5.1 Continuous pH measurements in low conductivity samples are sometimes required in pure water treatment using multiple pass reverse osmosis processes. Membrane rejection efficiency for several contaminants depends on pH measurement and control between passes where the conductivity is low.
5.2 Continuous pH measurement is used to monitor power plant cycle chemistry where small amounts of ammonia or amines or both are added to minimize corrosion by high temperature pure water and steam.
5.3 Conventional pH measurements are made in solutions that contain relatively large amounts of acid, base, or dissolved salts. Under these conditions, pH determinations may be made quickly and precisely. Continuous on-line pH measurements in water samples of low conductivity are more difficult (4, 5). These low ionic strength solutions are susceptible to contamination from the atmosphere, sample stream hardware, and the pH electrodes. Variations in the constituent concentration of low conductivity waters cause liquid junction potential shifts (see 3.2.1) resulting in pH measurement errors. Special precautions are required.
1.1 This test method covers the precise on-line determination of pH in water samples of conductivity lower than 100 μS/cm (see Table 1 and Table 2) over the pH range of 3 to 11 (see Fig. 1), under field operating conditions. pH measurements of water of low conductivity are problematic for conventional pH electrodes, methods, and related measurement apparatus.
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