qual: A Qualitative Analysis Scheme

Purpose: Chemical analysis can be divided according to quantitative analysis and qualitative analysis. In the next two experiments, we will be asked to find how much of a given species is present in a sample. These two experiments are quantitative analyses. However, sometimes a chemist is interested in what species are present in a sample, as opposed to the amount of species present in a sample. Thus, a qualitative analysis involves the determination of the types (not amounts) of ions present in a solution. A "qualitative analysis scheme" is then a systematic procedure for separating and identifying various ions present in an aqueous solution.

The procedures in most qualitative analysis experiments involve the use of using precipitation reactions to remove the cations sequentially from a mixture. A precipitation reaction is a reaction in which an insoluble solid compound is formed when solutions of two soluble compounds are mixed. The solid compound that forms during a precipitation reaction is referred to as the precipitate. An example of a precipitation reaction is the reaction between aqueous solutions of KBr and AgNO₃:

KBr (aq) + AgNO₃ (aq) ® AgBr (s) + KNO₃ (aq)

We know that the ionic compounds KBr, AgNO₃, and KNO₃ are soluble in water and AgBr is not soluble in water, see "General Solubility Guidelines of Ionic Compounds in Water" on page 60 of the Cooperative Chemistry Laboratory Manual.

First, to become familiar with this qualitative analysis scheme, you will be given a solution that contains six different cations. We will call this the "known" solution. You will perform a series of tests on this solution using some common test reagents that will sequentially remove these cations from the solution, since each cation will react differently towards this set of test reagents. Each time a cation is removed from solution either through a precipitate or a colored complex, you will observe what a positive test looks like for the presence of that particular cation. Second, you will be given an unknown solution that contains between 1 and 5 of the cations, and you will repeat the procedure to determine which cations are present in your unknown solution.

Background reading:

Cooperative Chemistry Laboratory Manual

Pages 53: The Bunsen Burner

Page 62: Analysis of Cations

From Chemistry and Chemical Reactivity

Section 4.2 Balancing Chemical Equations

Section 5.2 Precipitation Reactions

Section 18.7 Qualitative Analysis

From Chemical Principles, The Quest for Insight

Pages F61-F64 Chemical Equations

Pages F23, F25-F28 Compounds, Ionic Compounds

Pages F67-70 Precipitation Reactions

Pages 475-477 Qualitative Analysis

Equipment and Materials:

You will work alone for this experiment. You will use several different reagents throughout this experiment. The reagents are located in the common reagent area in the front of the lab. When you need a reagent, take it to your lab bench, use it, and then put it right back. Do not carry open solutions to the reagent bench (poor etiquette).Be sure you return it to its proper place. All reagents and salts are in alphabetical order according to their name, not formula.

The known solution is located in the common equipment area. You will only need 1 mL of the known solution. Since this is a qualitative experiment, 1 mL is approximately equal to 20 drops for most aqueous solutions. Each of you will be assigned an unknown solution that contains between 1 and 5 cations. You will need to determine which cations are present in your unknown solution. Be sure to indicate the number of your unknown solution in your data sheet.

 There are some general procedures in qualitative analysis that will be used throughout this experiment. Centrifuging is used to aid the separation of a precipitate from a solution in a test tube. There is a centrifuge at the end of each lab bench. To use the centrifuge, make sure the test tube containing the precipitate and solution (sample test tube) is not overly full. Place the sample test tube in one of the centrifuge tubes. Place a blank tube containing the same amount of water as you have in your sample test tube in the centrifuge tube that is opposite your sample test tube. Turn on the centrifuge and allow it spin for about 1 minute. Do not open the centrifuge until it has completely stopped. Do not try to manually stop the centrifuge from spinning. The blank tube filled with water is necessary to balance the instrument. Make sure to label your tubes. You wouldn't want to confuse your unknown with water!

 Once you have centrifuged your sample test tube, now you can decant (pour off) the supernatant liquid from the precipitate. Simply pour off the liquid that is above the precipitate (the supernatant). Warning: The supernatant may be used for further analysis, so make sure you understand the whole procedure.

Bunsen Burners: The Bunsen burner is a common device used for heating. It is limited in usefulness to warming aqueous solutions or other nonflammable substances. Care must be used when using a Bunsen burner. Your laboratory instructor will demonstrate the proper method for adjusting, lighting, and using a Bunsen burner.

Safety Precautions:

Wear your safety goggles at all times. You will be using several different acid, base and salt solutions. If you get any solution on you, wash immediately with lots of water. Dispose of all waste in the labeled containers in the common equipment area. Use a wash bottle to rinse glassware into the container.

Experimental Procedure:

Part A: Analysis of the Group A Ions ( Al³⁺, Fe³⁺)

 A1. Obtain 1 mL (approximately 20 drops) of the known solution in a small test tube.

 A2. Add 2 drops of 6M HNO₃ and then add 1 mL of 6M NH₄OH. The formation of a precipitate indicates the presence of Al⁺³ and/or Fe⁺³. Centrifuge the solution. Check for complete precipitation by adding 6 more drops of the 6M NH₄OH. If more precipitate forms in the solution then add 1 mL more of the ammonia solution and centrifuge again

A3. Decant the supernatant and save the solution for the analysis of Group B and C ions.

A4. To the precipitate obtained above, add 1 mL of the 3M NaOH and stir vigorously with a glass stirring rod. This will re-dissolve any Al⁺³ present in the precipitate. Centrifuge and decant the supernatant. Save the precipitate for verification of Fe⁺³ using step A6.

A5. To the solution obtained in step A4, dropwise, add 1 mL of 2M acetic acid (CH₃CO₂H), stir and check whether the solution is acidic. Continue adding acetic acid dropwise until either a precipitate is formed or the solution becomes acidic. The formation of a white precipitate indicates the presence of aluminum. If the solution becomes acidic, without a precipitate forming, then aluminum was not present in the original solution.

A6. To the test tube containing any solid from step A4, add 1 mL of water and then 6 M HNO₃ dropwise with stirring until the entire solid has dissolved. Add a spatula tip of KSCN and stir until it dissolves. The appearance of an intensely dark red colored solution indicates iron.

Part B: Analysis of Group B Ions (Ba²⁺, Ca²⁺, and Mg²⁺)

 B1. To the solution obtained in step A3, add 1 mL of 0.3 M (NH₄)₂CO₃ and stir. If a precipitate forms, Ba⁺² and/or Ca⁺² is/are present. Centrifuge and decant the supernatant, which may contain Mg⁺². Save this solution for analysis in step B5. If a precipitate formed, continue with step B2. If no precipitate forms upon the addition of the 0.3 M (NH₄)₂CO₃, neither Ba²⁺ nor Ca²⁺ are present and you should continue with step B5.

B2. To any precipitate obtained in step B1, add 1 mL of water and then slowly add a couple of drops of 6 M HCl to re-dissolve Ba⁺² and Ca⁺². This will cause the solution to effervesce vigorously. What gas is being formed? Stir the solution vigorously, adding further 6 M HCl dropwise, until the entire solid has dissolved.

B3. To the solution obtained in step B2, add 1 mL of 1M H₂SO₄ solution and stir. A white precipitate indicates barium. Centrifuge and decant the solution, which may contain Ca⁺². The solid BaSO₄ can be discarded.

B4. To the solution obtained in step B3, add dropwise 6M NH₄OH until it is neutral or slightly basic and then add 1mL 0.3M (NH₄)₂C₂O₄, stir and let stand 1 minute. The formation of a white precipitate indicates calcium. This solution and precipitate can be discarded.

B5. To the solution from step B1, add 1mL of K₂HPO₄ solution, stir and let stand 1 minute. The slow formation of a white precipitate indicates magnesium. Centrifuge and decant. The solid can be discarded, but save the remaining solution to test for Cu²⁺ in Part C.

Part C: Analysis of Group C Ions (Cu²⁺)

C1. Pour the solution from step B5 into an evaporating dish and heat the solution gently under the hood with a Bunsen burner. Try to avoid actively boiling the solution since this will tend to splatter the residue. Slowly evaporate the solution to dryness. 

C2. Once all the water has been evaporated, continue heating the evaporating dish gently for several minutes. If the residue was highly colored, continue heating until only a pale colored solid remains.

C3. Dissolve the residue in 2 mL of 1M H₃PO₄.

C4. To the above solution, add a spatula tip of KI and stir. The formation of a yellow/brown solution with a tan precipitate verifies the presence of copper.

 Part D: Analysis of an Unknown

Repeat the above procedure on an unknown that your instructor gives you. There will be anywhere from 1 to 5 ions in your unknown solution.

Data Sheet