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TRANSCRIPT
An Improved Approach for Fabricating Ag/AgCl
Reference Electrodes28th CITAC members’ meeting
Dr. Paul Brewer
14TH April 2013
Hotel Mercure Paris Porte de St Cloud, Paris
pH measurement
• Concept of pH was first introduced by SØrensen in 1909
• One of the most commonly used analytical determinations
- Chemical and drug manufacture
- Blood gas determinations
- Effluent discharge control
- Food and drink processing
- Water purity
• Essential to ensure the validity and traceability
IUPAC
Definition of pH and Harned cell
IUPAC
Definition of pH and Harned cell
NMIs
Primary buffers
NMIs
Primary buffers
Buffer manufacturers
Secondary buffers
Buffer manufacturers
Secondary buffers
Calibration labs
Glass electrode buffers
Calibration labs
Glass electrode buffers
Routine pH measurements
Commercial glass pH
electrodes
Routine pH measurements
Commercial glass pH
electrodes
Traceability
• Relating a measurement result to a
reference through an unbroken
chain of comparisons, all having
stated uncertainties
• Standards prepared at NMIs
traceable to the SI and measured
using a primary method
8 key comparisons in the last decade
(K9, K9.2 K17, K18, K18.1, K19, K19.1, K20)
International comparisons
CENAM
NIST
NMIJ
DFM
SMU
GUM
CMI
VNIIFTRI BIM
PTB
KRISS
INMETRO
NIM
UMTSLNE
INPL
OMH
7 6 5 4 3
Number of Key Comparisons
12
GUM
CMI
Harned cell – primary method
Pt|H₂|buffer, Cl⁻|AgCl|Ag
• electrochemical cell arrangement which does not contain a liquid
junction
• relies on well characterised Ag/AgCl reference electrodes for
operation
• potential to be a primary method for the absolute measurement
of pH, providing that it can conform to the accepted definition of
a primary method
E (V)
T (K)
Cl- (mol/kg)
HCl (mol/kg)
pH2 (Pa)
∆∆∆∆ Ag/AgCl (V)
lg γγγγ +/-
[Br-] (mol/kg) Buffer filled Cell
HCl filled Cell
0 1 E-4 2 E-4 3 E-4 4 E-4 5 E-4 6 E-4 7 E-4 8 E-4 9 E-4
Contribution to Uncertainty in Acidity Function u[p(aH+
γγγγCl-
)]
Influence of Ag/AgCl electrode potential
• Ag/AgCl is the 2nd largest uncertainty contributor
• Repeatability: potential must be < 100 µV from the mean
• Stability: potential < 30 µV/ h
Electrode fabrication
electrolytic
thermal electrolytic
20 µµµµm 20 µµµµm
porous open locked
more impurities
lower geometric surface area
lower exchange current at eqm
poorer stability
higher purity
higher geometric surface area
higher exchange current at eqm
good stability
3 structures proposed by Janz
Stability dependent on porosity
Electrode microstructure
• Thermal electrolytic electrodes offer several advantages:
- Fabrication less time consuming
- Reduced equilibrium times
- Low contamination risk
- Portability
• Annealing redistributes and destroys crystallographic defects,
producing an homogenous material
• Annealing should remove strains and improve structural uniformity,
improving performance
Thermal electrolytic electrodes
Requirement: improve long term stability and repeatability
AgCl distribution
P 37.2Anodisation – constant current density
ScheduleAnodisation – constant current
ParticipantsTransient anodisation signatures
ShippingTransient anodisation signatures
ShippingAg/AgCl potential measurements
ShippingImpedance spectroscopy
ShippingSEM – constant current / potential
Conclusions
• New self- limiting approach for fabricating Ag/AgCl electrodes
• Superior repeatability
• Improved conductivity due to microstructure
• Transient current data provides information in microstructure
• Far-reaching implications for pH measurement
Acknowledgements
• UK BIS Chemistry Biology Measurement Programme
• European Union
• Richard Brown (NPL)
• Paola Fisicaro and Daniela Stoica (LNE)
• Rebecca Leese (University of Southampton)
• CCQM EAWG