Measurement against hydrogen electrode

In the case of standard electrode potential, it is appropriate to have a standard electrode whose reversible potential is made arbitrarily zero and against which the potentials of other electrodes can be measured. The hydrogen electrode is an accepted standard. It is composed of a rod of platinum covered with platinum black saturated with hydrogen gas at atmospheric pressure. Electrode potential based on this zero are said to refer to the hydrogen scale. However, in experimental work, it is often more suitable to use another standard electrode. Calomel is a common example. It consists of a pool of mercury covered with calomel (mercurous chloride) and immersed in a solution of potassium chloride. 

Reference Electrode A specially chosen electrode which has a reproducible potential against which other electrode potentials may be measured. (See HYDROGEN ELECTRODE). 

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

As mentioned previously, absolute potentials cannot be measured. Rather, potentials are measured against a reference electrode. The hydrogen electrode is perhaps the most common, and at standard conditions, it is defined to be 0 V. The hydrogen electrode reaction is... 

It is impossible to measure the potential of a half-cell directly and a reference half-cell must be used to complete the circuit. The hydrogen electrode (Figure 4.3) is the standard reference electrode against which all other halfcells are measured and is arbitrarily attributed a standard electrode potential of zero at pH 0. Because it is difficult to prepare and inconvenient to use, the... 

Flg.1 Current density-potential curves for the anodic oxidation of two various reactants and finally of the solvent. The electrode potential is measured against a reference electrode (RE), here for example, the normal hydrogen electrode (NHE). 

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. 

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... 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

Standard reduction potential The potential of a halfreaction under standard state conditions, as measured against the potential of the standard hydrogen electrode. 

The correction for the increase of chloride ions due to the hydrolysis of the chlorine has largely eliminated the deviations between the observed and calculated values. 6. N. Lewis and F. F. Kupert find for the electrode potential of chlorine against the normal electrode to be —1 0795. F. Dolezalek measured the difference in the e.m.f. of two 5N- to 12A-hydrochloric acid cells of different strengths by the vap. press, method, and obtained satisfactory results. F. Boericke, G. N. Lewis and H. Storch found for the normal electrode potentials against hydrogen at 25°... 

The primary reference electrode for aqueous solutions is the standard hydrogen electrode (SHE), expressed by H+(a=l) H2(p=105 Pa) Pt (see 11 in Section 4.1). Its potential is defined as zero at all temperatures. In practical measurements, however, other reference electrodes that are easier to handle are used [24]. Examples of such reference electrodes are shown in Table 5.4, with their potentials against the SHE. All of them are electrodes of the second kind. The saturated calomel electrode (SCE) used to be widely used, but today the saturated silver-silver chloride electrode is the most popular. 

Fig. 6.12, in which potential Ex has been measured against a secondary reference electrode and it is necessary to convert it to the standard hydrogen electrode scale which defines the zero . Thus, if the potential measured against Ag/AgCl (sat.) KC1//... is —99 mV, it is —142 mV versus SCE, —451 mV versus Fc/Fc+ redox electrode, but +100 mV versus SF1E. 

The ORP (pe) can be expressed in units of electron activity as a negative logarithm of the electron activity (ae) in a solution pe = —log ae. The ORP can be also expressed in units of volts ( ),). As a matter of convention, the Eh measurements are referenced against a standard hydrogen electrode (SHE). 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

Hydrogen gas is bubbled over a platinum surface that is coated with platinum black, an electrolytically deposited coating of colloidal platinum, which is an excellent catalyst for the above equilibrium. The hydrogen electrode has been selected as the standard against which the potentials of other electrodes are measured. Equations of the type of reaction (I) are called half-cell reactions, because they include electrons. Reaction I is a reduction half-cell reaction. 

The potentials of both of these electrodes have been very accurately determined against the hydrogen electrode. The latter is seldom used in routine electrochemical measurements because it is more difficult to prepare the platinum surface has to be specially treated by preliminary electrolysis. Also, there is need for a supply of hydrogen gas which makes it somewhat cumbersome and hazardous. 

The potential of the catalyst slurry was measured during reaction with a Pt rod collector electrode against a Ag/AgCl/KCl(eat) reference electrode. More details of the method can be found elsewhere [6,14,15]. All the potentials in the text and Figures are referred to reversible hydrogen electrode. 

The potential of the saturated calomel electrode is +0-246 V at 25°C (measured against the standard hydrogen electrode). Ea, the potential of the glass electrode, on the other hand, depends on the pH of the solution. For the pH region 2-11... 

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