1、 TECHNICAL REPORT IEC TR 62434First edition 2006-03pH measurements in difficult media Definitions, standards and procedures Reference number IEC/TR 62434:2006(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
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7、mer Service Centre If you 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 62434First edition 2006-03pH measurements in difficult media Defin
8、itions, standards and procedures PRICE CODE IEC 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
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10、 2 TR 62434 IEC:2006(E) CONTENTS FOREWORD.3 1 Scope and object5 2 Normative references .5 3 General principles 5 3.1 Terms and definitions 5 3.2 Symbols 5 3.3 pH value5 3.4 Standard reference buffer solutions (primary and secondary pH standards) 7 3.5 Widths of normal pH scales or normal pH ranges i
11、n the general solvents Z .10 3.6 Electrodes and operating conditions12 4 Solvent media of applicability .14 5 Procedure for specification .14 6 Recommended standard values and ranges of influence quantities 14 7 Verification of values 14 8 Other difficult media for pH determinations.15 Annex A (info
12、rmative) Values of the Nernstian slope factor k = 2,3026 RT/F.16 Annex B (informative) .17 Annex C (informative) .22 Annex D (informative) .23 Annex E (informative) .26 Annex F (informative) .27 Bibliography28 Figure 1 Schematic structure of the hydrogen gas electrode and of the AgCl electrode formi
13、ng the cell (13) 9 Figure 2 Intercomparing widths and relative positions of normal pH scales (with neutral points indicated by halving dots) in different solvents .11 Table A.1 Values of the Nernstian slope factor k = 2,3026 RT/F.16 TR 62434 IEC:2006(E) 3 INTERNATIONAL ELECTROTECHNICAL COMMISSION _
14、pH MEASUREMENTS IN DIFFICULT MEDIA DEFINITIONS, STANDARDS AND PROCEDURES FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote inte
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23、is IEC Publication or any other IEC Publications. 8) Attention is drawn to the 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 th
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25、hen it has collected data of a different kind from that which is normally published as an International Standard, for example “state of the art“. IEC 62434, which is a technical report, has been prepared by subcommittee 65D: Analyzing equipment, of IEC technical committee 65: Industrial-process meas
26、urement and control. The text of this technical report is based on the following documents: Enquiry draft Report on voting 65D/121/DTR 65D/124/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. This publicat
27、ion has been drafted in accordance with the ISO/IEC Directives, Part 2. 4 TR 62434 IEC:2006(E) 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 th
28、e specific publication. At this date, the publication will be reconfirmed, withdrawn, replaced by a revised edition, or amended. A bilingual version of this Technical report may be issued at a later date. TR 62434 IEC:2006(E) 5 pH MEASUREMENTS IN DIFFICULT MEDIA DEFINITIONS, STANDARDS AND PROCEDURES
29、 1 Scope and object This Technical Report concerns analyzers, sensor units and electronic units used for the determination of pH in non-aqueous solvents and aqueous organic solvent mixtures using glass electrodes. IEC 60746-1 includes further definition of the scope and provides for the general aspe
30、cts of all electrochemical analyzers, including pH. It is worthwhile to remind that IEC 60746-2 contains specifications for simulators used for testing pH electronic units. This technical report specifies the terminology, definitions, methodology, requirements for statements by manufacturers and per
31、formance tests for analyzers, sensor units and electronic units used for the determination of pH value in non-aqueous and aqueous-organic solvent mixtures. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the
32、 edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60746-1, Expression of performance of electrochemical analyzers Part 1: General IEC 60746-2, Expression of performance of electrochemical analyzers Part 2: pH value 3
33、 General principles 3.1 Terms and definitions The required definitions will be given following on the order of appearance of the relevant physical quantities in the text, and they comply with the pertinent IUPAC documents 1,21and IEC 60746-2. 3.2 Symbols The meaning of each symbol used here is given
34、 immediately after its first appearance in the relevant equation and it is conform to the pertinent IUPAC documents 1,2 and IEC 60746-2. 3.3 pH value 3.3.1 General A measure of the conventional hydrogen ion activity a H+in solution given by the expression pH = log a H+= log(m H+ H+ ) (1) where H+is
35、the activity coefficient of the H +ion at the molality m H+(moles of H +per kg of solvent). pH is a dimensionless quantity; it is not correct to write the logarithm of a quantity other than a dimensionless number, and the full form of equation (1) is 1Numbers in square brackets refer to the bibliogr
36、aphy. 6 TR 62434 IEC:2006(E) pH = log a H+= log(m H+ H+ /m) (2) where m = 1 mol kg 1is the standard-state reference molality. This definition is in terms of the molal scale, which is that recommended by IUPAC for a key reason, i.e. the molality of a solution is temperature-independent, which saves m
37、uch repetitive work of cell construction and filling. However, if one wants to treat pH in terms of the amount-of-substance concentration c (formerly “molarity”) in mol dm 3 , the equation (2) would take the form pH c= log(a H+ ) c= log(c H+y H+ /c) (3) where y H+is the activity coefficient of H +at
38、 concentration c H+(moles of H +per dm 3of solvent). It is worthwhile to recall that pH and pH care interrelated by the equation pH c= pH log /(kg dm 3 ) (4) where is the relative density of the solvent. Although equation (2), or alternatively (4), can be used to give an interpretation to pH values
39、under certain limiting conditions, a H+cannot be rigorously obtained by any method, for example from potential difference measurements, because it involves such a non- thermodynamic quantity as the single-H + -ion activity coefficient y H+ , and instead an operational definition is adopted in terms
40、of pH values assigned to certain reference buffers (primary or secondary pH standards). The pH measurement is performed by measuring the potential difference (electromotive force) E Xbetween a pair of electrodes immersed in the sample at unknown pH Xin the (non-aqueous or aqueous-organic) solvent Z,
41、 according to the cell scheme: Reference electrode in solvent Z Concentrated equitransferent salt bridge in solvent Z Sample at unknown pH Xin solvent Z H + sensing electrode (hydrogen gas electrode, or glass electrode) (5) and measuring the potential difference E Swith the same electrode pair, the
42、same salt bridge of the same composition and solvent Z, and at the same temperature, in a reference buffer solution of known standard pH PSor pH SS , according to: Reference electrode in solvent Z Concentrated equitransferent salt bridge in solvent Z Standard pH PSor pH SSin solvent Z H + sensing el
43、ectrode (hydrogen gas electrode, or glass electrode) (6) E X , E S , etc. are all defined as the difference of the potential of the right-hand (glass electrode) minus the potential of the left-hand electrode (reference electrode). Considering the Nernstian expressions for E Xand E S , the sought pH
44、Xof the sample in question is given by: pH X= pH SS (E X E S )/k + (E JX E JS)/k (7) where k = 2,302 6 RT/F, and E JXand E JSare the liquid junction potentials (see 3.3.2) arising at the junctions between reference electrode and unknown pH Xand between reference electrode and the known standard ph P
45、S , respectively. The concentrated equitransferent salt bridge in solvent Z (see 3.6.5) duly minimizes E JXand E JS,so that their difference (E JX E JS ) (the so-called “residual liquid junction potential”) can be ignored, and the following operational equation is now internationally endorsed for th
46、e determination of pH X : pH X= pH SS (E X E S )/k (8) At extreme acidities or alkalinities, or with high salinities (ionic strengths) of the sample, the residual liquid junction potential may be significant and requires careful consideration for the assessment of the accuracy level of the measured
47、pH X . TR 62434 IEC:2006(E) 7 The cell diagrams (5) and (6), respectively, represent the well known “measure” and “calibration” configurations of the “pH operational cell”. Numerical values for k, the “Nernstian coefficient” or “theoretical slope factor”, at temperatures from (0 to 100) C, are given
48、 in Annex A. Upon aging, the glass electrodes show an irreversible decrease of the slope factor, which thus becomes the “practical slope factor” k k and should consequently be accounted for in the operational equation (8). This is currently accomplished by the “bracketing standards procedure” (or “t
49、wo standards calibration”). This requires use of two standards, one (pH PS1 ) below and one (pH PS2 ) above the expected pH X . The corresponding measurements of E X , E S1 , and E S2 , are then combined to give the following equations: k = (E S2 E S1 )/(pH S2 pH S1 ) (9) pH X= pH S1+ (E X E S1 )(pH S2 pH S1 )/(E S2 E S1 ) (10) 3.3.2 Liquid junction potential Electric potential difference arising across
