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Reference electrodes

OK yesterday was an interesting day the electric power went off during a thunderstorm at about 13:15 which brings me onto the topic of electrochemistry; I tried to write as long as the electrochemical cells in my laptop allow me to.

I was recently at a nuclear research centre where fuel is placed inside a research reactor to allow a range of different reactors to be simulated both under their normal conditions and under accident conditions. One of the things which I saw was the mention of a Fe/Fe3O4 reference electrode.

Before we go any further you need to know what a reference electrode is, it can be thought of as a stake driven into the ground and a landmark. This allows to locations of other things to be measured relative to the stake.

The classic reference electrode is the normal hydrogen electrode, this is the primary standard but it is a real dinosaur.

H2 → 2H+ + 2e

It requires a supply of good clean hydrogen (fire risk) and it is not easy to use. The potential is controlled by the proton concentration in the half cell and the partial pressure of the hydrogen. Much of the time people want to use electrodes which are more simple to use.

A classic which is an old friend of mine is the saturated calomel electrode; please note it is not a caramel electrode! Calomel is mercury(I) chloride (Hg2Cl2) which is a white solid with a low solubility. The electrode operates using the following equation.

2Hg + 2Cl→ Hg2Cl2 + 2e

The electrode consists of some mercury metal covered with some calomel which is then immersed in saturated potassium chloride solution. Hence as a result it is banned in Sweden, I can recall as a first year undergraduate at Imperial College making my own SCE in a teaching lab.

The potential of this electrode is largely controlled by the solubility product of calomel and the potassium chloride concentration.

Needless to say chemists have come up with some alternative electrodes which do not contain mercury and are suitable for aqueous media. One of these is the silver / silver chloride electrode which is used by many electrochemists.

A typical Ag/AgCl electrode consists of a silver wire with a coating of silver chloride which is immersed in a chloride solution. It operates by the following equation.

Ag + Cl→  AgCl + e-

Some other reference electrodes exist such as the copper / copper sulphate electrode which is quite popular for corrosion studies. This one is a copper wire which is immersed in copper sulphate solution. This forms part of the classic Daniel cell. I will let you write out the mechanism for this electrode.

When I saw description of a iron/iron oxide reference electrode which is designed for use under BWR conditions. This reference electrode is intended for use under the rather extreme conditions found in a boiling water reactor. It will be hot and unlike a PWR it will not be possible to maintain a constant concentration of hydrogen. The lack of hydrogen rules out the use of a platinum reference electrode.

I have not been able to find details of the construction of the iron/iron oxide electrode but I have found from the literature details of a copper/cuprous oxide electrode. This electrode is a mixture of copper powder and copper(I) oxide inside a yttrium stabilised zirconia (YSZ) tube.

The half cell will be Cu | Cu2O | ZrO2 | H2O

The reaction in the reference electrode will be

2 Cu + O2-→ Cu2O + 2e-

The YSZ is an oxide conductor, oxide anions are mobile in this solid. So the overall set of equations for the electrode will be

2 Cu + O2-→  Cu2O + 2e- on the central wire

H2O → 2H+ + O2- and HO → H+ + O2- on the surface of the YSZ tube

If we combine the equations we can get a single equation.

2 Cu + H2O + O2- → Cu2O + 2e + 2H+ + O2-

Remove the items which appear on both sides of the arrow.

2 Cu + H2O → Cu2O + 2e + 2H+

As the overall electrode equation has protons in it then the electrode potential will change as a function of pH, this is a disadvantage over electrodes such as the SCE. But on the other hand the SCE is dependant on the chloride concentration inside it.

PS. The power came back on about 1 hour and 15 minutes after it went off.


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