The working group's primary pH measuring device enables the measurement of the pH value of primary reference buffer solutions in the temperature range 5 °C–50 °C in accordance with the internationally agreed primary measurement procedure (
IUPAC Recommendation 2002, DIN 19266).
Test and calibration services offered
| Calibration Code | Buffer | Measuring Range (uncertainty) |
| AC-pH1 | oxalate buffer | 1.5 – 1.9 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH2 | tartrate buffer | 3.4 – 3.8 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH3 | phthalate buffer | 3.8 – 4.2 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH4 | phosphate buffer 1:1 | 6.8 – 7.2 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH5 | phosphate buffer 1:3.5 | 7.2 – 7.6 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH6 | borate buffer | 9.0 – 9.4 (± 0.003, k=2) (5 °C – 50 °C) |
| AC-pH8 | carbonate buffer | 9.8 – 10.2 (± 0.003, k=2) (5 °C – 50 °C) |
As a matter of principle, the pH value is measured of a buffer solution that is prepared in the Working Group using solid reference material sent in by the customer. The measurement of solutions prepared by the customer is subject to prior agreement.
Measuring principle
The pH value of the primary reference buffer solutions is determined by means of the potentiometric measuring arrangement as shown in Fig. 1. Since the pH value is temperature-dependent and the measurement is carried out at different pH values, the measuring cells are located in a thermostatic bath which is kept at a constant temperature of ±0.005 K. The platinum/hydrogen electrode and the silver/silver chloride reference electrode submerge directly in the measurement solution without the interposition of an electrolyte bridge or a diaphragm. This avoids diffusion voltages. The measuring cell voltage is measured directly between the silver/silver chloride reference electrode and the platinum/hydrogen electrode. The potential of the platinum/hydrogen electrode depends on the activity of the hydrogen ions, whereas the potential of the silver/silver chloride electrode is determined by the activity of chloride ions. Its use as a reference electrode therefore requires a defined addition of chloride ions. By measuring at different chloride concentrations and by extrapolation to zero chloride content, a so-called acidity function is obtained. Finally, the pH value is calculated on the basis of the acidity function by applying the activity coefficient of the chloride ion. Throughout the world, the activity coefficient is uniformly calculated by approximation on the basis of the Bates-Guggenheim convention which is, is turn, based on the Debye-Hückel theory of the electrolytes. This convention can be applied to buffer solutions with an ionic strength of ≤ 0.1 mol kg-1.
