Development of pH measurement system for legal traceability of pH standard solutions

The best reproducible technology of pH measurement for precise pH buffer solutions regulated by Japanese Industrial Standards (JIS) was studied. A pH meter was devised with a high resolution of +/- 0.0001 pH. An 18-bit analog-to-digital converter is used, one-bit resolution corresponding to 0.0019 mV (ca. 0.000032 pH) against an input electrode potential +/- 500 mV. Digital data were treated smoothly for some types of noise, a reproducibility of +/- 0.0002 pH being obtained with a potentiometer. A flow cell was devised to attain temperature control within +/- 0.03 degrees C and air-tight measurement prevented contamination with carbon dioxide. Also, the flow cell has a structure such that potassium chloride (KCl) inner solution effused from a ceramic junction of the reference electrode designed so as not to touch the glass membrane. A combination pH electrode (a glass electrode and a reference electrode) was assembled to minimize the dead volume of sample solution. This highly sensitive pH measuring system, consisting of a pH meter, a flow cell, a combination pH electrode, a circulating water thermostat and a peristaltic pump, was used for the certification of pH standard solutions in Japanese metrological law. The performance of this system was within +/- 0.0006 pH reproducibility and 20-30 min response time (5 min within +/- 0.0002 pH) at a sample flow rate of 3 ml min (-1).