Ordinarily pH measurements are made in water with 1,000 to 10,000 µS/cm conductivity. The conductivity comes from dissolved solids which are typically salts but also acid or base. These dissolved substances are in the solution in ion form which makes such solutions conducting. With conductivities of 1,000 µS/ cm or more, an ample supply of mobile ions provides pH electrodes with a good electrical circuit to yield fast, accurate pH readings. In pure water with low conductivity, standard pH electrodes exhibit less reliable results. They respond more slowly, drift, and do not reproduce calibration results between buffers and grab sample. In “Ultrapure” water, pH electrodes become jumpy, flow sensitive and inaccurate. To overcome these challenges, proper selection of the electrochemical cell elements in the pH sensor will provide stable, accurate measurements. This paper will outline these elements, their specific roles, as well as some application guidelines for on line Ultrapure Water (UPW) pH measurement.
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