Presentation on theme: "Qualitative Analysis Laboratory 30 points Name: For your laboratory report, please turn in just answers to the questions on the last slide. Handwritten."— Presentation transcript:
Qualitative Analysis Laboratory 30 points Name: For your laboratory report, please turn in just answers to the questions on the last slide. Handwritten responses will not be accepted.
Colorimetric methods of analysis have been developed for most of the elements and ionic species. These methods can be employed to quickly determine the chemical composition of an unknown substance or solution. Recall that in solution, ionic compounds can either separate into free ions (soluble) or form precipitates (insoluble salts). Before attempting to analyze an unknown it is useful to develop a logical process to follow. Care must be taken in sampling, collecting and washing precipitates, performing confirmatory tests and most importantly, recording data. It is also good laboratory practice to always analyze a known with each unknown as a double check on the reagents being used. Introduction
The halides (chlorine, bromine and iodine) all form insoluble salts (precipitates) with silver. Silver chloride is a white precipitate where as silver bromide is beige and silver iodide is yellow. The presence of silver chloride can be confirmed by dissolving the precipitate in ammonium hydroxide. Other silver halides are not soluble in ammonium hydroxide. Bromine can be confirmed using potassium permanganate and iodine can be confirmed with ferric nitrate reagent. Phosphate ions can be detected using ammonium molybdate reagent and stannous chloride. Nitrates can be detected using diphenylamine reagent, and sulfate using barium chloride reagent. Introduction (cont’d)
You will be provided with three unknown solutions containing any combination of six possible anions: Cl - Br - I - PO 4 2- NO 3 - SO 4 2- Determine which ions are present in each of the three unknown solutions. Objective
Test Solutions 100mL 0.1M NaBr 100mL 0.1M NaCl 100mL 0.1M Na I 100mL 0.1M NaNO 3 100mL 0.1M Na 3 PO 4 100mL 0.1M Na 2 SO 4 Materials Equipment disposable pipets test tubes / rack stoppers funnels filter paper scupula deionized water waste container(s) Other Solutions 3 Unknown Solutions 225mL 6M HNO 3 100mL 6M NH 4 OH 100mL mineral oil 25mL (NH 4 ) 6 Mo 7 O 24 (ammonium molybdate) 25mL SnCl 2 25mL (C 6 H 5 ) 2 NH (diphenylamine reagent) 25mL BaCl 2 25mL AgNO 3 25mL KMnO 4 25mL Fe(NO 3 ) 3 Safety: the nitric acid and ammonium hydroxide can cause severe burns and shortness of breath if inhaled. Wear goggles and wash you hands thoroughly after handling the chemicals!
Procedure Halides 1) Label one test tube for each test and unknown solution. Using a disposable pipet, add about 1mL (about 20 drops) of the 3 test solutions and the 3 unknown solutions to the appropriate test tubes. Add a few drops of acid to each tube. 2) Add about 10 drops of AgNO 3 reagent to each tube and mix. A precipitate (solid) will form in any tube containing halides. 3) Using filter paper and a funnel, filter the liquid from those tubes that have a precipitate and wash the precipitate with deionized water. Transfer (scrape) precipitates into new test tubes. 4) Add 0.5 ml (about 10 drops) 6M NH 4 OH to each precipitate. 5) Into new test tubes, place fresh 1.0 mL (20 drop) samples of the 3 test solutions and any unknown solutions that tested positive for halides. Acidify the samples as in step 1. 6) Add 1.0 mL Fe(NO 3 ) 3 reagent and mix. Add 1.0 mL of mineral oil, stopper the tube and shake to mix. Allow the layers to separate. A pink/purple color in the oil level confirms the presence of iodide. 7) Pour off the mineral oil layer and discard. Add 1.0mL of KMnO 4 reagent to the tube and mix. Add 1.0mL of mineral oil, stopper the tube and shake to mix. Allow the layers to separate. A yellow to brown color in the oil level confirms the presence of bromine.
Procedure (cont’d) Phosphate 8) Add about 1mL (20 drops) of the Na 3 PO 4 test solution and 3 unknown solutions to labeled test tubes. 9) Add 1.0mL of the ammonium molybdate reagent to the test tubes and mix. Add one to two drops of SnCl 2 reagent and mix. The formation of a blue color is a positive test for phosphate. A green or yellow color is negative. Nitrate 10) Label one test tube for the NaNO 3 test solution and the 3 unknown solutions. Add about 1mL of the test and unknown solutions to the appropriate test tubes. 11) Add 1.0mL of the diphenylamine reagent to the test tubes and mix. The formation of a blue color is a positive test for nitrate. Sulfate 12) Label one test tube for the Na 2 SO 4 test solution and the 3 unknown solutions. Add 1mL of the test and unknown solutions to the appropriate test tubes. 13) Add a few drops of nitric acid to each tube and mix. 14) Add 1.0mL of BaCl 2 solution to each tube and mix. The formation of a white precipitate is a positive test for sulfate.
Observations: Polyatomic Ions Test addedUnknown 1 Unknown 2 Unknown 3 Na 3 PO 4 NaNO 3 Na 2 SO 4 (NH 4 ) 6 Mo 7 O 24 + SnCl 2 green/ yellow green green/ yellow (C 6 H 5 ) 2 NHclear blueclearblueclear HNO 3 + BaCl 2 white pptno ppt white ppt
Laboratory Report Guidelines Data Analysis 1) (5pts) Explain why Cl -, Br -, I - are all called ‘halides.’ 2) (5pts) Give the chemical names for each of the unnamed chemicals (see Materials section). 3) (5pts) When you tested for the halides, you used silver nitrate to check for precipitation. Write the equations (complete or net ionic) showing the formation of these precipitates. Conclusion 1) (5pts) Which anions are present in which unknown solutions? Explain the details behind your reasoning. 2) (5pts) How does this experiment apply to the real world? Give two specific examples. Error Analysis 1) (5pts) What was the purpose of the 6 test solutions if we already knew what ions they contained?