1、 TECHNICAL REPORT IECTR 62432First edition 2006-03The rH index in aqueous and aqueous-organic media Reference number IEC/TR 62432:2006(E) Publication numbering As from 1 January 1997 all IEC publications are issued with a designation in the 60000 series. For example, IEC 34-1 is now referred to as I
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7、u have any questions regarding this publication or need further assistance, please contact the Customer Service Centre: Email: custserviec.ch Tel: +41 22 919 02 11 Fax: +41 22 919 03 00 TECHNICAL REPORT IEC TR 62432First edition 2006-03The rH index in aqueous and aqueous-organic media PRICE CODE IEC
8、 2006 Copyright - all rights reserved No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission, 3, rue de Varemb,
9、PO Box 131, CH-1211 Geneva 20, Switzerland Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmailiec.ch Web: www.iec.ch N For price, see current catalogue Commission Electrotechnique Internationale International Electrotechnical Commission 2 TR 62432 IEC:2006(E) CONTENTS FOREWORD.3 INT
10、RODUCTION.5 1 Scope.6 2 General principles 6 2.1 Redox couples, redox equilibria, redox potentials, redox systems6 2.2 The rH value .7 2.3 rH Standards for use in water and aqueous-organic solvent mixtures 11 2.4 Electrodes for the operational rH cell.12 2.4.1 General .12 2.4.2 The glass electrode .
11、12 2.4.3 The inert noble-metal electrode (Pt or Au) .12 2.5 rH Scales in diverse solvents 12 2.6 Pourbaixs diagrams for the triad rH pH E O|R 13 3 Instrumentation 13 Bibliography14 Figure 1 Pourbaixs diagram for the triad rH pH E O|R for some key redox systems .10 Table 1 6 Table 2 Some reference aq
12、ueous solutions proposed as rH-metric standards rH S8, 9 at 25 C and for the calibration of the redox electrode at E O|R .11 Table 3 Values of (E QHY E H + |H2 ) 6 with corresponding rH Svalues, at various temperatures, valid for any solvent (water W, or aquo-organic mixture Z = W + S compatible wit
13、h Quinhydrone) in non-alkaline solution 12 Table 4 Parallelisms between the aqueous pH-metric and rH-metric scales .11 TR 62432 IEC:2006(E) 3 INTERNATIONAL ELECTROTECHNICAL COMMISSION _ THE rH INDEX IN AQUEOUS AND AQUEOUS-ORGANIC MEDIA FOREWORD 1) The International Electrotechnical Commission (IEC)
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25、he art“. IEC 62432, which is a technical report, has been prepared by subcommittee 65D: Analyzing equipment, of IEC technical committee 65: Industrial-process measurement and control. The text of this technical report is based on the following documents: Enquiry draft Report on voting 65D/120/DTR 65
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28、ual version of this Technical report may be issued at a later date. TR 62432 IEC:2006(E) 5 INTRODUCTION The fundamental rationale for the rH index, extended to cover the pure aqueous and the aqueous-organic media, has been recently described critically 1 1 , but for the users convenience, the essent
29、ials will be recalled in the present Technical Report together with the application domains, the recommended procedures and operational details. 1Numbers in square brackets refer to the bibliography. 6 TR 62432 IEC:2006(E) THE rH INDEX IN AQUEOUS AND AQUEOUS-ORGANIC MEDIA 1 Scope This Technical Repo
30、rt concerns analyzers, sensor units and electronic units used for the determinations of the rH index in aqueous and aqueous organic media. This Technical Report identifies the terminology, definitions, theory and methodology used for the determination of rH values or redox systems in aqueous solvent
31、 or aqueous-organic solvent mixtures. 2 General principles 2.1 Redox couples, redox equilibria, redox potentials, redox systems An oxidation/reduction couple (“redox” couple) O|R, present in water or in an aqueous-organic solvent mixture involves the concurrence of an oxidant species O (ionic or unc
32、harged) and a reductant species R (ionic or uncharged) of the same chemical element, thereby establishing an oxidation/reduction equilibrium (redox equilibrium) O + ne = R, and an electrochemical oxidation/reduction potential (redox potential) E O|Rwhich is transmitted to the meter by an inert metal
33、 electrode (usually platinum or gold). This metal participates in the specific charge transfer which is going on throughout the solution and is called upon only to act as a donor or acceptor of electrons. When both the O and R species are at unit activity (standard state) the redox potential E O|Rbe
34、comes the standard redox potential, symbolized as E O|R . In the environmental, hydrological, biomedical, winery, dairy-farming, and corrosion domains of interest for rH measurements, only seldom is a single O|R couple present alone in the solvent medium. Instead, an undefined number of redox couple
35、s O|R, O|R, O”|R”,.O n |R noverlap, thus determining a mixed redox potential of very complex (not to say impossible) interpretation: therefore it is better to speak of a “redox system”, but this latter term is also legitimately applicable to a single redox couple. NOTE Some examples of familiar redo
36、x couples with related reaction equilibria and redox potential expressions, are given in Table 1. Table 1 - Examples of familiar redox couples with related reaction equilibria and redox potential expressions Redox couple Redox equilibrium Redox potential ferric|ferrous Fe 3 + + e = Fe 2 + E Fe3 + |F
37、e2 + = E Fe3 + |Fe2 + + k log(a Fe3 + /a Fe2 + ) H+ |H 2(hydrogen electrode) 2H+ + 2e = H 2E H + |H2= E H + |H2+ k log a H + (k/2)log p H2Cl 2 |Cl (chlorine electrode) Cl 2+ 2e = 2Cl E Cl2|Cl - = E Cl2|Cl - k log a Cl - + (k/2)log p Cl2O 2 |H 2 O (oxygen electrode) O 2 + 4e + 4H+ = 2 H 2 O E O2|H2O=
38、 E O2|H2O+ k log a H + + (k/4)log p O2 (k/2)log a H2OMnO 4 |Mn 2 + (permanganate electrode) MnO 4 + 5e + 8H+ = Mn 2 + + 4H 2 O E MnO4|Mn2 + = E MnO4|Mn2 + + (k/5)log(a MnO4|Mn2 + /a Mn2 + ) + + (8k/5)log a H + (4k/5)log a H2OSymbols: e = the electron; k = Nernstian coefficient = 2,303RT/F ; a = acti
39、vity; p = pressure. TR 62432 IEC:2006(E) 7 2.2 The rH value The notional definition of the rH index 2,3 for a given redox system in a given (aqueous or aqueous-organic) medium is rH = log p H2(1) where p H2is that pressure of hydrogen gas that would equalize the potential E H + |H2of the hydrogen ga
40、s electrode to the redox potential E O|Rof the system being studied (thus zeroing the pd of the cell resulting from the combination of these two electrodes). rH is an index of the reducing power of the redox system under consideration. The Nernstian expression for E H + |H2is (with k = 2,303RT/F): E
41、 H + |H2= E H + |H2 k pH + (k/2) rH (2) where E H + |H2is the standard electrode potential (which varies with the solvent but it is conventionally put equal to zero at any temperature in pure aqueous medium 4, 5). If the hydrogen gas electrode works at p H2= 1 bar (i.e. under standard state conditio
42、ns), then rH = 0 at any pH of the solution (see Figure 1, which describes the pertinent Pourbaixs E Redoxvs. pH diagram), and this is the nominal zero of the rH-metric scale to which corresponds the nominal maximum reducing power of a redox system One redox system of paramount importance is the equi
43、molal (quinoneQ + hydro- quinoneH 2 Q) system, commonly called the “quinhydrone”, of symbol QHY, whose electrode potential is expressed by E QHY= E QHY k pH (3) On consideration of equations (2) and (3), the potential difference E 4of the cell (4) below, where the quinhydrone electrode is combined w
44、ith the hydrogen electrode and the two solutions are at equal activity of the H + ion: Pt|H2 (p H2 )|H+ H + saturated with QHY|Pt (4) is clearly independent of pH. (In common practice, the two electrode compartments are kept separated by a porous glass frit or a closed stopcock, as indicated by the
45、symbol, to avoid the mutual diffusion of hydrogen gas and quinhydrone which would produce an irreversible chemical reaction and formation of a useless mixed electrode potential). Therefore, equating E H + |H2to E QHYin accord with the equation (1), i.e. putting E 4= 0, gives the related rH QHYvalue:
46、 E 4= E QHY E H + |H2= E QHY E H + H2 (k/2) rH QHY= 0 (5) from which rH QHY= 2 (E QHY E H + |H2 ) / k = rH S(6) As equation (6) shows, since the difference (E QHY E H + H2 ) is a well defined and accurately known quantity which is a function of temperature but is invariant upon passing from pure wat
47、er medium to most water-rich aqueous-organic media 1, the quinhydrone redox system constitutes the key standard rH Sfor reference in rH measurements, according to the operational equation (27) described later on. In the context of this invariancy, at 298,15 K, (E QHY E H + H2 ) = 0,699 75 V 6 and, t
48、herefore: (rH) QHY= rH S= 2 x 0,699 75 / 0,059 159 7 = 23,66 (7) 8 TR 62432 IEC:2006(E) For this reason, the cell (4) can be considered the archetype of the rH-metric calibration cell. Another important redox system for which it is important to known the rH value is that of the oxygen gas electrode,
49、 of reaction O 2+ 4H + + 4e = 2H 2 O. Thus, its redox potential E O2|H2O will, assuming unit H 2 O activity, be given by E O2|H2O = E O2|H2O k pH (k/4) rO (8) where we use the further definition rO = log p O2(9) The rO index, which is a quantity complementary to rH, is an index of the oxidizing power of the given redox system in the same medium (analogously, the pOH index of alkalini
