The Zinc-SHE Cell

Every oxidation must be accompanied by a reduction (i.e., the electrons must have somewhere to go). So it is impossible to determine experimentally the potential of any rfwg/e electrode. We therefore establish an arbitrary standard. The conventional reference electrode is the standard hydrogen electrode (SHE). This electrode contains a piece of metal electro-lytically coated with a grainy black surface of inert platinum metal (finely divided metals such as Pt often appear black), immersed in a 1 solution. Hydrogen, H2, is bubbled 

By international convention, the standard hydrogen electrode is arbitrarily assigned a potential of exaaly 0.0000. .. volt. 

As we shall see in Section 21-14, this arbitrary definition will provide a basis on which to describe the relative potential (and hence the relative spontaneity) of any half-ceU process. We can construct a standard cell consisting of a standard hydrogen electrode and some other standard electrode (half-cell). Because the defined electrode potential of the SHE contributes exactly 0 volts to the sum, the voltage of the overall cell is then attributed entirely to the other half-cell. 

This cell consists of a SHE in one beaker and a strip of zinc immersed in 1 M zinc chloride solution in another beaker (Eigure 21-9). A wire and a salt bridge complete the circuit. When the circuit is closed, the following observations can be made. 

As the cell operates, the mass of the zinc electrode decreases. The concentration of 

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The Zinc-Copper Cell 21-10 The Copper-Silver Cell Standard Electrode Potentials 21-11 The Standard Hydrogen Electrode 21-12 The Zinc-SHE Cell 21-13 The Copper-SHE Cell 21-14 Standard Electrode Potentials 21-15 Uses of Standard Electrode Potentials 21-16 Standard Electrode Potentials for Other Half-Reactions 21-17 Corrosion 21-18 Corrosion Protection... 

The lUPAC recipe for half-cell manipulations s first, half-cells are always written and tabulated as reductions when combined with the SHE and second, calculate the cell voltage as the right (reduction) half-cell voltage minus the left (oxidation) voltage, whether using the SHE or not. This is the opposite of the way we wrote equations (18.7) and (18.8) for the zinc-hydrogen cell, so we rewrite them (according to the first rule) as... 

Now considering the zinc-copper cell, which could be constructed by substituting a copper electrode in a cupric ion solution for the SHE in Figure 18.1, we could write... 

The Standard potential at the anode plus the standard potential at the cathode gives the standard cell potential. The potential of the SHE is 0.000 volt, and the standard cell potential is found to be 0.763 volt. So the standard potential of the zinc anode must be 0.763 volt. The Zn Zn2+(1.0 7W) H+(1.0 M), H2(l atm) Pt cell is depicted in Eigure 21-9. 

Note that in this cell the SHE is the cathode, and metallic zinc reduces H+ to H2. The zinc electrode is the anode in this cell. 

We can use the SHE to measure the potentials of other kinds of electrodes. For example. Figure 19.4(a) shows an electrochemical cell with a zinc electrode and a SHE. In this case the zinc electrode is the anode and the SHE is the cathode. We deduce this fact from the decrease in mass of the zinc electrode during the operation of the cell, which is consistent with the loss of zinc to the solution caused by the oxidation reaction ... 

A zinc electrode in a solution of zinc ion at unity activity exhibits a potential of -0.763 V when coupled with the SHE. The zinc electrode is the negative electrode in the galvanic cell, and its electrode potential is also negative. 

The Zn /Zn reduction potential is more negative than the H3 O /H2 reduction potential (-0.76 V vs. 0 V), so zinc is the anode in this cell. Zinc is oxidized and hydronium ions are reduced, causing electrons to flow from the more negative zinc electrode to the less negative SHE. Again, we reverse the direction of the half-reaction with the more negative potential and find E by subtracting the half-cell potentials ... 

The half-cell reactions are reversible. Depending on the conditions, any electrode can act either as an anode or as a cathode. Earlier we saw that the SHE is the cathode (H+ is reduced to H2) when coupled with zinc in a cell and that it becomes the anode (H2 is oxidized to H+) when used in a cell with copper. 

Example 19.7 shows that an electrochemical cell whose cell reaction involves ions can be used to measure [Ft+] or pH. The pH meter described in Section 15.3 is based on this principle, but for practical reasons the electrodes used in a pH meter are quite different from the SHE and zinc electrode in the electrochemical cell (Figure 19.6). 

Considering only standard conditions for now, if is the standard zinc halfcell potential and is the SHE potential, then by the second rule above, the cell potential is... 

This cell has a measured potential of —0.763 volts (i.e., the standard zinc electrode is 0.763 volts more negative than the SHE) so... 

Figure 19.4 (a) A cell consisting of a zinc electrode and a hydrogen electrode, (b) A cell consisting of a copper electrode and a hydrogen electrode Both cells are operating under standard-state conditions. Note that in (a) the SHE is the cathode, but in (b) it is the anode. 

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