Materials Included In Kit

Additional Materials Required

Prelab Preparation

Note: 0.2 M solutions are provided so unknown solutions can be easily made. The 0.2 M solutions are also easier to ship and take up less room. Diluting the 0.2 M solutions to 0.05 M solutions does not have to be 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, AgNO₃
• 0.2 M Copper(II) nitrate, Cu(NO₃)₂
• 0.2 M Zinc nitrate, Zn(NO₃)₂
• 0.2 M Iron(III) nitrate, Fe(NO₃)₃

Mix the solution, dispense in 12 clean 13 x 100 mm test tubes, and 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 following table.

Cation Unknowns 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, Na₂CO₃
• 0.2 M sodium nitrate, NaNO₃
• 0.2 M sodium sulfate, Na₂SO₄

Mix the solution, dispense in 12 clean 13 x 100 mm test tubes, and stopper the test tubes.

Unknown Anion Solutions
Prepare 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 clean 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.

Anion Unknowns 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. Copper(II) nitrate solution is mildly toxic. Zinc nitrate solution is mildly toxic and is irritating to body tissue. Iron(III) nitrate solution is irritating to body tissue. Barium hydroxide and barium chloride solutions are toxic by ingestion. Potassium thiocyanate 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 review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. 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 Suggested 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.

Lab Hints

• Students should have a set of reagents available to them in small (15–30-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 closest weights opposite one 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.
• The solution that is analyzed has a small total solution volume. Great care is needed in adding reagents, mixing solutions, precipitating the substances, separating 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 Prelab Questions

1. Test for Ag⁺, Cu²⁺ and Fe³⁺

   Some 6 M HCl is added to a solution that may contain the three ions. A white precipitate forms.
   Ions present: ___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 precipitate.
   Ions present: ______ Ions absent: ___Ag⁺, Cu²⁺___ Ions undetermined: ___Zn²⁺___

3. Test for Cu²⁺, Fe³⁺ and Zn²⁺

   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 H₂SO₄. The addition of 6 M NH₃ 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 H₂SO₄.
   Ions present: ___Fe³⁺___ Ions absent: ___Cu²⁺___ Ions undetermined: ___Zn²⁺___

4. Test for CO₃^2– and Cl^–

   Some 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: ___CO₃^2–___ Ions absent: ______ Ions undetermined: ___Cl^–___

5. Test for Cl^–, SO₄^2–, CO₃^2– and NO₃^–

   Addition of AgNO₃ causes no precipitate to form. Addition of BaCl₂ to fresh solution also causes no precipitate to form.
   Ions present: ______ Ions absent: ___Cl^–, SO₄^2–___ Ions undetermined: ___CO₃^2–, NO₃^–___
   Only one of each of the following pairs of reactants undergoes a reaction. Name the qualitive analysis procedure then complete and balance the equation for the reaction that occurs.

6. NaCl(aq) + BaCl₂(aq) → no reaction
   Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl^–(aq) (Confirmation of sulfate)
7. 3NO₃^–(aq) + 5OH^–(aq) + 8Al(s) + 18H₂O(l) → 3NH₃(aq) + 8Al(OH)₄^–(aq)
   CO₃^2–(aq) + OH^–(aq) + Al(s) → no reaction (Confirmation of nitrate)
8. K⁺(aq) + Cl^–(aq) → no reaction
   Ag⁺(aq) + Cl^–(aq) → AgCl(s) (Confirmation of chloride)

Sample Data

Cation Analysis Data Table

Anion Analysis Data Table

Answers to Questions

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: Ag⁺(aq) + Cl^–(aq) → AgCl(s)
   Dissolution: AgCl(aq) + 2NH₃(aq) → Ag(NH₃)₂⁺(aq) + Cl^–(aq)
   Reprecipitation: Ag(NH₃)₂⁺(aq) + Cl^–(aq) + 2H⁺(aq) → AgCl(s) + 2NH₄⁺(aq)

3. When Fe³⁺ and Cu²⁺ react with NH₃ 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) + 3NH₃(aq) + 3H₂O(l) → Fe(OH)₃(s) + 3NH₄⁺(aq)
   Cu²⁺(aq) + 4NH₃(aq) → Cu(NH₃)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) + 2NH₃(aq) → Ag(NH₃)₂⁺(aq) + Cl^–(aq)
   Reprecipitation: Ag(NH₃)₂⁺(aq) + Cl^–(aq) + 2H⁺(aq) → AgCl(s) + 2NH₄⁺(aq)

5. Write separate oxidation and reduction half-reactions for the procedure used in the test for nitrate ions.

   NO₃^–(aq) + 6H₂O(l) + 8e^– → NH₃(aq) + 9OH^–(aq) Reduction
   Al(s) + 4OH^–(aq) → Al(OH)₄^–(aq) + 8e^– Oxidation
   NO₃^–(aq) + Al(s) + 5OH^–(aq) + 6H₂O(l) → NH₃(aq) + Al(OH)₄^–(aq) 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 BaSO₄, but not BaCO₃. However, in step 3, the precipitation of BaCO₃ is the confirmatory test for carbonate ion. Why doesn’t BaCO₃ precipitate in step 4 but does in step 3?

   BaSO₄ will precipitate in acidic solution, but in acidic solution BaCO₃ will decompose to form CO₂ gas, H₂O and Ba²⁺(aq).

Introduction

Much of laboratory chemistry is focused on the question of how much of a given substance is contained in a sample. Sometimes, however, the focus shifts to what substances are in the sample, rather than their quantity. In this experiment, an analysis scheme for identifying both cations and anions in solution will be used in determining the ions present in known solutions.

Concepts

• Qualitative analysis
• Precipitation reactions

Background

Qualitative analysis is an analytical procedure in which the question “what is present?” is answered. In a systematic qualitative analysis scheme, each substance present is separated from the other substances. Then a confirmatory test is used to prove that the isolated substance is the expected one.

To begin the lab experiment, a solution containing four cations is analyzed using the techniques for qualitative analysis of cations. A second solution containing four anions is then analyzed using the qualitative scheme for anions.

Two solutions, one containing any combination of four different cations, and another containing any combination of four anions will be assigned to each student group. The two “unknown” solutions are then analyzed to determine which ions are present and which are absent.

This experiment is carried out on a semi-micro scale. Very small quantities of reagents are used. Cleanliness and a great deal of care are necessary to obtain good results.

While going through the steps of the analysis, keep a copy of the appropriate flow chart available for reference. The flow charts appear at the end of the experimental directions. The charts will help to give the “total picture” of where each analysis is and where it is heading. Read the directions carefully, and read the sections that give the notes or chemical theory for each step. Don’t just follow directions “cook book” style, but make an effort to understand the chemical principles behind the procedures.

General Techniques for Qualitative Analysis

Keep Good Records
It is necessary to keep good records so as not to get confused and forget what solutions are in which test tubes. Number the test tubes with pencil or permanent ink so the numbers do not come off in the hot water bath.

Maintain a current record of the work. Don’t trust the results to your memory. Below is an example from step 1 of the cation analysis.

Note: TT is short for test tube. Fill out the columns for the “Known Solutions” first. Then, when analyzing the “Unknown Solutions,” fill out the last two columns.

Be Orderly
Arrange the chemical reagents in a way so that it is easy to find the solutions needed. Put acids together, bases together, for example.

Avoid Contamination
Tap water is often a source of contaminating ions. Wash and rinse all glassware with distilled water.

A stirring rod is constantly used to mix solutions, and it also must be rinsed with distilled water so that it does not contaminate subsequent solutions. An easy way to do this is to fill a 400-mL beaker about ⅔ full of distilled water, and keep your stirring rods in this beaker. The small amount of contaminants present in this volume of water should cause no problem. Replace the distilled water as needed.

Droppers or plastic pipets should also be rinsed twice with distilled water after they are used. Get in the habit of rinsing them immediately after use.

Measuring Solutions
Generally, the volume of solutions added should be estimated. It is not necessary to use a graduated cylinder to measure solution volumes. A test tube can be calibrated in milliliters to give an idea of what volume a milliliter actually is.

Heating Solutions
Frequently it will be necessary to heat a solution to speed up a reaction. Do NOT heat small test tubes over Bunsen burner flames. A sudden steam bubble will cause the solution to shoot out of the test tube. Instead, heat test tubes in a boiling water bath. A good idea is to set up this bath when beginning work in the lab because it may take time to heat the bath to the appropriate temperature.

Stirring Solutions
Each time a reagent is added to a test tube, the solution needs to be stirred. It is important to mix the solutions at the top and the bottom of the test tube. A stirring rod that is flattened at the bottom can be used as a plunger to effectively mix solutions in narrow test tubes.

Separating Solids from Solutions
Centrifuge solutions so that the solid is packed at the bottom of the test tube. Don’t forget to counterbalance the test tubes in the centrifuge with similar test tubes holding equivalent volumes of liquid (see Figure 1). Let the centrifuge spin for about 30 seconds. Usually the supernatant liquid (the liquid above the precipitate) can be poured off of the precipitate. Sometimes precipitates tend to float on the surface of the solution. If this is the case, use a Beral-type pipet to draw off the supernatant liquid. It is better to leave a little liquid over the precipitate than to transfer some of the precipitate.

• Never fill centrifuge tubes to capacity. Keep liquid levels at least 1 cm from the top.
• Label all centrifuge tubes before inserting to avoid mix-up.
• Place tubes in a symmetrical fashion, the objective being to keep the rotor balanced.
• Fill all tubes to the same height.
• Follow manufacturer’s directions.
• If only one tube needs to be centrifuged, achieve balance by inserting an additional tube (labeled as a “blank”) containing the same volume of liquid.

Washing Precipitates
It is almost always necessary to wash precipitates to free them from ions that might cause confusion in later steps. To do this, add 1 or 2 mL of distilled water to the precipitate, stir, centrifuge and discard the wash water. Sometimes the directions will require a specific reagent in the wash water.

Checking the pH
To check the pH of a solution, put a piece of litmus paper or pH paper on a clean glass plate or watch glass. Dip the stirring rod into the solution in the test tube, and touch the stirring rod to the paper (see Figure 2). Do NOT dip the test paper into the test tube. This may cause some of the indicator dye to dissolve in the solution, and the indicator color may confuse subsequent tests.

Storing Solutions
To keep a solution until the next laboratory period, stopper the test tube with a rubber stopper. If a precipitate is present, put a few drops of distilled water on it before stoppering the test tube. Be sure to record a list of substances that are present in each test tube. Don’t rely on memory!

Materials

Prelab Questions

Use the flow charts at the end of the experimental procedure to answer the following questions. In each question, a test is carried out to determine the presence or absence of several ions. Only those listed may be present. State if the tests indicate if each ion is present, absent or undetermined.

1. Test for Ag⁺, Cu²⁺ and Fe³⁺

   Some 6 M HCl is added to a solution that may contain the three ions. A white precipitate forms.
   Ions present: ____________ Ions absent: ____________ Ions undetermined: ____________

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 precipitate.
   Ions present: ____________ Ions absent: ____________ Ions undetermined: ____________

3. Test for Cu²⁺, Fe³⁺ and Zn²⁺

   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 H₂SO₄. The addition of 6 M NH₃ to this acidic solution until it is basic results in a clear solution containing a dark precipitate. The dark precipitate completely dissolves in 6 M H₂SO₄.
   Ions present: ____________ Ions absent: ____________ Ions undetermined: ____________

4. Test for CO₃^2– and Cl^–

   Some 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: ____________ Ions absent: ____________ Ions undetermined: ____________

5. Test for Cl^–, SO₄^2–, CO₃^2– and NO₃^–

   Addition of AgNO₃ causes no precipitate to form. Addition of BaCl₂ to fresh solution also causes no precipitate to form.
   Ions present: ____________ Ions absent: ____________ Ions undetermined: ____________
   Only one of each of the following pairs of reactants undergoes a reaction. Name the qualitive analysis procedure then complete and balance the equation for the reaction that occurs.

6. NaCl(aq) + BaCl₂(aq) →

   Na₂SO₄(aq) + BaCl₂(aq) →

7. NO₃^–(aq) + OH^–(aq) + Al(s) →

   CO₃^2–(aq) + OH^–(aq) + Al(s) →

8. K⁺(aq) + Cl^–(aq) →

   Ag⁺(aq) + Cl^–(aq) →

Safety Precautions

Silver nitrate solution is mildly toxic and irritating to body tissue. It also stains skin and clothing. Copper(II) nitrate solution is mildly toxic. Zinc nitrate solution is mildly toxic and is irritating to body tissue. Iron(III) nitrate solution is irritating to body tissue. Barium hydroxide and barium chloride solutions are toxic by ingestion. Potassium thiocyanate 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. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Qualitative Analysis of Cations

Note that the following directions are written for a “known” solution that contains all of the cations. An “unknown” solution will probably not form all of the products described in this procedure. Make note of any differences in the “unknown” solution as it is analyzed.

In the directions that follow, a description of the physical properties and the chemistry of the substances appears before the instructions. Aqueous solutions of Ag⁺ and Zn²⁺ are colorless. Fe³⁺ has a yellow color and Cu²⁺ is blue in aqueous solutions.

1. Separation of the Silver from Iron, Copper and Zinc Ions

   Most chloride salts are soluble; however, Ag⁺ ions form an insoluble chloride. These Ag⁺ ions can be separated from the other ions present in this qualitative analysis scheme by precipitating them as chlorides. All of the other ions will stay in solution.
   Ag⁺(aq) + Cl^–(aq) → AgCl(s)

   1. Add 8 drops of 6 M HCl to the solution to be analyzed. Stir. A white precipitate indicates that the Ag⁺ ion is present.
   2. Centrifuge the solution and test to be sure that precipitation is complete by adding one more drop of 6 M HCl. No additional precipitate should form. If more precipitate does form, continue adding 6 M HCl until precipitation is complete.
   3. Centrifuge, decant (pour off) and save the clear liquid into a second test tube for Procedure 3. Alternatively, use a Beral type pipet to draw off the supernatant liquid to transfer it to another test tube.
   4. Wash the precipitate by adding 1 mL distilled water and stirring. Centrifuge and discard the wash water. Save the precipitate for Procedure 2.
2. Confirmation of Silver

   When 6 M NH₃ is added to AgCl, the Ag⁺ ion forms a colorless complex ion and goes into solution:

   {13828_Procedure_Reaction_1}

   Addition of hydrochloric acid to the Ag(NH₃)₂⁺ complex ion breaks apart the ion. The NH₃ combines with H⁺ to form NH₄⁺, and the Ag⁺ ion recombines with the Cl^– ion to precipitate as white AgCl.
   Ag(NH₃)₂⁺(aq) + Cl^–(aq) + 2H⁺(aq) → AgCl(s) + 2NH₄+(aq)

   1. To the precipitate from Procedure 1d, which is AgCl, add 1 mL 6 M NH₃.
   2. Stir until the precipitate completely dissolves.
   3. Add 15 drops of 6 M HCl to the solution. The solution will smoke and the reaction between the strong acid and the base will give off heat whether or not silver is present. The test tube may get very warm.
   4. Stir and test with pH indicator paper or litmus paper to be sure the solution is acidic. If it is not acidic, add more HCl. The reappearance of the white AgCl precipitate in the acidic solution confirms the presence of silver.
   5. Dispose of the silver compound as directed by the instructor.
3. Separation of Iron and Copper from Zinc

   In a basic solution, the amphoteric zinc will form a colorless complex ion and remain in solution, while the hydroxides of all the other ions will precipitate. The iron will precipitate as rust colored Fe(OH)₃, and the copper as blue Cu(OH)₂. The reactions are as follows:
   Fe³⁺(aq) + 3OH^–(aq) → Fe(OH)₃(s)
   Cu²⁺(aq) + 2OH^–(aq) → Cu(OH)₂(s)
   Zn²⁺(aq) + 4OH^–(aq) → Zn(OH)₄^2–(aq)

   1. To the solution saved from Procedure 1c, add, with stirring, 6 M sodium hydroxide, NaOH, until the solution is basic and then add 3 more drops.
   2. Stir and place the test tube in a hot water bath for 3 minutes. The formation of a precipitate indicates the presence of either copper or iron or both.
   3. Centrifuge the solution, and separate the clear solution from the solid. Save the clear solution, which may contain Zn(OH)₄^2– ions for Procedure 6.
   4. Wash the precipitate with a mixture of 10 drops of 6 M NaOH and 10 drops of water.
   5. Centrifuge and discard the wash water. Save the precipitate for Procedure 4.
4. Separation of Iron from Copper; Confirmation of Copper

   Both Copper(II) hydroxide, Cu(OH)₂, and Iron(III) hydroxide, Fe(OH)₃, readily dissolve in acid solution:
   Cu(OH)₂(s) + 2H⁺(aq) → Cu²⁺(aq) + 3H₂O(l)
   Fe(OH)₃(s) + 3H⁺(aq) → Fe³⁺(aq) + 3H₂O(l)
   Aqueous ammonia added to a solution in which Cu²⁺ is present, will cause the deep blue tetraammine copper(II) complex ion to form. The presence of this deep blue color confirms the presence of copper. At the same time, the basic ammonia solution will precipitate the hydroxides of iron.

   {13828_Procedure_Reaction_2}

   Fe³⁺(aq) + 3OH^–(aq) → Fe(OH)₃(s)
   An additional and very sensitive confirmatory test for copper is to precipitate the red-brown copper(II) hexacyanoferrate(II) [also called copper(II) ferrocyanide], Cu₂[Fe(CN)₆](s), from a Cu²⁺ solution.

   1. To the precipitate from procedure 3, add 5 drops of deionized water.
   2. Add 6 M H₂SO₄ dropwise until the solution is acidic when tested with litmus paper (about 6 drops). Stir to dissolve precipitate.
   3. To the solution, add 6 M aqueous NH₃ until the solution is basic to litmus, and then add 1 mL extra.
   4. Centrifuge and separate the supernatant liquid from the precipitate. Save the precipitate for Procedure 5. The presence of the blue Cu(NH₃)₄²⁺ ion is the confirmatory test for copper. Note: For an additional confirmatory test, add 6 M CH₃COOH, acetic acid, to the solution containing the Cu(NH₃)₄²⁺ until the blue color fades and the solution becomes acidic. Then add 2 drops of 0.1 M K₄[Fe(CN)₆]. A red-brown precipitate of Cu₂[Fe(CN)₆] reconfirms the presence of copper.
   5. Dispose of the copper solution as directed by the instructor.
5. Confirmation of Iron

   Iron(III) hydroxide will dissolve in sulfuric acid. Addition of the thiocyanate ion, SCN^–, forms a deep wine-red colored complex ion with iron that is a very sensitive test for the presence of iron.
   Fe(OH)₃(s) + 3H⁺(aq) → Fe³⁺(aq) + 3H₂O(l)

   {13828_Procedure_Reaction_3}
   1. Wash the precipitate of iron hydroxides from Procedure 4d.
   2. Add 6 M H₂SO₄ dropwise until the precipitate dissolves.
   3. Add 5 drops of 0.1 M KSCN solution to the solution. The deep red FeSCN²⁺ ion confirms the presence of iron.
   4. Dispose of the iron solution as directed by the instructor.
6. Confirmation of Zinc

   The confirmatory test for zinc is the formation of a precipitate of potassium zinc hexacyanoferrate(II), K₂Zn₃[Fe(CN)₆]₂. This precipitate is nearly white if pure, but if a trace of iron is present, it may appear light green or blue-green in color.
   Zn(NH₃)₄²⁺(aq) + 4H⁺(aq) → Zn²⁺(aq) + NH₄⁺(aq)
   3Zn²⁺(aq) + 2K⁺(aq) + 2Fe(CN)₆^4–(aq) → K₂Zn₃[Fe(CN)₆]₂(s)

   1. Make the solution from Procedure 3c slightly acidic by adding 6 M HCl dropwise.
   2. Add 3 drops of 0.1 M K₄[Fe(CN)₆] and stir.
   3. Centrifuge to see the confirmatory precipitate of K₂Zn₃[Fe(CN)₆]₂ that will be white to light green or blue green in color.
   4. Dispose of the zinc precipitate and solution as directed by the instructor.
7. Repeat steps 1–6 for the cation unknown sample. Be sure to record the results for each step.

Qualitative Analysis of Anions
Obtain both the anion standards sample and the unknown anions sample. As with the analysis of cations, record the results of each step in the Anion Data Table.

1. Separation of the Chloride (Cl^–); Confirmation of Chloride

   Chloride ion forms an insoluble silver compound. Silver chloride is a white solid.

   Cl^–(aq) + Ag⁺(aq) → AgCl(s)

   Silver chloride dissolves in 6 M ammonia, NH₃, forming the colorless ion Ag(NH₃)₂⁺. If nitric acid, HNO3, is added to a solution containing this ion, the ammonia in the complex reacts with hydrogen ions to form ammonium ions, and the silver recombines with the chloride ions that are still present in solution.
   AgCl(s) + 2NH₃(aq) → Ag(NH₃)₂⁺(aq) + Cl^–(aq)
   Ag(NH₃)₂⁺(aq) + Cl^–(aq) + 2H⁺(aq) → AgCl(s) + 2NH₄⁺(aq)

   1. Place 10 drops of the original test solution (or unknown solution) in a test tube. Test to see if the solution is acidic. If it is not, add 6 M acetic acid, CH₃COOH, dropwise with stirring until the solution is acidic.
   2. Add 10 drops of 0.1 M silver nitrate, AgNO₃. A precipitate of AgCl will form.
   3. Centrifuge and pour off the supernatant liquid.
   4. Wash the solid with 0.5 mL distilled water, centrifuge and discard the wash water.
   5. Add 0.5 mL 6 M ammonia, NH₃, to the precipitate. Stir to dissolve any AgCl.
   6. Centrifuge, and pour the supernatant liquid into another test tube to test for chloride ion.
   7. Add 1 mL 6 M nitric acid, HNO₃, to the solution containing the dissolved silver chloride. The solution will get hot and smoke from the reaction with the excess ammonia whether or not silver chloride is present.
   8. Test with litmus or pH paper to see if the solution is acidic. If it is not, add more HNO₃ until the solution is acidic. The appearance of the white precipitate of AgCl in the acidic solution confirms the presence of chloride.
2. Confirmation of Carbonate

   In acid solution, carbonate forms carbon dioxide gas and water. The carbon dioxide may be seen as a slight effervescence. Carbon dioxide is less soluble in hot water than cold water. When carbon dioxide gas is passed through a saturated solution of barium hydroxide, it readily forms a precipitate of white barium carbonate.
   CO₃^2–(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
   CO₂(g) + Ba²⁺(aq) + 2OH^–(aq) → BaCO₃(s) + H₂O(l)
   If any bubbles were formed when acid was added to the original solution, carbonate is probably present and carbon dioxide is being formed. A confirmation of the presence of carbonate involves reacting evolving carbon dioxide with barium hydroxide to form white, insoluble barium carbonate.

   1. Place 2 mL of clear, saturated Ba(OH)₂ solution in a test tube to be available for the test with carbon dioxide.
   2. Place 1 mL of the original test solution (or unknown solution) in a different test tube.
   3. Acidify this solution by adding 0.5 mL of 6 M HNO₃.
   4. Place the tube in a hot water bath and observe to see if any gas bubbles form.
   5. Take a dry Beral-type pipet and squeeze the bulb closed. Place the tip of the pipet close to (but not touching) the surface of the liquid in the test tube and slowly release the bulb to draw escaping carbon dioxide into the pipet.
   6. Put the pipet into the barium hydroxide solution, and slowly squeeze the bulb, causing the gas in the pipet to bubble through the barium hydroxide solution. This procedure may be repeated. The formation of a cloudy white precipitate of barium carbonate confirms the presence of carbonate ion in the original solution.
3. Confirmation of Sulfate

   The test for sulfate is the formation of white, insoluble barium sulfate. This solid is insoluble even in acidic solution.
   SO₄^2–(aq) + Ba²⁺(aq) → BaSO₄(s)

   1. Place 0.5 mL of the original test solution (or unknown solution) in a test tube.
   2. Add 6 M nitric acid, HNO₃, dropwise until the solution is acidic.
   3. Add 0.5 mL 0.2 M BaCl₂ solution. The formation of a white precipitate of BaSO₄ confirms the presence of sulfate.
4. Confirmation of Nitrate

   The test for nitrate involves the reduction of nitrate ions in a basic solution to ammonia, NH₃, using solid aluminum as the reducing agent. When the solution is heated, ammonia gas is liberated. The evolving ammonia gas will turn litmus paper from pink to blue.
   3NO₃^–(aq) + 8Al(s) + 5OH^–(aq) + 18H₂O(l) → 3NH₃(g) + 8Al(OH)₄^–(aq)

   1. Place 1 mL of the original test solution (or unknown solution) in a test tube.
   2. Add 6 M NaOH dropwise until the solution is basic, and then add 6 drops in excess.
   3. Use a Beral-type pipet to transfer the solution to the bottom of a dry test tube without getting the walls of the test tube wet with solution.
   4. Add the tip of a spatula containing aluminum granules.
   5. Place a small cotton wad loosely about halfway down the test tube, but not touching the solution. This is to prevent spattering of the solution onto the litmus paper.
   6. Hang a piece of moist red litmus paper (or pH paper) in the tube so that the bottom of the paper is close to (but not touching) the cotton.
   7. Warm the solution in a hot water bath until it starts bubbling strongly. Be sure that the solution and the cotton do not touch the litmus paper.
   8. Allow the solution to cool. A slow color change (within 3 to 5 minutes) of the litmus from pink to blue, starting at the bottom and spreading to the top, indicates the evolution of ammonia and confirms the presence of nitrate ion in the original solution.
5. Repeat anion Procedures 1–4 for the anion unknown sample.

Student Worksheet PDF

13828_Student1.pdf