Colour Changing Questions

An often indispensible item in redox titrations is the indicator, ferroin. According to Wikipedia, ferroin is a complex between Fe²⁺ and three 1,10-phenanthroline molecules. It changes colour from red to blue when endpoint is reached.

At work, I work with ferroin quite a bit, albeit always doing the same few reactions. My reagents, immediately prior to titration, are usually colourless to pale yellow/brown. (Sometimes the reagents foam when I collect them...which makes them look like beer. But I digress.)

Naturally, right after I add the indicator, the solution is vibrantly orange-red. Also naturally, the reacting mixture will deepen in colour (but remain bright red) as titrant is being added. Why is it that, at endpoint, the colours I end up with are noticeably different, depending on which analyte I started with?

Obviously, the analyte is the main variable here, as the titrant is always Ce(SO₄)₂, and the indicator is ferroin. But I am fairly sure that the molecules that we are testing for are colourless both before and after oxidation (not as sure about whether they form complexes, though...but with ferroin?). Also, some reagents have strikingly different initial colours (due to dyes and such) before adding the ferroin, and end up with the same colour at endpoint. However, the colours are always consistent for a particular type of analyte.

• For one reaction, I get a very pretty cyan, which is expected. Interestingly enough, the solution will actually become a dull grey before becoming cyan (colour change occurs with less than 1 drop difference of titrant)
  
• For a second reaction, I get pale green, which is different enough from the cyan to merit notice.
  
• For a third reaction, I get a very deep yellow-brown. Note that the molecule to be tested in this third reaction is the same as the molecule in the second; however, the preparation methods are different.

I have a feeling that the different colours are due to the numerous side reactions that take place, as all the analytes are mixtures containing many different species. And no, I don't really know what goes into the mixtures, since I am testing for a few specific molecules. Hmm, something to think about...can't really run experiments though *sad*.

Another really cool colour change occurs when sulfuric acid is added: the colours of certain solutions change quite a bit upon the addition of ~20 mL 9 M sulfuric acid. Examples: a dusky purple to pale pink, dark grey to pale magenta, orange-red to golden yellow. That might be due to the oxidizing abilities of sulfuric acid, or my eyes playing tricks on me, though I am not sure either way.

A third colour change is observed when washing out flasks used for iodometry. There are two types: titration of anionic sulfur oxide species (e.g. thiosulfate, sulfite) with a dilute iodine solution, or back-titration of the same by generating the iodine in situ and titrating the excess with thiosulfate. Obviously, the indicator is starch. The colour changed observed is either from clear/white (opaqueness due to starch) to dark blue-purple, or vice versa. The neat thing is, when flasks containing a clear mixture are washed out, everything becomes blue. The exact shade of blue depends on how much water was used to rinse the flask. At least the cause of this seems fairly clear: as the iodine-starch complex is very intense in colour, even very dilute solutions will appear blue. If there are any vestiges of iodine in the flask, washing the flask out will make the resulting wash water blue. Feel free to correct me if I am wrong, though.

Sorry for the lack of pictures; I can't exactly take them when I am being timed on how fast my lab skills are (answer: they aren't). Never let it be said that inorganic aqueous chemistry is boring.