ASTM D 2186 : 2005 : R2009

13.1 Ion-Exchange Degasser—An ion-exchange degasser consists of an ion-exchange resin that exchanges hydrogen ions for all cations in the sample, thereby eliminating all basic dissolved gases, including volatile amines. By converting mineral salts to their acid forms, it also increases the specific conductance of the impurities. As a result, the linear relationship between conductivity and impurity content is extended to a much lower level, depending on the carbon dioxide content. The test method is very useful for measuring low concentrations of impurities, such as condenser cooling water leakage, in steam condensate, and it is especially useful, for indicating small or intermittent changes in impurity content from some normal value. The test method is not satisfactory for the determination of impurities in steam condensate samples that contain acidic gases, such as carbon dioxide, large percentages of insoluble matter, or substances that ionize weakly. The sensitivity and accuracy of the method are decreased for samples in which hydroxides represent an appreciable percentage of the impurities, because hydroxides, which contribute to the formation of deposits, are converted to water by the ion-exchange resin. This characteristic is particularly significant when steam is generated at sufficiently high pressure to cause appreciable vaporization of sodium hydroxide from the boiler water.

13.2 Mechanical and Ion-Exchange Degasser—By combining mechanical and ion-exchange degassing of steam or condensed steam, or both, effective elimination of both acidic and basic dissolved gases is attained. This arrangement has the same advantages and limitations as the ion-exchange degasser alone, except that it will remove acidic gases, and the greater sensitivity afforded by measuring the conductance at atmospheric boiling water temperature extends the linear relationship between conductivity and the ionized impurity content down to about at least 0.005 mg/L (ppm). Although the relationship becomes somewhat nonlinear, the conductance is sensitive to concentration changes down to at least 0.005 mg/L (ppm).

20.1 The principal advantages of the sodium tracer test method are the freedom from interferences, the ability to measure extremely small concentrations of impurities, and the rapid response to transient conditions because of the absence of large stagnant sample volumes, such as reboil chambers. Either of two procedures may be employed for the sodium determination, as follows: Precise control of sample temperature is not required for the flame photometry test method. If the impurities are principally sodium compounds, impurity concentrations as low as 0.6 μg/L (ppb) may be detected by the flame photometry method and as low as 0.5 μg/L (ppb) by the sodium ion electrode test method. The sodium tracer test method is not recommended for samples having large percentages of impurities that do not contain sodium.

27.1 Silica and various metals are impurities that are occasionally found in steam and have definite tendencies to form deposits. Since these substances are not isolated when using Test Method A and are not detected when using Test Methods B and C, it is advisable to determine their concentrations separately when they are present in significant quantities.