title: Development of pH measurement system for legal traceability of pH standard solutions.
abstract: 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).
authors.affiliation: Applied R&D Department, DKK Corp., 4-13-14, Kichijoji Kitamachi, Musashino, Tokyo 180, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Ito
authors.forename: H
authors.initials: H
authors.lastname: Hachiya
authors.forename: K
authors.initials: K
authors.lastname: Baba
authors.forename: M
authors.initials: M
authors.lastname: Eto
authors.forename: Y
authors.initials: Y
authors.lastname: Asano
authors.forename: H
authors.initials: H
authors.lastname: Wada