CHM 101/102CHM 101/102

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Spot TestsSpot Tests: : Another Qualitative AnalysisAnother Qualitative Analysis

General Chemistry 101/102General Chemistry 101/102Laboratory ManualLaboratory Manual

University of North Carolina WilmingtonUniversity of North Carolina Wilmington

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Spot Tests: Spot Tests: Another Qualitative AnalysisAnother Qualitative Analysis• PurposePurpose

To identify several ions present in an unknown mixture: To identify several ions present in an unknown mixture: ClCl-- (chloride), (chloride), SCN SCN-- (thiocyanate),(thiocyanate), C C22HH33OO22

-- (acetate),(acetate),

POPO443-3- (phosphate),(phosphate), CO CO33

2-2- (carbonate),(carbonate), CrO CrO442-2- (chromate),(chromate),

SOSO442-2- (sulfate), and(sulfate), and NH NH44

++ (ammonium). (ammonium).

This analysis differs from other Qualitative Analysis experiments because we This analysis differs from other Qualitative Analysis experiments because we are not separating the ions from the mixture, therefore interferences between are not separating the ions from the mixture, therefore interferences between ions need to be considered.ions need to be considered.

• Safety ConsiderationsSafety Considerations

Do not allow acids or bases to come in contact with your Do not allow acids or bases to come in contact with your skin. Avoid skin contact with silver nitrate as it will leave a skin. Avoid skin contact with silver nitrate as it will leave a dark stain for several days.dark stain for several days.

Place all waste solutions in the appropriately labeled Place all waste solutions in the appropriately labeled containers in the hood.containers in the hood.

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Spot TestsSpot Tests• General ProceduresGeneral Procedures

Use clean glassware and deionized water to avoid contaminating your sample.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 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.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 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.given. Stir well after addition of reagent using a clean stirring rod or disposable transfer pipet.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 ManualPerform tests in the order given as described in the Laboratory Manual

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Spot TestsSpot Tests

• Test 1: Carbonate IonTest 1: Carbonate Ion

• You will first test the “known” You will first test the “known” solution by placing 5 drops of solution by placing 5 drops of the 1 M Nathe 1 M Na22COCO33 solution in a solution in a micro test tube (or spot plate) micro test tube (or spot plate) and add 5 drops of 6 M HCl. The and add 5 drops of 6 M HCl. The formation of bubbles indicates formation of bubbles indicates release of carbon dioxide gas.release of carbon dioxide gas.

• Repeat the test using 5 drops of Repeat the test using 5 drops of your unknown and adding 5 your unknown and adding 5 drops of 6 M HCl (do NOT add drops of 6 M HCl (do NOT add the 1 M Nathe 1 M Na22COCO33 solution as this solution as this is the “known” solution).is the “known” solution).

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Spot TestsSpot Tests• Test 2: Thiocyanate IonTest 2: Thiocyanate Ion

• This basis of the thiocyanate spot This basis of the thiocyanate spot test is to identify the thiocyanate test is to identify the thiocyanate ion, SCNion, SCN-- , by its reaction with , by its reaction with iron(III) to form a blood-red colored iron(III) to form a blood-red colored complex. Add 5 drops of 6 M HCl complex. Add 5 drops of 6 M HCl to 5 drops of 0.5 M KSCN (the to 5 drops of 0.5 M KSCN (the “known”) on the spot plate. Add 1 “known”) on the spot plate. Add 1 drop of 0.1 M Fe(NOdrop of 0.1 M Fe(NO33))33 to the to the solution in the spot plate. The solution in the spot plate. The solution will turn red if SCN- is solution will turn red if SCN- is present. present.

• Repeat the test on your unknown Repeat the test on your unknown solution solution withoutwithout adding the adding the “known” 0.5 M KSCN solution.“known” 0.5 M KSCN solution.

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Spot TestsSpot Tests• Test 3: Sulfate IonTest 3: Sulfate Ion

• Put 5 drops of the “known” Put 5 drops of the “known” 0.5 M Na0.5 M Na22SOSO44 solution in a solution in a

well of a spot plate. Add 5 well of a spot plate. Add 5 drops of 6 M HCl to the plate. drops of 6 M HCl to the plate. Now add 1 drop of 1 M BaCl Now add 1 drop of 1 M BaCl22

to the plate. A precipitate of to the plate. A precipitate of white BaSOwhite BaSO44 indicates the indicates the

presence of sulfate ion. presence of sulfate ion.

• Repeat the test on your Repeat the test on your unknown unknown withoutwithout using the using the 0.5 M Na0.5 M Na22SOSO44 “known” “known”

solution. solution.

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Spot TestsSpot Tests• Test 4: Phosphate IonTest 4: Phosphate Ion

• Place 5 drops of the “known” 0.5 M Place 5 drops of the “known” 0.5 M NaNa22HPO4 (sodium monohydrogen HPO4 (sodium monohydrogen phosphate) solution in a medium test phosphate) solution in a medium test tube. Add 1 mL of 6 M HNOtube. Add 1 mL of 6 M HNO33. Now . Now add 1 mL of Ammonium molybdate add 1 mL of Ammonium molybdate solution, (NHsolution, (NH44))22MoOMoO44 to the test tube. to the test tube. The presence of phosphate ions is The presence of phosphate ions is confirmed by the formation of a confirmed by the formation of a yellow precipitate.yellow precipitate.

• This precipitate is often slow to form This precipitate is often slow to form and heating in the hot water bath may and heating in the hot water bath may be required to initiate precipitation. be required to initiate precipitation.

• Repeat the test on your unknown. Repeat the test on your unknown.

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Spot TestsSpot Tests• Test 5: Chromate IonTest 5: Chromate Ion

• Solutions containing chromate ions Solutions containing chromate ions are yellow when neutral or basic and are yellow when neutral or basic and orange when acidic. The orange orange when acidic. The orange color is due to the formation of the color is due to the formation of the dichromate ion (Crdichromate ion (Cr22OO77

2-2-). ).

• Add 5 drops of “known” 0.5 M KAdd 5 drops of “known” 0.5 M K22CrOCrO44 to a spot plate. Add 5 drops of 3% to a spot plate. Add 5 drops of 3% HH22OO22. Now add 10 drops of 6 M HNO. Now add 10 drops of 6 M HNO33 to the plate. The formation of a blue-to the plate. The formation of a blue-green color, which appears but green color, which appears but rapidly disappears, indicates the rapidly disappears, indicates the presence of the chromate ion. presence of the chromate ion.

• Repeat this test on your unknown Repeat this test on your unknown withoutwithout adding the 0.5 M K adding the 0.5 M K22CrOCrO44 “known” solution.“known” solution.

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Spot TestsSpot Tests• Test 6: Chloride IonTest 6: Chloride Ion

• Place 5 drops of “known” 0.5 M NaCl Place 5 drops of “known” 0.5 M NaCl on a spot plate. Add 5 drops of 6 M on a spot plate. Add 5 drops of 6 M HNOHNO33. Now add 2 drops of 0.1 M . Now add 2 drops of 0.1 M AgNOAgNO33 to the plate. The presence of to the plate. The presence of chloride ion is indicated by the chloride ion is indicated by the formation of a white precipitate of formation of a white precipitate of AgCl. AgCl.

• If Test 2 indicated that your unknown If Test 2 indicated that your unknown contained SCNcontained SCN--, follow the lab , follow the lab manual procedure for removing the manual procedure for removing the SCNSCN-- ion before you test your ion before you test your unknown for Clunknown for Cl-- because SCN because SCN-- also also forms a white precipitate with Agforms a white precipitate with Ag++. .

• Repeat this test on your unknown Repeat this test on your unknown solution. Remember solution. Remember notnot to add the to add the “known” NaCl solution.“known” NaCl solution.

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Spot TestsSpot Tests• Test 7: Acetate IonTest 7: Acetate Ion

• Place 5 drops of “known” 0.5 M Place 5 drops of “known” 0.5 M NaCNaC22HH33OO22 (sodium acetate) on a spot (sodium acetate) on a spot plate. (The acetate ion is often plate. (The acetate ion is often abbreviated as OAcabbreviated as OAc--, or as CH, or as CH33COOCOO--). ). Add 5 drops of 3 M HAdd 5 drops of 3 M H22SOSO44 to the spot to the spot plate. The presence of acetate ion is plate. The presence of acetate ion is indicated by the smell of acetic acid indicated by the smell of acetic acid (a vinegar odor) coming from the (a vinegar odor) coming from the plate. If no odor is immediately plate. If no odor is immediately present, repeat the test in a micro present, repeat the test in a micro test tube, warming it in a hot water test tube, warming it in a hot water bath for 20 seconds and check the bath for 20 seconds and check the odor again. odor again.

• Repeat this test on your unknown Repeat this test on your unknown solution. Do solution. Do notnot add the “known” add the “known” NaCNaC22HH33OO2 2 solution.solution.

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Spot TestsSpot Tests• Test 8: Ammonium IonTest 8: Ammonium Ion

• This test also relies on your sense of This test also relies on your sense of smell to detect the presence of a smell to detect the presence of a substance. Add 5 drops of “known” 0.5 M substance. Add 5 drops of “known” 0.5 M NHNH44Cl to a spot plate or a micro test tube. Cl to a spot plate or a micro test tube.

Add 5 drops of 6 M NaOH to the plate and Add 5 drops of 6 M NaOH to the plate and check for the presence of an ammonia check for the presence of an ammonia odor by gentle wafting. If no odor is odor by gentle wafting. If no odor is immediately apparent, warm the test tube immediately apparent, warm the test tube and check again. and check again.

• As an additional test for NHAs an additional test for NH44++, place a drop , place a drop

of the unknown on a spot plate and add a of the unknown on a spot plate and add a drop of drop of Nessler’s ReagentNessler’s Reagent. A reddish-. A reddish-brown precipitate indicates the presence brown precipitate indicates the presence of the NHof the NH44

++ ion (see lower photo). ion (see lower photo).

• Repeat the test on your unknown. Repeat the test on your unknown.

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Spot TestsSpot Tests• Interfering IonsInterfering Ions• If you suspect that an ion is interfering with a particular test, If you suspect that an ion is interfering with a particular test,

make up a “known” solution that contains the ions you think make up a “known” solution that contains the ions you think you have in our unknown and repeat the spot tests which you have in our unknown and repeat the spot tests which gave you trouble.gave you trouble.

• Test 3 is most likely to have interference problems from the Test 3 is most likely to have interference problems from the BaBa2+ 2+ ion which also forms precipitates with the chromium and ion which also forms precipitates with the chromium and phosphate ions.phosphate ions.

• Test 6 is most likely to have interference problems from the Test 6 is most likely to have interference problems from the AgAg+ + ion which forms precipitates with the phosphate, sulfate, ion which forms precipitates with the phosphate, sulfate, carbonate, chromate, and thiocyanate ions. carbonate, chromate, and thiocyanate ions.

• Refer to the Laboratory Manual for specific instructions of Refer to the Laboratory Manual for specific instructions of how to remove these interfering ions.how to remove these interfering ions.