
C. Test for Lead Ion:|
1.Wash the precipitate from B.2 with 1 mL deionized water and 2 drops 1M NH4NO3. Centrifuge, and discard the supernatant. Now, add 15 drops 6M HNO3 to the precipitate and heat in a hot water bath for 5 minutes, stirring frequently. You may notice a yellow substance floating atop the solution. This is the free sulfur, which is normal. Centrifuge, and transfer the supernatant to an evaporating dish or crucible. Be careful not to get any free sulfur in with the supernatant.
2.Set up a bunsen burner in the fume hood. Add 5 drops concentrated H 2S04 to the solution and heat cautiously. As dense white S03 fumes appear remove the solution from the heat. There should be a moist residue remaining. If there is still too much solution, continue to heat cautiously until only a moist residue remains. Allow the residue to cool for a few minutes, then add 15 drops deionized water to it. Stir, and then carefully transfer the milky liquid to a clean, cetrifugable test tube. Centrifuge, and save the supernatant for the next test in part D.
3.Dissolve the precipitate with 10 drops 1M NH4C2 H302 and 2 drops 1M HC2H302. Heating may be required; if it is, use your hot water bath. Stir thoroughly, then add 2 drops 1M K2CrO4. A bright yellow precipitate confirms the presence of lead. Centrifuge if necessary.
D. Test for Copper Ion:
E. Test for Bismuth Ion:
All contents copyrighted (c) 1998
Peter Jeschofnig, Ph.D., Professor of Science, Colorado Mountain College
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