Instrumentation
Dr Michael Lange provides an overview of pH electrode technology and discusses how the latest advances in iodine/ iodide reference systems have changed the way pH measurements are performed.
Dr Michael Lange donne une vue d’ensemble de la technologie des électrodes pour le pH et examine la façon dont les tout derniers progrès dans les systèmes de référence à l’iode/au iodure ont changé la façon dont les mesures du pH sont effectuées.
Dr. Michael Lange liefert einen Überblick über die pH-Elektrodentechnologie und erörtert, wie die neuesten Fortschritte in Iod/ Iodid-Referenzsystemen die Ausführung von pH-Messungen verändert haben.
Innovative pH electrodes with iodine/iodide reference system
T
he core principles of pH electrodes technology have not changed significantly since were first introduced in 1908, although technological innovations have enabled the
development of a range of electrodes, including the most commonly used combination electrodes and electrodes with gelled electrolytes. Combination pH electrodes consist of two electrodes built into a single body, with one being a pH glass-measuring electrode and the other being a reference system. Using such a system, pH is measured by calculating the voltage difference between the two electrodes. There are certain parameters that affect the
performance of pH electrodes. The response time of an electrode is the length of time that is required to obtain a stable reading when the
electrode is moved between solutions that exhibit different pH or temperature. The response time depends on the selection of a type of electrode, the measuring sample, the temperature of the solution being measured, the magnitude and direction of concentration change and the presence of interfering substances. The thickness of the glass membrane of the
electrode is particularly important as it determines the resistance of the electrode and affects its efficiency. High concentrations of contaminating metal ions greatly decrease the lifetime of glass membranes while also resulting in measurement errors. Metal ions interact with the glass membrane and can partially or completely replace the hydrogen (H+) ions present, generating false pH readings. This effect increases with increasing temperature. The sensitivity of pH electrodes is typically dependent on the temperature of the solution being measured, with the sensitivity increasing linearly with temperature. The reference system also plays a crucial role
in determining the accuracy and efficiency of combination pH electrodes. pH electrodes with an iodine/iodide reference system overcome the aforementioned challenges and deliver fast and dependable pH measurements.
Technological advances pH electrodes with an iodine/iodide reference system, such as the IoLine electrode from SI Analytics, contain coloured solutions that are clearly visible in the pH glass measuring electrode and the reference system. The colours occur according to the specified ratio of iodine to iodide. The electrochemical voltage of the iodine/iodide ORP pair depends on the concentration ratio of both substances and not on their absolute concentration, as per the below equations. The ORP voltage can be used as the reference voltage in both the pH glass electrode and the reference system. A platinum wire is used in the control systems solely for determining the charge transport between the two iodine and iodide ORP partners and does not affect the voltage.
Nernst equation: E=E0 E0
=0.536 V; Reaction equation: I3
Fig. 1. Schematic structure of a pH electrode with iodine/iodide reference system.
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One key limitation of conventional iodine/iodide reference systems is that they continuously lose iodine from the reference electrolytes, thereby lowering
+ (UN /2) log ([I3 -]/[I- ]3 );
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