Corning; Fisher Scientific; The Foxboro Co.; Great Lakes Instruments, Inc.; HNU Systems, Inc.; Horiba Instruments, Inc.; Ingold Electrodes, Inc.; Leeds and Northrup; Orion Research, Inc., Radiometer; Rosemount Analytical, Inc.
Wastewater treatment plant effluents must be monitored for all ionic substances that are deleterious to humans or animal life in the receiving waters. These include cyanides, sulfides, lead, and other ionic substances. Other ionic substances are monitored as indicators of various water properties. For example, calcium is monitored to detect water hardness.
The ion-selective measurement theory and equipment are similar to those of pH. The electrodes are usually the same size and fit into the holder assemblies of the pH probes. These electrodes are also subject to fouling by oil or slimes, and can be cleaned by the probe cleaners discussed previously in this section.
Ion-selective electrodes comprise a class of primary elements used to obtain information related to the chemical composition of a process solution. These electrochemical transducers generate a millivolt potential when immersed in a conducting solution containing free or unassociated ions to which the electrodes are responsive. The potential magnitude is a function of the logarithm of measured ion activity (not the total concentration of that ion) as expressed by the Nernst equation (see Equation 7.7). The familiar pH electrode for measuring hydrogen ion activity is the best known ion selective electrode and was the first to be commercially available. The potential developed across an ion-selective membrane is related to the ionic activity as shown by the Nernst equation as follows:
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