1、 TECHNICAL REPORT IEC TR 62456First edition 2007-03An electrochemical reference system for use in different solvent media The decamethylated ferricinium/ferrocene redox couple Reference number IEC/TR 62456:2007(E) Publication numbering As from 1 January 1997 all IEC publications are issued with a de
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8、tion 2007-03An electrochemical reference system for use in different solvent media The decamethylated ferricinium/ferrocene redox couple PRICE CODE IEC 2007 Copyright - all rights reserved No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical
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10、e current catalogue Commission Electrotechnique Internationale International Electrotechnical Commission 2 TR 62456 IEC:2007(E) INTERNATIONAL ELECTROTECHNICAL COMMISSION _ AN ELECTROCHEMICAL REFERENCE SYSTEM FOR USE IN DIFFERENT SOLVENT MEDIA THE DECAMETHYLATED FERRICINIUM/FERROCENE REDOX COUPLE FOR
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20、he Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be hel
21、d responsible for identifying any or all such patent rights. The main task of IEC technical committees is to prepare International Standards. However, a technical committee may propose the publication of a technical report when it has collected data of a different kind from that which is normally pu
22、blished as an International Standard, for example, “state of the art“. IEC 62456 which is a technical report, has been prepared by subcommittee 65D: Analysing equipment, of IEC technical committee 65: Industrial-process measurement and control. The text of this technical report is based on the follo
23、wing documents: Enquiry draft Report on voting 65D/125/DTR 65D/127/RVC Full information on the voting for the approval of this technical report can be found in the report on voting indicated in the above table. TR 62456 IEC:2007(E) 3 This publication has been drafted in accordance with the ISO/IEC D
24、irectives, Part 2. The committee has decided that the contents of this publication will remain unchanged until the maintenance result date indicated on the IEC web site under “http:/webstore.iec.ch“ in the data related to the specific publication. At this date, the publication will be reconfirmed, w
25、ithdrawn, replaced by a revised edition, or amended. A bilingual version of this publication may be issued at a later date. 4 TR 62456 IEC:2007(E) INTRODUCTION The proposal of this technical report is justified by the difficulties met by electrochemists, electroanalysts, corrosionists, and biologist
26、s when electrode potentials measured in different non-aqueous or aqueous-organic solvents with reference to the familiar, commercially available, and very dependable, aqueous Saturated Calomel Electrode (SCE) are to be physically inter-compared on some “inter-solvental scale” of electrode potentials
27、. These difficulties are caused by the occurrence of two basic extrathermodynamic factors, namely, the primary medium effect 1 1on electrode potentials and the liquid-junction potentials between different solvent media. This report details the use of the half-wave potential of the ferricinium/ferroc
28、ene redox couple or the decamethyl derivatized ferricinium/ferrocene redox couple as the basis of a reference electrode system useful to compared measured electrode potentials to the aqueous saturated calomel electrode when used in various solvent systems. For an acceptable approach to the solution
29、of the above problem, superseding earlier attempts 2, Strehlow and co-workers 3, 4 proposed to use the redox couple ferricinium|ferrocene (in terms of its “half-wave potentials”, or “formal redox potentials”, obtainable from voltametric experiments in different solvents with the same supporting elec
30、trolyte and against the same working reference electrodes), for which, based on an extended experimental evidence, they assessed an “invariancy of potential” to within about 15 mV in a dozen different solvents, assuming the above redox couple to be unaffected by solvent interactions. Principles and
31、reasons for this choice were expounded in detail 3, 4 and need not be repeated here. Strehlows idea was later reassessed in nearly the same terms by Gritzner and K ta 5, but various authors later demonstrated the existence of specific interactions of the ferricinium| ferrocene couple with some of th
32、e solvent media considered, thus partially impairing the intersolvental invariancy of the above redox potentials. However, recently 6, 7, 8, 9, and 10, it was shown that such residual solvent effects could be drastically abated by permethylation of the aromatic rings of the redox couple. For instanc
33、e, the decamethyl- derivatized ferricinium|ferrocene couple, being chemically and electrochemically reversible, with half-wave potentials quite unaffected by the working solvent, lends itself as the best reference-potential redox couple known so far (besides other advantages with respect to the unme
34、thylated parent couple, such as a more general chemical stability, and a more advantageous potential window compared to the operating reference electrode SCE is more advantageously placed). Therefore, what remains to be done by the user is to calibrate the aqueous SCE electrode (which, in common pra
35、ctice, is the reference electrode blindly and invariably used in contact with the non-aqueous solvents studied) versus the half-wave potential of the decamethylferricinium|decamethylferrocene Me 10 Fec + |Me 10 Fec couple in the solvent studied, along the lines explained below, and to apply the cons
36、equent corrections to the measured potentials (see Table 1). The resulting corrected potentials will finally become intersolventally comparable. An additional bonus to the present methodology is provided by the parallel decamethyl- derivatized cobalticinium|cobaltocene Me 10 Coc + |Me 10 Coc redox c
37、ouple because it also was shown to be chemically and electrochemically reversible and its half-wave potential E* differs by a constant value from that of Me 10 Fec + |Me 10 Fec, in terms of E*Me 10 Coc + |Me 10 Coc = E*Me 10 Fec + |Me 10 Fec 1,407 V, at 298,15 K. _ 1Figures in square brackets refer
38、to the Bibliography. TR 62456 IEC:2007(E) 5 AN ELECTROCHEMICAL REFERENCE SYSTEM FOR USE IN DIFFERENT SOLVENT MEDIA THE DECAMETHYLATED FERRICINIUM/FERROCENE REDOX COUPLE 1 Scope The scope of this technical report is to present the background considerations which led to this compilation of potentials
39、of the aqueous saturated calomel electrode compared to the cyclovoltametric data for a glassy carbon electrode in various solvent media. A tabulation of working potentials of this electrode pair in various solvent media is presented. A comparison of the cyclovoltametric scans for the ferricinium/fer
40、rocene and deamethyl derivatized ferricinium/ferrocene redox couple system is presented and important features are discussed. 2 Tabulation of working potentials Clause 2 gives an explanation of the basis for the tabulation of working potentials of the aqueous saturated calomel electrode compared to
41、the ferricinium/ferrocene redox couple and the decamethyl derivatized ferricinium/ferrocene redox couple in various solvent media, as well as an identification of the voltametric range of these couples in various solvent media. 2.1 Working potentials of an SCE Table 1 compiles the working potentials
42、 of the aqueous saturated calomel electrode compared to the ferricinium/ferrocene redox couple and the deamethyl derivatized ferricinium/ferrocne redox couple in various solvent media. Table 1 Working potentials of the aqueous SCE compared to the half-wave potential of the decamethyl derivatized fer
43、ricinium/ferrocene redox couple in various solvent media Solvent E SCE / V aReferences Water 0,144 7 Methanol 0,124 7 Propylene carbonate 0,069 7 Methyl pyrrolidinone 0,055 8 to 10 Acetone 0,021 8 to 10 Acetonitrile 0,119 8 to 10 Methylene chloride 0,060 8 to 10 Dimethyl formamide 0,008 8 to 10 a Va
44、lues to be added to the potentials measured in the solvent studied versus the aqueous SCE to get them referred to the solvent-invariant half-wave potential of the Me 10 Fec + |Me 10 Fec redox couple. NOTE Table 1 gives values for the aqueous SCE at 298K working potential (including insolvental liqui
45、d junction potentials) when compared to the decamethyl derivatized ferricinium/ferrocene redox couple Ferrocene Fec is the dicyclopentadienyl ferrous complex; oxidation to the corresponding ferric complex (ferricinium, Fec + ) is a simple, chemically and electrochemically reversible one- electron pr
46、ocess. The metal ion is buried at the centre of a nearly spherical, big complex; hence, these substances undergo negligible chemical or steric changes when oxidation or reduction occurs. 6 TR 62456 IEC:2007(E) The half-wave potentials of Fec + |Fec were first measured polarographically by Strehlow 3
47、, 4 in concentrated aqueous sulfuric acid solutions with respect to the Hg 2 SO 4electrode in the cell (1): Pt|Fec + |Fec in aq.H 2 SO 4 aq.H 2 SO 4 |Hg 2 SO 4 |Hg|Pt (1) combined with cell (2): Pt|H 2(1 bar)|aq.H 2 SO 4 |Hg 2 SO 4 |Hg|Pt (2) It is readily shown that E 2 E 1= EHg 2 SO 4 EH 2 E*Fec+|
48、Fec + k pH (3) Now, since in Equation (3) the constant term (E Hg2SO4 E H2 E* Fec+|Fec ) is determinable, pH is here a true pH = log(a H+ ) in superacid H 2 SO 4solutions, a quantity not attainable by the conventional IUPAC procedure, which is reliable only in the range 1 pH 13, due to the increasin
49、g effect of residual liquid junction potentials at extreme pH. This interesting scheme can also be applied to other ultra-concentrated acid solutions (HCl, HBr, HI). However, definitely, the most interesting and important opportunity is to use the Fec + |Fec couple in its improved decamethylated form 6, 7, 8, 9, and 10, Me 10 Fec + |Me 10 Fec, as one tool of
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