Spot Tests: Another Qualitative Analysis General Chemistry 101/102 Laboratory Manual University of North Carolina Wilmington

Spot Tests: Another Qualitative Analysis Purpose To identify several ions present in an unknown mixture: Cl- (chloride), SCN- (thiocyanate), C2H3O2- (acetate), PO43- (phosphate), CO32- (carbonate), CrO42- (chromate), SO42- (sulfate), and NH4+ (ammonium). This analysis differs from other Qualitative Analysis experiments because we are not separating the ions from the mixture, therefore interferences between ions need to be considered. Safety Considerations Do not allow acids or bases to come in contact with your skin. Avoid skin contact with silver nitrate as it will leave a dark stain for several days. Place all waste solutions in the appropriately labeled containers in the hood.

Spot Tests General Procedures Use clean glassware and deionized water to avoid contaminating your sample. Perform tests on “known” solutions and your unknown solution simultaneously. Keep in mind that your unknown is diluted compared and may be colored by the presence of several ions. Therefore, positive results might have slightly different appearances between the known and your unknown. Some spot test reactions occur slowly. If no reaction is apparent after the addition of reagent, heat the reaction mixture in a hot water bath according to instructions given. Stir well after addition of reagent using a clean stirring rod or disposable transfer pipet. Perform tests in the order given as described in the Laboratory Manual

Spot Tests Test 1: Carbonate Ion You will first test the “known” solution by placing 5 drops of the 1 M Na2CO3 solution in a micro test tube (or spot plate) and add 5 drops of 6 M HCl. The formation of bubbles indicates release of carbon dioxide gas. Repeat the test using 5 drops of your unknown and adding 5 drops of 6 M HCl (do NOT add the 1 M Na2CO3 solution as this is the “known” solution).

Spot Tests Test 2: Thiocyanate Ion This basis of the thiocyanate spot test is to identify the thiocyanate ion, SCN- , by its reaction with iron(III) to form a blood-red colored complex. Add 5 drops of 6 M HCl to 5 drops of 0.5 M KSCN (the “known”) on the spot plate. Add 1 drop of 0.1 M Fe(NO3)3 to the solution in the spot plate. The solution will turn red if SCN- is present. Repeat the test on your unknown solution without adding the “known” 0.5 M KSCN solution.

Spot Tests Test 3: Sulfate Ion Put 5 drops of the “known” 0.5 M Na2SO4 solution in a well of a spot plate. Add 5 drops of 6 M HCl to the plate. Now add 1 drop of 1 M BaCl2 to the plate. A precipitate of white BaSO4 indicates the presence of sulfate ion. Repeat the test on your unknown without using the 0.5 M Na2SO4 “known” solution.

Spot Tests Test 4: Phosphate Ion Place 5 drops of the “known” 0.5 M Na2HPO4 (sodium monohydrogen phosphate) solution in a medium test tube. Add 1 mL of 6 M HNO3. Now add 1 mL of Ammonium molybdate solution, (NH4)2MoO4 to the test tube. The presence of phosphate ions is confirmed by the formation of a yellow precipitate. This precipitate is often slow to form and heating in the hot water bath may be required to initiate precipitation. Repeat the test on your unknown.

Spot Tests Test 5: Chromate Ion Solutions containing chromate ions are yellow when neutral or basic and orange when acidic. The orange color is due to the formation of the dichromate ion (Cr2O72-). Add 5 drops of “known” 0.5 M K2CrO4 to a spot plate. Add 5 drops of 3% H2O2. Now add 10 drops of 6 M HNO3 to the plate. The formation of a blue-green color, which appears but rapidly disappears, indicates the presence of the chromate ion. Repeat this test on your unknown without adding the 0.5 M K2CrO4 “known” solution.

Spot Tests Test 6: Chloride Ion Place 5 drops of “known” 0.5 M NaCl on a spot plate. Add 5 drops of 6 M HNO3. Now add 2 drops of 0.1 M AgNO3 to the plate. The presence of chloride ion is indicated by the formation of a white precipitate of AgCl. If Test 2 indicated that your unknown contained SCN-, follow the lab manual procedure for removing the SCN- ion before you test your unknown for Cl- because SCN- also forms a white precipitate with Ag+. Repeat this test on your unknown solution. Remember not to add the “known” NaCl solution.

Spot Tests Test 7: Acetate Ion Place 5 drops of “known” 0.5 M NaC2H3O2 (sodium acetate) on a spot plate. (The acetate ion is often abbreviated as OAc-, or as CH3COO-). Add 5 drops of 3 M H2SO4 to the spot plate. The presence of acetate ion is indicated by the smell of acetic acid (a vinegar odor) coming from the plate. If no odor is immediately present, repeat the test in a micro test tube, warming it in a hot water bath for 20 seconds and check the odor again. Repeat this test on your unknown solution. Do not add the “known” NaC2H3O2 solution.

Spot Tests Test 8: Ammonium Ion This test also relies on your sense of smell to detect the presence of a substance. Add 5 drops of “known” 0.5 M NH4Cl to a spot plate or a micro test tube. Add 5 drops of 6 M NaOH to the plate and check for the presence of an ammonia odor by gentle wafting. If no odor is immediately apparent, warm the test tube and check again. As an additional test for NH4+, place a drop of the unknown on a spot plate and add a drop of Nessler’s Reagent. A reddish-brown precipitate indicates the presence of the NH4+ ion (see lower photo). Repeat the test on your unknown.

Spot Tests Interfering Ions If you suspect that an ion is interfering with a particular test, make up a “known” solution that contains the ions you think you have in our unknown and repeat the spot tests which gave you trouble. Test 3 is most likely to have interference problems from the Ba2+ ion which also forms precipitates with the chromium and phosphate ions. Test 6 is most likely to have interference problems from the Ag+ ion which forms precipitates with the phosphate, sulfate, carbonate, chromate, and thiocyanate ions. Refer to the Laboratory Manual for specific instructions of how to remove these interfering ions.