pH Measurement

Arterial blood gas analysers measure [H+] and PCO₂ using potentiometric electrodes and PO₂ via an amperometric technique. [H+] is measured using a power of hydrogen (pH) electrode, which is an ion-sensitive electrode whose operation depends upon the ion-sensitive glass at its tip. pH electrode: silver/silver chloride electrode within a buffer solution of hydrochloric acid. The tip of the electrode is composed of pH-sensitive glass. [H+] is held constant around the pH electrodes by the buffer solution and so any potential difference across this electrode is due to [H+] within the blood sample. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH has been established by international agreement. The system is calibrated using two of these standard buffer solutions of known pH. A pH indicator is a substance that will change colour at a particular pH value.