sulfite and acid -- continued

From: Wilco Oelen (photo_at_woelen.nl)
Date: 07/13/04 

Date: 13 Jul 2004 16:11:04 -0700

Hello,

Still working on the acid/sulfite puzzle (see postings "iodine and
sulfite in acidic medium" and "unexplained yellow color in
acid-iodine-sulfite system"). Till now, this is not yet solved.

I think I can contribute a little to the answer. It looks indeed as if
SO2 is oxidizing in acidic medium, but only to a little extent. This
theory was posted in a previous posting already, but the following
experimental data seems to confirm this. With iodide it looks as if
some iodine is formed (which then immediately combines to tri-iodide
with excess iodide), now a similar observation can be made with copper
(I) at the place of iodide and copper (II) at the place of iodine.

Aqueous copper (I) solutions mostly are colorless. Copper (I) cannot
exist as plain aqua ion, but in moderately concentrated HCl it can
exist as colorless complex CuCl2(-). Copper (II) exists as green
CuCl3(-) and/or yellow CuCl4(2-) in moderately concentrated HCl. When
a solution contains both copper (I) and copper (II), then it has an
intense brown color. This intense color is (as far as I know) caused
by a copper (I)/copper (II)/chloride complex. The dark color probably
is due to the mixed oxidation state in the copper-chloride complex,
but the ligand and charge transfer specialists in this group probably
can tell more about the cause of this intense color.
Observing the dark complex is not difficult: Take some concentrated
HCl and dissolve some CuSO4.5H2O or some CuCl2.2H2O in it. Add some
copper wire to the resulting yellow/green solution and voila, the
liquid becomes dark brown very quickly. When an excess amount of
copper is used and air is excluded carefully, then after a long time
(many hours), the liquid becomes colorless (all copper (II) is reduced
to copper (I) by the metallic copper, the metal itself being oxidized
to copper (I) as well). As soon as some air is allowed to reach the
liquid, it becomes brown again.

I did the following experiments with sulfite:

Dissolve some copper sulfate or copper (II) chloride in HCl (appr. 10%
by weight). The liquid becomes green. Add an excess amount of sodium
ascorbate or ascorbic acid to this. When this dissolves, the liquid
becomes almost colorless. Cu(2+) is reduced to Cu(+). The very faint
yellow color which remains I think is due to an oxidation product of
ascorbic acid. I have seen that color often, when working with
ascorbic acid. If some sodium sulfite is added to this liquid, then
the liquid becomes gold/yellow/brown, looking like a dilute
copper(I)/copper(II)/chloride complex. Even large excess amount of
ascorbic acid is not capable of removing this color.

O.t.o.h., if I add sodium sulfite to a green copper(II)-solution in
dilute HCl, then it hardly changes color. The green becomes a little
less bright, but that's all. Apparently, sulfite is not capable of
reducing copper (II) to copper (I) in strongly acidic medium, at least
not instantaneously. If I add an excess amount of ascorbic acid to
this, then the liquid becomes gold/yellow/brown again on dissolving
the ascorbic acid.

This behaviour looks very much like the behaviour with iodide. In the
presence of sulfite, with copper-salts, most of the copper is present
as copper (I) in strongly reducing environments, but a small amount
remains present as copper (II), otherwise there would not be such a
gold/brown color. With iodide I think that most is present as iodide,
but a small amount is present as iodine, causing the yellow color, as
mentioned in previous postings.

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