Standard hydrogen electrode half-cell

The voltage of any half cell can be recorded against a standard hydrogen electrode (half cell or reference electrode). Table 2.1 gives the standard reference electrode or half cell potentials that are of interest to us as we evaluate corrosion problems. 

The voltage of any half cell can be recorded against a standard hydrogen electrode (half cell). Table 2.1 gives the standard half cell potentials that are of interest to us as we evaluate corrosion problems. Half cell potentials are a function of concentration as well as the metal and the solution. A more concentrated solution is generally) more corrosive than a dilute one so a current will flow in a cell made up of a single metal in two different concentrations of the same solution. We can consider the corrosion of steel in concrete as a concentration cell. 

Half-cell potential The potential of an electrochemical half-cell measured with respect to the standard hydrogen electrode. Half-life, fi/i The time interval during which the amount of reactant has decreased by one half. 

Reversible potential( qu(7thnum potential) It is the potential of an electrode where the forward rate of reaction equals to the reverse rate of reaction. Standard electrode potential (E°) It is reversible potential of an electrode measured against a standard hydrogen electrode (SHE) consisting of a platinum specimen immersed in a unit activity acid solution through which H2 gas at 1 atm pressure is bubbled. The potential of the hydrogen electrode (half cell) is taken to be 2ero. 

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

When the two electrodes are connected, current flows from M to X in the external circuit. When the electrode corresponding to half-reaction 1 is connected to the standard hydrogen electrode (SHE), current flows from M to SHE. (a) What are the signs of ° of the two half-reactions (b) What is the standard cell potential for the cell constructed from these two electrodes ... 

Schematic diagram of a cell for measuring E ° of the Cu / Cu half-reaction relative to the standard hydrogen electrode (SHE).

Defining a reference value for the SHE makes it possible to determine E ° values of all other redox half-reactions. As an example. Figure 19-14 shows a cell in which a standard hydrogen electrode is connected to a copper electrode in contact with a 1.00 M solution of C U . Measurements on this cell show that the SHE is at higher electrical potential than the copper electrode, indicating that electrons flow from the SHE to the Cu... 

Ab initio atomic simulations are computationally demanding present day computers and theoretical methods allow simulations at the quantum mechanical level of hundreds of atoms. Since an electrochemical cell contains an astronomical number of atoms, however, simplifications are essential. It is therefore obvious that it is necessary to study the half-cell reactions one by one. This, in turn, implies that a reference electrode with a known fixed potential is needed. For this purpose, a theoretical counterpart to the standard hydrogen electrode (SHE) has been established [Nprskov et al., 2004]. We will describe this model in some detail below. 

In the subsequent text the half-cell reactions will be used to characterize the electrode potentials instead of the cell reactions of the type of Eq. (3.1.42) under the tacit assumption that such a half-cell reaction describes the cell reaction in a cell with the standard hydrogen electrode on the left-hand side. 

The expression for the potential of electrodes of the second kind on the hydrogen scale can be derived from the affinity of the reaction occurring in a cell with a standard hydrogen electrode. For example, for the silver chloride electrode with the half-cell reaction... 

The half-cell reduction potential of the standard hydrogen electrode (SHE) was set arbitrarily to 0.000... V by international agreement. Since it is impossible to determine the potential of a single half-cell without comparing it to another, an arbitrary standard was established. 

The electrochemical potential in a potentiometric cell is inevitably measured with respect to a standard electrode. Several types of electrodes are commonly used. The standard hydrogen electrode (SHE) is a hydrogen half-cell in which the cell reaction is as follows ... 

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. 

Redox half-reactions are often written for brevity as, for example, Li+ + e - Li. with the state symbols omitted. The electrode system represented by the half-reaction may also be written as Li+ /Li. The standard redox potentials for ion-ion redox systems can be determined by setting up the relevant half-cell and measuring the potential at 298 K relative to a standard hydrogen electrode. For example, the standard redox potential for the half-reactions... 

Because you can measure potential differences, but not individual reduction potentials, all values in the table are relative. Each half-cell reduction potential is given relative to the reduction potential of the standard hydrogen electrode, which has been assigned a value of zero. The design of this electrode is shown in Figure 11.12. 

You have learned that the standard hydrogen electrode has an assigned standard reduction potential of exactly 0 V, and is the reference for all half-cell standard reduction potentials. What would happen to cell potentials if a different reference were used You will address this question in the following ThoughtLah. 

In this section, you learned that you can calculate cell potentials by using tables of half-cell potentials. The half-cell potential for a reduction half-reaction is called a reduction potential. The half-cell potential for an oxidation half-reaction is called an oxidation potential. Standard half-cell potentials are written as reduction potentials. The values of standard reduction potentials for half-reactions are relative to the reduction potential of the standard hydrogen electrode. You used standard reduction potentials to calculate standard cell potentials for galvanic cells. You learned two methods of calculating standard cell potentials. One method is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. The other method is to add the standard reduction potential of the cathode and the standard oxidation potential of the anode. In the next section, you will learn about a different type of cell, called an electrolytic cell. 

The oxidation-reduction potential or redox potential ( h) is a measure of the tendency of a solution to be oxidizing or reducing. Oxidation and reduction are basically electrical processes that are readily measiued by an electrode potential. All measurements are referred to die standard hydrogen electrode, the potential of which is taken as 0.00 V at 298 K, the H2 pressure as 101325 N/m (1 atm) and activities of H2 and as unity. When the half-cell reaction is written as an oxidation reaction ... 

Standard hydrogen electrode (SHE) The standard against which redox potentials are measured. The SHE consists of a platinum electrode electroplated with Pt black (to catalyse the electrode reaction), over which hydrogen at a pressure of 1 atm is passed. The electrode is immersed in a solution containing hydrogen ions at unit activity (e.g. 1.228 mol dm of aqueous HCl at 20°C). The potential of the SHE half cell is defined as 0.000 V at all temperatures. 

The of this standard cell is +0.76 V. By international convention, the half-cell potential of the hydrogen reduction is assigned a value of exactly OV. Thus, the half-cell potential of the zinc oxidation is equal to K.n (i.e., +0.76 V). This voltage is called the standard half-cell potential and is represented by the symbol 1, to indicate that it was determined against a standard hydrogen electrode. 

SHE, standard hydrogen electrode The electrode used as a standard against which aU other half-cell potentials are measured. The following reaction occurs at the platinum electrode when immersed in an acidic solution and cormected to the other half of an electrochemical cell 2H (aq) -H 2e —> H2(g). The half- cell potential of this reaction at 25°C, 1 atm and 1 m concentrations of aU solutes is agreed, by convention, to be OV... 

When two interval scales are used to measure the amount of change in the same property, the proportionality of differences is preserved from one scale to the other. For example. Table 1.4 shows reduction potentials of three electrochemical half-cell reactions measured in volts with reference to the standard hydrogen electrode (SHE, E°) and in millivolts with reference to the standard silver-silver chloride electrode (Ag/AgCl, ). For the SHE potentials the proportion of differences between the intervals +0.54 to +0.80 and +0.34 to +0.80 is... 

Electrochemical half-cell potentials vs. the standard hydrogen electrode (SHE, E°) and vs. the standard silver-silver chloride electrode (Ag/AgCl, E). 

E is the standard equilibrium potential, i. e. the potential corresponding to unit activity and RTF. The dissolution reaction leads to the development of an electrical double layer at the iron-solution interface. The potential difference of the Fe/Fe " half cell cannot be measured directly, but if the iron electrode is coupled with a reference electrode (usually the standard hydrogen electrode, SHE), a relative potential difference, E, can be measured. This potential is termed the single potential of the Fe/Fe electrode on the scale of the standard hydrogen couple H2/H, the standard potential of which is taken as zero. The value of the equilibrium potential of an electrochemical cell depends upon the concentrations of the species involved. 

To determine the E° of different cell arrangements, chemists use what are called standard reduction potentials for halfcells. A standard reduction potential is the electrical potential under standard conditions of a cell compared to the standard hydrogen electrode. The standard hydrogen electrode is a special half-cell that has been chosen as a reference to measure electrical potential. Just as sea level is a logical elevation for measuring gravitational potential,... 

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