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Qualitative Analysis of Some Common Ions

Qualitative Analysis of Some Common Ions - 3 Qualitative...

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Unformatted text preview: 3 Qualitative Analysis of Some Common Ions Performance Goals 13—1 Conduct tests to confirm the presence of known ions in a solution. 13—2 Analyze an unknown solution for certain ions. CHEMICAL OVERVIEW When we analyze an unknown solution, two questions come to mind: (1) what ions are present in the solu- tion and (2) what is their concentration? The first question can be answered by performing a qualitative analy- Sis, and the second by a quantitative analysis. These two broad categories are known collectively as analytical chemistry. In this experiment, you will perform a qualitative analysis. The general approach to finding out what ions are in a solution is to test for the presence of each pos— sible component by adding a reagent that will cause that component, if present, to react in a certain way. This method involves a series of tests, one for each component, carried out on separate samples of solution. Difficulty sometimes arises, particularly in complex mixtures, because one of the species may interfere with the analytical test for another. Although interferences are common, many ions in mixtures can usually be identified by simple tests. In this experiment, you will analyze an unknown mixture that may contain one or more of the follow- ing ions in solution: (:03 or sort 5043’ 1903’ Cu2+ A13+ First, you will perform the various tests designed to detect the presence of individual ions. Once you have observed these Specific reactions, you will obtain the unknown solution from your instructor. Then, taking small portions of this solution, you will run each reaction again to determine which ions are present and which are absent. This experiment is designed to test the behavior of only a few ions. More complex schemes are used for a more complete qualitative analysis. SAFETY PRECAUTIONS AND DISPOSAL METHODS In some tests you will be required to use fairly concentrated acids and bases. When in contact with skin, most of these chemicals cause severe burns if not removed promptly. Wear goggles when working with any of the reagents required in this experiment. Discard all solutions in the container provided. 22 Qualitative Analysis OF Some Common Ions PROCEDURE A boiling water bath is required for some of the tests you are to perform. Pour about 100 mL of deionized water into a lSO-mL beaker and heat it to boiling. Maintain it at that temperature throughout the experiment, replenishing the water from time to time as it becomes necessary. 1. TEST FOR THE CARBONATE ION. c032“ Cautiously add about 10 drops of 1 M HCl to 10 drops of 1 M NaZCO3 in a small—size test tube. Bubbles of a colorless and odorless gas, carbon dioxide, usually appear immediately in the presence of the carbonate ion. If the bubbles are not readily apparent, warm the solution in the hot-water bath and stir. 2. TEST FOR THE SULFATE ION, 5042' Cautiously add about 10 drops Of l M HCl to 10 drops of 0.5 M Na2804. Then add 3 to 4 drops of 1 M BaClz. A white, powdery precipitate of BaSO4 indicates the presence of 8042* ions in the sample. 3. TEST FOR PHOSPHATE ION. POf‘ Add 1 M HNO3 tO 10 drops of 0.5 M Na3PO4 until the solution is acidic. (Test by dipping a stirring rod into the solution and touching the wet rod to a strip of blue litmus paper. The solution is acidic if the color changes to red.) Then add 5 drops of 0.5 M (NH4)2MOO4 and heat the test tube in a hot—water bath. A powdery, light yellow precipitate indicates the presence of P0430 ions. Caution: The molybdate solution is yellow. Be sure you see a precipitate before concluding that P043“ is present. 4. TEST FOR THIOCYANATE ION. SCN‘ Add 10 drops of 3 M HC2H302 to about 10 drops of 0.5 M KSCN and stir with a glass rod. Add 1 or 2 drops of 0.1 M Fe(NO3)3. A deep red color formation is proof of the presence of SCN ions. 5. TEST FOR CHLORIDE ION, CI— A. Add 5 drops of 1 M HNO3 to about 10 drops of 0.5 M NaCl. Add 2 or 3 drops of 0.1 M AgNO3. A white precipitate ongCl confirms the presence Of chloride ion. B. If thiocyanate ion is present, it will interfere with this test, since it also forms a white precipitate with AgNO3. If the sample contains SCN ion, put 10 drops of the solution in a medium-size test tube and add 10 drops of 1 M HNO3. Boil the solution gently until the volume is reduced to half. This procedure will oxidize the thiocyanate and remove the interference. Then perform the chloride ion test as previously explained. ' 5. TEST FOR ALUMINUM ION. A|3+ Add 1 M NH3 dropwise to about 10 drops of 0.5 M A1C13 until the solution is basic (red litmus turns blue). A white, gelatinous precipitate, Al(OH)3, will form. Add 3 M HC2H302 dropwise until the solid dissolves. Stir and add 2 drops of cathecol violet reagent. A blue solution indicates the presence of A13+ ions. 7. TEST FOR THE COPPER(II) ION, Cu2+ Add concentrated ammonia, NH3, drop by drop to about 10 drops of 0.5 M CuSO4. The development Of a dark blue color is proof Of the presence Of copper(II) ion. Qualitative Analysis of Some Common Ions 23 8. ANALYSIS OF AN UNKNOWN When you have completed all of the tests, obtain an unknown from your instructor. Be sure to record the unknown number. Analyze your solution by using 10-drop portions of the unknown and then applying the various tests to separate portions. Substitute the unknown for the compound that contains the ion you are testing for. For example, if you are testing for the C0327 ion, do not add NaZCO3; use your unknown in its place. The unknown will contain more than one ion, so your test for a given ion may be affected by the pres- ence of others (see Step 5B). 24 Qualitative Analysis of Some Common Ions Qualitative Analysis of Some Common Ions 25 Experiment 13 NAME ........................................ Advance Study Assignment DATE ................ SECTION ............ 1. An unknown that might contain any of the seven ions studied in this experiment (but no other ions) has the following properties: a) On addition of I M HCI, bubbles form. b) When 0.1 M BaCl2 is added to the acidified unknown, a white precipitate results. (3) When 0.1 M AgNO3 is added to the unknown, a clear solution results. On the basis of the preceding information, classify each of the following ions as present (P), absent (A), or undetermined (U) by the tests described: 0032‘ _; 8042‘ ___; P043“ ; scrxr ; Cl” __; A13+ _; (3112* ; OPTIONAL ASSIGNMENT Write net ionic equations for the reactions in this experiment in which the following ions are detected: a) (3032‘: W b) 3042‘: _——__**___* 0) Cl‘: W 26 Qualitative Analysis ofSome Common Ions Experiment 13 Work Page Unknown number Qualitative Analysis of Some Common Ions NAME Ions present 28 Qualitative Analysis of Some Common Ions Experiment 13 Report Sheet Unknown number Qualitative Analysis ofSnme Common Ions NAME Ions present 29 50 Qualitative Analysis of Some Common Ions ...
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