The Qualitative Analysis Process

Klein High School - Chemistry 1 Final Laboratory Investigation: Identification of Unknown Solution

The Qualitative Analysis Process: 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, using a series of chemical reactions. You will begin with a sample of seven metallic ions in aqueous solution, then add substances that will react to create new, insoluble products in order to separate the ions from one another. Confirmatory tests are used to prove the presence of a suspected ion in the isolated substance.

In this investigation, you will first perform a step by step analysis of a solution that is “known” to contain all of the ions of interest. You will use this analysis to learn the techniques of the procedure and to observe the positive reactions of each ion of interest. Recording Observations: As you go through the steps of the analysis, you will develop a flow chart that will serve as your reference for the later analysis of the unknown solution. This flow chart will help you to see the total picture of the separation and identification. Read the directions for each step carefully and be sure that you fully understand each result before moving on to the next.

Keep good records of everything that you do and every observation as you proceed. Record in your own words exactly what the products look like so that you will be able to identify a positive result when necessary. Record each step as it happens, don’t trust your results to memory – write them down. Procedural Notes: Heating: It may be necessary to heat a solution to speed up the reaction. Do NOT heat small test tubes over Bunsen burner flames. A sudden steam bubble could cause the solution to shoot out of the test tube. Instead, heat the test tubes in a boiling water bath. Separating Solids from Solutions: Centrifugation will separate solids by packing the high density solid into the bottom of the test tube. When centrifuging, remember that the centrifuge must be counter balanced with similar tubes holding equivalent volumes opposite one another in the rotor. For the solutions we will be using, a short centrifugation period is generally all that is necessary. Time may vary from one type of centrifuge to another, check with your teacher for exact timing instructions. Once the tubes are removed from the centrifuge, the supernatant (the liquid above the precipitate) can be poured off, leaving the precipitate (solid) in the bottom of the tube. It is always better to leave a little liquid over the precipitate than to accidentally transfer some of the precipitate. Washing Precipitates: In order to completely remove any possible contaminates from the supernatant, it is advisable to wash the precipitate with distilled water before continuing with the confirmation test. To do this, add 1-2 ml of distilled water to the precipitate, stir, centrifuge, and discard the wash water. Checking pH: To determine the acidity or alkalinity of a solution, use a piece of litmus paper. Dip the stirring rod into the solution in the test tube, and touch the stirring rod to the paper. Do NOT dip the test paper into the test tube, as this could cause some of the indicator dye to dissolve in the solution and interfere with later tests. Storing Solutions: At the end of each lab period, be sure to stopper the test tube with a cork stopper. If a precipitate is present, put a few drops of distilled water on it before stoppering the test tube. Be sure to include in your notes a list of what substances are present in each test tube. Don’t just trust your memory until the next lab period. Disposal of Solutions: Many of the ions to be tested are poisons or are hazardous to the environment. Solutions containing silver and lead in particular should not be washed down the sink. Use a few drops of water to suspend Qual Lab precipitates containing these ions in solution and dispose of them in the chemical waste disposal container in the hood.

Safety Precautions: Acids and bases used in this investigation are relatively concentrated and can cause chemical burns if spilled. If you come into contact with acid or base, wash the affected area with copious amounts of water. Small spills can be cleaned up with paper towels. If a large spill occurs, notify the teacher immediately for clean-up instructions. Solutions containing silver ions cause stains which do not appear for several hours. If you think you spilled a silver containing solution on yourself, wash thoroughly with soap and water. As always, goggles and lab apron should be worn at all times in the lab.

Procedure: Prior to beginning the investigation, you and your lab partner will need to provide the following supplies: A roll of paper towels One gallon of distilled water A roll of blue painters tape A small bottle of dishwashing liquid

In preparation for beginning the investigation, obtain 5 test tubes. Wash them with soap and water, rinsing thoroughly with tap water, followed by a final rinse with distilled water. Once dry, label the tubes 1-5 and place them in a test tube rack. To begin the investigation, take a clean test tube to your instructor and obtain about 2 ml of a mixture containing the seven cations of interest – Ag+, Pb+2, Fe+3, Cu+2, Mn+2, Co+2, and Ni+2. Transfer 25 to 30 drops of the solution to another test tube for analysis and stopper the remaining known solution. This will be your back-up solution should you need to repeat any tests or start over. If you completely exhaust your known solution and need more it will result in a point deduction from your final grade. Analyzing the Known Solution: 1) Observe your known solution: In aqueous solution, Ag+ and Pb+2 are colorless. Fe+3 has a yellow color, Cu+2 is blue, Mn+2 is an extremely pale pink, Co+2 is a deeper pink and Ni+2 is green. 2) Separation of the Silver Group (Ag+ and Pb+2) from manganese, cobalt, iron, copper and nickel: - Although most chloride salts are soluble, Ag+ and Pb+2 form insoluble chlorides, thus these can be precipitated while the others remain in solution

Ag + (aq) + Cl- (aq)  AgCl (s) +2 - Pb (aq) + Cl (aq)  PbCl2 (s) Add 8 drops of 6M HCl to the sample of known solution. Stir. A white precipitate indicates that one or both of the ions Ag+ or Pb+2 are 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 6M HCl until precipitation is complete. Centrifuge and decant (pour off) the clear liquid (supernatant) into a second test tube and save for step 5. 3) Separation and Confirmation of Lead: Add 1 ml of distilled water to the precipitate from step 2 and heat in a boiling water bath for about 3 minutes with occasional stirring. Lead chloride is considerably more soluble in hot water than cold water, so this will cause the lead chloride to return to solution, while the silver chloride remains in the precipitate.

+2 - PbCl2 (s)  Pb (aq) + 2Cl (aq) +2 -2 Pb (aq) + CrO4 (aq)  PbCrO4 (s) Centrifuge the hot solution and quickly pour off the supernatant. Save the precipitate for step 4 Qual Lab

To the clear liquid, add 5 drops of K2CrO4. a yellow precipitate of PbCrO4 confirms the presence of lead. Dispose of the lead chromate in the chemical waste container. 4) Confirmation of Silver: To the precipitate from step 3, add 1 ml of 6 M NH4OH. The precipitate should redissolve.

+ - AgCl (s) + 2NH3 (aq)  Ag(NH3)2 + Cl (aq)

Centrifuge and decant discarding any remaining precipitate. Add 15 drops of 6 M HCl to the clear solution. The solution will smoke and give off heat as the acid breaks apart the silver ammonia base complex. The test tube may get very hot. Stir and test with litmus paper to be sure the solution is acidic. Add more HCl drop by drop until it is acidic. + - + + Ag(NH3)2 (aq) + Cl (aq) + 2H (aq)  AgCl (s) + 2NH4 (aq) The reappearance of a white precipitate confirms the presence of silver. Discard the silver compound in the chemical waste container. 5) Separation of Mn+2, Fe+3, and Cu+2 from Ni+2 and Co+2: To the liquid remaining from step 2, carefully add 6M NH4OH until basic - a precipitate should form that does not redissolve. Stir well and re-check for basicity, then add 1 ml excess NH4OH. Stir well and centrifuge. Decant the liquid into a clean, labeled test tube to save for step 10. In a basic solution, hydroxides of the Mn+2,Fe+3 and Cu+2 will precipitate, while the complex amines of Ni+2 and Co+2 remain in solution.

+2 +2 Ni (aq) + 4 NH3(aq)  Ni(NH3)4 (green) (blue) +2 +2 Co (aq) + 6NH3(aq)  Co(NH3)6 (pink) (brown) +3 - Fe (aq) + 3 OH (aq)  Fe(OH)3 (s) (yellow) (brown) +2 - Mn (aq) + 2OH (aq)  Mn(OH)2 (s) +2 – Cu (aq) + 2OH (aq)  Cu(OH)2 (s) (blue) (blue)

6) Separation of Mn+2 from Fe+3 and Cu+2: Manganese (IV) oxide does not dissolve well in acid or base unless it is oxidized or reduced. The other hydroxides that are present will readily dissolve in acid solution.

+ +3 Fe(OH)3 (s) + 3H (aq)  Fe (aq) + 3 H2O (l) + +2 Cu(OH)2 (s) + 2H (aq)  Cu (aq)+ 2 H2O (l)

Add 5 drops of water to the precipitate from step 5. then add 6 M H2SO4 dropwise until the solution is acidic when tested with litmus paper (it should take about 6 drops). Centrifuge and separate the precipitate, which may be dark brown to black, from the supernatant liquid, which will contain the yellow Fe+3 and blue Cu+2 ions, and save the precipitate for step 7. Save the supernatant for step 8.

7) Confirmation of Manganese: To the precipitate from step 6, add 1 ml of water and 1 ml of 6 M H2SO4. Add 1 ml of 3% H2O2 and heat in a boiling water bath with stirring. The precipitate should dissolve quickly leaving very little residue. Manganese oxide will dissolve in acid solution when a reducing agent is present. + +2 MnO2(s) + H2O2(aq) + 2H (aq)  Mn (aq) + O2 (g) + H2O (l) Qual Lab

To the solution, add 1 ml of 6M HNO3, and a spatula of solid sodium bismuthate, NaBiO3. Some excess solid sodium bismuthate should remain. Add more if needed. Stir and centrifuge to - determine the color of the supernatant liquid. The purple color of the MnO4 ion in the solution confirms the presence of manganese. Dispose of the manganese in the chemical waste container provided.

+2 + - - +3 2Mn (aq)+14H (aq) 5BiO3 (aq)2MnO4 (aq) +5Bi (aq)+7H2O (l) 8) Separation of Iron from Copper and confirmation of Copper: To the solution from step 6, add 6 M NH4OH until the solution is basic when tested with litmus, then add 1 ml more. Centrifuge and decant. Save the precipitate for step 9. The bright blue color indicates the possible presence of copper, absence of a blue color does not mean copper is not

present. To confirm, add 6M HC2H3O2 until the blue color fades and/or the solution tests acidic. Add 2 drops of 0.1M K4(Fe(CN)6). A red-brown precipitate reconfirms the presence of copper. Dispose of the copper solution in the chemical waste container.

+2 +2 Cu (aq) + 4NH3 (aq)  Cu (NH3)4 (aq) (bright blue) +3 - Fe (aq) + 3OH (aq)  Fe(OH)3 (s) 9) Confirmation of Iron:

Wash the precipitate from step 8. Add 6 M H2SO4 dropwise until the precipitate dissolves. Add 5 drops of 0.1M KSCN solution. The deep red FeSCN+2 ion confirms the presence of iron. Dispose of the iron in the chemical waste container provided.

+ +3 Fe(OH)3 (s)+ 3H (aq)  Fe (aq) + 3H2O (l) Fe+3(aq)+ SCN- (aq)  FeSCN+2 (aq) 10) Confirmation of Nickel and Cobalt The solution from step 5 should be colored if either nickel or cobalt is present in large quantities. To the solution, add 6M HCl until litmus tests acidic. Stir well after each addition. The addition of HCl will re-dissolve the amine complexes

+2 + +2 + Ni(NH3)4 (aq) + 4H (aq)  Ni + 4NH4 +2 + +2 + Co(NH3)6 (aq) + 6H (aq)  Co + 6NH4 Divide the solution in half. To one half add NH4OH until basic according to litmus test, then add 2 ml DMG, dimethylglyoxime. The formation of a red/pink precipitate confirms Ni+2. +2 Ni + C8H14N4O4(aq)  Ni C8H14N4O4(s) (pink/red) To the other half of the solution, without mixing, add 2 ml NH4SCN (ammoniumthiocyanine) in acetone. The formation of a blue to blue green solution confirms Co+2. Dispose of both solutions properly. +2 - +2 Co (aq) + 4SCN (aq)  Co(SCN)4 (aq) (pink) (blue)