1. Improvement in the Reliability and Comparability of pH and Electrolytic Conductivity Measurement Results
San Diego, 06 August 2002
Petra Spitzer, Ralf Eberhardt, Janine Giera, Uwe Sudmeier
Physikalisch-Technische Bundesanstalt (PTB) Germany

2. How to Provide Reliable Measurement Results?
What are the key elements?
Traceability to “stated references”
Measurement uncertainty evaluation
Target values of uncertainty
Concordance: values lie within each other´s uncertainty
How to realize?
Calibration
Validation
Inter-laboratory Comparisons
[1] Paul de Bievre, Nineta Majcen, Proceedings Slovenian chemical event, Maribor 2001

3. Electrolytic conductivity
Overview pH
Primary procedure
Traceability chain
International comparisons
Standards and procedures
Future projects
Electrolytic conductivity
International comparison

4. Quantity pH Definition a = activity (“effective concentration”)
Problem  = activity coefficient  not measurable
Answer  Electrochemical sensor Pt|H2 electrode EH = f (aH)
 Reference electrode Ag/AgCl:
 Cl ! Approximation - Bates-Guggenheim Convention
Common “stated reference“
Reference buffer solutions (pH standards)
primary secondary commercial (CRM´s)

5. Primary-set-up for pH
Harned-cell

6. CCQM WG on Electrochemical Analyses
Terms of references related to pH
To establish the degree of equivalence
of national measurement standards for pH
To support the dissemination of traceability to the RMO´s
To develop and improve measurement methods
at the primary level

7. CCQM K-9 Key Comparison on pH

8. Secondary standards Primary level: cell without liquid junction
Pt | H2 | buffer PS, Cl-mCl | AgCl |Cl
Secondary level: cell with liquid junction
Pt | H2 | buffer PS ¦ ¦ buffer SS | Pt | H2

9. Reference | KCl(conc.) ¦ Buffer pH(S), sample pH(X) | glass electrode
pH Measurement
Affected by:
Sample / standards
Reference-/glass electrode
Meter/ temperature sensor
Environmental conditions
Operator ....
Reference | KCl(conc.) ¦ Buffer pH(S), sample pH(X) | glass electrode
Calibration: pH-reference buffer solutions
primary secondary commercial (CRM´s)
Calibration of the meter
Calibration of the temperature sensor

10. pH cause and effect analysis
Standard E(S)
Standard pH(S)
Response time
CO2
stirring
standard
calibration certificate
pH electrode
pH meter
temperature
temperature
repeatability
liquid junction potential
Sample pH(X)
Response time
CO2
stirring
sample
pH electrode
liquid junction potential
temperature
pH meter
calibration
repeatability
Sample E(X)
Temperature sensor

11. Traceability chain for pH in Germany

12. A New Approach for pH
IUPAC (provisional) recommendation
The measurement of pH- definition, standards and procedures
Part I primary methods and primary standards
Part II secondary methods and secondary standards
Part III calibration procedures
Part IV other media, high and subzero temperatures
 New project
Traceability of pH measurement for applications in fundamental and applied science
Traceability is established throughout the entire sequence of measurement.

13. A New EUROPEAN (CEN) Standard for pH
Scope
How to establish traceability between the sample pH and the primary reference buffer solutions
Part 1 general aspects, terminology
Part 2 certification of reference materials for pH measurements
Part 3 calibration of pH measuring equipment
and practical pH measurement
Problem: ~ 150 pH related standards (milk, juice, soil...)

14. Improved Reliability of pH Values
State-of-the-art
Internationally agreed primary method  Primary standards
for aqueous solutions at Ionic strength  0,1 mol/kg 
IUPAC- WP on pH  Recommendation
CEN TC 332  European standard ISO CCQM - K9, K17, P37  Comparisons
Problem
Lack on reliability for pH in environmental monitoring, process monitoring, laboratory medicine...
 Uncertainty pH  if sample and standards are similar (matrix, T, ionic strength...)

15. Improved Reliability of pH
How to realize?
 Extended primary measurement procedure
Extended temperature range > 50°C to 80°(140°C)
Primary standards at higher ionic strength
Improved approximation for single ionic activity coefficient
 Improved secondary measurement procedures
 Potentiometric  spectroscopic results for pH

16. Electrolytic Conductivity - Important Ranges
0,05 µS/cm
Ultrapure water
1 µS/cm
Pure water
100 µS/cm
Drinking water
Juice, milk, beer, wine
10 mS/cm
Waste water
Process water
100 mS/cm

17. Electrolytic Conductivity
Non specific! depends on concentration and charge of ions in solution
Definition
Primary
Kcell traceable to meter
Homogeneous field distribution!
Secondary
k of primary standards 
calibration of Kcell of secondary cells
secondary cells  traceable measurement of k in the sample
R = resistance of the solution in the cell
Kcell = cell constant in cm-1

18. PTB´s Primary (Piston Type) Cell
1 µS cm mS cm -1
100 W  R(X)  300 kW
400 Hz  f  5 kHz
Side effects are eliminated by altering the electrode spacing l A

19. Primary Standard Measurement Device

20. CMC entries of PTB MRA: Mutual Recognition Arrangement
for National Measurement Standards

21. Traceability chain for
electrolytic conductivity
in Germany
Traceability
Example: kP ~ 1408 µS cm-1

23. Electrolytic Conductivity
State-of the art
First international comparison (CCQM-P ),
Restricted number of primary reference solutions
Problem
Ultra pure water (0,05 µS/cm -1µS/cm)
for microelectronics, pharmaceuticals, water for laboratory use....
 e.g. FDA specifications (USP 24):  1,3 µS/cm
 None fully reliable calibration procedure
Demand
Traceability of conductivity (0,05 µS/cm -1µS/cm) to the SI
U < 1% (k=2)

24. Metrology in Electrochemistry
To measure means to compare.
Vision...
Comparability of results obtained by (Bio)chemical sensors. pH
Electrolytic conductivity
Ion activity by ISE
Redox potential...