OH- spread more than H+ ions?

W

WAYNEL

Guest
I've conducted a simple experiment on a glass sample that has two clean
copper electrodes biased with 20v DC. Between the two electrodes I
have placed a drop of de-ionised water with Universal Indicator added.

Over a period of time the electrodes changes colour, as expected, with
the cathode going purple (OH-) and the anode going red (H+).

After a longer period of time the colours start to spread out toward
the opposing electrodes, pH gradient. However, the amount the purple
(OH-)spreads is apx three times greater that that of the red (H+).

I have repeated this 10 times in random positions and I get the same
results.

I would have thought that the H+ ions would have more mobility than the
OH- ions and thus I would have expected the opposite to happen.

Can anyone help and through some light on this phenomena, or have I
missed something?


Cheers

WayneL
 
Hi Mike

Thanks for your detail response. I have re-run the test using graphite
electrode and 18.3MG pure water with Universal Indicator. I get the
same results. From what you are saying I guess the H+ ion and the OH-
ions are attaching themselves to something in the Universal Indicator?

Cheers

Wayne
 
The reason is probably because H+ *does* move faster than OH-. H+ has
a limiting ionic conductivity of about 350 S.cm^2/eq and OH- has a
limiting ionic conductivity of about 200 S.cm^2/eq. Limiting ionic
conductivities are directly proportional to the mobility (consult any
physical chemistry text). They are 4-7x more mobile than any other ion
due to the Grotthuss transport mechanism. Glass provides plenty of -OH
groups, and even more importantly, loads of adsorbed water (which I
doubt you controlled for), to allow this mechanism to occur.
 
Oops I misread..OH- more than H+, so this has nothing to do with ion
mobility. Also, there is a lot more going on here than H+ and OH-
migration if you add universal indicator, which usually contains a brew
of various indicators that are quite large molecules. I think you need
to consider the mechanism of reaction between the relevant indicator(s)
in acid/base. Just for example, methyl orange is a large diimide
contains a sulfonic acid group that changes from red to yellow on
deprotonation. Red/purple sounds like two different indicators to me -
you could easily have two different charged large indicator molecules
moving towards each electrode.
 
I thought that cation have grater mobility than anions?

WayneL
 
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