1801
Anal. Chem. 1981, 53, 1691-1695
an added advantage in that solutions after extraction being very clear need no filtration unlike with ‘TEA-molecular sieve 13X. Further experiments with this sorbent are worth pursuing. In the determination of nitric oxide, analysis of the sorbent for nitrite by ion chromatography showed more than 85% recoveries with good linearity in the concentration range of 10-50 pg of NO collected. One interesting fact observed in this study is the absence of any nitrate peak in the ion chromatograms. If NO is converted to NO2 after passing through the catalyst, the hydrolysis reaction with water should produce nitrite and nitrate according to the equation
2N02 + H2O
+
“02
t HNOB
Detection of nitrite only suggests that the catalyst oxidizes NO to dinitrogen trioxide (N203)rather than to NO2under anhydrous conditions.
N203
+ H20
-+
2HN02
Because the first section of sorbent collects any NOz and SO2 present in the sample, it should be possible to collect NO, NO2,and SO2 in a single sampling tube and analyze the three species in two ion chromatographic runs. The determination of nitrite by ion chromatography may be subject to error when working with nearly expended suppressor column. This is because of the oxidation and ion exclusion of nitrite ion in the suppressor column. Deaeration of the eluent and frequent calibration with standards would alleviate this problem (11). When collecting samples at humidities greater than 50%, a granular P206tube upstream could solve problems in collection capacity loss (2). Alternately, standards could be generated at the same humidity as samples. Also, when collecting atmosphericaerosol samples, a pH-neutral quartz fiber filter could effectively remove acid mist and particulate matter (12). Our studies show that in
using TEA-MS 13X as the solid sorbent, a spectrophotometric method for SO2also could be adapted if the collected sample is analyized within 24 h. Here the SO2is extracted from the sorbent with tetrachloromercurate(I1) and stabilized as dichlorodisulfitomercurate(I1). It is then made to react with formaldehyde and pararosaniline hydrochloride and determined spectrophotometricdy as pararosaniline methylsulfonic acid at 1560 nm (13).
ACKNOWLEDGMENT We gratefully acknowledge W. E. Koerner, W. E. Dahl, J. L. Ogilvie, and B. G. Ward for their guidance and support of this WOI k.
LITERATURE CITED Wllley, Maurine A.; McCammon, Charles S., Jr.; Doemeny, Lawrence J. Am. Ind. Hyg. Assoc. J. 1977, 38, 358-363. NIClSH Manual of Analytical Methods, 204, 1976. Black, Marilyn S.; Herbst, Richard P.; Hitchcock, Dian R. Anal. C h m . 1978, 50, 848-851. Axellrod. Herman D.; Hansen, Steven G. Anal. Chem. 1975, 47, 2480-2462. Schlnakenberg, G. H., Jr. Tech. Pmg. Rep. 1, U . S . Bur. Mines 1978, No. 95. Mullk, J. D.; Todd, G.; Estes, E.; Puckett, R.; Sawlckl, E.; Williams, D. In “Ion ChromatograpahlcAnalysis of Environmental Pollutants”; Sawlckl, E., Mullk, J. D., Wittgensteln, E., Eds., Ann Arbor Science Publish ers: Ann Arbor, MI, 1978; Chapter 3. Tonipklns, Frederick C., Jr.; Goldsmith, Robert L. Am. I d . Hyg. AsSOC. J. 1977, 38, 371. Bruiio, P.; Caselli, M.; Della Monica, M.; DlFans, A. Talsnta 1979, 28, 1011-1014. Blacker, J. H. Am. Ind. Hyg. Assoc. J . 1973, 34, 390-395. Gold, Avram Anal. Chem. 1978, 50, 1448-1450. Koch, Wllllam F. Anal. Chem. 1979, 51, 1571-1573. Pierson, Wllllam R.; Hammerle, Robert H.;Brachaczek, Wanda W. Anal. Chem. 1978, 48, 1808-1811. Scaringelli, F. P.; Saltzman, B. E.; Frey, S. A. Anal. Chem. 1987, 39, 1709-1 7 11,
RECEIVE~D for review February 23, 1981. Accepted June 2, 1981.
Single-Coluimn Ion Chromatography for the Determination of Chloride and Sulfate in Steam Condensate and Boiler Feed Water K. M. Roberts AECI Ltd., North Rand, Transvaal, Republic of South Africa
D. T. Gjerde and J. 8. Fritz” Ames Laboratory and Department of Chemi:my, Iowa State University, Ames, Iowa 5001 1
An effectlve analytlcal method Is descrllaed for determinatlon of parts-per-bllllon conlcentratlons of chloride and sulfate In very pure water such als steam condensates. Sample anlons are concentrated on a short precolumn, separated on an anIon-exchange column of very low capacity, and detected by an electrical conductivity detector placed lmmedlately after the separatlon column. No “suppressor” column Is employed. Chlorlde and sulfate caln be determlned at concentratlons as low as 1-2 ppb in the water samples. Several real water samples have been analyzed successfully by the new procedure.
Ion chromatography provides the means for the rapid determination of mixtures of ions in aqueous media. Small et
al. (1) pioneered this technique which is basically liquid chromatography using conductometric detection. After separation on a low-capacity ion-exchange column, the ions pass through a suppressor column which converts eluent ions into a low conducting species. With the eluent background effectively neutralized, the ions of interest are detected conductometrically. However, the suppressor column must be frequently regenerated in order to remove unwanted ions which have accumulated from the eluent stream. Gjerdt:, Fritz, and Schmuckler ( 2 , 3 )have described a simpler appi:oach to anion analysis which eliminates the need for a suppressor column. By use of low-capacity resins and eluents of very low conductivity, no eluent suppression is required. Excellent separations have been reported, and the sensitivity is good enough to make the routine direct analysis of anions at less than 1 ppm.
0003-2700/81/0353-1691$01.25/00 1981 American Chemical Society
1892
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
An important industrial application is the investigation of steam quality in power generation plants. Stress corrosion cracking of turbine blades is a major problem, and it is essential that a high level of steam purity is maintained. Ion chromatography provides a simple means of monitoring corrosion-forming anions such as chloride and sulfate at the low parts-per-billion concentration level. Stevens, Turkelson, and Albe (4) have reported on the determination of anions in boiler blowdown water using ion chromatography with eluent suppression. Typical levels were in the low parts-per-million range and the data were used to monitor boiler feed water treatment. Wetzel, Anderson, Schleicher, and Crook (5) have described the application of concentrator columns to ion chromatography with eluent suppression. The concentrator column can be used either on-line with the separator column or separately for field sampling. Analysis of anions in water a t concentrations as low as 2 ppb was reported, but the analyses were limited to standards made up in very pure distilled water. Fulmer, Penkrot, and Nadalin (6) published a similar procedure in which actual water samples were analyzed successfully for parts-per-billion concentrations of chloride and sulfate. Although ion chromatography with a concentrator column has provided a long-awaited answer to the problem of chloride and sulfate analysis in steam condensate, the chloride peak is not separated or is poorly separated from the “water dip”. This is a negative peak that occurs because the concentration of ions in the water sample is so much lower than the eluent. Interference from the water dip can be avoided by either of the following: (1)using only about 50% regeneration of the suppressor column or (2) using a special suppressor column that is smaller than usual. The difficulty with these methods is that the suppressor columns require more frequent regeneration. The preferred procedure seems to be one in which two small suppressor columns are used. This paper reports on the determination of chloride and sulfate in boiler feed water and steam condensate a t the low parts-per-billion concentration level using single-column ion chromatography with a concentrator column. The method is simple, reliable, and inexpensive. The problems relating to a suppressor column including regeneration are eliminated, and the apparatus can be assembled from common liquid chromatographic equipment. Several water samples have been analyzed successfully for low parts-per-billion concentrations of chloride and sulfate, both in the U.S.A. and in South Africa. EXPERIMENTAL SECTION Sampling Procedures. To avoid contamination of samples, we took great care at all stages of sample collection, handling, and analysis. Samples were taken directly from sampling taps in plastic bottles, and exposure to the atmosphere was avoided. Polystyrene and polypropylene are the recommended plastics for containers (6). Sampleswere loaded directly onto the concentrator column by means of a sample loading pump. This prevented any contamination by syringes. After the sample ions were loaded, the concentrator column was placed in the eluent stream in the same way as a sample loop. Sample ions were then eluted onto and separated on the separator column. “Load” and “inject” eluent paths in the sample injector for single anion analysis are shown in Figure 1. The concentrator column used for simultaneous chloride and sulfate determinations contained a relatively high capacity anion exchanger. Problems of irreversible ion exchange of sample ions with the concentrator column were avoided by loading in one direction and then stripping the sample ions by eluent flow in the opposite direction. This was accomplished by reversing the two samplingconnectionsthat are shown in the figure. Samples were loaded at a 2.5 mL/min flow rate at volumes up to 38 mL. Reagents. All solutions were made up in “pure” water which had been doubly distilled and then deionized. Standards were prepared by dilution of 500 ppm stock solutions of reagent grade
Ten+ separating column
8
,precolumn ouxiliory pump for loading samples Concentrator column
‘sample1 volume J
LOAD ---I NJECT -
meosurement
Flgure 1. Sampling valve wkh a concentrator column. Sample loadlng
positions are for a concentrator column containing a low-capacity Ion-exchange resin. salts. A microliter syringe was used instead of a pipet for preparation of standards at parts-per-billion level. All glassware and plasticware were thoroughly rinsed and soaked in “pure” water before use. Equipment. A schematic representation of the single-column ion chromatographwas shown in a previous publication (2). For most of the work, a Waters M45 solvent delivery system and a Valco Model CV-6 six-portinjection valve were used. The detector was a Model 213 conductivity detector made by Wescan Instruments (Santa Clara, CA). A 500 mm X 3 mm i.d. separator column, a 70 mm X 2 mm i.d. concentrator column, and a 50 mm x 2 mm i.d. catex pretreatment column were used for the single-ion analysis work. Three coupled 50 mm X 2 mm i.d. separator columns and a 30 mm X 2.5 mm i.d. concentrator column were used for simultaneous chloride and sulfate determinations. Altex 200-28 fittings were used for all column connectionsexcept where a low-dead-volumeconcentrator column was needed. In this case, the column was made from low-dead-volumestainless-steel connections. The column was made from two Altex 200-41 ‘/le in. coupling adapters and a machined Kel-F column. Altex 250-21 Kel-F column frits were turned down to make them fit inside the coupling adapters to hold the ion exchange resin in place. The eluent flow rate was 1.0 mL/min, detector output was 10 mV, and recorder input was 1-10 mV full scale. Most of the work was done with the detection at a range of 0.1 p(i2-l) full scale. The cell was insulated in a box lined with 1in. thick polystyrene. Room temperature changes of 2-3 “C did not significantly affect the base line. Resins. The single-ion analysis work was done with concentrator and separator columns containing XAD-10,0175mequiv/g 44-57 pm anion exchanger. The separator columns contained 0.013 mequiv/g XAD-1 anion exchanger and the concentrator column contained 0.5 mequiv/g XAD-4 anion exchanger for the simultaneous anion determinations. Synthesis of the anion exchangers has been studied in other work (2, 7,8). The sample pretreatment column contained Dowex 50x8 or Dowex 50x16 in the hydrogen form. RESULTS AND DISCUSSION Chromatographic Method. Some preliminary work was carried out using a resin of very low exchange capacity (0.003 mequiv/g) and a 7.5 x lo4 M solution of benzoic acid as the eluent. It has been shown that unusually good sensitivity can be obtained in single-columnanion chromatography using this eluent (9). To increase the sensitivity further, we increased the size of the sample loop to 500 HLfrom the usual 100 &. This method adequately separated chloride and sulfate with detection limits of 10 ppm chloride and 100 ppm sulfate. However, a prolonged dip in the base line occurred sometime after the sulfate peak and made this procedure inefficient for repetitive analyses. Further experimentation showed that extremely dilute water samples could be concentrated very simply and effectively using a small column containing an anion-exchange resin. This column is positioned on a sampling valve in the same way as a sample loop, and the concentrated ions are eluted from the concentrator column to the separator column by placing the concentrator column in the eluent stream.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
Previous work indicated (2,3)that chloride and sulfate can be separated on an XAD-1 anion-exchange resin using a dilute solution of potassium phthalate as the eluent. However, attempts to separate chloride, sulfate, and other anions in a single chromatogram were not immediately successful for extremely dilute sampletr because of the relatively large dead volume in the concentrator column which caused the “pseudopeak” or water diip to interfere with the chloride peak. With another eluent, potassium benzoate, chloride elutes later and is nicely separated from the pseudopeak, but no sulfate peak is observed. One ]proceduredeveloped uses the same concentrator and separation column but requires two discreh runs with different eluents. For chloride determination the columns are in the benzoate form and a potassium benzoate eluent is used. For determination of sulfate the columns are converted to the citrate form and a citrate eluent is employed. Equipment used in these analyses is inexpensive and it is feasible to have two complete systems: one for chloride and one for sulfate. Down tiime for the conversion of the column from one form to another can be avoided in this way. Alternately, two sets of columns could be used. Chloride must be separated from the water dip before it can be analyzed reliably in the same run with sulfate. Thin was accomplished by carefully constructiing a concentrator column and by careful ch,oice of the column dimensions,resin capacity, and eluent strength. The width of the watw dip was reduced by using a small concentrator and low dead-volumefittings. The small column contains only 85 mg of resin; therefore, a high capacity anion exchanger is needed to trap effectively the sample. In this work, a resin of 0.5 mequiv/g was used. The sample ions were eluted from the concentrator column with an eluent flow opposite to that of the sample flow. This back-flushingmode is needed because the concentrator column may have a column capacity that is higher than the column capacity of the separator column. In this situation, an eluent could not elute sample ions through the added capacity in a reasonable amount of time. A back-flush mode does not limit the capacity of the resin used in the concentrator column. It is more difficult to overload or use all the sites of a relatively high capacity concentrator column. The effect of column and eluent parameters on anion retention times has been shown in previous work (3, IO). If‘ eluent flow rate and anion selectivity coefficients are taken to be constant then the effects of column resin weight, w,resin capacity, c, and eluent concentration, [E], on the adjusted retention time of anions, t’, are shown by t3q 1where y is the log t’ = log w y / x log c - y / x log [E] - constant (1)
+
sample anion charge and x: is the eluent anion charge. It can be seen that the log of clolumn resin weight can be directly proportional to log adjusted retention timle regardless of the anionic charges. Increasing the weight of resin in a column will shift both chloride and sulfate to later retention times. However, lowering the resin capacity and f or increasing the eluent concentration will shift the sulfate faster relative to chloride to shorter retention times. These parameters were adjusted until chloride eluted away from the water dip and sulfate still eluted in a reasonable amount of time. The determination of chloride anid sulfate in one run greatly reduced the time needed to do the analysis. Sample concentrations for which peaks were at least three times the background signal were 3-5 ppb chloride and 1-2 ppb sulfate with this procedure. This compares with 1-2 ppb chloride and sulfate detection limits for the Eiingle-anion analytical procedures. Chloride Analysis. A. typical chromatogram is shown in Figure 2. The experimental conditions are listed with the figure caption. Samples that contain larger mounts of carbon dioxide, bicarbonate, or phosphate give a larger peak that
I
0
U
l0SS
I
5
IO
TIME, min
Flgure 2. Steam condensate sample containing 9.3 ppb chloride concentrated from 8.4 mL: eluent, 2.0 X IO4 M potasslum benzoate, pH 6.2; concentrator and separator column resin XAD-1 0.0175 mequlv/g, 44-57 Fm.
Table I. {CalibrationData for Chloride, Corrected to 15 mL Load Volume peak height, peak height, pph c1mm ppb C1mm
15.0
1.8 5.7 8.6 11.4
213.0
14.7
ID
5.0 10.0
30.0
22.5
50.0
70.0
36.0 47.3
90.0
60.0
comes before the chloride peak; however, the chloride may be shifted to a longer retention time and further separated from this early peak by using a slightly more dilute eluent. Nitrate, if present, elutes approximately 4 min after chloride and is easily determined. One sample, Iowa State University return condensate 9/23/80, was found to contain 3.2 ppb of nitrate. By use of conditions established for chloride separation, a standard addition calibration graph was obtained by analyzing samples prepared by adding chloride to “pure” water in concentrationsfrom 5 to 90 ppb. The data obtained are given in Table I, A straight line of the form y = 0.649~ 2.209 was obtained, with a correlation coefficient of 0.9994. Extrapolation of this plot gave a chloride concentration of 3.4 ppb for the pure water. Amounts of 7-30 mL of a 5 ppb standard of chloride were loaded and separated. A straight line of the form y = 1.321% + 0.867 was obtained, with correlation coefficient 0.9962 obtained from a plot of peak height vs. sample volume loaded. Since the concentration process showed a linear isotherm, it was not necessary to load an identical volume on every run. For the calibration graph, peak data were corrected to a 15-mL load volume, this being a convenient amount. The precision of chloride analysis was tested by repeated analysis of a 10 ppb chloride standard, corrected to 10-mL sample volume. The results of ten analyses were as follows: mean retention time 8.3 min with a standard deviation of 0.08; mean peak height 8.2 mm with a standard deviation of 0.43 mm; mean peak area 55.9 mm2 with a standard deviation of 5.8 mm2. The relative standard deviation using peak height was only 5.3% and was roughly half that obtained using peak area. Sulfate Analysis. Potassium phthalate or potassium citrate elutes chloride so quickly that it is apparently lost in the large negative dip. Sulfate elutes within a few minutes as a well-resolved positive peak. With 2.0 X lo4 M potassium citrate eluent, pH 6.2, the retention time for sulfate on a column containing 0.017 mequiv/g resin was 5.8 min. With 2.0 X lo4 M potassium phthalate, pH 6.7, under identical conditions the retention time for sulfate was 12.4 min and the peak was less sharp. Samples of 20 mL worked well when citrate was used, but larger samples are needed with a potassium phthalate eluent. Potassium citrate was used for all analyses of actual samples. Figure 3 shows a typical chromatogram of sulfate and an unknown in steam condensate.
+
1694
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
Table 11. Sample pH before and after Catex Column Treatment
m
!I! e 0
c
2
L-&+-++ TIME, min
Boiler feed sample containing 12.9 ppb sulfate concentrated from 24.7 mL: eluent, 2.0 X potassium citrate, pH 6.0; concentrator and separator column resin, XAD1 0.0175 mequiv/g, 44-57 I.Lm. Flgure 3.
pH before
pH after
catex
catex
boiler feed
8.2
5.8
return condensate mixed bed 100 bar steam boiler blowdown
8.0 7.4 7.6 8.9
5.1
5.3 5.4 3.9a
a Due to formation of phosphoric acid, since the sample has 8 ppm phosphate,
Table 111. Results of Analyses on Samples with Separate Chloride and Sulfate Determinations date 1
0
I! 3
I
6
,
j
9 1 2 1 5
T I M E , min
Standard containing 19 ppb chloride and 22 ppb sulfate potassium phthalate, concentrated from 20.0 m L eluent, 2.0 X pH 6.2; concentrator column resin, XAD-4 0.5 mequiv/g; separator column resin, XAD-1 0.013 mequiv/g, 44-57 pm. Flgure 4.
A standard addition calibration graph was plotted, with sulfate concentrations ranging from 5 to 90 ppb. A straight line of the form y = 0.390~+ 0.182 was obtained, with a correlation coefficient of 0.9990. The background of “pure” water was less than 2 ppb sulfate. The detection limit for sulfate was 2 ppb for a 25-mL load volume. The relationship between the volume loaded and peak height gave a straight line of the form y = 0.382~+ 0.181 with correlation coefficient 0.9985. Since a linear isotherm was obtained, calibration data were corrected to a 25-mL sample volume, this being the smallest amount possible to give the 2 ppb detection limit. A 10 ppb sulfate standard, corrected to 25 mL sample volume, was analyzed 10 times. As with chloride, the retention times for sulfate were highly reproducible and the precision using peak heights was much better than that obtained from peak areas. The mean peak height was 7.1 mm with a standard deviation of 0.31 mm and a relative standard deviation of 4.3 %. Simultaneous Chloride a n d Sulfate Analysis. A chromatogram of the determination of chloride and sulfate in a single run is shown in Figure 4. Data from 20-mL injections of 5-100 ppb chloride and sulfate in “pure” water were used to plot standard addition calibration graphs. A straight line of the form y = 0.401~ 1.56 and a correlation coefficient of 0.997 were obtained for chloride, and y = 0.516~ 3.70 and a correlation coefficient of 0.995 were obtained for sulfate. Extrapolation of these plots showed a 4.0 ppb chloride concentration and 6.5 ppb sulfate concentration for this sample of “pure” water. Detection limits under these conditions were 3-5 ppb. Plots of peak height vs. sample volume were not linear for chloride a t volumes greater than 20 mL. The reason for this is unknown, but perhaps chloride is not taken up quantitatively by the concentrator column under these conditions. The peak heights of chloride from standard 30 ppb chloride solutions were the same with either 30 or 100 ppb sulfate present, and it was shown that calibration curves are linear. The same loading volume should be used for all standard and sample solutions. Analysis of Actual Samples. A number of water samples were taken from an industrial plant in South Africa and from
+
+
8/8/80 8/8/80 8/8/80
8/8/80 8/15/80 8/15/80 8/15/80 8/15/80 8/15/80 8/15/80
sample
ppb C1-
Modderfontein Factory return condensate
