Separation and Qualitative Determination of Cations and Anions
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 which 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. Following is an example from step 1 of the cation analysis.
{13907_Background_Table_1}
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.
{13907_Background_Figure_1_How to safely load and use a centrifuge}
To safely load and use a centrifuge, keep the following in mind.
- 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.
{13907_Background_Figure_2_Using pH test paper}
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
Acetic acid solution, CH3COOH, 6 M, 10 mL Aluminum, Al, granules Ammonia solution, NH3, 6 M, 10 mL Barium chloride solution, BaCl2, 0.2 M, 10 mL Barium hydroxide solution, Ba(OH)2, saturated, 10 mL Cupric nitrate, Cu(NO3)2, 0.05 M† Ferric nitrate, Fe(NO3)3, 0.05 M† Hydrochloric acid solution, HCl, 6 M, 10 mL Nitric acid solution, HNO3, 6 M, 10 mL Potassium ferrocyanide solution, K4Fe(CN)6, 0.1 M, 5 mL Potassium thiocyanate solution, KSCN, 0.1 M, 5 mL Silver nitrate, AgNO3, 0.05 M† Silver nitrate solution, AgNO3, 0.1 M, 10 mL Sodium carbonate, Na2CO3, 0.05 M* Sodium chloride, NaCl, 0.05 M* Sodium hydroxide solution, NaOH, 6 M, 8 mL Sodium nitrate, NaNO3, 0.05 M* Sodium sulfate, Na2SO4, 0.05 M* Sulfuric acid solution, H2SO4, 6 M, 10 mL Zinc nitrate, Zn(NO3)2, 0.05 M† Beaker, 250-mL Beaker, 400-mL Centrifuge Hot plate Litmus paper or pH paper Microscope slide or watch glass Pipets, Beral-type, 6 Spatula Stirring rods Stoppers to fit test tubes, 6 Test tubes, 13 x 100 mm, 6 Test tube rack Wire test tube holder *Anion solutions, 5 mL each †Cation solutions, 5 mL each
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.
- 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: ____________
- 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: ____________
- 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: ____________
- Test for CO32– 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: ____________
- Test for Cl–, SO42–, CO32– 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.
- NaCl(aq) + BaCl2(aq) →
Na2SO4(aq) + BaCl2(aq) →
- NO3–(aq) + OH–(aq) + Al(s) →
CO32–(aq) + OH–(aq) + Al(s) →
- 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. 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 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 and 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 in italics. For example:
Aqueous solutions of Ag+ and Zn2+ are colorless. Fe3+ has a yellow color and Cu2+ is blue in aqueous solutions.
- 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)
- Add 8 drops of 6 M HCl to the solution to be analyzed. Stir. A white precipitate indicates that the Ag+ ion is present.
- 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.
- 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.
- Wash the precipitate by adding 1 mL distilled water and stirring. Centrifuge and discard the wash water. Save the precipitate for procedure 2.
- Confirmation of Silver
When 6 M NH3 is added to AgCl, the Ag+ ion forms a colorless complex ion and goes into solution:
{13907_Procedure_Equation_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)
- To the precipitate from procedure 1d, which is AgCl, add 1 mL 6 M NH3.
- Stir until the precipitate completely dissolves.
- 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.
- 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.
- Dispose of the silver compound as directed by the instructor.
- 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)42–(aq)
- 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.
- 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.
- Centrifuge the solution, and separate the clear solution from the solid. Save the clear solution, which may contain Zn(OH)42– ions for procedure 6.
- Wash the precipitate with a mixture of 10 drops of 6 M NaOH and 10 drops of water.
- Centrifuge and discard the wash water. Save the precipitate for procedure 4.
- Separation of Iron from Copper; Confirmation of Copper
Both cupric hydroxide, Cu(OH)2, and ferric 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.
{13907_Procedure_Equation_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.
- To the precipitate from procedure 3, add 5 drops of deionized water.
- Add 6 M H2SO4 dropwise until the solution is acidic when tested with litmus paper (about 6 drops). Stir to dissolve precipitate.
- To the solution, add 6 M aqueous NH3 until the solution is basic to litmus, and then add 1 mL extra.
- 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, to the solution containing the Cu(NH3)42+ add 6 M CH3COOH, acetic acid, 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.
- Dispose of the copper solution as directed by the instructor.
- Confirmation of Iron
Ferric 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)
{13907_Procedure_Equation_3}
- Wash the precipitate of iron hydroxides from procedure 4d.
- Add 6 M H2SO4 dropwise until the precipitate dissolves.
- To the solution add 5 drops of 0.1 M KSCN solution. The deep red FeSCN2+ ion confirms the presence of iron.
- Dispose of the iron solution as directed by the instructor.
- 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)64–(aq) → K2Zn3[Fe(CN)6]2(s)
- Make the solution from procedure 3c slightly acidic by adding 6 M HCl dropwise.
- Add 3 drops of 0.1 M K4[Fe(CN)6] and stir.
- Centrifuge to see the confirmatory precipitate of K2Zn3[Fe(CN)6]2 which will be white to light green or blue green in color.
- Dispose of the zinc precipitate and solution as directed by the instructor.
- 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.
- 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)
- 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.
- Add 10 drops of 0.1 M silver nitrate, AgNO3. A precipitate of AgCl will form.
- Centrifuge and pour off the supernatant liquid.
- Wash the solid with 0.5 mL distilled water, centrifuge and discard the wash water.
- Add 0.5 mL 6 M ammonia, NH3, to the precipitate. Stir to dissolve any AgCl.
- Centrifuge, and pour the supernatant liquid into another test tube to test for chloride ion.
- 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.
- 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.
- 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. CO32–(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.
- Place 2 mL of clear, saturated Ba(OH)2 solution in a test tube to be available for the test with carbon dioxide
- Place 1 mL of the original test solution (or unknown solution) in a different test tube.
- Acidify this solution by adding 0.5 mL of 6 M HNO3.
- Place the tube in a hot water bath and observe to see if any gas bubbles form.
- 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.
- 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.
- Confirmation of Sulfate
The test for sulfate is the formation of white, insoluble barium sulfate. This solid is insoluble even in acidic solution. SO42–(aq) + Ba2+(aq) → BaSO4(s)
- Place 0.5 mL of the original test solution (or unknown solution) in a test tube.
- Add 6 M nitric acid, HNO3, dropwise until the solution is acidic.
- Add 0.5 mL 0.2 M BaCl2 solution. The formation of a white precipitate of BaSO4 confirms the presence of sulfate.
- 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)
- Place 1 mL of the original test solution (or unknown solution) in a test tube.
- Add 6 M NaOH dropwise until the solution is basic, and then add 6 drops in excess.
- 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.
- Add the tip of a spatula containing aluminum granules.
- 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.
- 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.
- 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.
- 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.
- Repeat anion procedures 1–4 for the anion unknown sample.
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