Water quality is critical to the efficiency and performance of a steam generation plant and to the longevity of its associated equipment. Historically, simple conductivity measurement has been used extensively to monitor the quality of water but for many plant chemists this has become insufficient to ensure the protection of high cost capital equipment. Today sodium concentration has become one of the most important indexes for water quality throughout the steam and water cycle in power plants. However measurement of this parameter can be difficult in practice. For example, improvements in ion exchange resins over the past decade have now resulted in post mixed bed sodium levels below 0.03 ppb (1) therefore there is a need not simply to measure low levels of sodium but rather levels that continuously remain at sub ppb levels.
Several laboratory methods are available for sodium analysis; they include electrode potentiometry, ion chromatography, atomic absorption (ICP-AA (2), and mass spectroscopy. Although laboratory analysis forms a part of the normal quality control process it can require high demands of manpower and, by its nature, can only provide information for a specific point in time. Therefore, most power plants will also employ on-line continuous monitoring.
The most economical and best practiced on-line method used for high purity water analysis is the potentiometry method which uses a sodium ion specific electrode (ISE). In practice, temperature and pH fluctuations, fall-off in response in very low sodium concentrations and difficulties maintaining calibration are all potential issues that must be addressed with an on-line analyzer. In fact, several techniques can be used to maximize accuracy and efficiency of ISE analysis at a level as low as 20 parts per trillion. This paper discusses the importance of sodium analysis and describes the key problems that need to be resolved in order to provide trouble free, sub ppb measurement.