ISO 12183:2024

International
Standard
ISO 12183
Fourth edition
Nuclear fuel technology —
2024-05
Controlled-potential coulometric
measurement of plutonium
Technologie du combustible nucléaire — Dosage du plutonium
par coulométrie à potentiel imposé
Reference number
© ISO 2024
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 2
7 Procedure . 8
7.1 Plutonium determination .8
7.2 Analysis of subsequent test samples . .14
8 Expression of quantity values . 14
8.1 Calculation of the electrical calibration factor .14
8.2 Calculation of the blank . 15
8.3 Fraction of electrolysed plutonium . 15
8.4 Plutonium, amount of substance and mass .16
8.5 Quality control .16
9 Characteristics of the method . 17
9.1 Repeatability .17
9.2 Confidence interval .17
9.3 Analysis time .17
10 Interferences . 17
11 Procedure variations and optimization .21
11.1 Accountability measurements and reference material preparation .21
11.2 Process control measurements .21
11.3 Measurement cell design . 22
11.4 Electrolyte and electrode options . 22
11.5 Test sample size . 22
11.6 Background current corrections . 23
11.7 Correction for iron . 23
11.8 Control-potential adjustment .24
11.9 Calibration methodologies . 25
12 Traceability to SI units .25
Annex A (informative) Purification by anion-exchange separation .26
Annex B (informative) Determination of formal potential, E .28
Bibliography .29

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies, and
radiological protection, Subcommittee SC 5, Nuclear fuel cycle, in collaboration with the European Committee
for Standardization (CEN) Technical Committee CEN/TC 430, Nuclear energy, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 12183:2016), which has been technically
revised.
The main changes are as follows:
— Figures 1 and 2 have been revised to resolve errors introduced in the third edition of this document;
— quantity values and uncertainties values have been reformatted to comply with requirements for
properly stating these values with SI units;
— editorial changes were made throughout the document to ensure clarity of the instructions;
— words with optional spellings were corrected to match ISO/IEC guidance;
— an additional key step was added to Clause 4 to indicated that the moles of plutonium obtained by
controlled-potential coulometry is multiplied by the molar mass of plutonium obtained by other means,
such as mass spectrometry or process knowledge;
— a formula has been added to 8.4 to calculate the amount of substance of plutonium in millimoles in
addition to the mass of plutonium in milligrams;
— Clause 12 has been added to discuss traceability to SI units.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
International Standard ISO 12183:2024(en)
Nuclear fuel technology — Controlled-potential coulometric
measurement of plutonium
1 Scope
This document specifies an analytical method for the electrochemical measurement of pure plutonium
nitrate solutions of nuclear grade, with an expanded uncertainty not exceeding ±0,2 % at the confidence
level of 0,95 for a single determination (coverage factor, k = 2). The method is applicable for aqueous solutions
containing plutonium at more than 0,5 g/l and test samples containing plutonium between 4 mg and
15 mg. Application of this technique to solutions containing plutonium at less than 0,5 g/l and test samples
containing plutonium at less than 4 mg requires experimental demonstration by the user that applicable
data quality objectives will be met.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principle
The key steps and their purposes are outlined below:
— test samples are prepared from homogenous solutions by weighing and then fuming to dryness with
sulfuric acid to achieve a stable anhydrous plutonium sulfate salt that is free from chloride, fluoride,
nitrate, nitrite, hydroxylamine, and volatile organic compounds;
— if needed to remove interferences, dissolve test samples and purify by anion exchange, then fume the
eluted plutonium solution in the presence of sulfuric acid to obtain the anhydrous plutonium sulfate salt;
— measure the supporting electrolyte blank and calculate the background current correction applicable to
[1]
the electrolysis of the test sample from charging, faradaic, and residual currents ;
— dissolve the dried test sample in the previously measured supporting electrolyte (the blank);
3+
— reduce the test sample at a controlled potential that electrolyses the plutonium to a Pu amount of
substance fraction greater than 99,8 % and measure the equilibrium solution potential at the end of this
[2]
step by control-potential adjustment ;
4+
— oxidize the test sample at a controlled potential that electrolyses the plutonium to a Pu amount fraction
greater than 99,8 % and measure the equilibrium solution potential at the end of this electrolysis by
control-potential adjustment;
— correct the integrated current (integrator output from the test sample) for the background current,
including the residual current corrections, and for the amount fraction of plutonium not electrolysed;

— calibrate the coulometer using traceable electrical standards and Ohm’s law;
— use the measured value of the electrical calibration factor and the Faraday constant to convert the
integrator output to coulombs and then to moles of plutonium measured by the coulometer;
— calculate the mass of plutonium by multiplying the moles of plutonium determined by controlled-
potential coulometry times a molar mass of plutonium determined by other means, such as thermal
ionization mass spectrometry, magnetic sector inductively coupled plasma mass spectrometry, or
process knowledge.
— use quality-control standards with traceable plutonium quantity values to demonstrate independently
the performance of the measurement system;
— periodically measure the formal potential of the plutonium couple, E which is user-specific based on the
0,
cell design, connections, reference electrode type, acid-type and molarity of the supporting electrolyte,
and the presence of any complexing agents in the electrolyte.
These steps ensure that test samples are taken from reproducible and stable sample solutions and prepared
for measurement. The test samples are measured using a protocol based upon first principles and a traceable,
electrical calibration of the coulometer. Further details are provided in Clauses 10 and 11.
5 Reagents
Use only analytical grade reagents.
All aqueous solutions shall be prepared with double-distilled or distilled, demineralized water with a
[3]
resistivity greater than 10 MΩ⋅cm, i.e. ISO 3696 Grade 1 purified water.
5.1 Nitric acid solution, c(HNO ) = 0,9 mol/l.
NOTE Refer to 11.4 for alternative electrolyte options.
5.2 Amidosulfuric acid solution, c(NH HSO ) = 1,5 mol/l.
2 3
5.3 Sulfuric acid solution, c(H SO ) = 3 mol/l.
2 4
NOTE The concentration of the sulfuric acid solution used to fume the plutonium test samples is not a critical
parameter, provided the sulfate ion concentration remains in large excess (above 50) compared to the plutonium ion
in order to avoid the formation of colloidal Pu complexes.
5.4 Pure argon or nitrogen, (O amount of substance fraction less than 10 μmol/mol).
5.5 Pure air (optional reagent), free of organic contaminants.
6 Apparatus
Usual laboratory equipment found in a medium-activity-radiochemical laboratory suitable for work with
plutonium should be used.
6.1 Analytical balance, installed in radiological containment unit and shall be capable of weighing a mass
of 1 g, with a standard uncertainty of ±0,1 mg, k = 1. This represents a relative standard uncertainty of 0,01 %.
— Weighing less than 1 g will increase the relative uncertainty to >0,01 %, in an inversely proportional manner.
— If the uncertainty of the balance, as installed, does not meet the criterion of ±0,1 mg, then test samples
greater than 1 g should be used.

6.2 Weighing bottle, glass or plastic, the material selection is not critical provided it is chemically inert,
maintains a stable mass (tare weight), and static charge is controlled as described in 7.1.1.
6.3 Equipment for test sample evaporation in the coulometric cell, comprising of an overhead radiant
heater or hot-plate with controls to adjust temperature. Design requirements and optional features for
effective evaporation and fuming include:
— providing settings that allow both a rapid and well-controlled rate of initial evaporation, followed by
fuming the remaining sulfuric acid solution to dryness at a higher temperature;
— preventing mechanical loss of the test sample solution from boiling and/or spattering;
— preventing contamination by extraneous chemicals, such as those which may be used to neutralize acid
vapours;
— heating of the coulometer cell wall to optimize fuming and minimize refluxing of sulfuric acid by placing
the cell inside an optional aluminium tube (inner diameter 1 mm to 3 mm larger than the outer diameter
of the cell, tube height 1 mm to 5 mm shorter than the cell) placed around the cell during the fuming step;
NOTE An aluminium block with holes bored to a similar specification for inserting the coulometer cell can be
used instead of the aluminium tubes.
— addition of an optional air supply with the delivery tube directed towards the surface of the liquid to
optimize the evaporation rate and disperse the acid fumes, with appropriate controls and feature that
will depend upon facility design and ventilation system requirements;
— addition of an optional vapour capture and local neutralization to control acid fumes, with appropriate
controls and features that will depend upon facility design and ventilation system requirements.
See Figure 1.
Dimensions in millimetres
Figure 1 — Sample evaporation system

6.4 Controlled-potential coulometer.
See Figure 2.
6.4.1 Coulometer cell assembly, comprising the following:
-1 −1
a) A stirrer motor with a rotation frequency of at least 16,7 s (1 000 min ).
NOTE 1 Adjustable-speed motors allow users to optimize the rate of rotation to the individual cell designs.
Stirrer motors powered by isolated DC power supplies are recommended as they prevent electrical noise from
being superimposed on the blank and test sample electrolysis current signals sent to the integrator.
b) A cylindrical or tapered glass coulometric cell of capacity 50 ml, or less.
c) A tight-fitting lid made from chemically and electrochemically inert material [e.g. polytetrafluoroethylene
(PTFE)], that includes an O-ring seal, and with openings to insert the following internal equipment:
— an inlet tube for humidified, inert gas to displace dissolved and atmospheric oxygen from the solution
and the electrolysis cell, respectively;
— a stirrer with blade and shaft made from chemically and electrochemically inert materials
[e.g. polytetrafluoroethylene (PTFE)], and designed to prevent splashing; the shaft of the stirrer is
typically located in the centre of the cell and connected directly to the stirrer motor;
— a working electrode made of gold [mass fraction (purity) 999,9 g/kg or greater] and consisting of
a gold wire welded or machined to a cylindrical gold wire frame, a nominal height of 15 mm and a
diameter of 20 mm, around which is welded or machined a very fine gold mesh, which is typically
several layers (e.g. four layers);
NOTE 2 Refer to 11.4 for other working electrode options.
— a glass salt bridge tube plugged at the bottom end with a sintered-glass disc (typical thickness of
2,5 mm and pore size of <0,01 μm), the tube is filled with nitric acid (5.1) and the tip of the sintered-
glass end is positioned within the ring of the working electrode;
NOTE 3 The diameter of the glass salt bridge tube and sintered-glass disc containing the auxiliary
(counter) electrode can be larger than that of the glass salt bridge tube and sintered-glass disc containing the
reference electrode. The desired flow rate of the solution through both glass discs is 0,05 ml/h, or less.
— a reference electrode, saturated calomel electrode (SCE), or other reference electrodes as described
in 11.3, is inserted into the glass salt bridge tube;
— another glass salt bridge tube, similar to the first one, also filled with nitric acid (5.1), and the tip of
the sintered-glass end positioned within the ring of the working electrode;
— an auxiliary (counter) electrode consisting of a platinum wire [mass fraction (purity) 999,5 g/kg or
greater] with a diameter of 0,5 mm to 3,0 mm, is inserted into the second glass salt bridge tube;
NOTE 4 Coiling the platinum wire increases the surface area submerged in the supporting electrolyte,
as illustrated in Figure 2.
d) A gas washer bottle, filled with reagent water as described in Clause 5, to humidify the inert gas before
it is introduced into the coulometer cell assembly.
e) A thermocouple or resistance thermometer installed in the coulometer cell assembly for measuring the
temperature of the test sample solution during the measurement process is an optional feature. The solution
temperature should be measured either during the oxidation of the test sample or immediately following
the analysis. A goal for the standard uncertainty of the temperature measurement is ±0,2 °C, k = 1.
NOTE 5 The purge gas is cooled by expansion causing the solution temperature to decrease relative to the
ambient temperature; the extent of this decrease is a function of the inert-gas flow rate and the cell design.

NOTE 6 If it is not possible to insert a temperature sensor into the electrolysis cell or not desirable to measure
the temperature of the test sample solution immediately after the electrolysis is completed, then the solution
temperature can be estimated from the ambient air temperature or the reagent temperature. The measured air
or reagent temperature value is then corrected for this cooling effect and a higher standard uncertainty of ±1 °C,
k = 1, is expected in the calculated solution temperature.
f) For optimum potential control, position the sintered-glass discs of the reference and auxiliary electrodes’
glass tubes in order to meet the following requirements:
— the closest distance from the reference electrode sintered-glass disc to the working electrode is
2 mm or less;
— the distance between the two sintered-glass discs containing the auxiliary and reference electrodes
is less than the distance between the auxiliary electrode disc and the nearest point on the working
electrode.
g) The hole through which the stirrer shaft is inserted serves as the primary escape vent for the inert gas.
Except for this hole, all other insertions are tight fitting. The inert-gas flow rate shall be sufficiently
high such that it removes oxygen quickly from the supporting electrolyte and the test sample solution.
Furthermore, it shall prevent leakage of air into the cell assembly during the electrolysis. A practical
guide for adjusting the flow rate is to direct all or part of the inert gas supply toward the solution, such
that a dimple is formed on the surface with a depth of 2 mm to 4 mm without causing the solution to
3 -1
splash. An inert gas flow rate of 0,000 1 m s is sufficient for the coulometer cell assembly illustrated in
Figure 2.
NOTE 7 Cell assemblies with an optimized design, an adequate inert-gas flow rate, and a tight fit, will remove
oxygen from nitric acid supporting electrolyte in 150 s or less. Due to the variabilities of factors involved (e.g. cell
geometry, volume of electrolyte), the time required to remove oxygen from the solution can be established by
users based on testing of their cell assembly under routine conditions.

Key
1 computer monitor 8 auxiliary (counter) electrode in salt bridge tube filled with
supporting electrolyte
2 printer (optional)
3 control computer 9 reference electrode in salt bridge tube filled with supporting
electrolyte
4 keyboard 10 inert gas inlet tube
5 potentiostat and integrator 11 stirrer
6 digital voltmeter (DVM) 12 working electrode
7 AC/DC power for stirring motor 13 cell
Figure 2 — Coulometric cell assembly connections

[4][5]
6.4.2 Instrumentation, comprising the following :
a) Potentiostat with the desired range of electrolysis potentials for plutonium measurement and the
following capabilities:
— a power amplifier with a current output capability of 250 mA, or greater;
— a quick-response control-potential circuit, with a maximum rise-time of 1 ms from zero volts to the
desired control potential, with a voltage overshoot not exceeding 1 mV;
— a control amplifier with a common-mode rejection of 90 dB, or greater;
— automatic control-potential adjustment, with a resolution of 0,001 V, or less;
— a voltage-follower amplifier, to isolate the reference electrode (electrometer), with a minimum input
impedance of 10 Ω;
— capability to monitor the electrolysis current, including charging current from -500 mA to +500 mA,
with a detection capability of ±0,5 μA, or less.
NOTE This procedure assumes that the coulometer has two accurate potentiometers, one for selecting the
oxidation potential and the other for the reduction potential, although this is not a system requirement.
b) Coulometric integrator capable of integrating blank and test sample electrolysis currents from at least
150 mA down to 1 μA, or less, with a readability of ±10 μC (refer to 7.1.4 for integrator capabilities and
calibration requirements):
— The control-potential system should not drift more than ±1 mV and the current integration system
should not drift more than 0,005 % during routine measurements (between electrical calibrations),
over the range of temperatures to which the control-potential circuitry will be exposed. If the room
temperature varies excessively, the instrumentation should be located in a cabinet with temperature
controls sufficient to limit electronic drift within these specifications.
— An electronic clock, with a standard uncertainty of ±0,002 %, k = 1, for determining the duration of
electrical calibrations and electrolyses.
— A system for generating a known constant current, stable to ±0,002 % over the range of temperatures
to which the constant-current circuitry will be exposed. This system will be used for electrical
calibration of the integration circuit of the coulometer, as described in 7.1.4.
— The cable connecting the potentiostat to the cell should be a three-wire conductor, twisted-shielded
cable, preferably with the shield grounded at the potentiostat. Gold-plated connectors at the cell are
recommended as these are not susceptible to corrosion.
— The charging-current peak maximum observed during the first 25 ms to 50 ms of the blank and test
sample oxidations shall be within the instrument specification for the integrator circuit. The surface
area of the working electrode can be decreased to reduce the charging current peak maximum. An
oscilloscope or a voltmeter with high-speed data acquisition is required to measure the amplitude
of this peak, which has a typical width at half the maximum of 10 ms to 20 ms.
6.5 Digital voltmeter (DVM), with an input impedance of 10 Ω or greater and having a standard
uncertainty within ±0,001 %, k = 1, for voltages in the range 0,5 V to 10 V, and within ±0,01 %, k = 1, for
voltages in the range 100 mV to 500 mV. These uncertainties are required for electrical calibration of the
instrumentation, as described in 7.1.4.
6.6 Regulated power, instrumentation should be protected with an uninterruptable power supply that
provides a regulated voltage within ±1 % of the standard for the country in which the analysis is performed,
and provides appropriate surge protection.

7 Procedure
7.1 Plutonium determination
7.1.1 Weighing the test sample, with a standard uncertainty of ±0,01 %, k = 1.
The test sample may be weighed after delivery into a tared coulometer cell, and the apparent mass corrected
for the air buoyancy effect using either Formula (1) or Formula (2), as described below.
Alternatively, a known mass of test sample may be delivered into the coulometer cell, as described in steps
a) through f).
NOTE 1 For test samples at high plutonium concentrations (e.g. 15 g/l or more), it is recommended that the solution
be diluted to achieve a standard uncertainty of ±0,01 %, k = 1 for the overall mass measurement process.
NOTE 2 If a weight bottle made of polyethylene, or other material susceptible to static electricity, is used, then the
problem of static electricity is eliminated by contact between the weighing bottle and a copper plate connected to the
ground, or a similar arrangement.
a) Fill a weighing bottle with the solution to be analysed.
b) Weigh the bottle to ±0,1 mg.
c) Deliver a test sample of at least 1 ml, drop by drop, into a coulometric cell, ensuring that at least 4 mg of
plutonium has been delivered.
d) Weigh the bottle again to ±0,1 mg.
e) The mass difference gives the apparent mass, M , of the test sample in the cell.
a
f) The real mass of the test sample, M , is obtained by correcting the apparent mass of the test sample
real
for the air buoyancy effect using Formula (1):
−1
MM=⋅ 11−⋅DD//−DD (1)
() ()
real aa ba s
where
D is the density of air, which is a function of room temperature, atmospheric pressure, and
a
relative humidity. When the room temperature is 22 °C ± 5 °C, the atmospheric pressure is
-3 -3
1 000 kPa ± 40 kPa, and the relative humidity is 45 % ± 15 %, D is 1,18 kg m ± 0,07 kg m
a
-3
D is the density of the stainless-steel weights used in modern analytical balances, 8 000 kg m
b
D is the density of the test sample, in kilograms per cubic metre
s
NOTE 3 Equations for calculating the density of air from the room temperature, atmospheric pressure, and the
[6]
relative humidity are available from several sources including the International Organization of Legal Metrology ,
which is based on guidance from the International Committee for Weights and Measures (CIPM).
In addition to applying an air buoyancy correction to the apparent mass of the test sample, air buoyancy
corrections should be applied to all mass measurements (including any bulk material mass measurements).
This correction is required to eliminate systematic errors that can approach 0,1 % for solutions. The
correction is less for a solid test sample, but can still be significant.
For plutonium metal and alloy test samples, an additional buoyancy correction term for self-heating from
[7]
radioactive decay, as detailed in Formula (2) is also appropriate for the apparent mass of metal or alloy .
−1 −23/ −1
′
MM=⋅()11−⋅DD//()−⋅DD [(1−⋅ΔmM) ⋅ P ]] (2)
()
()
real aa ba sa u,heat
where
M is the apparent mass of the metal or alloy, in grams
a
Δm’ is the mass coefficient for the heat buoyancy term, with a value of
1/3 −1 1/3 −1
0,000 03 g mW ± 0,000 01 g mW (1σ) for test samples ranging from 1 g to 15 g
P is the specific heat of the plutonium, in milliwatts per gram, calculated from the plutonium
u,heat
241 −1
isotopic abundance and the isotope mass fraction of Am. [This value is nominally 2 mW g
−1
to 3 mW g for plutonium metal whose origin is a spent nuclear fuel with a burn up ranging
−1 −1
from 2 MW⋅d⋅kg to 8 MW⋅d⋅kg . The specific heat increases with higher reactor burn up and
238 241
increased isotope mass fractions of Pu and Am.]
7.1.2 Preparation of the test sample
a) Add 1 ml of sulfuric acid solution (5.3) to the coulometric cell containing the test sample.
b) Place the cell containing the test sample into the sample evaporation system and carefully evaporate the
liquid in the test sample so as to avoid splashing.
c) Evaporate the remaining liquid in the test sample at a temperature sufficient to evolve fumes of nitrous
oxide (N O) and sulfur trioxide (SO ), and continue until SO fumes are no longer observed and a residue
2 3 3
of anhydrous plutonium sulfate salt (pink/orange-coloured precipitate) is formed. Do not allow the
solution to boil or splash as this will cause mechanical loss of the sample.
NOTE 1 The colour of the anhydrous plutonium sulfate salt is dependent on the type of lighting used in the
laboratory. Under fluorescent lighting the dried sulfate appears coral pink.
NOTE 2 Degradation of anhydrous plutonium sulfate salt to plutonium oxide is not expected even after baking
the residue unless subjected to extremely high temperatures.
NOTE 3 Failure to use 1) high purity reagents, 2) anion-exchange resins washed free of resin fines, and 3)
heating equipment that is well maintained and clean will impact the fuming operation adversely. Any or all of
these failures can produce a visible black residue in combination with the dried sulfate powder. These residues
could be mistaken for plutonium oxide, and depending on their composition might interfere in the coulometric
measurement.
d) Allow the test sample to cool to room temperature.
6+ 2+ 3+ 4+
e) If Pu (PuO ) is known to be present, reduction to lower oxidation states (Pu and/or Pu ) is
required prior to coulometric measurement by the addition of either hydrogen peroxide, nitrite ion, or
ferrous ion. The excess reducing agent shall be removed by purification or destroyed prior to coulometric
measurement, and the test sample again fume to dryness in sulfuric acid as detailed in steps a) through
d), above. Refer to Clause 10 for details.
6+ 2+
NOTE 4 If the presence of Pu (PuO ) is suspected, the test sample can be treated with one of the reducing
agent and processed appropriately. Alternatively, the test sample electrolysis can be monitored and the result
6+ 2+
rejected if the reduction step is slow, indicating the presence of Pu (PuO ).
6+ 2+
NOTE 5 When Pu (PuO ) is reduced using hydrogen peroxide in 8 mol/l nitric acid, then step e) can be
performed promptly after weighing the test samples, prior to fuming the test sample in sulfuric acid, as described
in steps a) through d).
f) If the presence of significant amounts of impurities is suspected, dissolve and purify the dried test
sample to eliminate the interfering elements. Repeat the sulfuric acid fuming step as detailed in 7.1.2.
Anion-exchange is an effective purification process; it is outlined in Annex A.
NOTE 6 The interfering elements gold, iridium, palladium, and platinum, along with the elements that do not
interfere: cerium, lanthanum, niobium, silver, tantalum, thallium, and thorium are not separated from plutonium
using the anion exchange purification in Annex A. Refer to Clause 10 for additional information on interferences.

7.1.3 Electrode pre-treatment
Electrode conditioning is critical to ensuring reproducibility. The following storage and treatment
techniques may be used individually or in combination to condition the working and auxiliary electrodes:
— storing in 8 mol/l nitric acid when the electrodes are not in use (this storage technique is recommended
as the general practice);
— soaking in concentrated nitric acid;
— soaking in concentrated sulfuric acid containing 10 % hydrofluoric acid, followed by 8 mol/l nitric acid;
— soaking in aqua regia (limited to several minutes to prevent damage to the working electrode);
— boiling in nitric acid;
— flaming the platinum auxiliary electrode to white or red heat.
Electrode treatment may be performed on a preventative basis, at the beginning and/or at the end of the
day of electrode use. Alternatively, treatment may be on an “as needed” basis, particularly needed in case
of failure to obtain optimum electrode performance in either the blank or the test sample measurements.
The background current values (total mC, charging current mA maximum, and residual current μA) should
be reproducible for a given coulometer cell assembly and are normally used as indicators of satisfactory
performance.
Each day, or more frequently if desired, before performing the actual blank determination, a further
conditioning of the electrodes is performed through the following sequence of electrolyses:
a) Assemble the cell lid, complete with the electrodes and other internal equipment (6.4.1).
b) Take a clean dry coulometric cell and add nitric acid solution (5.1) in sufficient quantities to immerse the
working electrode, and the sintered-glass discs at the bottom of the salt bridge tubes for the reference
and auxiliary electrodes.
c) Add one drop of amidosulfuric acid solution (5.2).
d) Firmly fit the cell under the lid.
e) Start the stirrer at the desired speed. This speed should be selected in order to maximize the stirring
rate, while avoiding splashing or the formation of an excessive vortex that would interrupt electrical
connections.
f) Open the gas inlet and maintain a sufficient flow of inert gas throughout the electrolysis period,
as described in 6.4.1 g). Inadequate purging to remove oxygen can be mistaken for an electrode-
conditioning problem.
g) Preselect the oxidation potential at E +0,32 V and the reduction potential at E –0,36 V.
0 0
h) After degassing for 150 s, start the oxidation and oxidize at E +0,32 V until a residual current of 10 μA is
obtained.
i) Start the reduction and reduce at E –0,36 V until a residual current lower than 10 μA is obtained.
j) Oxidize at E +0,32 V.
k) Stop the electrolysis when the current is lower than 10 μA.
l) Rinse the electrolysis cell and the outside wall of the salt bridge tubes with fresh supporting electrolyte.
m) Based upon electrode performance,
— perform further electrode conditioning according to 7.1.3, until the desired performance is observed, or

— measure the supporting electrode blank determination according to 7.1.6, in preparation for the
subsequent measurement of plutonium test samples.
7.1.4 Electrical calibration of the current integration system
The electrical calibration factor of the coulometer is measured by using a high accuracy, highly stable
constant current in place of the electrolysis cell. Detailed instructions for the calibration of a current
integration system are highly dependent upon the design of the specific integration circuit. However, the
following general principles and specifications apply toward determining the calibration factor within a
standard uncertainty not exceeding ±0,01 %, k = 1.
— Generate a constant current (stable and known to within ±0,002 %, k = 1) in a manner that is electronically
equivalent to the process by which the electrolysis current from the test sample and the blank are
integrated.
NOTE Typically, the potentiostat is converted into a constant current source with the current flowing through
a standard resistor, instead of the cell assembly. The voltage drop across the standard resistor is measured to
determine accurately the actual calibration current. Alternatively, if a constant current source is used instead of
the potentiostat, then this external source requires periodic calibration to ensure consistency and traceability.
— Determine the duration of calibration (i.e. current flow) within ~0,002 %, k = 1.
The linearity of the integrator response shall be demonstrated for the range of currents observed during
plutonium measurement from the maximum current at time equals 0 seconds to the current when the
control-potential adjustment begins (e.g. 100 mA to 50 μA). Ensure that the impact of the integrator
nonlinearity on the plutonium measurement is 0,005 %, or less, k = 1.
A typical sequence for performing an electrical calibration is:
a) configure the instrumentation for electrical calibration and set to the desired constant current, for
example 10,000 mA;
b) set the integration time to an appropriate duration, for example 300 s;
c) reset the integrator;
d) allow time for the electronics to stabilize;
e) initiate the calibration and record the constant current used, I , mA;
c,
f) at the completion of the calibration, record the output signal from the integrator, Q (in the units
C
appropriate for the specific measurement system) and the actual calibration time, t , in seconds;
C
Electrical calibration should be performed at least daily and in the same laboratory where the plutonium
measurements are performed. An automated coulometer should perform the electrical calibration without
the user needing to reconfigure the instrumentation. Refer to 8.1 for further details.
7.1.5 Formal potential determination
4+ 3+
The formal potential, E , of the Pu /Pu couple should be measured at regular intervals (as described
in Annex B), especially when electrodes have been replaced, if the electrodes have been out of use for a
considerable time, or if the studied solution is liable to contain a different amount of Pu-complexing agents
than that of solutions previously studied. Before performing this measurement, ensure that the working and
auxiliary electrodes have been properly pre-treated and conditioned. Also ensure that the SCE is filled with
saturated potassium chloride solution and contains a few free-flowing salt crystals, but is not clogged by
excessive amounts of salt crystals.
When the control potentials for reduction, E , and oxidation, E , are measured during the analysis of the
3 4
test sample, as described in 7.1.7, these potentials are approximately equal to E -0,17 V and E +0,17 V,
0 0
respectively. Thus, the average of E and E is highly correlated with E . The average of E and E may be
3 4 0 3 4
plotted on a control chart and used as an indicator of the stability of the electrolysis cell and the reference
electrode between periodic E determinations.
The formal potential is close to +0,668 V vs SCE when 0,9 mol/l nitric acid is used as the supporting
electrolyte but small variations can be expected because different calomel electrodes exhibit slightly
different potentials. The formal potential is also moderately dependent on the concentration
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