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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