Anal. Chem. 1995,67,1204-1209
Determination of Carbonic Acid in Steam-Condensate Cycle Samples Using Nonsuppressed Ion Chromatography Sylvie Charbonneau, Roland Gilbert,* and Louis Lbpine lnstifut de recherche dHydro-Qu6bec(IREQ), 1800 montee Sainte-Julie, Varennes, Quebec, Canada J3X 1S1
The main limitations for the determination of low-level carbonic acid (HzCO3) in the steam-condensate cycles of nuclear and fossil fuel generating plants are the use of techniques with insufficient sensitivity and the risk of sample contamination by atmospheric C02. To avoid contamination, a measuring chamber and an analytical protocol, including procedures for container preconditioning, sampling, and sample conservation, have been developed and validated. This system was used to establish the performance of two nonsuppressed ion chromatographic techniques implemented on plastic-free systems for carbonic acid determination. The best results were obtained with a high-capacity anion-exchange resin, an eluent with a high basic ionic strength, and direct conductivity detection of the species. This approach shows good linearity up to 860 pg/L H2CO3, with a detection limit of 0.4 pg/L for a preconcentrated volume of 40 mL, which represents an absolute limit of 0.016 pg of H2C03. The precision (relative standard deviation) for the lower and upper ranges of the calibration plot, as established with laboratory samples, was 10% and 3% respectively, with an accuracy of +2% and +5%. The precision and accuracy of the technique were further validated using a field sample containing 28 pg/L HzC03; in this case, the precision was 6.3% and the accuracy as measured by spiking recovery was +6.8%. Combined with reliable knowledge of the system operating conditions, this new technique can be used to determine the severity of air ingress, makeup water contamination, feedwater conditioning agent contamination, and some condensate polisher operating problems. Interest in methods of quantifying carbonic acid (HzCO3) in modem high-pressure power plants has revived in recent years because this species has been identified as one of the contaminants contributing to stress corrosion cracking of low-pressure turbine disks, low-pH erosion-corrosion of steam and return lines, and corrosion in the air-cooling zone of water-tube condensers used in nuclear and fossil fuel generating plants.’ The major sources of introduction of HzC03in steam-condensate cycles are the presence of bicarbonate and carbonate ions in the makeup water and additives used for chemical conditioning, air inleakage in the vacuum regions of the cycle, and thermal decomposition (1) Bursik, A Carbon dioxide and fossil plant cycle chemistry. Proceedings of the 52nd International Water Conference, Pittsburgh, PA, October 21-23, 1991; Engineer’s Society of Western Pennsylvania: Pittsburgh, PA, 1991; pp 163-173.
1204 Analytical Chemistry, Vol. 67, No. 7, April 7, 7995
of additives and organic impurities present in the feedwater. Knowledge of the amount of carbonic acid present is important not only for corrosion control, but also for prevention of scaliig and foaming in boilers, for operation and control of various water treatment processes, and for validation of chemical controls. Carbonic acid can be determined by ion chromatography with direct conductimetric detection2q3 or indirect photometric d e t e ~ t i o n ~using - ~ an anion-exchange low-capacity resin for the separation. The diluted salt of a weak acid used as the eluent has a sufficiently low conductivity that a suppressor column is not needed. The possibility of indirectly detecting carbonic acid by conductimetry after passage through a low-capacity anionexchange resin has also been explored by different gr0ups.~-1~ In this case, the eluent used is converted to highconductivity acid in a suppressor column and produces a large background response in the conductivity cell. Bicarbonate and carbonate ions are converted by the suppressor to carbonic acid, causing a drop in the background conductivity proportional to the original concentration of the species in the sample. HzC03 can also be determined by ion-exclusion chromatography using a highcapacity sulfonated ion-exchange re sir^.'^-'^ This mode has been (2) Gjerde, D. T.; Schmuckler, G.; Fritz, J. S. ]. Chromatogr. 1980,187, 3545. (3) Dogan, S.; Haerdi, W. Chimia 1981,35, 339-342. (4) Small, H.; Miller,T. E. Anal. Chem. 1982,54, 462-469. (5) Fung, Y. S.; Dao, K L. Anal. Sci. 1991,7, 161-164. (6) Brandt, G.; Matuschek, G.; Kettrup, A Fresenius’ 2.Anal. Chem. 1985, 321, 653-654. (7) Brandt, G.; Kettrup, A Fresenius’ 2.Anal. Chem. 1985,320, 485-489. (8)Hayakawa, K; Kitamoto, S.; Okubo, N.; Nakamura, S.; Miyazaki, M. ]. Chromatogr. 1989,481, 323-330. (9) Pinschmidt, R K, Jr. Ion chromatographic analysis of weak acids using resistivity detection. In Ion chromatographic analysis of enoironmentul pollutants; Mulik, J. D., Sawicki, E., Eds.; Vol. 2, Ann Arbor Science: Ann Arbor, MI, 1979; pp 41-50. (10) Sandmann, H.; Vogeli, A; Romanelli, S. Von Wasser 1988,70, 43-49. (11) Svoboda, R; Sandmann, H.; Romanelli, S.; Bodmer, M. Volatility of anions in steam-water systems of power plants. Proceedings of the International Conferenceon Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, October 23-27, 1989 British Nuclear Energy Society: London, 1989 pp 229-234. (12) Pohlandt, C. The separation of some weak-acid anions by ion-exclusion chromatography; Report No. NIM-2107, National Institute for Metallurgy, Analytical Chemistry Division, The National Institute For Metallurgy: Randburg, South Africa, 1981, p 14. (13) Byers, W. A; Anderson, S. L.; Hickam, W. M. Organic acids in steam condensate by ionexclusion chromatography. Proceedings of the 44th International Water Conference, Pittsburgh, PA, October 24-26, 1983; Engineer’s Society of Western Pennsylvania: Pittsburgh, PA, 1983;pp 436441. (14) Gjerde, D. T.; Fritz, J. S.; Schmuckler, G.]. Chromatogr. 1979,189,509519. (15) Tanaka, K; Fritz, J. S. J. Chromatogr. 1986,361, 151-160. (16) Kreling, J. R; DeZwaan, J. Anal. Chem. 1986,58, 3028-3031. 0003-2700/95/0367-1204$9.00/0 8 1995 American Chemical Society
Tq water
Drierite/Ascarite
Molecularsieves
1/8" stainless steel transfer line to the 590 programmablepumps
or the W6OOEP pump
1/16"stainless steel transfer line to the TEP pumping system
i First-stage airpurifLingsystem (2% < 2 ppm (v/v)
Figure 1. Schematic of the measuring chamber.
simplified by employing an eluent with a low background conductance such as deionized water or a very diluted acid solution to avoid the use of a suppressor column. On the other hand, different COSevolution techniques based on partial or total removal of the C02 from the liquid phase by gas stripping or static headspace sampling have also been developed for determining small amounts of carbonic acid in water by gas chromatography20-23 or infrared s p e c t r o ~ c o p y . ~ ~ - ~ ~ However, the precision, accuracy, and sensitivity of all these techniques need to be improved in order to quantify H2C03 in specific parts of the steam-condensate cycle, i.e., polisher outlet, deaerator outlet, high-pressure heater outlet, etc., while the literature reports the dif6culty of avoiding atmospheric C02 contamination at the different steps of application of the techniques. In view of these considerations, this paper describes the development of a measuring chamber and the validation of an analytical protocol including procedures for container preconditioning, sampling, sample conservation, etc. The performance of two nonsuppressed ion chromatographic techniques implemented on plastic-free systems will be discussed. EXPERIMENTAL SECTION
Apparatus. The ion-exclusion technique was implemented on a Waters automated dual column coupled system equipped (17) Tanaka, IC; Fritz, J. S. Anal. Chem. 1987,59,708-712. (18) Hayakawa, IC; Nomura, IC; Miyazaki, M. Anal. Sci. 1992,8,111-113. (19) Zenki, M.; Nabekura, T.; Kobayashi, A; Iwachido, T.; Shimoishi, Y. Analyst 1993, 118,273-276. (20) Birmingham, B. C.; Colman, B. Plant Physiol. 1979, 64,892-895. (21) Rudenko, B. A. J. Anal. Chem. 1982,37,790-795. (22) Pradeau, D.; Postaire, M.; Postaire, E.; Prognon, P.; Hamon, M. J. Chromatogr. 1988, 447,234-238. (23) Rhiile,W.; Hoffmann, W.; Civin, V. VGB Krafiwerktech. 1990, 70, 708711. (24) Joyce, R J.; Takahashi, Y.; Wirth, L. F. Application of TOC analyzer to the ppblevel measurement of COP trapped in ammoniated steam condensate. Proceedings of the 44th International Water Conference, Pittsburgh, PA, October 24-26, 1983; Engineer's Society of Western Pennsylvania: Pittsburgh, PA, 1983; pp 430-435. (25) Schumacher, T. E.; Smucker, A. J. M. Plant Physiol. 1983, 72, 212-214. (26) Roberts, D. G.; Smith, D. M. Limnol. Oceanogr. 1988,33, 135-140.
with two Model 590 programmable pumps, two switching valves (analyte transfer valve and eluent transfer valve), and a conductivity detector, Waters Model 431. Sample preconcentration is achieved by passing a large volume of sample through a Waters IC-Pak A concentrator cartridge using a TEP pumping system (Waters). A 6 mM solution of a sodium salt of octane sulfonic acid (eluent 1) is used to remove the anions from the concentrator cartridge, while the eluent transfer valve allows deionized water (eluent 2) to continue the chromatography on a IC-Pak ion exclusion column (7.8 x 300 mm); both eluents were used at a flow rate of 1 mL/min. The anion-exchange technique was implemented on a Waters action analyzer (Waters, Division of Millipore, Milford, MA) equipped with a W6OOEP multisolvent pumping unit, one switching valve (analyte transfer valve), and a conductivity detector, Waters Model 431. The anions were removed from the concentrator cartridge and separated onto a Waters IC-Pak A HC column (4.6 x 150 mm) using a solution of 3 mM NaOH at a flow rate of 2 mL/min. The conductivity signals from both chromatographic systems were recorded using the MAXIMA-820 chromatographic workstation (Waters). The conductivity measurements performed in order to validate the measuring chamber were done on a CDM83 radiometer (Radiometer A/S, Copenhagen, Denmark) equipped with a CDC104 immersion conductivity cell. Measuring Chamber. A Model DG001-SP glovebox equipped with an automatic pressure control, Model HE-63P from Vacuum/ Atmosphere Co. (Hawthorne, CA), was used to develop the measuring chamber. This setup, schematized in Figure 1, was equipped with a two-stage air-pullfying system. A Balston unit Model 75-60 (Lexington, MA), which generates air with a residual C02 concentration below 2 ppm (v/v), is used as the first stage. The effluent of this system passes through a Model L68GP gas purifier (Hammond Drierite Co., Xenia, OH) filled with Ascarite before it penetrates into the glovebox. The second stage consists of a vacuum/pressure pump (Millipore, Bedford, MA), which allows the air of the glovebox to be recirculated through two gas purifiers connected with stainless steel tubing, one filled with Analytical Chemistry, Vol. 67, No. 7, April 1, 7995
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molecular sieves, and the other containing Ascarite at the top and Drierite at the bottom. The glovebox is supplied with deionized @I) water obtained from a two-stage tap water polishing system: a primary Millipore system, Model Milli-RO 60 PLUS, which comprises one Milli-Pak filter, one activated-carbon cartridge, and two RO cartridges, and a secondary Millipore unit, Model Milli-Q Plus water system, equipped with one Super-C carbon cartridge and two Ion-Ex cartridges. Reagents. The sodium carbonate salt used to prepare the standard solutions was certified ACS grade from Fisher Scientific (Montreal, Canada). The eluents were prepared using 50%w/w sodium hydroxide of low carbonate content from Fisher Scientific and sodium salt of octane sulfonic acid of HPLC grade from Eastman Kodak Co. (Rochester, NY). Argon used for pressurization of the eluent and for preparation of containers was Ultrapure carrier grade (99.999%) from Air Products (Montreal, Canada). Ascarite 11, Drierite, and molecular sieves were obtained from Fisher Scientific, as were the morpholine and ammonia of certified ACS grade used for the preparation of some standards. LaboratoryProcedure. Eluents and standard solutions were prepared in the measuring chamber using DI water obtained from the two-stage tap water polishing system. In this case, the Milli-Q secondary system operating in standby mode during the night was turned on to continuous recirculation and flushed for 20 min every morning before water was collected. The eluent was degassed by filtering the solution through a 0.45pm Millipore filter under vacuum directly into a 2-L HPLC solvent delivery system (Kontes, Vineland, NJ) and kept under argon. This reservoir located in the measuring chamber was connected to the chromatograph via a stainless steel transfer line. A standard stock solution of 1000 mg/L C032- was prepared in a volumetric flask by dissolving a weighed amount of salt in DI water. This solution, kept in a 02/CO~resistantbottle (Nalgene PETG bottle, Canlab Division, Catalog No. B7539-125P), was stored in the measuring chamber for a maximum of 2 weeks. A 5WmL polyethylene bottle (Canlab Division, Catalog No. B7532- 16) filled with DI water was successively spiked with an aliquot of the standard stock solution or substandards of 10 and 100 mg/L C032- to obtain a series of calibration points. These samples were transferred to the chromatographic system via a stainless steel transfer line (Figure 1). Field samples were collected in 125 or 250-mL gas-sampling tubes equipped with two stopcocks and a septum screw cap (Canlab Division, Catalog No. 6531500125 or 6531500250). The special procedure developed for the preparation of these containers will be described in the next section. Field Sampling Procedure. The analytical protocol was verified by collecting samples from difEerent parts of the Gentilly 2 (685 Mw Canadian deuterium uranium-pressurizedheavy water reactor) steam-condensate cycle: composite steam-generator blowdown (CSGB) , main steam line, moisture-separator reheater drains (MSRD), condensate-extraction pump discharge (CEPD) , low-pressure heater outlet (LPHO) ,deaerator outlet (DO), highpressure heater outlet (HPHO), and makeup water. All samples were taken as quickly as possible to minimize the effect of transient plant conditions. They were routed directly from the sample system panel lines via Tygon tubing to the collecting device schematized in Figure 2. The tubes were filled vertically from the bottom until they overflowed and were flushed for at least 5 volumes before the stopcocks were closed. Special care was taken to avoid headspace and air bubbles. 1206 Analytical Chemistry, Vol. 67, No. 7, April 1, 1995
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Figure 3. Variation in the conductivity of DI water vs time in ambient air (0)and in the measuring chamber with the first-stage air-purifying system (0)and both stages (0) in operation.
RESULTS AND DISCUSSION
Validation of the Measuring Chamber. This system was developed in an attempt to limit atmospheric C02 contamination during the preparation of blanks, standard solutions, and eluents, sample handling, and transfer to the analytical instrument. The quality of the glovebox air was established prior to use by a series of conductivitymeasurements performed on DI water. The latter have a very small buflering capacity so that absorption of carbon dioxide results in a fast increase in the conductivity and, at equilibrium, the carbonic acid concentration should be proportional to the amount of COS in the air in contact with the solution. Time-related variations in the conductivity were obtained from samples (1) directly in contact with ambient air, (2) in the measuring chamber with the first stage of the air-purifyingsystem in operation, and (3) in the measuring chamber with both airpurifying stages in operation. The results in Figure 3 show that the values measured in ambient air increase rapidly in the first few minutes of exposure. The lower value measured at the beginning of the test was about 0.09 pS/cm at 25 "C. After 120 min of exposure, the sample had a conductivity of 0.784 pS/cm. Injected into the chromatograph, this sample shows a co32-peak area corresponding to 150 pg/L H2CO3. The results obtained in the measuring chamber were quite difEerent: the lowest conductivity value for DI water measured at the beginning of the test
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20
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Figure 5. Typical nonsuppressed ion-exchange chromatograms: (a) 28 mL of DI water and (b) 20 mL of 3 yg/L H2C03.
04 0.2
00
1
Figure 4. Typical nonsuppressed ion-exclusion chromatograms: (a) 25 mL of DI water; (b) 41 mL of 20 yg/L H2C03; and (c) 20 mL of 100 pglL formic, acetic, propionic, and carbonic acids.
was also about 0.09 ,pS/cm, but the increase observed even after 120 min of exposure was very small, and even smaller when both air-purifying stages were in operation. The respective conductivities were 0.164 and 0.111 pS/cm for a pH value near neutrality with chromatographic peak areas corresponding to 44 and 16 pg/L HzCO3, respectively. Considering the data obtained with both airpurifying stages in operation, an average contamination rate of 0.1 pg/L HzCO3/min was observed over a period of 120 min for a 200-mL sample kept in a beaker with an open surface of 38 cm2. As may be seen, the use of this system does not eliminate all precautions for the handling of blanks, standards, and samples, but contamination can be reduced to a negligible level by limiting the sample contact area and exposure time; these two practices were applied for the rest of the study. Analytical Performance. The ion-exclusion mode was performed on a high-capacity, totally sulfonated cation-exchange resin of controlled cross-link. Figure 4 shows typical chromatograms obtained under the chromatographic conditions given in the Experimental Section. The first chromatogram (Figure 4a) was obtained by passing 25 mL of DI water through the concentrator cartridge. The second and third chromatograms (Figure 4b,c) show the signals obtained by passing 41 mL of a solution of 20 pg/L carbonic acid and 20 mL of a 100 pg/L standard of formic, acetic, propionic, and carbonic acids. The formate and acetate peaks are not resolved under these conditions, but bicarbonate can be detected without interference from these two ions or from
propionate. The calibration plot established in the range of 5-1000 pg/L is linear, with a slope of 0.25 x lo-' V*s/pgL-l, an intercept of -0.5 x lo-' V-s, and a correlation coefficient of 0.999 35 (n = 7 ) . The detection limit (DL) calculated using the relation DL = 3Sb/m, where s b is the standard deviation of blank noise measured statistically in the vicinity of the peak and m is the slope of the regression line, is 0.9 pg/L for a preconcentrated sample volume of 40 mL, which corresponds to an absolute detection limit of 0.035 pg of HzC03. The precision (relative standard deviation) measured at the lower and upper ends of the concentration range was 7%and 2%,with an accuracy of -3% and +2%, respectively; these values were established by performing five consecutive measurements on solutions of 30 and 300 pg/L HzCO3. The anion-exchange mode investigated was performed on a high-capacity anion-exchange resin. Figure 5 shows typical chromatograms obtained under the chromatographic conditions given in the Experimental Section. The first chromatogram (Figure 5a) shows the signal recorded after 28 mL of DI water was passed through the concentrator cartridge. Very low contamination is observed, with a C032- peak area corresponding to 0.5 pg/L HzCO3, which mostly represents the residual concentration found at the effluent of the two-stage tap water polishing system. The second chromatogram (Figure 5b) shows the signal obtained by concentrating 20 mL of a standard solution of 3 pg/L HzCO3. It is important in this mode to optimize the ionic strength of the eluent in order to minimize the interference from the sulfate ions. The best resolution between the C032-/S042-peaks (R = 1) for a sample containing 50 pg/L carbonic acid and 100 pg/L sulfate was obtained with an eluent of 3 mM NaOH. Under these conditions, there is also no possible interference with phosphate ions, which are detected about 10 min after the sulfate ions (peak not shown in Figure 5). The calibration plot was established in the range of 2-860 pg/L. A good linearity is observed with a slope of 0.26 VdpgL-l, an intercept of 2 Vas, and a correlation coefficient of 0.999 23 (n = 10). An example of the fit obtained Analytical Chemistry, Vol. 67, No. 7, April 1, 1995
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1
12
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10
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Table 1. Gentilly 2 Steam-Condensate Cycle Conditions
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steam generator analytical data pH at 25 "C
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