Standard hydrogen electrode pressure dependency

The SHE. The H" " H2 couple is the basis of the primary standard around which the whole edifice of electrode potentials rests. We call the H H2 couple, under standard conditions, the standard hydrogen electrode (SHE). More precisely, we say that hydrogen gas at standard pressure, in equilibrium with an aqueous solution of the proton at unity activity at 298 K has a defined value of of 0 at all temperatures. Note that all other standard electrode potentials are temperature-dependent. The SHE is shown schematically in Figure 3.3, while values of Eq r are tabulated in Appendix 3. 

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

The hydrogen electrode potential depends on the hydrogen activity and the partial hydrogen pressure. Under standard conditions, the hydrogen pressure is one atmosphere thus, its activity is equal to 1 and, therefore, according to lUPAC convention, the hydrogen reference electrode potential is zero. The schematic of a SHE is shown in Fig. 2.3. 

Thus, these relationships can be used to define a pH scale for non-aqueous protic media, consistent with the pH scale for aqueous solutions. For standard hydrogen pressure, the potential of the hydrogen electrode depends on the pH(s) according to the relationship... 

The value of the constant V, and hence the values of standard potentials, depend on the choice of the reference electrode and on the character of electrode reaction, which takes place on it With the reference electrode potential conventionally taken as zero, we can choose, for example, the normal hydrogen electrode (NHE), i.e., an electrode, for which the equilibrium at the interface is attained due to the reversible redox reaction H+ + e = H2, provided the activity of H+ ions in the solution is 1 mol/liter and the pressure of gaseous hydrogen above the solution is 1 atm. Many of the measured potentials are given below relative to the saturated calomel electrode (SCE) its potential relative to the NHE is 0.242 V. 

Thus the potential of the hydrogen electrode depends on the hydrogen-ion concentration in the solution and on the pressure of hydrogen gas above the solution. The standard potential, E9, can be measured in a system where the activity of hydrogen ions is unity in the solution and the pressure of hydrogen gas over the solution is 1 atm. We have seen that the standard potential, B, of such an electrode is by definition 0. 

As we mentioned, the cell voltage depends on the activities of all the ions and compounds in the cell reaction in this case it depends not only on the hydrogen gas pressure and but on acu ao,2+ as well. Standard conditions is defined as u = 1 for all products and reactants in the cell reaction, and so if the hydrogen electrode is operating under SHE conditions (uh2( ) = /njfe) = 1 bar), the copper electrode is pure Cu (acu(s) = 1) and the cupric ion concentration and activity coefficient are adjusted to give 00,2+ = 1, the ceU voltage will be the standard cell voltage, S°. 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

The emf of a cell depends on the concentrations of ions and on gas pressures. For that reason, cell emfs provide a way to measure ion concentrations. The pH meter, for example, depends on the variation of cell emf with hydrogen-ion concentration. You can relate cell emfs for various concentrations of ions and various gas pressures to standard electrode potentials by means of an equation first derived by the German chemist WalthCT Nanst (1864—1941). ... 

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