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EXPERIMENT C5: QUALITATIVE ANALYSIS OF ANIONS
Learning Outcomes
Upon completion of this lab, the student will be able to:
- Analyze a given sample of an ionic compound and identify which of the following anions is present: chloride, bromide, iodide, carbonate, sulfate, phosphate, oxalate, or nitrate
Introduction
The chemical identity of a given sample must often be analyzed in various circumstances. In the case of ionic compounds, the identification involves tests to determine the exact cation and anion in the compound. These analyses are routinely conducted in medical (blood/urine tests), environmental (water/sewer/waste), pharmaceutical (drugs), and chemical research laboratories. While the exact analytical technique may be different from that described here, the basic principles of these analyses are often the same. In this experiment, the identification of some common anions will be explored. The underlying principle of this analysis has to do with the solubility properties of ionic compounds. As has been previously discussed several ionic substances are only sparingly soluble in water. The solubility of a particular ionic compound depends on the solubility product constant. The solubility rules are a good guideline indicating the solubility of anions. An abbreviated set of these rules is given in Table 1 below. Rule Compounds containing Exceptions 1 Group 1A cations Soluble None 2 NH 4 +^ Soluble None 3 NO 3 −^ Soluble None 4 CH 3 COO−^ Soluble None 5 Cl−, Br−, I−, Soluble If the cation is: Ag+, Pb2+, Cu+, Hg 2 2+ 6 SO 42 -^ Soluble If the cation is: Ca2+, Ba2+, Sr2+, Ag+, Pb2+ 7 OH−^ Insoluble Rule 1, 2 & if the cation is Ca2+, Ba2+, Sr2+ 8 CO 32 - , PO 43 -^ Insoluble Rule 1, 2 TABLE 1 The differences in the solubility of ions and the characteristic colors of their precipitates will be used to identify the anions. The anions explored in this experiment are: chloride (Cl−), bromide (Br−), iodide (I−), carbonate(CO 32 −), phosphate (PO 43 −), sulfate (SO 42 −), oxalate (C 2 O 42 −), and nitrate (NO 3 −). The tests for
the identity of an anion is usually twofold: 1) an initial reaction to predict the possibility of a particular ion and 2) a confirmatory test specific to the particular anion in question. The following flow-chart is a depiction of the tests that will establish the identity of an anion. Cl-, Br-, I-, CO 3 2-, SO 4 2-, PO 4 3-, C 2 O 4 2-, NO 3 - Add 6M HNO 3 Gas formation: CO 3 2-^ present Add 0.1M BaCl 2 White precipitate: Con^irms CO 3 2- No gas formation: Cl-, Br-, I-, SO 4 2-, PO 4 3-, C 2 O 4 2-, NO 3 - Add 0.1M AgNO 3 Precipitate:Cl- (white), Br- (pale yellow), I- (yellow) Add 6M NH 3 Precipitate dissolves: Cl- Add 6M HNO 3 White precipitate: Con^irms Cl- Precipitate does not dissolve: Br-, I- Add Fe3+, followed by hexane Reddish brown color: Con^irms Br- Purple color: Con^irms I- No Precipitate: SO 4 2-, PO 4 3-, C 2 O 4 2-, NO 3 - Add 1M BaCl 2 White precipitate: Con^irms SO 4 2- Add 0.4 M (NH 4 ) 2 MoO 4 Yellow precipitate: Con^irms PO 4 3- Add 1M FeSO 4
- H 2 SO 4 Brown ring: Con^irms NO 3 -
Bromide is a strong reducing agent. It will become oxidized to bromine in a red-ox reaction with Fe3+. The reddish brown color of the bromine may be observed in a layer of hexane. 2Br−(aq) + 2Fe3+(aq)! Br 2 (l) + 2 Fe2+(aq) Test for iodide (I−) According to Table 1, silver salts of bromide ions are insoluble. Ag+(aq) + I−(aq)! AgI(s) Iodide is a strong reducing agent. It will become oxidized to iodine in a red-ox reaction with Fe3+. The purple color of the iodine may be observed in a layer of hexane. 2I−(aq) + 2Fe3+(aq)! I2(s) + 2 Fe2+(aq) Test for sulfate (SO 42 - ) The sulfate ion is the conjugate base of a strong acid (sulfuric acid). As a result, a precipitate of sulfate is insoluble in strong acids. According to Table 1, sulfate salts are generally soluble except when the cation is Ca2+, Ba2+, Sr2+, Ag+, Pb2+. When aqueous barium chloride is added to an acidified solution containing sulfate ions, an insoluble precipitate of BaSO 4 results. This precipitate is white in color and confirms the presence of sulfate. Ba2+(aq) + SO 42 - (aq)! BaSO4(s) The remaining anions at this point are phosphate, oxalate, and nitrate (refer to flow- chart above). Both phosphate and oxalate are conjugate bases of weak acids (phosphoric acid and oxalic acid, respectively) and therefore their precipitates will dissolve in a strong acid. Ba 3 (PO 4 )2(s) + 2H+(aq)! 3Ba2+(aq) + 2HPO 42 - (aq) Ba(C 2 O 4 )(s) + H+(aq)! Ba2+(aq) + HC 2 O 4 - (aq) Since nitrates are always soluble (refer to Table 1), the precipitate formed from the addition of aqueous barium chloride cannot be nitrate. Test for phosphate (PO 43 - ) Addition of aqueous ammonium molybdate to an aqueous solution containing phosphate ions results in a yellow precipitate. Other anions may also result in a precipitate with this reagent, but those precipitates are white in color.
2 PO 43 - (aq) + 6 H+(aq) + 3 (NH 4 ) 2 MoO4(aq)! 2(NH 4 ) 3 PO4(aq) + 3MoO3(s) + 3H 2 O(l) Test for nitrate (NO 3 −) Nitrate is reduced to nitric oxide in the presence of Fe2+^ in an acidic medium. NO 3 −(aq) + Fe2+(aq) + H+(aq)! Fe3+(aq) + NO + H 2 O(l) NO + Fe2+(aq)! [FeNO]2+ The nitric oxide forms a complex with the excess Fe2+. This appears as a brown ring at the interface of the concentrated acid layer and the aqueous medium containing Fe2+.
Procedure
PART 1: ANALYSIS OF KNOWN SAMPLES
CARBONATE
- Add 6 M nitric acid to a pinch of the solid carbonate salt. Record the observation.
- Dissolve a pinch of the solid carbonate in a small amount of (~10 mL) deionized water.
- In a small test tube, combine 10 drops of the above solution (from Step 2) with five drops of 0.1 M BaCl 2. Record the observations. HALIDES
- In three separate labeled test tubes, dissolve a pinch of the respective halide salt in a small amount of (~10 mL) deionized water.
- In small test tubes, combine 10 drops of the test solution (each of the three solutions from step 4 above) with five drops of 6 M HNO 3.
- Check the pH of the acidified test solution from Step 5 to ensure that the solution is acidic and add more HNO 3 if needed. Add five drops of 0.1 M AgNO 3 to the acidified test solutions. Record the observations.
- Chloride test: Centrifuge the mixture containing the white precipitate from Step
- Discard the supernatant. Add 10 drops of deionized water and two drops of 6 M NH 3 to dissolve the precipitate. The precipitate only dissolves if it is a silver chloride precipitate. To the dissolved solution, add 6 M HNO 3 until the solution is acidic. The reappearance of a precipitate confirms the presence of chloride ions.
- Bromide/Iodide test: If the precipitate in Step 6 is pale yellow or darker yellow, it likely contains a bromide or iodide. Combine 10 drops of the test solution with five drops of 0.1 M Fe3+, followed by 6-8 drops of hexane. a. Bromide: appearance of reddish brown color in the hexane layer confirms the presence of bromide ions. b. Iodide: appearance of purple color in the hexane layer confirms the presence of iodide ions. SULFATE
- Dissolve a pinch of the sulfate salt in a small amount of (~10 mL) deionized water.
- To five drops of the above solution from Step 9, add two drops of 6 M HCl followed by two drops of 0.1 M BaCl 2. Record the observations. PHOSPHATE
- Dissolve a pinch of the phosphate salt in a small amount of (~10 mL) deionized water.
- Take 10 drops of the above solution (from Step 11) and add five drops of 6 M HNO 3 and five drops of 0. 4 M (NH 4 ) 2 MoO 4. Record the observations. NITRATE
- Dissolve a pinch of the nitrate salt in a small amount of (~10 mL) deionized water.
- Add five drops of 1 M FeSO 4 solution to 10 drops of the above solution (from Step 13) and mix thoroughly. Add 2-3 drops of 3 M H 2 SO 4 to this solution until it is acidic. Incline the test tube at a 45° angle and gently add 1 mL of concentrated H 2 SO 4 using a dropper along the sides of the test tube. Make sure that the contents of the test tube do not mix. Allow the test tube to stand for a few minutes and look for the appearance of a brown ring at the interface of the sulfuric acid layer (the bottom) and the test solution layer. Record the observations.
- Phosphate test: Take 10 drops of the original test solution (from Step 1) and add five drops of 6 M HNO 3 and five drops of 0. 4 M (NH 4 ) 2 MoO 4. Formation of a yellow precipitate confirms the presence of phosphate. All the other anions form a white precipitate.
- Nitrate test: Add five drops of 1 M FeSO 4 solution to 10 drops of the original test solution (from Step 1) and mix thoroughly. Add 2 - 3 drops of 3 M H 2 SO 4 to this solution until it is acidic. Incline the test tube at a 45° angle and gently add 1 mL of concentrated H 2 SO 4 using a dropper along the sides of the test tube. Make sure that the contents of the test tube do not mix. Allow the test tube to stand for a few minutes and look for the appearance of a brown ring at the interface of the sulfuric acid layer (the bottom) and the test solution layer. Formation of the brown ring confirms the presence of nitrate ions in the original solution.
- Oxalate test: The presence of oxalate in the unknown is determined by exclusion. If the unknown tests negative to all the above confirmatory tests, then based on the process of elimination, the unknown must be oxalate.
Data Table
PART 1: ANALYSIS OF KNOWN SAMPLES
Step Experiment Observation 1 Add 6 M nitric acid to a pinch of the solid carbonate salt 3 Carbonate + 0.1 M BaCl 2 6 Chloride + HNO 3 + 0.1 M AgNO 3 6 Bromide + HNO 3 + 0.1 M AgNO 3 6 Iodide + HNO 3 + 0.1 M AgNO 3 7 Chloride test 8a Bromide test 8b Iodide test 10 Sulfate + 6 M HCl followed by two drops of 0.1 M BaCl 2 12 Phosphate + five drops of 6 M HNO 3 and five drops of 0. 4 M (NH 4 ) 2 MoO 4 14 Nitrate + 1 M FeSO 4 + 3 M H 2 SO 4 + concentrated H 2 SO 4
Results
List the relevant observations and the identity of the unknown below: Unknown Number Relevant Observations Identify Name Formula 1 2 3
