Environ. Sci. Technol. 1985, 79, 450-454
Chemiluminescent Analysis of Chlorine Dioxide with a Membrane Flow Cell Deanna J. Saksa and Ronald 8. Smart”
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045 The measurement of chlorine dioxide (CIOz)has been investigated by using a chemiluminescent method combined with a membrane flow cell. A homogeneous membrane separated the luminol (5-amino-2,3-dihydro-1,4phthalazinedione) reagent from the CIOz. The effect of reagent and sample pH, flow rate, and temperature was studied. Adjusting the sample and reagent pH to 10 was effective in eliminating interferences from hypochlorous acid (HOC1) and hypochlorite ion (OC1-). The response was linear from 0.050 to 1.50 ppm of C102with a relative standard deviation of 2.3% at 0.20 ppm of C102. This method was used to measure CIOz in tap water, river water, and sewage effluent and compared with several other methods for CIOz. Introduction
The presence of chlorinated organics in drinking water supplies has become a major concern to water treatment facilities. Interest in these chlorinated organics stems largely from the results of two independent studies done by Rook (1)and Bellar et al. (2) in 1974. Because some of these compounds have been found to be carcinogenic (3-6),the U S . Environmental Protection Agency has imposed regulations on the maximum contaminant level (MCL) for trihalomethanes (THM) in drinking water (7). The current regulations set a limit of 100 pg/L (ppb) on total TMH concentration for every public water system serving over 10000 persons. Imposition of this MCL has led to numerous investigations for procedures which will reduce THM concentrations in finished water. The use of an alternative disinfectant to chlorine has been suggested as one method of THM reduction (8). Chlorine dioxide (CIOz)is a likely possibility because (1) it is a more potent disinfectant than chlorine, (2) it will destroy phenols and chlorophenols, and (3) it will remain effective over a larger pH range than chlorine (9). In general, a disinfectant will be most useful if its residual can be easily measured. A current problem with C102 is the lack of a sensitive, selective, and easily automated analytical method for the determination and continuous monitoring of concentrations at the sub-partper-million level. Many of the methods for the determination of CIOz are subject to interference from C12,HOC1, and OC1-. We have developed a chemiluminescent (CL) method of analysis using a membrane flow cell which is free from these interferences. Isacsson and Wettermark (10, 1 1 ) first reported a CL method for C102using luminol(5-amino-2,3-dihydro-1,4phthalazinedione) as the luminescing agent. C102oxidized the luminol, causing light emission from an excited-state product, the aminophthalate ion. This technique, however, suffered from interference due to OC1- which could also oxidize the luminol. This method was later modified (12) by performing the CL analysis within a membrane flow cell where the CL reagent was separated from the analyte. We used a membrane flow cell similar to that described by Nau and Nieman (13) but utilized a homogeneous membrane. The selectivity expected was in part due to this membrane, because the CIOz must first dissolve in the membrane and then diffuse through the membrane into the reagent portion of the cell. The permeability of ClOz is a function of both the solubility of CIOz in the membrane 450
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and the rate of diffusion. Dissolved electrolytes, bacteria, and colloidal particles as well as interfering ions such as OCl- are isolated from the reagent by the membrane. This membrane flow cell technique is a continuous method of analysis where analyte samples enter the cell, CL intensity is allowed to achieve a steady state, and the cell is then rinsed with distilled water. Optimum experimental conditions for the membrane flow cell CL method were determined, and its application to the measurement of C102 in tap water, river water, and sewage effluent was investigated. Experimental Section
Apparatus. Light emisison was detected without wavelength discrimination with an 1P 28 photomultiplier tube (PMT) powered by an Aminco 10-267 microphotometer (SLM; Aminco, Urbana, IL). The PMT and flow cell were housed in a light-proof box of our own construction which has been described elsewhere (14). CL intensities were recorded with a Fisher 5000 Recordall strip chart recorder. Sage Model 249-3 or 341A syringe (Sage Instruments, White Plains, NY) and Gilson Minipuls 2 peristaltic pumps (Gilson Medical Electronics, Middleton, WI) were used to pump solutions through the flow cell. A Fisher refrigerated water bath, Model 90, was used to maintain constant sample temperature for a study of temperature effects. Samples flowed from the water bath to the flow cell in less than 1 min, and change in temperature was assumed to be minimal. Membrane Flow Cell. The flow cell has been described earlier (12). It was machined from plexiglass and consisted of two plates with a glass window inlaid into the top plate and secured with plexiglass strips glued along each edge of the glass. The membrane was placed between the two plexiglass plates which were then screwed onto a platform within the light-proof box in order to keep the cell stationary and positioned as close as possible to the PMT. The homogeneous membrane was a 0.054 mm thick silicone-polycarbonate copolymer, MEM-213 (General Electric Co., Schenectady, NY). Reagents and Solutions. All solutions were prepared from chlorine demand-free water which had been passed through a Fisher Ultrapure deionizing column and double glass distilled. Chlorine dioxide stock solutions were prepared by acidification of NaC1OZ with subsequent scrubbing of CIOz through a saturated NaC102solution to remove any C12formed in this process (15). Stock solutions were stored in the dark to prevent photodecomposition of C102(16). Temporary stock solutions were prepared daily by dilution of the stock solution and stored in a “shrinking bottle”, described elsewhere (14,17),which prevented loss of CIOz to the air above a stock solution. Hypochlorous acid solutions were prepared by dilution of Chlorox bleach (5% NaOC1). Both C102and HOC1 stock solutions were standardized by iodometric titration (15). Luminol (MCB reagent grade) was dissolved in 0.01 M NaOH. H202 was prepared by dilution of a 30% Fisher reagent. Buffer solutions from pH 6 to 13 were prepared according to standard methods (18). For comparison of the CL method with standard methods for CIOz analysis, river water and secondary sewage effluent samples were collected in polyethylene
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Table I. Determination of CIOz
sample distilled water' tap water" river waterb sewage effluentb
CL method 0.505 f 0.017 0.250 f 0.004 0.496 f 0.024 0.256 f 0.018 0.476 f 0.010 0.162 f 0.009 0.544 f 0.020 0.155 f 0.020
CIOz concentration, mg L-' iodometric titration DPD 0.515 f 0.016 0.208 f 0.050 0.813 f 0.011 0.274 f 0.006 1.26 f 0.03 1.11f 0.07 3.41 f 0.84 2.82 f 0.33
CPR 0.550 f 0.062 0.230 dz 0.040 0.533 f 0.033 0.394 f 0.032 0.614 f 0.005 0.550 f 0.016 0.533 f 0.033 0.123 f 0.005
0.505 f 0.031 0.242 f 0.035 0.507 f 0.10 0.305 f 0.008 1.46 f 0.01 1.19 f 0.03 1.61 f 0.04 1.25 f 0.01
[CIOz]added: 0.500 mg L-l, 0.250 mg L-I. bCIOzadded until a residual was detected.
bottles, tap water was used directly, and distilled water was chlorine demand free. For distilled water and tap water, C102was added as indicated in Table I. Since river water and sewage effluent contain compounds that will react with C102, it was added to these samples until a residual concentration was detectable with the CL method. Procedure for CL Analysis. The reagent solution was composed of 6.00 X loa M luminol and 2.00 X M H202 as these were the concentrations previously used with the membrane flow cell (12). Analyte solutions were prepared by dilution of stock C102and HOC1 solutions. Prior to a sample analysis,the analyte reservoir was flushed with 100 mL of 10 ppm of CIOz to ensure thorough cleansing of the cell and oxidation of any material in the flow lines which could react with C102. The analyte reservoir was then flushed with distilled water, and the reagent reservoir was filled. CL intensities were measured as the change in millivolt output to the Fisher 5000 strip chart recorder. The PMT high voltage was initially set at 800 V but could be lowered if C102samples above 0.6 ppm were to be analyzed. Recalibration was required at each high-voltage setting.
40r '."\ 0.1 ppm CIO,
A
1.0
ppm HOCI
Results and Discussion CL intensities were monitored as a function of time. Background intensity was measured at zero relative intensity units with the luminol reagent solution in the reagent reservoir and distilled water flowing through the analyte reservoir. CL response to C102was linear from 0.05 to 1.5 ppm (R = 0.9992) at optimum experimental conditions of reagent pH 10, sample pH 10, reagent flow rate 0.078 mL/min, and analyte flow rate 9.0 mL/min. Concentration of interest for water treatment applications will usually be less than 1.0 ppm, since C102doses of 1.0 ppm or less are recommended (19) due to some of the unresolved health effects of C102- (20). The detection limit of the method was estimated to be 0.005 ppm which was the response of twice the background signal. Seven replicate measurements of 0.200 ppm of C102gave an average relative intensity of 141 f 3 with a relative standard deviation (RSD) of 2.3%. Response Time. A quick response time is important to any analytical method used in a continuous monitoring application where the analyte concentration could be fluctuating. The average 100% steady-state response time was 30 s, and the time required for CL intensity to decay to the background level as the cell was flushed with distilled water was 44 s. The average 90% response time was only 11.2 s, and the time required for completion of one sample analysis was estimated at 5 min. Effect of Reagent pH. Luminol reagent solutions were prepared a t pH 7-13. C102 calibration curves were recorded for each reagent pH, with the least-squares slope of each curve considered to be the sensitivity. The relationship between reagent pH and analytical sensitivity is
Flgure 1. CL response for 0.1 ppm of C102 and 1.00ppm of HOC1 as a functlon of analyte pH. Reagent pH 10, reagent flow rate = 0.082 mL min-I, and analyte flow rate = 4.7 mL min-'.
shown in Figure 1. Clearly pH 10 is the optimum reagent pH for maximum C102sensitivity, and this pH was used for all subsequent studies. Effect of Analyte pH. The CL response for C102was determined for samples at pH 6-12 with luminol reagent maintained at optimum pH 10. Response from 0.1 ppm of C102is shown in Figure 2, and the data indicated that the optimum CL intensity occurred at analyte pH 9. As shown in Figure 2, as the analyte pH was increased, CL intensity decreased markedly, which was probably the result of C102disproportionation in alkaline solution (16). Effect of Flow Rates. The CIOz flow rate was varied from 2.0 to 11mL min-l in order to determine the effect of analyte flow rate on CL response. The CL intensity increased at higher flow rate which was the result of increased mass transfer of CIOz through the membrane. At flow rates above 9.5 mL min-l, the CL steady-state response fluctuated as a result of flow cell turbulence. An optimum analyte flow rate of 9.0 mL m i d was selected due to its high CL response and stability. The luminol reagent flow rate was varied from 0.078 to 3.32 mL min-l. A low reagent flow rate is preferable in continuous monitoring for economic reasons and is useful provided excess reagent concentration is maintained; however, the kinetics of the CL reaction must also be considered. For example, if the reaction kinetics are slow and the flow rate is too fast, CL emission can occur after Environ. Sci. Technol., Vol. 19, No. 5, 1985
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Y
z -1
5.2
I
I
3.3 t/T, 7
8
9
IO
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Figure 2. CL sensitivity as a function of reagent pH
the emitting species has left the cell. The optimum flow rate has been previously described to be a function of reaction kinetics and the geometry of the flow cell (21). Since the CL response remained constant over the reagent flow rate range studied, the residence time of the emitting product, the aminophthalate ion, was apparently sufficient for complete CL detection. A flow rate of 0.078 mL m i d was selected for subsequent analysis in order to conserve reagents. Effect of Temperature. The permeability coefficient of a membrane is the product of the distribution coefficient and the diffusion coefficient of the diffusing species. The effect of temperature on these parameters is well-known, and the linear relationship between the logarithm of the response and the inverse of the absolute temperature is readily established (22). The CL response for 0.500 ppm of C102 was monitored at temperatures between 10 and 30 "C, and the data are shown in Figure 3. This temperature effect would make it necessary to correct the CL response when seasonal variations caused a change in water temperature; however, automatic temperature compensation could be easily established. Investigation of Chlorine. Since 83 of the 84 water treatment facilities in the U.S.utilizing CIOz treatment also use C12 as a disinfectant (9),the method of C102 generation usually involves oxidation of C102-by Clz. This production of CIOz in the presence of chlorine necessitates that CIOz measurement exhibit no interference from C12, HOCl, or OC1-. Since OC1- has been previously shown to effectively cooxidize luminol (IO),the reaction of chlorine with luminol in the membrane flow cell was investigated. In addition, the selection of optimum analyte pH which would eliminate chlorine as an interference was determined. Effect of Reagent pH on HOCl CL. Reagent (luminol) solutions were prepared from pH 7 to pH 13 to examine their effect on CL from HOCl. Samples were maintained at pH 6 where 97% of the acid is in the membrane-permeable form of HOC1. Linear calibration curves for HOCl were obtained at each reagent pH, and the least-squares data were calculated to give the CL sensitivity for HOCl as shown in Figure 4. The optimum sensitivity was achieved at pH 9. These data correlated Environ. Sci. Technol., Vol. 19, No. 5, 1985
3.5 1
13
30
PH
452
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3.4 i IO3 20
T,
IO
OC
Flgure 3. Effect of temperature on CL response to 0.500 ppm of CIO,. I
0
7.0
,
11.0
9.0
O
13.0
PH
Figure 4. CL sensitivity for HOC1 as a function of reagent pH. Sample pH of 6.
quite well with that obtained by using a direct mix nonmembrane method (IO). The increase in sensitivity noted above pH 11 was reproducible; however, no explanation for the increase can be given. Isacsson and Wettermark ( I O ) did not study this pH effect above pH 12. Effect of Analyte pH on HOCl CL. CL response for HOCl was determined for samples at pH 6-12 while the reagent pH was held at 10. Data for 1.00 ppm of HOCl are shown in Figure 1 and compared to results observed for 0.1 ppm of C102 As the sample pH was raised to 10, the HOCl response decreased to zero. A t this pH 99.7 % of the hypochlorous acid present in the sample was in the OC1- form, and since the membrane is impermeable to that species, it cannot diffuse into the reagent reservoir to oxidize luminol. Below pH 10, a larger percentage of HOCl will be present in the sample, and as a neutral molecule, it can diffuse through the membrane. After the HOCl
~
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._ c
c
3 0
._ +
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_I
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I
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1.0
2.0
I
I
3.0
4.0
I
5.0
HOC1 ADDED, pprn
Figure 5. Effect of HOC1 additions on CL sensitivity for analyte pH 9 (0)and anaiyte pH 10 (m). RSD = 2.3%. CL sensitivities indicated are for C102 analysis.
entered the reagent reservoir (pH lo), it apparently dissociated to OC1- which produced light emission. A 0.1 ppm C102 sample produced a much higher CL response than a 1.00 ppm HOCl sample. Isacsson and Wettermark (10) earlier reported that 0.525 ppm of OC1produced a CL response twice that of 0.608 ppm of C102 in a reaction cell where reagent and analyte were mixed directly. The use of the membrane flow cell has reversed this CL sensitivity, indicating that membrane is probably more permeable to C102 than to HOC1. The analyte pH for all subsequent analysis was fixed at either 9 or 10 where HOCl gave little or no CL response. The C102response remained relatively high and stable at either pH with no noticeable effect from disporportionation. Chlorine as an Interference. HOCl is a stronger oxidant than C102 in aqueous solution, and the following reaction has been given for aqueous mixtures of C102and HOCl (16): Cl02 + HOCl
+
C10,-
+ H+ + O.5C12
(1)
Mixtures of C102and HOCl were analyzed to determine the influence of chlorine on CL sensitivity. As shown in Figure 5, at pH 9 HOCl additions up to 3.00ppm increased CL response. This increase was probably the result of the small amount of diffusable HOCl present. The CL response decreased markedly for a 5.00 ppm HOCl addition probably due to reaction with C102 (eq l),which would cause a decrease in both the C102 and HOCl concentrations. At analyte pH 10, CL sensitivity was relatively constant for HOCl additions up to 3.00 ppm but again decreased for a 5.00 ppm addition due to the oxidation of C102by HOC1. Analyte pH 10 was selected for subsequent analysis to eliminate chlorine interference. Application of Water Analysis. The membrane flow cell CL method was compared to several other methods (iodometric titration, diethyl-p-phenylenediamine (DPD), and chlorophenol red (CPR)) for C102analysis in distilled water, tap water, river water, and secondary sewage effluent. Of the three standard methods chosen, only CPR
is stated to be selective for C102in the presence of other chlorine species and oxidizing agents (23). The iodometric titration and DPD methods are subject to interference from other chlorine species and oxidants such as Fe, Mn, and Cr (15). The colorimetric methods, DPD and CPR, also suffer from low sensitivity for C102. The results of these water analyses are summarized in Table I. Distilled water samples gave comparable results for all methods, with C102concentrations in agreement within the stated standard deviation of each method. Tap water samples produced significantly higher results by iodometric titration where residual chlorine can act as a positive interference. The colorimetric methods, DPD and CPR, also gave higher results than CL for tap water, which could be due to the presence of either chlorine or other oxidants. The iodometric titration and DPD method both gave higher results than CL for river water and secondary sewage effluent, indicating that the CL method is not as affected by interferences. The iodometric titration and DPD methods are prone to interference by trace metals as well as other compounds present in river water and sewage effluent. CPR data were not as notably affected for the river and sewage samples, and although this method has no listed interferences (23),it may be subject to less accurate results because of the low sensitivity. Conclusions
The membrane flow cell CL method has been shown capable of selectively measuring C102 in the presence of chlorine. Results have shown that this method provides suitable results in tap water as well as river water and sewage effluent where standard methods are subject to a variety of interferences. In addition, the CL method could be easily used for the continuous, automated monitoring of CIOz residuals in water treatment applications. We are currently investigating the effects of phenol and the chloramines on this method. Acknowledgments
We would like to acknowledge Carl Wise and Robert Victor for their work on the construction of the flow cell and the “shrinking bottle”. We also acknowledge Charles McNemar for preliminary experimental work and for his design of the light-proof box. Registry No. CIOz, 10049-04-4; HOC1, 7790-92-3; water, 7732-18-5.
Literature Cited (1) Rook, J. J. Water Treat. Exam. 1974, 23, 234. (2) Bellar, T. A.; Lichtenburg, J. J.; Kroner, R. C. J.-Am. Water Works Assoc. 1974, 66, 703. (3) Trussell, R. R.; Umphres, M. D. J.-Am. Water Works Assoc. 1978, 70,604. (4) Jolley, R. L.; Gorchev, H.; Hamilton, D. H., Jr., E&. “Water (5) (6) (7) (8)
(9)
Chlorination Environmental Impact and Health Effects”; Ann Arbor Science: Ann Arbor, MI, 1978; Vol. 2. Page, N. P.; Saffiotti, U. S. “Report on Carcinogenesis Bioassay of Chloroform”; National Cancer Institute: Bethesda, MD, 1976. Simmon, U. F.; Kauhanen, K.; Tardiff, R. G. Dev. Toxicol. Environ. Sci. 1977, 2, 249. Fed. Regist. 1983, 48, 8460. Symons, J. M.; Carswell, J. K.; Clark, R. M.; Love, 0. T.; Miltner, R. J.; Stevens, A. A. “Interim Treatment Guide for the Control of Chloroform and Other Trihalomethanes“; Municipal EnvironmentalResearch Laboratory: Cincinnati, OH, 1976. Katz, J., Ed. “Ozone and Chlorine Dioxide Technology for Disinfection of Drinking Water”; Noyes Data Corporation: Park Ridge, NJ, 1980. Environ. Sci. Technol., Voi. 19, No. 5, 1985
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Isacsson, U.; Wettermark, G. Anal. Chim. Acta 1978,83,
(17) Silverman, R. A.; Gordon, G. Anal. Chem. 1974,46, 178. (18) “CRC Handbook of Chemistry and Physics”,60th ed.; CRC Press: Boca Raton, FL, 1980. (19) Aieta, E. M.; Roberts, P. V.; Hernandez, M. J.-Am. Water Works Assoc. 1984, 76, 64. (20) Bull, R. J. J.-Am. Water Works Assoc. 1980, 72, 299. (21) Pilosof, D.; Nieman, T. A. Anal. Chem. 1980, 52, 662. (22) Crank, J.; Park, G. S. ”Diffusion in Polymers”; Academic Press: New York, 1968. (23) Wheeler, G. L.; Lott, L. F. Microchem. J . 1978, 23, 160.
227.
Isacsson, U.; Wettermark, G. Anal. Lett. 1978, 11, 13. Smart, R. B. Anal. Lett. 1981, 14, 189. Nau, V.; Nieman, T. A. Anal. Chem. 1979,51,424. Saksa, D. J. M.S. Thesis, West Virginia University,Morgantown, WV, 1984. “Standard Methods for the Examination of Water and Wastewater”, 15th ed.; American Public Health Assoc., American Water Works Assoc., and Water Pollution Control Federation: Washington, DC, 1980. Masschelein, W. J. “Chlorine Dioxide”;Ann Arbor Science: Ann Arbor, MI, 1979.
Received for review August 24, 1984. Accepted November 21, 1984.
Laboratory Evaluation of Chemical Dispersants for Use on Oil Spills at Sea J. W. Anderson,* D. L. McQuerry, and S. L. Kiesser Batteiie, Marine Research Laboratory, Sequim, Washington 98382
Data on toxicity and effectiveness of 14 chemical dispersants were combined in a straightforward equation to provide an overall assessment of the relative merits of the oil spill chemicals. When a decision is made by regional response authorities to mitigate the damage of spilled oil to the shoreline, our findings should aid in the selection of an effective low toxicity product. Products were evaluated by using standard toxicity tests with a mysid shrimp (Mysidopsis bahia) and a standard effectiveness test using the Mackay-Nadeau-Steelman (MNS) apparatus. Ratios of dispersant to oil required to maintain 90% dispersions of oil in seawater (15 “C and 30%) with a standard mixing energy (1.0 in. of water pressure) of air flow were derived for each chemical by using Prudhoe Bay crude oil. Toxicity tests with M . bahid were conducted at 25 OC and 25%0 by using freshly hatched juveniles (15 per concentration times 5 concentrations) in small dishes in an incubator.
then application could proceed. In many cases, however, it will be important to consider the relative effectiveness and toxicity of the candidate chemical dispersants. In this event, two different points in a decision tree illustrated (Figure 1) provide for the evaluation of experimental data on effectiveness, cost, and toxicity of the candidate dispersant. The response authority can make much more accurate predictions regarding dispersant application if these data are presented in a clear comparative manner and if the analyses of individual factors have been combined to produce an overall ranking. The objective of this paper is to present values for relative effectiveness and toxicity and show how they can be combined with cost data to provide potential users a means of selection. Materials and Methods
Introduction
A relatively sensitive marine organism (mysid, Mysidopsis bahia) and an effectiveness system called the Mackay-Nadeau-Steelman (MNS) test were used ( 4 , 5 ) .
There is still considerable controversy regardig the use of oil spill chemicals on spills in U.S. coastal waters. While it is recognized that the magnitude of toxicity produced from the chemical products has decreased considerably with the development of new chemicals, state and federal regulatory agencies generally require approval on a caseby-case basis. Field tests have been conducted to determine the effectiveness of arid application and the resulting concentrations of petroleum hydrocarbons in the water (1). Mackay and Wells (2) have attempted to model the potential impacts of dispersant and dispersed oil under actual spill conditions. We feel that both regulatory agencies and spill response authorities need a sound base of comparative data on a variety of chemical dispersants to properly evaluate the ability to disperse a spill and the outcome of the application. The decision process for responding to oil spill begins with an evaluation of the response options available to authorities, and these have been described elsewhere (3). For the purposes of this paper, it will be assumed that all chemical dispersants tested are available for use with the suitable application equipment. In actuality the response authorities might select one or more of the chemicals to stockpile at strategic locations. The possibility exists that the conditions and location of the spill will not require careful assessment to select a dispersant. If there is little possibility of damage to marine life from the use of available equipment and chemicals,
The mysid has been an important species in tests by the U S . EPA regarding acute and chronic toxicity of several different toxic chemicals (6). Freshly hatched juvenile mysids were placed separately in plastic Petri dishes with 100 mL of the five different dilutions of dispersant (in 25% seawater) and these (15 per concentration) were placed in an incubator at 25 OC. Since the animals have a low capability to store energy reserves, a few (20-50) brine shrimp nauplii were added to each dish at 48 h. While mortality was recorded on a daily basis, only the 96-hvalues are reported, and logit analysis was used to produce 96-h LCm values and the 95% fiducial limits. The MNS test has been used extensively throughout the U.S., Canada, and Europe, and an intercalibration (round robin) exercise has been conducted (Cashion, personal communication). This testing apparatus is now the designated method for use in Canada to determine the acceptability of oil spill dispersants (7), while a different testing method is used in the United States (8). Our toxicity and effectiveness test results are presented, recognizing that other types of tests will likely produce somewhat different findings. When this research project was initiated (Oct 1981) there were 16 products designated as dispersants on the EPA Acceptance List. Subsequently, our purchase requests to the manufacturers produced the 12 oil spill dispersants used in the majority of testing. In 1983, two dispersants (products N and P) were sent directly from the manufacturers after discussions with the
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