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Revival of ion selective appl covers.indd - Endress+Hauser

Revival of ion selective appl covers.indd - Endress+Hauser

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instability of previous technology forced the design of complex analyzers with pH adjustment, Ion Strength Adjustment, and Single Known Addition in applications requiring high accuracy. Today, with a more stable design and a better understanding of interferences, ISE electrodes can be mounted in-situ to measure nutrients such as Ammonium (Ammonia) and Nitrate. The increased repeatability permits their use for automatic process control to maintain the highest efficiency at the lowest cost. INTRODUCTION “Wastewater treatment plants that achieve very low effluent nutrient will need fail-safe design, extensive high-quality instrumentation” (J. Barnard, interview with WERF 2009) Manufacturers are working hard to provide better and easier to use instruments to measure nutrients. Multiple technologies are available such as colorimetry, UV absorption, Gas Sensitive Electrodes and Ion Selective Electrodes. Each technology has its advantage and specific range of applications. Over the last few years many improvement have taken place to increase accuracy, improve reliability, decrease maintenance and reduce the cost of ownership. Ion Selective Electrodes may well be the technology that has seen more changes. In this paper we will provide information on the evolution of this technology and present today’s status with references to the potential use and misuse of that technology. REVIEW OF STANDARD TECHNICS THEORY Ion Selective Electrodes are potentiometric sensors. They are equivalent to a standard pH electrode using a reference and an active measuring cell. The potential between those 2 components is proportional to the concentration of the component being measured in an analyte. Distributed with permission of author(s) by ISA 2009 Presented at ISA 4 th WWAC Symposium 2009; http://www.isa.org

ISE are available for many ions such as: Fluoride, Hardness, Bromide, Potassium, Chloride and of more direct interest for this paper Ammonium and Nitrate. Compared with other types of measurements the advantage of ISE is the large measuring range typically 0.1 to 1,000 mg/l or more. For a very long time, ISE have required a complex installation to provide reliable data. This was due to the instability of the electrodes with drift of the zero point, change in sensitivity (slope) and interferences from other ions. To compensate for those, a relatively complex sample preparation is required. It could include Ionic Strength Adjustment, Known Addition or in some cases only a pH adjustment. For high accuracy the Known Addition Method remains in use. In this method a measurement is taken of the sample to be measured. Then a known quantity of the ion to be measured is added to the sample and a second measurement is taken. The difference in measurement due to the added quantity allows to determine the slope and thus the real concentration of the ion in the sample. Distributed with permission of author(s) by ISA 2009 Presented at ISA 4 th WWAC Symposium 2009; http://www.isa.org

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