In the Laboratory
Turbidimetric Analysis of Water and Wastewater Using a Spectrofluorimeter
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Jason J. Evans Chemistry Department, Dickinson College, Carlisle, PA 17013-2896;
[email protected] Student interest in environmental science continues to grow, and as a result, many chemistry departments are offering new courses in environmental chemistry. Environmental analysis has become increasingly instrumental, and it is important to introduce the students to the instruments that are most commonly used for the analysis of environmental samples. However, many of these instruments are not available in most undergraduate laboratories—purge-and-trap GC–MS, graphitefurnace AA, ICP–MS, particle counters, and nephelometers to name just a few. This experiment illustrates that a spectrofluorimeter can be used to perform a turbidimetric analysis that is normally performed in quality-control environmental laboratories using a nephelometer. Although nephelometers are relatively inexpensive, many colleges and universities are utilizing spectrofluorimeters in other laboratory courses. Thus, this experiment can be readily adapted to use in an environmental chemistry laboratory course without the need to purchase new instrumentation. For programs that do not already possess a spectrofluorimeter, I certainly encourage the purchase of a few nephelometers, given their relatively low cost. In the spring semester of 1999 the chemistry department at Dickinson College offered an environmental chemistry course designed for environmental science majors. Previously, this course has been offered only sporadically, and this was the first time a laboratory component was included. One of the major goals of the new laboratory component is to give the students more hands-on experience with a variety of instrumentation. Experiments included determination of alkalinity and hardness, determination of heavy metals in acidmine drainage samples using flame AA, GC–MS of volatile organics, and a few others. This paper describes a turbidity experiment developed for this course. Overview of Experiment Turbidity measurements are used as a quality-control parameter for drinking water mostly for aesthetic reasons. The standard method for the turbidimetric analysis of water samples utilizes an instrument called a nephelometer (1). This instrument measures the intensity of light scattered from colloidal particles at a 90° angle from an incident beam. The optical geometry is similar to that of a spectrofluorimeter. The major differences between the two instruments have to do with wavelength selection, path length, and stray light. Spectrofluorimeters are actually more complex, because there is no need for wavelength selection in turbidity measurements. As a result, low-cost nephelometers can be constructed from supplies available at the local electronics store (2, 3). They can also be purchased for under $1000. The design of the typical nephelometer is such that light from a source enters a 3-inch tube from the bottom. As a result, the path length of the incident beam is about 8 cm, and scattering from the tube
is minimal because the light beam strikes the glass only at the bottom of the tube, which is far removed from the detector. For these reasons one may expect the nephelometer to give a better S/N ratio than a typical spectrofluorimeter. Nevertheless, we found that the detection limit of our spectrofluorimeter for turbidimetric analysis is 0.03 nephelometric turbidity units (NTU), which is more than sufficient for the analysis of most water samples. According to EPA standards the maximum allowable turbidity in drinking water is 0.5–5.0 NTU (1). Experimental Procedure In accordance with standard methods (4 ), hydrazine sulfate, hexamethylene-tetramine, and reverse osmosis (R.O.) water were used to prepare a 500.0 NTU standard. This standard was freshly prepared two days before the experiment. The students prepared diluted standards ranging from 0.05 to 50 NTU. The R.O. water served as the dilution water. Six samples were acquired from the Carlisle Water System: three from the wastewater treatment plant and three from the water treatment plant. The three samples from the wastewater treatment plant were taken from three different stages of the treatment process, namely, raw sewage (the plant’s intake), the second-degree influent, and the final effluent that is discharged into the local stream. The second-degree influent is water that has been through the processes of coagulation and flocculation, but not bioremediation or filtration. The three samples taken from the water treatment plant consisted of the intake from the local stream, filter influent, and final effluent. The filter influent is creek water that has undergone pH adjustment, coagulation, and some bioremediation. To obtain the final product, the water is chlorinated and filtered through 6 feet of sand and activated carbon to remove organics. We also analyzed tap water and water taken from a fresh sample of melted snow. Prior to measurement all samples were tightly sealed and refrigerated. The instrument used in this experiment is a Hitachi F-2000 fluorescence spectrophotometer. For this experiment, the excitation monochromator was set to allow all incoming radiation from the tungsten filament to pass through the sample. The emission monochromator was also set to allow scattered radiation of all wavelengths (200–800 nm) to reach the detector. The software on the Hitachi F-2000 enables one to select these conditions easily. We used a quartz cuvette for the analysis; however, it is only necessary that the cuvette be clear and unfrosted on all four sides. All samples were brought to room temperature before analysis. We set up a 3-minute delay time in an effort to be reasonably sure that the settleable solids did not lead to erroneously high results. Longer delay times were briefly investigated, but no significant changes in the signals were found. The spectrofluorimeter was operated in a mode that processes and prints out the signal at 5-second intervals. The signal for each sample was the average of 25 measurements.
JChemEd.chem.wisc.edu • Vol. 77 No. 12 December 2000 • Journal of Chemical Education
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In the Laboratory
Hazards Hexamethylenetetramine and hydrazine sulfate are the only two chemicals used in this experiment. They are used to make the formazine standard, and since this takes 48 hours to develop, I suggest that the laboratory instructor prepare these standards in advance. Hexamethylenetetramine: Avoid heat and open flames and avoid contact with oxidizers or acids. Contact can cause eye irritation (flush with water for 15 minutes), and inhalation or ingestion can cause irritation of the respiratory tract (drink large quantities of water). Hydrazine sulfate: Avoid ammonia, oxides of sulfur, and oxides of nitrogen. Can cause irritation to the skin and eyes (flush with water for 15 minutes). Can cause chronic damage to the organs if ingested. See MSDS sheets for further detail. Students should wear gloves when making the dilutions of the formazine standard and when measuring the turbidity. Dispose of formazine standards in an organic waste container. Alternatively, one can now purchase nontoxic turbidity standards; see http://www.apsstd.com/amco/advantages.html.
second-degree influent (27 vs 6.2 NTU). Filtration lowered the turbidity further to 2.7 NTU, which is less than the turbidity of the creek (3.9 NTU), where it is being discharged. We were certainly comforted by this comparison, as it indicates that the wastewater treatment plant is functioning well. Conclusion The students enjoyed this experiment because they were analyzing “real” samples from the water treatment facilities. The data obtained with the nephelometer in the qualitycontrol laboratory at the plant provided validation of their results. Students were able to prepare fresh formazine standards and obtain results that closely matched those from the plant. I believe this helped to develop self-confidence in their analytical skills. The instrument was not treated as a black box that magically produced data. The students were encouraged to think about the geometry of the instrument and why the spectrofluorimeter has a sensitivity advantage over a spectrophotometer that has a 180° geometry. Finally, the experiment provided excellent reinforcement of the lecture topics that cover the processes involved with water and wastewater treatment.
Results
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A typical calibration curve is shown in Figure 1, and the turbidities of the eight samples discussed above are shown in Table 1. The plot shows standards ranging from 0.05 to about 6 NTU, although data were obtained from 0.05 to 50 NTU. I choose to show a truncated plot to better show the data points at the low turbidities. The equation of the regression line used to calculate the turbidities of the unknown samples was obtained from the plot of all the points. The correlation coefficient of the plot is .999 (including all points). Also shown in the table are measurements made using a nephelometer at the Carlisle Water Treatment Plant. The agreement is excellent, and our data appear to have a higher degree of precision. The errors were calculated using standard error analysis for linear regression plots (5). The turbidity data from the nephelometer were obtained from one measurement per sample. Thus, the greater precision likely resulted from the greater number of measurements that were taken per sample using the spectrofluorimeter. Using the calibration plot, a turbidity of ᎑0.19 ± 0.06 NTU was calculated for the effluent from the water treatment plant. This indicates the turbidity of the outgoing municipal water supply is less than that of the R.O. water that we used as the dilution water. Water picks up a significant degree of turbidity as it flows through the city water lines into the lab, and the process of reverse osmosis certainly is not very effective at removing particles suspended in the water. The turbidity of the dilution water can be estimated from the x intercept of the calibration plot (0.32 NTU). This complication provided an opportunity to discuss the problems associated with centuryold water lines that are still in use in many communities throughout the United States. It is also apparent from the data that a significant degree of turbidity is removed from water at each stage of wastewater and water treatment. Most of the turbidity found in raw sewage was removed by coagulation and flocculation, as can be seen by comparing the turbidity of raw sewage and the
The lab handout used for this experiment is available in this issue of JCE Online.
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Material
120
Scattering Intensity
100
80
60
40
20
0
0
1
2
3
4
5
6
Turbidity / NTU Figure 1. Standard curve for turbidimetric analysis using a spectrofluorimeter.
Table 1. Comparison of Results Obtained with the Spectrofluorimeter and with a Nephelometer Sample
Turbidity/NTU Fluorimeter
Nephelometer
Creek
3.89 ± 0.07
3.7
Filter influent
0.65 ± 0.03
0.80
Plant effluent (city water)
᎑0.20 ± 0.03
Raw sewage
27.2 ± 0.3
0.10 33
Second-degree effluent
6.2 ± 0.1
5.2
Effluent from WWT plant
2.78 ± 0.05
3.1
Snow Tap water
24.6 ± 0.4 0.22 ± 0.03
Journal of Chemical Education • Vol. 77 No. 12 December 2000 • JChemEd.chem.wisc.edu
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In the Laboratory
Literature Cited 1. Sawyer, C. N.; McCarty, P. L.; Parkin, G. F. Chemistry for Environmental Engineering, 4th ed.; McGraw Hill: New York, 1994; pp 439–442. 2. Hetterton, F. A. J. Chem. Educ. 1991, 68, 254. 3. Das, I.; Das, S. S.; Pushkarna, A. J. Chem. Educ. 1987, 64, 729.
4. Standard Methods for the Examination of Water and Wastewater, 20th ed.; Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D., Eds.; American Public Health Association: Washington, DC, 1998; pp 2–20. 5. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1999; p 100.
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