posted by David on .
IN my experiment, I was doing a qualitative analysis of Group I and II anions using a flowchart. IN the chart I added 5 drops of 0.1M AgNO3 and several drops of 3M HNO3 to 5 drops of solution and found that a precipitate was formed. Thus it was classified as a Group I anion. Next, to 5 drops of fresh sample I added 5 drops of 0.1 M AgNO3 and observed a precipitate once again. Then I centrifuge the solution and discarded the liquid. Then I added 1mL concentrated NH4OH and 1mL water and stirred well. I once again observed another precipitate and then to 3 drops of fresh sample, add 3M HNO3 until just acidic. Add 3 drops of NaOCl solution and then 1mL hexane. Then I had to observe the color of the hexane layer. If it turned yellow it was Br- and if it was brown or purple then it was I-.
My question is: What is going on in the hexane layer at the end of the Group I anion experiment?
The Group I anion has been oxidized to its molecular form: Br2 or I2 in the earlier steps. When you shake two layer mixture, the molecular (as opposed to ionic) Br2 or I2 are extracted into the hexane layer and give it the characteristic color or of one of those molecular forms.
The NaOCl has oxidized any bromide or iodide in the unknown to free bromine (the element) or free iodine (the element) respectively. Neither bromine nor iodine are appreciably soluble in water but they are quite soluble in hexane (hexane is a non-polar solvent and bromine and iodine are non-polar elements while water is a polar solvent). The yellow color (if it is bromine) and the violet color (if iodine) are caused by London dispersion forces (instantaneous dipole-induced dipole) between Br or I atoms since these are the only forces acting in the non-polar solvent. (The black color for I2 PROBABLY is the result of a large excess of I2 and the violet color can look almost black if it is very intense).I hope this explanation is what you are looking for. By the way, in this particular question I could tell what was going on; however, not all qualitative schemes are the same and sometimes it is helpful if we know what anions are in group I. It is interesting to me that you are doing a qual run on ANIONS. I did several runs on Groups I, II, III, IV, and V of anions (in addition to the usual group I thorough V of cations. But later years have seen the decline of anion analyses, almost to the point of extinction. I'm glad to see it coming back. There's a lot of good chemistry in anion qual tests.