Lab: Spectroscopy and Qualitative Analysis in theDetermination of an Unknown Iron Salt

Procedure

Part A:

Obtain one of the provided kits and inspect for completeness(see available list in the laboratory). Ask your instructor forreplacement items if necessary. Select one of the available unknowniron salts. All of the iron salts (which may or may not behydrated) are in the +2 oxidation state and will have one of thefollowing anions: Cl^-, Br^-, SO4^2- and CH3COO^- (acetate).Finally, obtain ~ 30 mL of the standard iron(II) solution (recordthe concentration).

2. Using the 10 mL beaker, weigh out ~ 0.0400 to 0.0500 g(±0.0001 g) of your salt. Record the mass dispensed. Transfer thesalt to a 100.00 mL volumetric flask using a small amount of water.Rinse the beaker 3 to 4 times; transferring the washings to theflask each time. Allow the iron salt to completely dissolve andthen dilute to the mark.

3. Using the pipette from your kit, transfer 1.00 mL of thesolution from step 2 into a 100.00 mL volumetric flask. To theflask add 1 mL each of: 1% hydroxylamine hydrochloride solution, 1M sodium acetate and 1% 1,10-phenanthroline solution from theprovided burettes. Dilute to the mark and mix thoroughly. This isthe unknown solution you will measure the absorbance of.

4. Rinse and fill a burette with the standard iron solutionobtained in step 1. Prepare the standard solutions as follows:accurately dispense – recording initial and final burette readings(±0.02 mL) – approximately 1.00, 2.00, 3.00 and 4.00 ml of thestandard Fe^2+ solution into separate volumetric flasks. As in step3, add 1 mL each of: 1% hydroxylamine hydrochloride solution, 1 Msodium acetate and 1% 1,10-phenanthroline solution from theprovided burettes. Dilute to the mark and mix thorougly.

5. Using the final 100.00 mL volumetric flask, prepare the blanksolution by adding 1 mL each of: 1% hydroxylamine hydrochloridesolution, 1 M sodium acetate and 1% 1,10-phenanthroline solutionfrom the provided burettes. Dilute to the mark andmixthoroughly.

6. To analyze your samples ask the lab demonstrators forinstruction on the proper use of the spectrophotometer. Obtain theabsorbance of each of the standards and the unknown at 508 nm.

Part B:

Qualitative Analysis

Transfer some of your salt (tip of a scoopula) into a beaker anddissolve it in ~ 30 mL distilled water. If you are finding itdifficult to dissolve your salt, add two to three drops of 6M HNO3and mix.Transfer equal portions (about 2-3 cm in height) of thissolution into two test tubes:

1. Chloride and Bromide. Add 3 drops of 6M HNO3 to the solutionin the first test tube and shake well. Add 5 drops of 0.1M AgNO3and mix well. If a precipitate has formed upon the addition ofsilver nitrate a positive test for chloride or bromide hasoccurred. If no precipitate formed you do not have either thechloride or bromide salt and can move on to the sulfate test. Todifferentiate between chloride and bromide, add 10 drops of 6M NH4OH to the mixture (do not mix). If two distinct layers formimmediately, then the chloride anion is present.

2. Sulfate. Add 3 drops of 6M HNO3 to the solution in the secondtest tube and shake well. Add 5 drops of 0.1M BaCl2. The formationof a white precipitate indicates the presence of SO4^2-.

3. Acetate. Place a small amount of the solid salt (~ the sizeof a pea) into a test tube. Add 20 drops of concentrated H2SO4(CARE!!!) and 20 drops of 1-pentanol from the dropper bottles inthe fume hood to the test tube and mix with a glass stirring rod.Place the test tube in the water bath provided (making a note ofwhich one is yours) for about 5 minutes. Remove the test tube fromthe bath and check the odour of the solution by bringing your noseslowly to the test tube and waving your hand over the top of thetest tube towards your nose. You can also pour it onto a watchglass to make it easier to pick up the odour. A fruity odour (itoften reminds people of fake banana smell) indicates the presenceof the acetate ion.

_______My Data Below

Concentration of standard iron(II) solution = 50.0 ppm

Weight of unknown salt: 0.0413 g (+- 0.0001 g)

Fe^2+ Solution (mL)	Volume (final)-Volume (initial) = Change in Volume (+-0.04 mL)
1.00	1.01
2.00	1.89
3.00	3.02
4.00	4.01

Fe^2+ Solution (mL)	Calculated Concentration (ppm)	Absorbance (a.u.
1.00	0.505	0.079
2.00	0.945	0.198
3.00	1.51	0.304
4.00	2.005	0.410
Unknown solution	1.35	0.267

The positive ion test determined that the unknown salt wassulphate (SO4^2-).

Question: Using the mass of the sampleanalyzed, the dilutions you performed and the positive anion testdetermine: the identity of your unknown salt, its molecularweight and the water of hydration.