Some time ago I purchased a solution of 57% HI from a chemical supplier. It was colourless when bought, but quickly discoloured to the dark red-brown described above. The iodide ion, I
-, is colourless, but HI acts as a reducing agent by itself being oxidised to iodine, I
2, which has a rather distinct colour. With pure HI as starting material, iodine will be the source of any coloured impurities.
The same phenomenon occurs with alkyl iodides; the ethyl iodide I have has also taken on a brownish colour. Alkyl iodides are often stabilised by the addition of metals such as copper or silver, which seem to inhibit iodine formation. Unfortunately I can find no references for this working in the presence of a strong acid such as HI, as adding metals to acids generally results in the production of hydrogen and a metal halide:
2 HI + M
__> H
2 + MI
2 (where M is a generic transition metal such as Cu or Ag)
From a chemical perspective, there is no point in removing the colour, except maybe to inhibit any potential side reactions from the action of iodine on (pseudo)ephedrine. The initial dark HI solution I had was used in an HI/P reduction, giving a 76% yield of product after a 36 hour reflux (see the final step of
Post 475219
(Kinetic: "Benzene -> methaephetamine", Novel Discourse)). Very quickly on heating, the phosphorus present reduced the iodine back to iodide, making the reaction beautifully colourless for its entirety.
If you want to reduce any iodine present to colourless iodide simply to keep others happy, a literature search for suitable reducing agents indicates that formic acid, among other reagents, is suitable. The ancient reference given is
Monatsh. Chem.; Vol. 25; (1904); 591 - 591, a journal I don't have access to. The reaction is simply:
I
2 + HCOOH
__> 2 HI + CO
2Since the only byproduct from this reduction is carbon dioxide, there should be no problem with interference in the subsequent reduction of (pseudo)ephedrine.
Other reagents that should keep HI colourless are phosphine, sulfur dioxide, red phosphorus, phosphorous acid and hypophosphorous acid. Although some may require heating to actually
remove any colour, the formation of coloured iodine is relatively slow, so all of these reagents should supress the reformation of iodine from HI. As a guide, the chemical suppliers Lancaster stabilise their HI with 1.5% hypophosphorous acid.
After finding the reference about the use of formic acid to produce HI from iodine, I couldn't help but wonder if it could be a viable alternative to red phosphorus/phosphorous acid/hypophosphorous acid as a recycling agent in the HI reduction. I can't see any way in which it could hinder the reduction. Maybe someone could give it a try.