Anal. Chem. 1981, 5 3 , 361-362
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Sample Containers for Trace Analysis of Dissolved Oxygen, Hydrogen, and Carbon Dioxide in Water Alfred Pebler Westinghouse Research & Development Center, Pittsburgh, Pennsylvanla 75235
The mass spectrometric and gas chromatographic analyses of gas mixtures rely heavily on the sampling, transfer, and temporary storage of grab samples. Commercially available Type 304 stainless steel (SS) sample cylinders are frequently used to contain the samples. The chemical integrity of a gas sample in contact with the container material is acceptable for most gases containing little or no moisture. However, this situation in the presence of liquid water may differ significantly. We found that the determination of oxygen, carbon dioxide, and hydrogen in the presence of liquid water cannot be relied on if the samples are stored in standard unconditioned 304 SS containers. In a particular application, steam and feedwater were sampled at power plants for subsequent analysis of dissolved gases, including hydrogen, nitrogen, oxygen, carbon dioxide, and hydrocarbons. Two-port 304 SS sample containers used for this purpose were flushed with the sample streams at elevated temperature until an aliquot of steam or water was isolated within the container. A mass spectrometric technique for trace gas analysis in steam and water batch samples has been described before (I, 2). Questions raised in regards to the validity of the analytical results using this technique led us to conduct certification tests in our laboratory. In preliminary experiments, oxygen in the presence of water was completely removed from standardized gas samples in 304 SS sample Cylinders, apparently by reacting with the containment material. At the same time, appreciable quantities of hydrogen and hydrocarbons were generated. A test program was then undertaken to search for less reactive container materials and test pretreatment processes that inactivate container surfaces toward reaction with water and oxygen.
EXPERIMENTAL SECTION Materials. The tests were carried out with double-ended sampling cylinders of 150-mL nominal capacity made of 304 SS (Hoke No. 4HD150), 304L SS (a low carbon grade 304 SS) (Hoke No. 4LD150), 316 SS (Hoke No. 4HDY150), and Monel (Hoke No. DHM150). Type 303 SS, packed needle valves (Hoke No. 3712M4S) were assembled to the 304 SS sample cylinders by using Teflon as a thread sealant. Likewise, the 316 SS and Monel sample cylinders were equipped with the respective 316 SS and Monel valves (Hoke No. 3712M4Y and No. 1715M4M). The valves were installed with their packing on the outside of the containment closure. The leak rate of the assembled sample cylinders was tested with a helium leak detector and certified to be cm3/s. Pretreatments. As-received sample cylinders and disassembled valve bodies were surface treated by (1) preoxidation in supercritical steam at 400 "C and 140 bar for 5 days in an autoclave, (2) preoxidation in alkaline water (0.001 M LiOH) at 360 "C for 5 days in an autoclave, and (3) electroplating a gold layer on the internal parts after sandblasting and electropolishing the surface and preplating a thin layer of nickel. Procedures. The sample cylinders were filled with deionized water that was saturated with pressurized air or a 10% oxygen in nitrogen mixture. The latter gas mixture was used to facilitate the detection of C 0 2and H 2 that may form during exposure. The deionized water was circulated through a gas saturator and the sample cylinder for several hours by use of a peristaltic pump. Unless specified otherwise, the water-filled containers were held for approximately 1 day at room temperature. The containers 0003-2700/81/0353-0361$01.00/0
were briefly heated to as high as 250 "C to simulate the conditions during hot water or steam sampling. Before the cylinders were heated, a portion of the water content was quickly withdrawn by suction to allow for thermal expansion of the water. The cylinders were then placed in a preheated furnace, removed after reaching the furnace temperature, and allowed to cool to room temperature. The apparatus and procedures for determining the dissolved gases were essentially the same as described earlier (I). Briefly, the gases were separated from the liquid water by vacuum extraction, preconcentrated with a dry ice cooled trap, and quantitatively analyzed with a DuPont 21-104 gas mass spectrometer. The analytical values for the various gases were ratioed to that of nitrogen used as an internal standard in order to make the results independent of the sample size. It was established that the nitrogen balance was unaffected by the exposure to the container materials. The analytical gas/N2 ratios were compared with the ratios expected on the basis of the composition of dry air and the known solubility coefficients (3).
RESULTS In fresh cylinders, oxygen was effectively removed from air-saturated water. The removal rate of oxygen is expected to drop as a protective oxide layer on the steel surface builds up from repeated exposure to oxygenated water. Yet after 10 exposures, more than 80% of the initial oxygen was still lost. Hydrogen was generated only during the first exposure to water and dropped below the reliable experimental detection level in subsequent tests. Argon and carbon dioxide were recovered at about the expected level in fresh, untreated containers. After passivating the 304 SS sample cylinder in superheated steam, the stability toward oxygen improved dramatically. After a second exposure to an air-saturated solution, the expected amount of oxygen was nearly recovered. However, appreciable amounts of COz and hydrocarbons were released after the steam-treated 304 SS cylinder was exposed to water. The formation of COz and hydrocarbons subsided after the second water exposure but was still significantly above the expected level. Gold plating (-2.5 pm) was only partially successful to inactivate the steel surface; however, it prevented hydrogen and COP formation. The investigation was extended to include other container materials. The sample containers were used as-received or following passivation in hot alkaline water or gold plating (12.5 pm). In addition, the water-filled sample containers were briefly heated to temperatures as high as 250 "C. The experimental 02/Nz ratios for the test matrix of the various levels of materials, p'retreatments, and maximum temperatures of exposure to water saturated with a 10% Oz/Nzgas mixture are given in Table I. Hydrogen and carbon dioxide in all cases were below the detection limits. It is seen that 304 SS, after treating in hot alkaline water or after gold plating, was satisfactorily stabilized toward oxygen at room temperature. The protection was increasingly lost, however, when the container was briefly heated to 125 and 150 "C. Low carbon 304L SS proved to be more corrosion resistant than the standard 304 SS, as evidenced by the quantitative recovery of oxygen, even after heating the container to 125 "C. Still greater protection is provided by a 316 SS container passivated in hot alkaline water. The best overall performance, however, was achieved with Monel. Even un0 1981 American Chemical Society
Anal. Chem. 1981, 53, 362-363
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Table I. Exposure of Various Sample Container Materials to Water Saturated with a 10%0,-90% N, Mixturea
material
pretreatment
25 "C
125 "C
OJN, ratio 150 "C
304 SS
M LiOH at 300 "C Au plated
0.20
0.19
0.14
0.20
0.10
200 "C
250 "C
none 0.16 l o m 3M LiOH at 360 "C 0.21 0.17 316 SS none 0.20 0.16 M LiOH at 360 "C 0.20 0.13 Monel none 0.21 0.21 0.20 M LiOH a t 360 "C 0.21 0.16 0.015 The expected O,/N, ratio was 0.210. The HJN, and CO,/N, ratios were
