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WAYNEL183
science forum beginner

Joined: 20 Aug 2005
Posts: 26

Posted: Tue Jun 06, 2006 6:25 pm    Post subject: Re: Help with Pourbaix diagrams

You are correct, I forgot to mention that the electrodes are in pure
water. I has assumed, correctly or incorrectly that because it is
biased as a "convential" anode that it is an anode.
I am not a true electrochemist rather an electronic engineer. This may
explain some of the confusion.

WayneL

Dieter Britz wrote:
 Quote: WAYNEL wrote: I have a cell biased with 5v. Both electrodes are copper and the cathode is at 0v and the anode is at +5v. I am trying to read a Pourbaix diagram for copper on the following paper from IBM http://www.research.ibm.com/journal/rd/291/ibmrd2901E.pdf, Can I disregard the -ve part of the diagram as my ref. point is zero and should I only consider the anodic part of the diagram i.e. the +E? If not then it implies that the -E(cathode??) is immune. I suspect there is something here you have not mentioned, that the two electrodes are probaby sitting in pure water (?). People have wondered how you can polarise the anode at +5V; well, most of that is probably iR drop. Am I right? -- Dieter Britz, Kemisk Institut, Aarhus Universitet
Dieter Britz
science forum beginner

Joined: 04 May 2005
Posts: 45

Posted: Tue Jun 06, 2006 7:53 am    Post subject: Re: Help with Pourbaix diagrams

WAYNEL wrote:

 Quote: I have a cell biased with 5v. Both electrodes are copper and the cathode is at 0v and the anode is at +5v. I am trying to read a Pourbaix diagram for copper on the following paper from IBM http://www.research.ibm.com/journal/rd/291/ibmrd2901E.pdf, Can I disregard the -ve part of the diagram as my ref. point is zero and should I only consider the anodic part of the diagram i.e. the +E? If not then it implies that the -E(cathode??) is immune.

I suspect there is something here you have not mentioned,
that the two electrodes are probaby sitting in pure water (?).
People have wondered how you can polarise the anode at
+5V; well, most of that is probably iR drop. Am I right?
--
Dieter Britz, Kemisk Institut, Aarhus Universitet
Robert Copcutt
science forum beginner

Joined: 16 Jun 2005
Posts: 9

Posted: Mon Jun 05, 2006 9:57 pm    Post subject: Re: Help with Pourbaix diagrams

WayneL wrote:
 Quote: Thanks, not so confused now. Get warm in here in the summer. Is there a way of using Pourbaix diagrams with only two electrodes as I an investigating electrochemical corrosion on two electrode systems? Thanks for your patience. WayneL

Without a reference electrode you can only make intelligent guesses
about the chemical potentials of your 2 electrodes. You could do quick
and rough comparisons of coatings using 2 electrodes but to do work
worth publishing a third electrode is essential.
WayneL
science forum beginner

Joined: 03 May 2005
Posts: 33

Posted: Mon Jun 05, 2006 8:48 pm    Post subject: Re: Help with Pourbaix diagrams

Thanks, not so confused now. Get warm in here in the summer.
Is there a way of using Pourbaix diagrams with only two electrodes as I an
investigating electrochemical corrosion on two electrode systems?
Thanks for your patience.

WayneL

<nagy@anl.gov> wrote in message
news:1149522756.570190.230120@c74g2000cwc.googlegroups.com...
 Quote: Wayne, Read about reference electrodes, etc in Electrochemistry Dictionary http://electrochem.cwru.edu/ed/dict.htm Good luck :ZN Robert Copcutt wrote: WayneL wrote: I'm confused, have I misunderstood? Both electrodes are copper. One of them is connected to ground (0V) and the other electrode to a more positive (+5V), hence anode - in water. My question is; on the Pourbaix diagram which part of the diagram corresponds to the Cathode and which side corresponds to the Anode? On my simple cell is the 0v electrode -E of the Pourbaix diagram and the +5v +E? If so where is the 0v E point? WayneL You sound very confused and need to read up on reference electrodes. The hydrogen reference electrode has been assigned a chemical potential of zero volts and Pourbaix diagrams measure all potentials against this standard. Do not get chemical and electrical potentials confused. Just because you have decided to ground the negative terminal does not mean that it is anywhere near 0V as used in the diagram. In order to measure chemical potentials you need at least 3 electrodes - the 2 electrodes that carry the current and a reference electrode that carries negligible current and which is built to have a well defined electrochemical reaction occur on it. You also need to compensate for IR losses in all parts of your system that carry current (think ohms law). Look at figure 7 of the pdf you referenced. The working electrode is the copper plate bottom left. The counter electrode is the dotted bar top left. These 2 electrodes carry the current. They then have a salt bridge to a beaker and top right they have clearly drawn a reference electrode.
nagy@anl.gov
science forum beginner

Joined: 18 Jul 2005
Posts: 16

Posted: Mon Jun 05, 2006 3:52 pm    Post subject: Re: Help with Pourbaix diagrams

Wayne,

Read about reference electrodes, etc in Electrochemistry Dictionary

http://electrochem.cwru.edu/ed/dict.htm

Good luck :ZN

Robert Copcutt wrote:
 Quote: WayneL wrote: I'm confused, have I misunderstood? Both electrodes are copper. One of them is connected to ground (0V) and the other electrode to a more positive (+5V), hence anode - in water. My question is; on the Pourbaix diagram which part of the diagram corresponds to the Cathode and which side corresponds to the Anode? On my simple cell is the 0v electrode -E of the Pourbaix diagram and the +5v +E? If so where is the 0v E point? WayneL You sound very confused and need to read up on reference electrodes. The hydrogen reference electrode has been assigned a chemical potential of zero volts and Pourbaix diagrams measure all potentials against this standard. Do not get chemical and electrical potentials confused. Just because you have decided to ground the negative terminal does not mean that it is anywhere near 0V as used in the diagram. In order to measure chemical potentials you need at least 3 electrodes - the 2 electrodes that carry the current and a reference electrode that carries negligible current and which is built to have a well defined electrochemical reaction occur on it. You also need to compensate for IR losses in all parts of your system that carry current (think ohms law). Look at figure 7 of the pdf you referenced. The working electrode is the copper plate bottom left. The counter electrode is the dotted bar top left. These 2 electrodes carry the current. They then have a salt bridge to a beaker and top right they have clearly drawn a reference electrode.
Robert Copcutt
science forum beginner

Joined: 16 Jun 2005
Posts: 9

Posted: Sun Jun 04, 2006 5:31 pm    Post subject: Re: Help with Pourbaix diagrams

WayneL wrote:
 Quote: I'm confused, have I misunderstood? Both electrodes are copper. One of them is connected to ground (0V) and the other electrode to a more positive (+5V), hence anode - in water. My question is; on the Pourbaix diagram which part of the diagram corresponds to the Cathode and which side corresponds to the Anode? On my simple cell is the 0v electrode -E of the Pourbaix diagram and the +5v +E? If so where is the 0v E point? WayneL You sound very confused and need to read up on reference electrodes. The

hydrogen reference electrode has been assigned a chemical potential of
zero volts and Pourbaix diagrams measure all potentials against this
standard. Do not get chemical and electrical potentials confused. Just
because you have decided to ground the negative terminal does not mean
that it is anywhere near 0V as used in the diagram. In order to measure
chemical potentials you need at least 3 electrodes - the 2 electrodes
that carry the current and a reference electrode that carries negligible
current and which is built to have a well defined electrochemical
reaction occur on it. You also need to compensate for IR losses in all
parts of your system that carry current (think ohms law).

Look at figure 7 of the pdf you referenced. The working electrode is the
copper plate bottom left. The counter electrode is the dotted bar top
left. These 2 electrodes carry the current. They then have a salt bridge
to a beaker and top right they have clearly drawn a reference electrode.
WayneL
science forum beginner

Joined: 03 May 2005
Posts: 33

Posted: Sun Jun 04, 2006 3:10 pm    Post subject: Re: Help with Pourbaix diagrams

I'm confused, have I misunderstood? Both electrodes are copper. One of
them is connected to ground (0V) and the other electrode to a more positive
(+5V), hence anode - in water.
My question is; on the Pourbaix diagram which part of the diagram
corresponds to the Cathode and which side corresponds to the Anode? On my
simple cell is the 0v electrode -E of the Pourbaix diagram and the +5v +E?
If so where is the 0v E point?

WayneL

"Oscar Lanzi III" <ol3@webtv.net> wrote in message
news:25601-44823C1C-56@storefull-3258.bay.webtv.net...
 Quote: You can't call the anode +5v and the cxathode 0v on the Pourbaix diagram. You CAN assume that they differ by 5 V (if there is no loss due to ohmic resistance), but the cathode does not necesarily match ground. The actal electrode potentials relative to the standard hyydrogen electrode will be such that the anodic anc cathodic reaction currents are in blanace. Thus electrochemical kinetics is involved, and things ca nget a little tricky. What would happen if you measured say the cathode potential against a reference electrode? --OL
Oscar Lanzi III
science forum Guru Wannabe

Joined: 30 Apr 2005
Posts: 176

 Posted: Sun Jun 04, 2006 1:49 am    Post subject: Re: Help with Pourbaix diagrams You can't call the anode +5v and the cxathode 0v on the Pourbaix diagram. You CAN assume that they differ by 5 V (if there is no loss due to ohmic resistance), but the cathode does not necesarily match ground. The actal electrode potentials relative to the standard hyydrogen electrode will be such that the anodic anc cathodic reaction currents are in blanace. Thus electrochemical kinetics is involved, and things ca nget a little tricky. What would happen if you measured say the cathode potential against a reference electrode? --OL
guvenay@gmail.com
science forum beginner

Joined: 03 Jun 2006
Posts: 1

Posted: Sat Jun 03, 2006 2:13 pm    Post subject: Re: Help with Pourbaix diagrams

waynel
i am sorry but could you explain how can u polarize the anode and
cathode.
at 5v polarization at the anode means your electrode --copper-- will be
dissolved. but how can willbe zero at catode. can u explain your system
briefly. the pourbaix diagram at your referance is a true diagram for
cu-h20 system.
if you use a dc regulator for polarized the system u can use only the
cahtode or anode side of the diagram but they are related each other.
regards

WAYNEL wrote:
 Quote: I have a cell biased with 5v. Both electrodes are copper and the cathode is at 0v and the anode is at +5v. I am trying to read a Pourbaix diagram for copper on the following paper from IBM http://www.research.ibm.com/journal/rd/291/ibmrd2901E.pdf, Can I disregard the -ve part of the diagram as my ref. point is zero and should I only consider the anodic part of the diagram i.e. the +E? If not then it implies that the -E(cathode??) is immune. Cheers WayneL
WAYNEL183
science forum beginner

Joined: 20 Aug 2005
Posts: 26

 Posted: Fri Jun 02, 2006 8:08 pm    Post subject: Help with Pourbaix diagrams I have a cell biased with 5v. Both electrodes are copper and the cathode is at 0v and the anode is at +5v. I am trying to read a Pourbaix diagram for copper on the following paper from IBM http://www.research.ibm.com/journal/rd/291/ibmrd2901E.pdf, Can I disregard the -ve part of the diagram as my ref. point is zero and should I only consider the anodic part of the diagram i.e. the +E? If not then it implies that the -E(cathode??) is immune. Cheers WayneL
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