E-Zine August 2013
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A closer look at the overall circuit for a pH sensor using a glass measurement half-cell provides insight into the various mechanisms involved in the circuits of all pH and ORP sensors. In these circuits, current flows using three different mechanisms --- solid conduction, interfacial charge transfer, and liquid diffusion. Electrons move from higher to lower energized states in individual solid atoms in the electronic unit, the wires to the electrodes and the electrodes. Charge transfer occurs leading to the development of an electrical potential at the electrode surfaces in contact with the electrolytic solutions and at the glass membrane in contact with the internal chloride buffer and the process fluid.
In conductive solutions, individual ions move through the solution from one electrode to the other and the overall composition of the conductive solution does not change because the small amount of current passed to effect the measurement does not perceptibly change the concentration gradients established in the conductive solution.
The reference half-cell includes an electrode immersed in an electrolytic solution. The electrolytic solution is contained within a cavity and, this generally impervious cavity, is penetrated in at least one location by a micro porous material. This porous material allows a small amount of bulk flow and diffusion between the internal electrolytic solution and the process fluid. The direction of the bulk flow depends upon the relative pressures of the electrolyte solution and the process fluid. The flow resulting from diffusion depends upon the relative concentrations of each ion in the electrolytic solution and in the process fluid. Even if pressure is applied to the electrolytic solution to achieve a small flow rate of electrolyte into the process fluid, ions in the process fluid will diffuse into the electrolyte solution.
Excerpted from The Consumer Guide to Industrial pH and ORP Instrumentation