SHE half-cell

The internationally accepted primary reference is the standard hydrogen electrode (SHE). The potential of the SHE half-cell is dehned as 0.000 V at all temperatures. We say the schematic for the half-cell is... 

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. 

Standard zinc half - cells and SHE half-cells are used to set Zn - SHE chemical cells. 

In addition, the Pt serves as the electrical conductor to the external circuit. Under standard state conditions, that is, when the H2 pressure equals 1 atm and the ideal concentration of the HCl is 1 M, and the system is at 25°C, the reduction potential for the reaction given in Eq. (15.8) is exactly 0 V. (The potential actually depends on the chemical activity of the HCl, not on its concentration. The relationship between activity and concentration is discussed subsequently. For an ideal solution, concentration and activity are equal.) The potential is symbolized by where the superscript zero means standard state conditions. The term standard reduction potential means that the ideal concentrations of all solutes are 1 M and all gases are at 1 atm other solids or liquids present are pure (e.g., pure Pt solid). By connecting the SHE half-cell with any other standard half-cell and measuring the voltage difference developed, we can determine the standard reduction potential developed by the second half-cell. 

When a SHE is connected to the Cu/Cu " " half-cell from the model, the Cu/Cu half-cell exhibits a stronger pull on electrons than does the SHE half-cell. Thus, the following reaction takes place at the Cu electrode (cathode) ... 

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 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 ... 

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. 

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 potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

Figure 7.12 Schematic depiction of the standard hydrogen electrode (SHE). The half-cell schematic is therefore Pt H2(a = l) H+(a = 1)...

Measurement with the hydrogen electrode The SHE is the primary reference electrode, so other half-cell potentials are measured relative to its potential. In practice, if we wish to determine the value of E M then we construct a cell of the type... 

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 ... 

The SHE is chosen as the ultimate reference electrode since its value is defined. By simply making a cell in which one half cell is the SHE, then straightaway we also know the potential of the second half cell. For this reason, we say that the SHE is a reference electrode. Since all potentials are ultimately cited with respect to the SHE, the latter is the reference electrode from which all other electrode potentials are derived we say that the SHE is the primary standard. It is also called the primary reference electrode. 

A reference electrode is defined as a constant-potential device . In other words, if we make a cell in which one half cell is the SHE and then measure the emf. we then simply employ the equation emf = Eright-hand side — ieft-hand side (equation (3.3)) to determine the other half-cell potential. 

Electrode potential, E The energy, expressed as a voltage, of a redox couple at equilibrium. E is the potential of the electrode when measured relative to a standard (ultimately the SHE). E depends on temperature, activity and solvent. By convention, the half cell must first be written as a reduction, and the potential is then designated as positive if the reaction proceeds spontaneously with respect to the SHE. Otherwise, E is negative. 

Reference electrode (RE) A constant-potential device (e.g. an SHE or SCE) used as a half cell of known potential. 

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. 

Chapter 2) apply. The standard reference half-cell is reaction 15.6, the standard hydrogen electrode (SHE), and the standard conditions are those listed in Section 2.3, although for our purposes the molar concentration scale (mol L 1) can generally be used without significant loss of precision. We will simplify matters further, for illustrative purposes, by equating activities with molar concentrations our numerical results will therefore be only approximate, except where concentrations are very low. A thermodynamically acceptable treatment would require the calculation or measurement of ionic activities or, at the very least, maintenance of constant ionic strength, as outlined in Section 2.2. 

By international agreement, the algebraic sign of E° for a half-cell is chosen to be the same as its electrical sign relative to the SHE. This means, in effect, that we must write the half-reactions with the electrons on the left-hand side in other words, E° values are taken to be reduction potentials. Consequently, a reagent such as chlorine that is more oxidizing than aqueous H+ (— H2) under standard conditions will have a positive E°... 

Since in experiments such as the one we have just discussed, it is only possible to determine potential differences between two electrodes (and not the absolute potential of each half cell), it is now useful to choose a reference system to which all measured potential differences may be related. In accord with the IUPAC 1953 Stockholm convention, the standard hydrogen electrode (SHE) is commonly selected as the reference electrode to which we arbitrarily assign a zero value of electrical potential. This is equivalent to assigning (arbitrarily) a standard free energy change, ArG°, of zero at all temperatures to the half reaction ... 

Because (like AG) refers to a difference in a state property, it can be evaluated in additive fashion along many alternative pathways. For this purpose, it is convenient to assign conventional ° values to each half-cell reaction [e.g., standard oxidation potentials as compiled in W. M. Latimer. Oxidation Potentials, 2nd edn (Prentice-Hall, New York, 1952)], such that the algebraic sum of the two half-reaction potentials equals the overall cell °. Such half-reaction ° values can in turn be obtained by choosing some standard electrode reaction as the conventional zero of the scale [such as the standard hydrogen electrode (SHE) for the l/2H(g) —> H+ aq) + e oxidation reaction, with she = 0]. Sidebar 8.2 illustrates a simple example of this procedure. 

The American convention would assign a positive value to E° for the Zn Zn2+(aq) half cell written as an oxidation, but a negative sign if written as a reduction. It is seen that the European convention refers to the invariant electrostatic potential of the electrode with respect to the SHE, whereas the American convention relates to the thermodynamic Gibbs free energy which is sensitive to the direction of the cell reaction. 

It was thought that the Fe203 in chlorites was the result of secondary oxidation of FeO however, Foster found that in many chlorites much of the Fe3+ was necessary to maintain a charge balance. The conversion of OH" to O2 is necessary for the conversion of Fe2+ to Fe3+. She found no relation between Fe203 and O content in excess of 10.0 ions per half cell and concluded that Fe203 is a normal constituent of many chlorites this would be particularly true of low-temperature clay chlorites. Hey (1954) divided the chlorites on the basis of more or less than 4% Fe203. Those with the higher values were considered to be oxidized. Those with the smaller values, which... 

Although the standard half-cell reactions are all referenced to the standard hydrogen electrode (NHE or SHE), this is an exceedingly awkward reference... 

As indicated previously, it is desirable to consider the individual electrode reactions independently. One might suppose that this could be achieved by characterizing the individual electrodes as described in Section 3.1.3. However, for reasons of sound thermodynamics, another method has been established. It was decided to relate all electrode reactions to one common reference electrode. Electrochemists have chosen the H+/H2 reaction under standard conditions (ct 1+ = 1M p 12 = 1 bar) as such a general reference electrode. It is termed the normal hydrogen electrode or the standard hydrogen electrode (SHE). Thus, whenever E and E° values are presented for individual electrode reactions (half cells), it is understood that these values pertain to a complete cell in which the SHE constitutes the second electrode. 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

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