Thioglycolic acid?

Question

Because thioglycolic acid is diprotic (it has two H+ to donate) am I right in assuming that its redox potential is dependent on pH even though the half reaction does not have H+ or OH- transfer when it is reducing?

Answer 1

Yes it is.

Standard potential will generally refer to standard conditions, i.e. [H+] = 1, when it will be the free acid. The SH group is weakly acidic, and under physiological conditions will be partly dissociated (the carboxylic acid will of course be fully dissociated), so that the potential will be pH-dependent in a rather tricky way.

Standard acid: reducing agent is free glycollic acid

pH anywhere near 7.1 or so, reducing agent is mixture of -O2C.CH2SH and -O2C.CH2S-

High pH: reducing agent is the dianion.

Just to confuse you, the real reaction may well be H atom transfer, in which case the pH dependence of the reducing couple will in part be cancelled out by a shift in the oxidizing couple.

For reason, it may actually help to consider the total reaction rathr than the two half-reactions.

Hope this helps.

Answer 2

Are you absolutely sure that there are no H+ involved if the half reaction is balanced in acid?

And you're right -- if there is no H+ involved when the half reaction is balanced in acid, then the standard potential *for thioglycolic acid* should be, to a first approximation, independent of pH. But the standard potential of thioglycolate monoanion might be different. So if that is the main species present at a particular pH, you could have a more strongly reducing system.