Experiment No. 43 – Qualitative Analysis of the Group I Cations Chemistry 1412 – General Chemistry II South Texas Community College Spring Semester 2004 For: Professor Alberto A. Alvarez, Jr. By: Raul Mercado, Jr. Objective: A sample containing only the Group I cations will be analyzed for the presence of silver(I), mercury(I), and lead(II) ions. Summary of Procedures: A. Precipitation of the Group I Cations i. Transfer one mL of the sample solution into a test tube and add 10 drops of 6 M HCl. ii. Stir then centrifuge until firmly packed into the bottom. iii. Add one more drop of HCl to cause the precipitation of all the Group I cations, watching the supernatant liquid in the test tube for the appearance of more precipitate. iv. If more precipitate forms on the addition of one drop of HCl, recentrifuge and retest with addition single drops of HCl until it is certain that all the Group I chlorides have been precipitated. v. For Group I known, discard the centrifugate. If doing unknown samples, do not discard, keep for further possible analysis. vi. Wash the silver/mercury/lead precipitate in the centrifuge tube by adding 1 mL of water and two drops of 6 M Hcl. Stir with a glass rod and centrifuge. vii. Remove the wash water and discard. Repeat step five and again discard the wash liquid. B. Separation and Confirmation of Lead Ion i. Add 2-3 mL of distilled water to the precipitate from Part A. ii. Heat the mixture in a boiling water bath for 3-4 minutes and stir iii. Centrifuge and remove the centrifugate to a separate test tube using a dropper. iv. To the centrifugate containing lead ions, add 2-3 drops of 6 M acetic acid and 5- 6 drops of potassium chromate solution. Allow the sample to stand for a few minutes. A yellow precipitate of lead chromate will appear. C. Separation and Confirmation of Mercury(I) and Silver Ions i. To the remaining Group I precipitate, add 2-3 mL of hot water and 1-2 drops of 6 M HCl. ii. Discard the supernatant liquid and add a 2-3 mL portion of hot water and 1-2 drops of 6 M HCl. iii. Stir the mixture and heat in the hot water bath. Centrifuge, remove, and discard the supernatant liquid. These several washing steps are necessary to remove all traces of lead ion from the silver/mercury precipitate. iv. Add 2-3 mL of 6 M aqueous ammonia to the silver/mercury precipitate. v. Stir the mixture, centrifuge, and remove the centrifugate which contains dissolved silver ions. The presence of a gray-black precipitate in the centrifuge tube at this point confirms the presence of mercury(I) in the original sample. vi. Divide the centrifugate containing complexed silver ion into two portions in separate clean test tubes. vii. To one portion, add sufficient 6 M nitric acid to make the solutino acidic when tested with pH paper. The appearance of a white precipitate of AgCl confirms the presence of silver ions in the original sample. viii. Add a few crystals of potassium iodide to the second portion of complexed silver ion. The appearance of a creamy yellow precipitate serves as an additional confirmation of silver ion. Pre-Laboratory Questions: 1. The separation of lead chloride from the chlorides of silver and mercury(I) is based on the differing solubilities of these substances in cold and hot water. Use a handbook of chemical data to find the specific solubilities (in g/100 g H2O) in both cold and hot water for AgCl, PbCl2, and Hg2Cl2. Cold Water Hot Water Reference Handbook of Chemistry and Silver chloride 0.00084 10/100 g H2O 0.0021 100/100 g H2O Physics 50th Edition; Pg. B-159, B-121, B-129 Handbook of Chemistry and Lead(II) chloride 0.99 20/100 g H2O 3.34 100/100 g H2O Physics 50th Edition; Pg. B-159, B-121, B-129 Handbook of Chemistry and Mercury(I) chloride 0.00020 25/100 g H2O 0.001 43/100 g H2O Physics 50th Edition; Pg. B-159, B-121, B-129 2. Because a qualitative analysis may often consist of a rather long series of manipulations, precipitations, centrifugations, and separations, flow charts are often prepared to summarize graphically the steps to be undertaken. For this analysis of Group I, prepare a simple flow chart showing what ions are present at each point in the procedure, what reagents are to be added, and the result to be expected at each point. One mL Solution of Ag+, Hg22+, Pb2+ Add one drop of HCl and if more Add 10 drops precipitate forms, recentrifuge and of 6M HCL test with m ore drops of HCl. Centrifuge Precipitate of AgCl(s), Hg2Cl2(s), PbCl2(s) Discard centrifugate Wash the precipitate by adding one mL Add 2-3 mL water of water and two drop sof 6 M HCl. Stir Heat and centrifuge with glass rod and centrifuge. Precipitate of Solution of Pb2+ AgCl(s), Hg2Cl2(s) Add 2-3 drops of 5-6 drops Add 2-3 mL of water 6M acetic acid K2CrO4 Add 1-2 drops of 6M HCl Heat 3-4 minutes and centrifuge Precipitate of Clean precipitate of PbCrO4(s) AgCl(s), Hg2Cl2(s) (yellow) Add 2-3 6M NH3(aq) Centrifuge and remove centrifugate Precipitate of Hg(l) (black) Centrifugate with HgNH2Cl(s) (white) Centrifugate Ag+, Cl- ½ solution ½ solution ½ Centrifugate ½ Centrifugate with Ag+, Cl- with Ag+, Cl- Add 6M nitric acid Add KI crystals until acidic Precipitate of Precipitate of AgCl(s) (white) AgK2 (yellow) Results/Observations: On adding HCl to the initial sample (Part A) Group I known A white precipitate which confirms the presence of silver, mercury, and lead ions in the original sample appears. Unknown A white precipitate which confirms the presence of silver, mercury, and lead ions in the original sample appears. On adding potassium chromate (test for lead, Part B) Group I known A yellow precipitate which confirms the presence of lead ions in the original sample appears. Unknown A yellow precipitate which confirms the presence of lead ions in the original sample appears. On adding aqueous ammonia (Part C) Group I known A gray-black precipitate which confirms the presence of mercury(I) in the original sample appears. Unknown A gray-black precipitate which confirms the presence of mercury(I) in the original sample appears. On reacidifying with dilute nitric acid Group I known A white precipitate of AgCl which confirms the presence of silver ions in the original sample appears. Solution gets cold, thus, is said to be exothermic. Unknown A white precipitate of AgCl which confirms the presence of silver ions in the original sample appears. Solution gets cold, thus, is said to be exothermic. On adding potassium iodide Group I known A creamy yellow precipitate which again confirms the presence of silver ions in the original sample appears. Solution gets hot, thus, is said to be endothermic. Unknown A creamy yellow precipitate which again confirms the presence of silver ions in the original sample appears. Solution gets hot, thus, is said to be endothermic. Based on your results, which Group I cations are present in your unknown sample? Ions Silver (Ag+), mercury (Hg2+2), and lead (Pb+2) Unknown code number #36 Post-Lab Questions: 1. The analysis of Group II (which follows in a later experiment) involves precipitation of certain cations as the insoluble sulfides. Why are the Group I cations removed as the insoluble chlorides, rather than as the sulfides? The Group I cations are removed as the insoluble chlorides rather than as the sulfides because the ions present in the Group I cations can easily combine with chloride to form precipitates which can in turn easily prove the presense of any of these ions with just observing the color of the precipitate. If sulfide were used to prove the presence of silver, mercury, and lead ions in Group I instead of chloride, it would be much more difficult to prove these ions are there because the sulfide would not combine with these ions to create a precipitate. 2. Why is HCl added on a drop-by-drop basis to ensure complete precipitation of Group I chlorides initially, rather than just added all at once? HCl is added on a drop-by-drop basis to ensure complete precipitation of Group I chlorides initially because if it was to be added all at once, there would be a very high chance of losing many of the ions upon discarding the centrifugate, giving us a much less effective qualitative analysis of the Group I cations. 3. Why is acetic acid added before testing for lead with chromate ion? Acetic acid is added before testing for lead with chromate ion because if it were to be added after the chromate ion, one would not be able to see the change that would occur in the test tube because of the cloudy addition of the chromate. Furthermore, if the chromate ion would have been added before the acetic acid, there is a possibility of different unwanted reactions or changes occuring. 4. Why is the silver/mercury precipitate washed to remove lead ion before addition of aqueous ammonia? The silver/mercury precipitate is washed for traces of lead ion before the addition of aqueous ammonia to get rid of lead ion so that upon the addition of the ammonia, the results of the reaction between it and the precipitate will be more accurate than if there were to be lead ions present. The presense of lead ions in this step would cause the experiment to be less effective. 5. Why must a centrifuge always be balanced during use? A centrifuge must always be balanced during using to prevent excessive vibration and cause the precipitate to not be centrifuged properly as it should. 6. What is the identity of the creamy yellow solid produced when potassium iodide is added to a portion of the final centrifugate? The creamy yellow solid produced when potassium iodide is added to a portion of the final centrifugate identifies silver chloride (AgCl).