1. Petra Spitzer, Ralf Eberhardt, Janine Giera PTB, Germany Barbara Werner, Anett Czysch ZMK, Germany Traceable calibration procedures in chemistry Example: pH measurement DKD-K-06901

2. 2 Table of contents – How to provide reliable measurement results ? – pH measurement - are there any problems ? – Traceability chain for pH – Contribution of the NMI - PTB – Contribution of the Calibration laboratory - ZMK – Calibration and pH measurement procedure – How to ensure comparable pH values ?

3. DKD-K-06901 3 BIPM - NMI: primary measurement systems + devices primary reference materials accredited calibration laboratories, reference laboratories Certified Reference Materials (CRMs) chemical test laboratories, which have to proove their traceability (ISO 17025) Based on national structure  international comparability

4. DKD-K-06901 4 How to provide reliable measurement results ? What are the key elements? – Traceability to international recognized “stated references” – Measurement uncertainty evaluation – Target values of uncertainty How to realize? – Calibration – Validation – Inter-laboratory Comparisons / Proficiency tests

5. Traceability Dissemination Traceability chain for pH in Germany

6. From the primary level to the calibration service

8. CCQM-K17: link to EUROMET 696 Link to EUROMET

9. CMC entries of PTB

10. From the calibration service to the customer

11. DKD-K-06901 11 The measurement of pH The pH measurement is a simple operation There is a variety of meters, electrodes and buffers with specified pH on the market Are there any problems? There are 150 pH related standards (blood, water, soil milk, beer...)

12. DKD-K-06901 12 Cause and effect diagram

13. The pH electrode Electrode response is affected by – temperature – pH of the sample – streaming conditions – ionic strength – high salt concentration – pure water sample – viscosity of the sample – ageing of the electrode... Reference electrode |KCl ¦sample pH(X) |glass electrode

14. DKD-K-06901 14 The measurement of pH - calibration step – A calibration is required to match the pH meter to the electrode – Certified pH standards and provisions for their dissemination to the field level are the prerequisite for the comparability of sample pH pH measurements are only as accurate as the buffers used for calibration

15. DKD-K-06901 15 –two point (bracketing) calibration –three point calibration –five point calibration (to derive reference buffers) Calibration procedure Depends on the sample always the same wide variety on the target uncertainty

16. DKD-K-06901 16 The right electrode for each application – high pH - (temperature dependent alkali error !) – high temperature > 80 °C (response time, reference system) – high temperature and high pH and/or high salt concentration – pure water samples – interference with K + or Cl - – emulsions, surface measurement –.... ZMK calibration service offers custom made solutions for numerous measuring problems DKD-K-06901

17. 17 One example - the alkali (sodium) error - interference by alkali ions at high pH - reading is lower than the actual pH standard glass

18. DKD-K-06901 18 One example - the alkali error - alkali error depends on T alkali-resistant glass type

19. DKD-K-06901 19 Do you trust in these statements? pH buffers are certified to ±0.01 accuracy at 25°C and carry individual expiration dates These solutions are standardised at 25°C and are accurate to ± 1.0%. All are directly traceable to N.I.S.T standard reference materials. pH buffers are made of Potassium Dihydrogen Phosphate and Disodium Hydrogen Phosphate and are standardized with a pH meter that is calibrated to NIST primary standard buffer solutions (NIST 186Ie & 186IIe and NIST 158g S.R.M.).

20. DKD-K-06901 20 Calibration certificate Key elements – CRM measured mean value – expanded uncertainty (+ coverage factor) – composition of the CRM – batch number – reference of the pH standards used to calibrate the CRM – calibration procedure – pH variation with temperature – signature Additional information – expiry date – recommendation for handling according to GLP – storage conditions

21. DKD-K-06901 21 How to provide comparable pH values ? Contribution of the metrology institute - PTB – primary measurement procedures and reference materials – demonstrate competence: international comparisons – calibration service – embedded in world-wide collaboration and national net-work – offer consulting and training Metrology institute - calibration laboratory - test laboratory Sharing knowledge in metrology - calibration, research, consulting

22. DKD-K-06901 22 How to provide comparable pH values ? Contribution of the accredited calibration lab - ZMK –prove traceability to national standards (ISO 17025) –demonstrate competence: comparisons / proficiency tests –certification of pH reference materials and buffers –custom made solutions close to application –in-house and on-site calibration of test equipment –consulting and training DKD-K-06901

23. 23 Comparison between PTB and DKD on pH Measurement methods Diff-pot - differential potentiometric cell 5-point - glass combination electrode / calibration with 5 CRMs

24. DKD-K-06901 24 How to provide comparable pH values ? – knowledge about the measurement problem – definition of a target uncertainty for each application – regular control of the test equipment: meter, electrode, temperature sensor - in-house or extern calibration – CRMs traceable to primary pH(PS) buffers for calibration – pH value stated + uncertainty at measurement temperature  SOPs for each application Contribution of the test laboratory

25. DKD-K-06901 25 Outline Objectives  primary level – buffer for higher temperature > 50 °C – buffer at higher ionic strength  secondary level – improved secondary methods – comparability of potentiometric  spectroscopic results – improved evaluation of calibration results Uncertainty pH  s ample and calibration buffers similar (matrix, T, ionic strength...)  Improved comparability of pH values on the field level

26. Thank you for your attention Zentrum für Messen und Kalibrieren GmbH Physikalisch-Technische Bundesanstalt Braunschweig and Berlin