Anal. Chem. 1983, 55, 1173-1 175
noise should ultimately limit the detection of optical absorption using the OG effect (18). Further improvement in the design of an OG resonance detector for molecules lis possible. The demountable hollow cathode used hers was originally intended for atomic resonance detection. Both1 the anode and cathode can be enlarged to produce molecular OG spectra with higher signal-to-background ratios. Optical quality windows should be added to permit transverse excitation. The position of the laser beam and electrode separation can also be optimized. The addition of a fill gas such as He has already been shown to greatly increase the OG signal (13). Refinement of the simple OG detection circuitry should also contribute to a wider dynamic range for the detector and permit lower limits of detection. Optogalvanic spectra have been reported for a limited number of molecules (19). Therefore it is difficult to predict the generality of‘this detection technique. Optogalvanic detection of molecular absorption should be readily adaptable to simple molecules such as NO2 and Cob, known pollutants. Optogalvanic spectra using visible dye laser excitation have been observed for both of these molecules (12,19).Absorption measurements a t two or more wavelengths will be required for unequivocal identification. It may ble possible to detect larger molecules by employing an electrical discharge as the absorption cell. In this way, the absorlbing species will be equivalent to the species in the detector, even if fragmentation occurs. Presumably, any decomposition in the absorption and detector cells will result in production of identical fragments. Broad-band dye laser excitation may be used for OG detection of narrow absorption transitions. If several absorption transitions are overlapped by the dye laser bandwidth, a narrowband dye laser with wavelength scanning may be necessary to introduce selectively into the measurement. Pattern recognition techniques could be used to match the analyte’s absorption spectrum with the OG “template” spectrum. With large molecules, computer data acquisition and management will be essential to extract useful information from the data comparisons. Since OG detection of molecular absorption may be used with a flowing system, gas chromatographic separation of complex samples prior to detection will be possible. In any case, a gas chromatograph provides a simple, quantitative method of sample introduction.
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ACKNOWLEDGMENT The authors acknowledge helpful discussions with R. A. Keller and C. T. Rettner. Registry No,, Iodine, 7553-56-2.
LITERATURE CITED (1) Green, R. B.; Keller, R. A.; Luther, G. G.; Schenck, P. K.; Travis, J. C. A d . Phvs. Lett. 1076. 29. 727-729. (2) Foote, R:A.; Mohler, F.’L. Phys. Rev. 1925, 2 6 , 195-207. (3) Green, R. B.: Keller, R. A.; Luther, 0. 0.; Schenck, P. K.; Trevls, J. C. I€€€ J. Quantum Electron. 1978, QE-17, 63-64. (4) King, D. S.;Schenck, P. K.; Smyth, K. C.; Travis, J. C Appl. Opt. 1977, 16, 2617-2619. (5) White, J. R.; Freeman, R. R.; Llao, P. F. Opt. Lett. 1980, 5 , 120-122. (6) Smyth, K. C.; Schenck, P. K. Chem. Phys. Lett. 1978, 55, 466-472. (7) Zalewskl, E. F.; Keller, R. A.; Engleman, R., Jr. J. Chem. Phys. 1979, 70, 1015-1026. (6) Ausschnitt, C. P.; BJorklund, G. C.; Freeman, R. R. Appl. Phys. Lett. 1978, 33, 651-653. (9) Lawler, J. E : Ferguson, A. I.; Goldsmith, J. E. M.; Jackson, D. J.; Schawlow, A. L. Phys. Rev. Lett. 1979, 42, 1046-1049. IO) Travis, J. C.; Turk, G. C.; Green, R. B. Anal. Chem. 1982, 5 4 , 1006A-1018A. 11) Schenck,-P:K.; Mallard, W. G.; Travis, J. C.; Smyth, K. C. J. Chem. Phvs. 1978. 69. 5147-5149. 12) Fddman, D.’ Opt. Commun. 1070, 2 9 , 67-72. 13) Demuvnck. C. A.: Destombes. J. L. I€€€ J. Quantum Electron. 1981, ’ Q E - i 7 , 575-577. (14) Rettner, C. T.; Webster, C. R.; Zare, R. N. J. Phys. Chem. 1081, 85, 1. 105. - - 1. 107. .- .. (15) Nippoldt, M. A.; Green, R. B. Appl. Opt. 1981, 2 0 , 3206-3210. (16) Zalewski, E. F.; Keller. R. A.; Apel. C. T. Appl. Opt. 1981, 2 0 , 1584-1507. (17) Sullivan, J. V.; Walsh, A. Appl. Opt. 1988, 7 , 1271-1280. (16) Keller, R. A.; Zaiewskl, E. F. Appl. Opt. 1080, 19, 3301-3304. (19) Webster, C. R.; Rettner, C. T. Laser Focus 1083, 19, 41-52. ’Current address: Rlker Laboratories, 19901 Nordhoff St., Northridge, CA 91324.
Mark A. Nippoldt’ Robert B. Green* Department of Chemistry University of Arkansas Fayetteville, Arkansas 72701 RECEIVED for review October 4,1982. Accepted February 16, 1983. The authors acknowledge the support of the National Science Foundation EPSCOR program, NSF CHE-8105000, and BRSG SO7 RR07101-03, awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health.
AIDS FOR ANALYTICAL CHEMISTS Automated Analysls for Water Alkallnlty Raymond B. Wlllis” and Gregory L. Mulllnsl USDA -Forest
Sewice, Northeastern Forest Experiment Station, Rf. 2, Highway 2 1, East, Berea, Kentucky 40403
As a result of governmental regulationsi and environmental interest, determination of alkalinity in natural water has become a routine analysis in many laboratories. Alkalinity usually consists of carbonate and bicarbonate and is most commonly measured by titration with acid (1-4). Ion selective electrode (5), ion exchange (6),and gel chromatography (7) methods are also available. Automated methods of measuring alkalinity are typically based on the change of pH of a buffer containing methyl orange (8). The methyl orange is buffered ‘Present address: Department of Agronomy, Purdue University, West Lafayette, IN 47907.
at a pH in the acid end of the transition range of methyl orange. Additions of alkalinity cause the color of the methyl orange to gradually change from orange to yellow as the pH increases within the transition range. Two instruments that are used to automate the measurement of alkalinity are the Technicon AutoAnalyzer (TAA) made by Technicon Industrial Systems, Tarrytown, NY, and the Coulter Kem-O-Lab, Industrial Model (IKL), made by Coulter Electronics Inc., Hialeah, FL. Coulter Electronics has published methods for the IKL that are useful for concentrations of alkalinity from 10 to 500 mg/L as CaCOS,whereas published methods for the TAA are useful only in the range
This artlcle not subject to U S . Copyright. Published 1963 by the Amerlcan Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Table I. Reagents and Instruments Used for Different Alkalinity Determinations
a
reagenta
instrument
mT. of buffer
A B C D E
TAA TAA TAA IKL IKL
60 125 43.5 392
10
mL of
methyl orange 85 85 50 32.6 32.6
range, mg/L
mL of Brij-35
linear
usable
2
10-90
2
50-250
2 0 0
200-650 10-90 160-900
4-140 10-600 10-900 4-150 50-1000
Each reagent also contains enough water to make a total volume of 500 mL.
of 100-500 mg/L. Since this laboratory analyzes many samples from streams with concentrations less than 50 mg/L, tests were performed to find conditions suitable for analysis of alkalinity at both low and high concentrations. Results using the IKL and the TAA were compared with those obtained by titration. EXPERIMENTAL SECTION Reagents. Methyl Orange Stock Solution. Methyl orange was 0.05% in water. p H 3.1 Buffer. The buffer consisted of 5.1 g of potassium acid phthalate and 87.5 mL of 0.1 N hydrochloric acid. Dilute to 500 mL with distilled water and adjust the pH to 3.1 with 1 N hydrochloric acid. Methyl Orange Reagents. Varying amounts of methyl orange stock solution and buffer (Table I) are mixed and diluted with distilled water. Reagent A must be prepared fresh daily. If the reagent was to be used with the TAA, 2 mL of Brij-35 (obtained from Technicon Instrument) was also added. Alkalinity Standards. Alkalinity standards of approximately 10000 mg/L as CaC03 were prepared by dissolving 1.060 g of sodium carbonate in 1000 mL of distilled water. The exact concentration was determined by titrating with 0.02 N HCl. Instrumentation Used. TAA. The Technicon AutoAnalyzer 11,Industrial Model, was used with the same manifold as described by Technicon for the measurement of alkalinity. A 15 X 2.0 mm flow cell and a 550-nm filter were used. The electronic gain on the colorimeters can be varied from 0 to 100. For the purpose of comparison the gain was set at 10.0. IKL. The Coulter Kem-0-Lab was used following manufacturer’s recommended procedure for measuring alkalinity. The IKL automatically takes 1 mL of sample and mixes it with 0.5 mL of distilled water and 1.0 mL of methyl orange reagent. The absorbance is measured with a 2-cm flow cell and a wavelength of 540 nm 12 min after the reagent is added. Titration. The titration was performed with a Brinkman pH meter, Model 506, to monitor the pH and a Dosimat E535 (Brinkman) to deliver the titrant. A 25-mL sample was titrated to a preliminary end point of pH 5.6 with 0.02 N H2S04. With this volume of titrant as a measure of the approximate alkalinity, the sample was further titrated to a final end point based on the relationship of equivalence point pH to total alkalinity as described by Thomas and Lynch (6). RESULTS AND DISCUSSION Selection of Reagent. The lower the concentration of buffer in the methyl orange reagent, the more sensitive the method should be; but at the same time the methyl orange composition should be such that the final absorbance of the solution as it passes through the flow cell is about 0.7. After many different reagents with varying buffer concentrations and methyl orange compositions were studied, it was concluded that three reagents would be sufficient to measure alkalinity at concentrations ranging from 5 to 700 mg/L CaC03 on the TAA. Solutions with concentration from 5 to 550 mg/L CaC03 will be within the linear range of a t least one reagent. The reagents used are listed in Table I along with the range of linear and usable concentrations. The reagents that were used with the IKL are the same as those described in the IKL manual and are included in Table I for completeness. The calibration curves have a linear portion in the middle and a curved portion on each end. The usable
100
200
1 300
I 400
I 600
500
700
800
Concentration, m g l l
Figure 1. Calibration curve for different reagents used on TAA (the composition of each reagent is described in Table I).
100
200
300
400
500
600
700
Concentration, m g l l
Figure 2. Calibration curve for different reagents used on IKL (the composition of each reagent is described in Table I).
range includes the linear portion as well as some of the curved portion at each end. Plots of the calibration curves are shown in Figures 1 and 2. Comparison of Results. Several water samples obtained from small streams in Breathitt and Jackson Counties, Kentucky, were analyzed by automated methods on the IKL and the TAA and by titration. The results are listed in Table 11. The titration method has greater reproducibility than either of the two automated methods, but it is much slower. About 8 h are required for a set of 80 samples, whereas both the IKL and the TAA take less than 2 h for a set of 80. Also, once the standardization is completed, the automated instruments require almost no attention. The IKL and the TAA are equally reproducible, i.e., the coefficient of variation was 2% for four or more samples analyzed on separate days. The largest errors with the TAA occur when a sample containing
Anal. Chem. 1983, 55, 1175-1176
Table 11. Comparison of Results of Alkalinity Tests by Titration, IKL, and TAAa sample no. 1103 1111
1112 1113
1114 1116 1118
1119 1120 av
titration, IKL, mg/L TAA, mg/L mg/L __-mean std dev mean std dev mean std dev 26.1 44.6 14.9 13.4 20.3 56.6 147 104 266
0.9 0.3 0.7 0.7 0.7 0.3 0.3 0.3 1.6
26.5 44.1 15.2 13.8 19.6 57.3 144 102 259
0.6
0.5 0.5 0.9 1.3
0.6 0.3 2.5 3.6 2.4 1.4
25.4 44.3 13.9 13.4 18.7 57.8 141 104 264
0.2 0.1
1.2 2.3 1.5 0.9 1.4 2.9 1.2 1.3
a Based on five measurements for each determination of mean or standand deviation.
Table 111. Appa.rent Result of Successive Samples with a Concentration of 8 0 mg/L Which Immediately Followed Samples with a Concentration of 550 mg/LQ sample no.
apparent result as a % of equilibrium value condition 1 condition 2
1
119
167
2 3 4 5
106 102 100
118
105 103 100
See text for case description. low alkalinity follows one of high alkalinity (Table 111, case 1). Successive samples with a concentration of 550 mg/L were analyzed with the TAA until equal results were obtained. They were immediately followed by samples with a concentration of 80 mg/L. When preceded by a sample containing 550 mg/L, an 80 mg/L sample was overestimated by approximately 19%. The error on the TAA results from intersample mixing between any sample and the adjacent sample in the segmented flow trystem. The problem of intersample mixing on the TAA is worse where the tubing is dirty. The experiment described was conducted with a short length of slightly dirty tubing following the sample probe. In this case, the first 80 mg/L sample that followed the 550 mg/L sample was 67% high. Succeeding samples were also higher (Table 111, case 2). Another problem with the TAA is a slight drift of the bass line. A typical drift for bicarbonate over a span of 50 samples is 2 mg/L. This amount of drift is enough to account for all the deviation shown in Table 11.
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When the IKL was used to measure samples with an 80 mg/L concentration that followed samples with a 550 mg/L concentration, the alkalinity measured in the first 80 mg/L sample was slightly higher than the actual concentration. The largest error observed was 10%. Intersample mixing cannot occur on the IKL since samples are kept in separate cups. Any error in the IKL system results from carryover from one sample to the following on the sample probe, stirring paddle, or colorimeter tubing. The stirring paddle and sample probe are made of a nonwetting material to minimize any carryover.
CONCLUSIONS The primary objective of this study was to develop reagents that can be used to measure low concentrations of alkalinity in water samples using the TAA. The reagents in Table I allow accurate determinations of alkalinity in the 10 to 700 mg/L range. The IKI, and TAA were about equally accurate provided one watched for the possibility of intersample mixing on the TAA. If the approximate alkalinity of the samples is known, one can immediately choose the appropriate reagent for the sample. If the approximate alkalinity is not known, it is recommended that one start with the reagent useful for the highest concentrations (reagent C for the TAA, reagent E for the IKL). Accurate results will be obtained for samples within the usable range of this reagent (Table I). Approximate results will be obtained for samples with a concentration below this range. Such samples should be separated from the remainder and rerun by using the reagent with the usable range appropriate for these samples. This procedure also eliminates the problem of intersample carryover (Table 111). Registry No. Water, 7732-18-5.
LITERATURE CITED (1) McKay, D. K.; et al. Clin. Chim. Acta 1965, 12, 75-79. (2) Larson, T. E.; Henley, L. Anal. Chem. 1955, 2 7 , 851. (3) Thomas, J. F. J.; Lynch, J. J. J . Am. Water Works Assoc. 1960, 5 2 , 259-268. (4) American Public Health Assoclation; American Water Works Associatlon; Water Pollution Control Federation "Standard Methods for the Examlnation of Water and Wastewater", 15th ed.; Amerlcan Publlc Health Association: Washington, DC, 1980; p 253. (5) Rechnitz, G. A. Report 238490/7 GA; U.S. National Technical Information Service, 1974, Chem. Abstr. 1975, 83, 152043. (6) Small, H.; et al. Anal. Chem. 1976, 4 7 , 1801. (7) Deguchi, T.; et 81. J. Chromatogr. 1977, 133, 173. (6) United States Environmental Protection Agency, EPA-600/4-79-020, 1979; p 310.2.
RECEIVED for review October 13,1982. Accepted January 24, 1983. The use of trade, firm, or corporation names in this paper is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture or the Forest Service of any product or service to the exclusion of others that may be suitable.
Glass-Sample-Tube Breaker Wllllam E. Caldwell and Jerome D. Odom" Department of Chemistty, 1Jniversity of South
