Separation and Qualitative Determination of Cations and Anions Instructor’s Guide
Master Materials List (complete for 24 students working in pairs) Silver nitrate, AgNO 3, 0.2 M, 100 mL* Silver nitrate, AgNO 3, 0.1 M, 100 mL* Cupric nitrate, Cu(No 3)2, .02 M, 100 mL* Zinc nitrate, Zn(NO 3)2, 0.2 M, 100 mL* Ferric nitrate, Zn(NO 3)2, 0.2 M, 100 mL* Hydrochloric acid, HCl, 6 M, 125 mL* Nitric acid, HNO3, 6 M, 125 mL* Ammonia, NH3, 6 M, 125 mL* Sulfuric acid, H 2SO4, 6 M, 125 mL* Acetic acid, CH 3COOH, 6 M, 125 mL* Sodium chloride, NaCl, 0.2 M, 100 mL*
Sodium carbonate, Na 2CO3, 0.2 M, 100 mL* Sodium hydroxide, 6 M, 100 mL* Sodium sulfate, Na 2SO4, 0.2 M, 100 mL* Sodium nitrate, NaNO 3, 0.2 M, 100 mL* Barium chloride, BaCl 2, 0.1 M, 100 mL* Barium hydroxide, Ba(OH) 2, 5 g* Potassium thiocynate, KSCN, 0.1 M, 100 mL* Potassium ferrocyanide, K 4Fe(CN)6, 0.1 M, 140 mL* Aluminum granules, Al, 50 g* Beral-type pipets, 120*
*Materials included in kit
Additional Materials Needed Beakers, 400-mL, 12 Beakers, 250-mL, 5-14 Centrifuges, 3 Dropper bottles, 15-30 mL, 96 Hot plates, 3-12 Litmus paper or pH paper Microscope slides or watch glasses, 12
Spatulas, 12 Stirring rods, 12 Stoppers to fit test tubes, 72 Storage bottle, with cap, 125 mL Test tubes, 13 X 100 mm, 72 Wire test tube holders, 12
Preparation of Solutions (for 12 student groups) Note: 0.2 M solutions solutions are provided so unknown unknown solutions can be easily made. The 0.2 M solutions solutions are also easier to ship ship and take up less room. Dilution the 0.2 M solutions to 0.05 M solutions does not have to quantitative. quantitative. Use the markings on the beaker for measurements.
Solution of Four Cations Prepare 100 mL of a 0.05 M solution of all the cations to be tested by adding 25 mL of each of the following solutions to a clean 250-mL beaker. 0.2 M silver nitrate, AgNO 3 0.2 M cupric nitrate, Cu(NO) 3 0.2 M zinc nitrate, Zn(NO 3)2 0.2 M ferric nitrate, Fe(NO 3)3
Mix the solution, dispense in 12 clean 13 X 100 mm test tubes, and a stopper the test tubes. Label as known cation solution.
Unknown Cation Solutions Prepare unknown cation solutions for the students to analyze from the four 0.2 M stock solutions of cations. Any combination of these four cations may be prepared. Using a graduated Beral-type pipet, add 2 mL of each ion selected for the unknown to a clean 13 X 100 mm test tube, then add distilled water to create a total volume of 8 mL. Stopper the test tube. Some suggested combinations appear on the table below.
Unknown No. 1 2 3 4 5 6 7 8 9 10 11
Ag + X X X X
X X X
Cu2+ X X
Zn2+ X X X
X X X X
X
Fe3+ X X X X X
X X X X
X X
Solution of Four Anions Prepare 100 mL of a 0.05 M solution of all the anions to be tested by adding 25 mL of each of the following solutions to a clean 250-mL beaker. 0.2 M sodium chloride, NaCl 0.2 M sodium carbonate, Na 2CO3 0.2 M sodium nitrate, NaNO 3 0.2 M sodium sulfate, Na 2SO4
Mix the solution, dispense in 12 clean 13 X 100 mm test tubes, and stopper the test tubes.
Unknown Anion Solutions Prepare the unknown anion solutions for the students to analyze from the four 0.2 M stock solutions of anions. Any combination of these four anions may be prepared. Using a graduated Beral-type pipet, add 2 mL of each ion selected to a 13 X 100 mm test tube. Add distilled water to give a total volume of 8 mL. Stopper the test tube. A matrix of combinations, similar to the cation unknowns, can be generated for the anions.
Unknown No. 1 2 3 4 5 6 7 8 9 10 11
Cl X X X X
X X X
CO32X X
NO3X X X
X X X X
X
SO42X X X X X
X X X X
X X
Barium Hydroxide Solution Prepare 100 mL of a saturated barium hydroxide solution by dissolving 5.0 g of barium hydroxide in 100 mL of deionized or distilled water. Transfer the 100 mL of solution to a stoppered or capped bottle and label, “Ba(OH) 2” (saturated.)
Safety Precautions The 0.05 M solutions are very dilute and hazards are greatly reduced. However, dilute silver salts will still stain the skin. Silver nitrate solution is mildly toxic and irritating to body tissue. It also stains skin and clothing. Cupric nitrate solution
is mildly toxic. Zinc nitrate solution is mildly toxic and is irritating to body tissue. Ferric nitrate solution is irritating to body tissue. Barium hydroxide and barium chloride solutions are toxic by ingestion. Potassium thyocyanate solution is slightly toxic by ingestion. Avoid contact with concentrated acids since toxic hydrogen cyanide gas may be liberated. Concentrated ammonia (ammonium hydroxide) solution and hydrochloric acid solutions are toxic by inhalation, ingestion, and are corrosive to all body tissues. Sulfuric acid solution and sodium hydroxide solution are severely corrosive to eyes, skin, and other tissue. Nitric acid solution is severely corrosive, a strong oxidant, and toxic by ingestion and inhalation. Acetic acid solution is a corrosive liquid. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Have students wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Material Safety Data Sheets for additional safety information.
Dispoal Provide a separate container for cations and for anions into which the students may empty their waste solutions. Filter the solids from the liquid. Each liquid may be disposed of according to Flinn Suggested Disposal Method #26b. The cation solids may be disposed of according to Flinn Suggestion Disposal Method #26a. For barium solutions and solids from anion steps 2 and 3, place in a separate waste container. This waste may be disposed of according to Flinn Suggested Disposal Method #27h. Please consult your current Flinn Scientific Catalog/Reference Manual for proper disposal procedures.
AP Chemistry Standards This lab fulfills the requirements for the College Board recommended AP Experiment #14: Separation and Qualitative Analysis of Cations and Anions.
Lab Hints •
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Four to five 50-minute lab periods are needed to analyze the known and unknown solutions for both cations and anions. One hour of prep time will be needed to prepare the known and unknown solutions. Students should have a set of reagents available to them in small (15-39 mL) dropping bottles. To simplify preparation of the sets of dropper bottles, have each student group clean and label a set of dropper bottles for the reagents. Dispense the reagents from the stock solutions. Use great caution in dispensing the strong, concentrated acids and bases from the stock solutions. Wear chemical splash goggles and chemical-resistant gloves. If a complete set of dropper bottles is not available for each student group, a set may be shared between groups. Check centrifuges to make sure that they are balanced and spin quietly while empty. It may be necessary to weigh the centrifuge tubes and place those with the closest weights opposite on another. The test tubes should not drop so far into the centrifuge tubes that they cannot be easily removed. If necessary, place a small cork in the bottom of each centrifuge tube so that the test tubes extend just a little above the centrifuge tubes. It is not necessary to stopper the test tube when centrifuging the solution. A test tube must be counterbalanced by placing a similar tube with an equal volume of water opposite it. Caution students not to stop the centrifuge while it is spinning. Water baths can be shared. Enough solution is provided for each lab group to perform multiple tests. Encourage students to run tests until they get clear and reproducible results.
Teaching Tip •
The solution that is analyzed has a small total solution volume. Great care is needed in adding reagents, mixing solutions, precipitating the substances, separating the liquids from solids, washing precipitates, adjusting pH, and following the laboratory procedures. It is essential to maintain a good record of the results of each step because the procedure will be carried out over a number of laboratory periods. Students should be able to demonstrate an understanding of the chemistry involved in the steps as well as the procedures used.
Answers to Pre-Lab Questions 1. Test for Ag+, Cu2+, Fe3+ Some 6 M HCl is added to a solution that may contain the three ions. A white precipitate forms. Ions: ____ Ag+___ Ions absent: __________ Ions undetermined: __ Cu2+, Fe3+ __ 2. Test for Cu2+, Ag+, and Zn2+ Some 6 M HCl is added to a solution that may contain the three ions. No precipitate forms. The addition of 6 M NaOH until the solution is basic results in no formation of the precipitate. Ions present: ___ ______ Ions absent: __ Ag+, Cu2+ __ Ions undetermined: ____ Zn2+___ 3. Test for Cu2+, Fe3+, and Zn3+ 6 M NaOH is added to a clear solution that may contain the three ions until the solution is basic. A dark precipitate forms. The precipitate totally dissolves in 6 M H 2SO4. The addition of 6 M NH 3 to the acidic solution until it is basic results in a clear solution containing a dark precipitate. The dark precipitate completely dissolves in 6 M H 2SO4. Ions present: ___ Fe3+____ Ions absent: ____ Cu2+____ Ions undetermined: ____ Cl-____ 4. Test for CO32- and ClSome 6 M HCl is added to the solution that may contain the above ions. Formation of bubbles is noted as the solution is heated. Ions present: ___ CO32- ____ Ions absent: __________ Ions undetermined: ___ Zn2+___ 5. Test for Cl-, SO42-, CO32-, and NO3Addition of AgNO 3 causes no precipitate to form. Addition of BaCl 2 to fresh solution also causes no precipitate to form. Ions present: _______ Ions absent: ___ Cl-, SO42-___ Ions undetermined: _ CO32-, NO3 __ Only one of each of the following pairs of reactants undergoes a reaction. Complete and balance the equation for the reaction which occurs. 6. NaCl(aq) + BaCl2(aq) → no reaction Na2SO4(aq) + BaCl 2(aq) → BaSO4(s) + 2Na+(aq) + 2Cl-(aq) 7. NO3-(aq) + OH-(aq) + Al(s) → no reaction 3NO3-(aq) + 5OH-(aq) + 8Al(s) + 18 H 2O(l) → 3NH 3(aq) + 8Al(OH) 4-(aq) CO32-(aq) + OH-(aq) + Al(s) → no reaction 8. K +(aq) + Cl -(aq) → no reaction Ag+(aq) + Cl-(aq) → AgCl(s)
Sample Data Tables Cation Analysis Data Table Step 1
2
Procedure Add HCl to known solution in TT1. Centrifuge. Pour colored solution into TT2. Add NH3 to precipitate in TT1. Dissolve precipitate. Add HCl. Precipitate forms.
(Student answers will vary) Known Solution Results Conclusion White ppt forms
Precipitate dissolves in NH3, white ppt with HCl.
Ag+ present
Cu2+ and Fe3+ may be present
3
To solution in TT2, add NaOH. Solution basic. Heat. Precipitate forms. Centrifuge. Pour solution in TT3. Add NaOH and water to ppt in TT2. Centrifuge, discard liquid.
Precipitate forms
4
To TT2, add H2SO4 until solution is acidic. Stir to dissolve ppt. Add NH3. Precipitate forms. Solution blue. Centrifuge, discard blue solution.
Basic solution blue
5
Wash ppt in TT2. Add H2SO4 until ppt Acidic solution dissolves. Add 5 drops KSCN. turns deep red Solution turns deep red. Add HCl to solution in TT3 (step 3). White to light green Add K 4[Fe(CN)6]. Stir. Centrifuge. ppt White to light green ppt forms.
6
Anion Analysis Data Table Step
Cu2+ present
Fe3+ present
Zn2+ present
(Student Answers will vary.)
Procedure Add CH3COOH to known solution in TT1. Add 10 drops of 0.1 M AgNO3. Centrifuge. Discard water. Add 0.5 mL of NH3. Centrifuge. Acidify solution with 6 M HNO3. White ppt forms. Add 0.5 mL of 6 M HNO3 to known solution in TT2. Heat TT2. Capture gas in pipet. Add 2 mL of Ba(OH)2 solution. Cloudy white ppt forms
Known Solution Results Conclusion
White ppt forms
Cl- present
White ppt forms
CO32- present
3
Add 0.5 mL of known solution to TT4. Add 0.5 mL of 6 M HNO3. Solution acidic. Add 0.5 mL BaCl2. White ppt forms.
White ppt forms
SO42- present
4
Add 1.0 mL of known solution to TT5. Add 6 M HNO3. Solution acidic. Transfer solution to dry TT6. Heat. Litmus gradually turns blue.
Litmus paper turns from pink NO3- present to blue
1
2
Unknown Solution Results Conclusion
Unknown Solution Results Conclusion
Post-Lab Questions (Student answers will vary.) 1. What is the precipitating reagent for silver (Ag +)? Would a solution of NaCl work as well? Why or why not? The chloride ion is the precipitating reagent for the silver ion. A solution of NaCl would also work as well, since it provides the chloride ion in solution. 2. In the analysis scheme, Ag+ is precipitated as AgCl, the precipitate is dissolved, and then AgCl is precipitated again in the confirmatory step. Explain the chemistry of each of these steps by showing a balanced equation for each. Precipitation: Dissolution: Reprecipitation:
Ag +(aq) + Cl -(aq) →AgCl(s) AgCl(aq) + 2NH 3(aq) → Ag(NH 3 )2(aq) + Cl -(aq) Ag(NH 3 )2+(aq) + Cl -(aq) + 2H +(aq) → AgCl(s) + 2NH 4+(aq)
3. When Fe3+ and Cu2+ react with NH 3 solution they form two different types of products. One is a precipitate and one a complex ion in solution. Write equations for these two reactions. Fe3+(aq) + 3NH 3(aq) + 3H 2O(l) → Fe(OH) 3(s) + 3NH 4+(aq) Cu2+(aq) + 4NH 3(aq) → Cu(NH 3 )42+(aq) 4. The confirmatory test for chloride ion with silver ion is the same chemical reaction used to confirm silver in the cation analysis scheme. Explain what the reaction is and how the initial precipitate is dissolved and reprecipitated. Use equations in your explanation. Precipitation: Ag +(aq) + Cl -(aq) → AgCl(s) Dissolution: AgCl(aq) + 2NH 3(aq) → Ag(NH 3 )2+(aq) + Cl -(aq) Reprecipitation: Ag(NH 3 )2+(aq) + Cl -(aq) + 2H +(aq) → AgCl(s) + 2NH 4+(aq) 5. Write separate oxidation and reduction half-reactions for the procedure used in the test for nitrate ions. NO3-(aq) + 6H 2O(l) + 8e- → NH 3(aq) + Al(OH) 4-(aq) Al(s) + 4OH -(aq) → Al(OH) 4-(aw + 8e -
NO3-(aq) + Al(s) + 5OH -(aq) + 6H 2O(l) → NH 3(aq) + Al(OH) 4-(aq)
reduction oxidation
overall
6. In the nitrate test, why must care be taken to keep the moist litmus from coming in contact with the cotton or the solution? The solution being tested is basic. If the litmus comes in contact with the solution or cotton moist from the solution, it will turn blue from the solution contact, not from the evolving ammonia. 7. In step 4, Ba2+ is added to the solution containing all four of the anions and precipitates BaSO 4, but not BaCO 3. However, in step 3, the precipitation of BaCO 3, is the confirmatory test for carbonate ion. Why doesn’t BaCO 3 precipitate in step 4 but does in step 3? BaSO4 will precipitate in acidic solution, but in acidic solution BaCo 3 will decompose to form CO 2 gas, H 2O, and Ba2+(aq).
