INTRODUCTION

For compendial purposes, pH is defined as the value given by a suitable, properly calibrated, potentiometric sensor and measuring system. By definition, pH is equal to −log₁₀[a_H⁺] where a_H⁺ is the activity of the hydrogen (H⁺) or hydronium ion (H₃O⁺), and the hydrogen ion activity very closely approximates the hydrogen ion concentration.