IEC TR 62432:2006

TECHNICAL IEC
REPORT TR 62432
First edition
2006-03
The rH index in aqueous
and aqueous-organic media
Reference number
IEC/TR 62432:2006(E)
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TECHNICAL IEC
REPORT TR 62432
First edition
2006-03
The rH index in aqueous
and aqueous-organic media
 IEC 2006  Copyright - all rights reserved
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– 2 – TR 62432  IEC:2006(E)
CONTENTS
FOREWORD.3
INTRODUCTION.5

1 Scope.6
2 General principles .6
2.1 Redox couples, redox equilibria, redox potentials, redox systems.6
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 .12
2.4.3 The inert noble-metal electrode (Pt or Au) .12
2.5 rH Scales in diverse solvents .12
2.6 Pourbaix’s diagrams for the triad rH – pH – E .13
O|R
3 Instrumentation .13

Bibliography.14

Figure 1 – Pourbaix’s diagram for the triad rH – pH – E for some key redox systems .10
O|R
Table 1 .6
Table 2 – Some reference aqueous solutions proposed as rH-metric standards rH [8, 9]
S
at 25 °C and for the calibration of the redox electrode at E .11
O|R
Table 3 – Values of (E – E ) [6] with corresponding rH values, at various
QHY H + |H2 S
temperatures, valid for any solvent (water W, or aquo-organic mixture Z = W + S
compatible with 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
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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 65D/123/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 publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 4 – TR 62432  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 the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
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A bilingual 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] , but for the user’s
convenience, the essentials will be recalled in the present Technical Report together with the
application domains, the recommended procedures and operational details.

———————
Numbers 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 Report 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 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 uncharged) 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 which is transmitted to the meter by an
O|R
inert metal 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 becomes the standard redox potential, symbolized as E .
O|R 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
n n
solvent medium. Instead, an undefined number of redox couples O|R, O’|R’, O”|R”,.O |R
overlap, 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 redox 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
3 + 2 +
ferric|ferrous Fe + e = Fe E = E + k log(a /a )
Fe3 + |Fe2 + Fe3 + |Fe2 + Fe3 + Fe2 +
+ +
H |H (hydrogen electrode) 2H + 2e = H
E = E° + k log a − (k/2)log p
2 2
H + |H2 H + |H2 H + H2
− −
Cl |Cl (chlorine electrode) Cl + 2e = 2Cl
E - = E - − k log a - + (k/2)log p
2 2
Cl2|Cl Cl2|Cl Cl Cl2
+
O + 4e + 4H = 2 H O
O |H O (oxygen electrode) E = E° + k log a + (k/4)log p − (k/2)log a
2 2
2 2 O2|H2O O2|H2O H + O2 H2O
− 2 + − +
MnO |Mn MnO + 5e + 8H = E = E° + (k/5)log(a /a ) +
4 4 MnO4|Mn2 + MnO4|Mn2 + MnO4|Mn2 + Mn2 +
2 +
(permanganate electrode) Mn + 4H O
+ (8k/5)log a − (4k/5)log a
H + H2O
Symbols: e = the electron; k = Nernstian coefficient = 2,303RT/F ; a = activity; 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 (1)
H2
where p is that pressure of hydrogen gas that would equalize the potential E of the
H2 H + |H2
hydrogen gas electrode to the redox potential E of the system being studied (thus zeroing
O|R
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 is (with k = 2,303RT/F):
H + |H2
E = E − k pH + (k/2) rH (2)
H + |H2 H + |H2
where E is the standard electrode potential (which varies with the solvent but it is
H + |H2
conventionally put equal to zero at any temperature in pure aqueous medium [4, 5]). If the
hydrogen gas electrode works at p = 1 bar (i.e. under standard state conditions), then
H2
rH = 0 at any pH of the solution (see Figure 1, which describes the pertinent Pourbaix’s
E vs. pH diagram), and this is the nominal zero of the rH-metric scale to which
Redox
co
...