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

• Published in: Talanta (Oxford) 1996-09, Vol.43 (9), p.1421-1428
• Main Authors: Ito, Satoshi, Hachiya, Hiromitu, Baba, Keiko, Eto, Masayoshi, Asano, Yasukazu, Wada, Hiroko
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Exact sciences and technology; General, instrumentation; Legal traceability; pH measurement; pH standard solutions
• ISSN: 0039-9140; 1873-3573
• DOI: 10.1016/0039-9140(96)01904-2

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 °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.