Evaluation of a Visual Color Comparator Method for Determination of Atmospheric Nitrogen Dioxide n preliminary, widespread field surveys of atmospheric pollution concentrations, equipment for sampling and analysis that does not rely upon a source of a x . electric power is advantageous. Low-volume battery-powered air pumps suitable for aspirating air samples through gas bubblers or filtration units and visual color comparators employing uniformly tinted glass filters mounted in a revolvable disk are commercially available. A method for measuring the concentration of oxidants in air by the phenolphthalein method using a visual color comparator has been developed (Haagen-Smith and Brunelle, 1958 ; Tintometer Ltd.). Because nitrogen dioxide is a common air pollutant and a satisfactory colorimetric method of analysis is available (Saltzman, 1954); U. S. Department of Health, Education, and Welfare. Public Health Service, 1965), it was thought desirable to apply this procedure to visual colorimetry using the visual color comparator. T o our specifications the manufacturer of this apparatus prepared a color disk containing nine colored glass filters having appropriate spectral characteristics. This apparatus was evaluated in a comparison study of data obtained by use of the visual color comparator and data obtained spectrophotometrically. A ten-member untrained observer panel was used t o obtain measurements by means of the visual color comparator.
I
Experimental
Apparatus. A standard Lovibond Comparator with special NO, color disk, having glass filters corresponding to 0.5, 1.0, 1.5, 2.033.0, 4.0, 5.0. 6.0, and 7.0 pg. of NOs gas complete with 10-ml. capacity 13.5-mm. diameter cuvettes, is available from Hayes G. Shimp. Inc., 866 Willis Ave., Albertson, L. I . , N.Y. A Bausch and Lomb, Spectronic 20 spectrophotometer with 0.5-inch diameter test tubes was used in the spectrophotometric analysis. Reagents. The diazotizing, coupling: and absorbing reagents used are the same as those described by Saltzman (1954). An NO, test solution (1 ml. t o 10 pg. of NOn) was prepared from sodium nitrite. The empirically determined factor of 0.72 mole of sodium nitrite per mole of NO? gas was used.
Procedure. Azo dye sample solutions produced by reaction of diluted sodium nitrite test solutions and absorbing reagent were evaluated spectrophotometrically and by use of the visual color comparator. I n the spectrophotometric method, absorbancy of the liquid samples was determined at 550 mp and these data were converted to NOs concentration by reference to a calibration curve. N O p data were obtained by the visual color comparator method by visual matching of the color of the liquid sample and reagent blank against the color of stained glass filters. T o accomplish this, the disk was revolved until the color filter matching the sample appeared. If the color of the sample lay between colors of the filters, the NO, concentration was estimated to the nearest 0.1 pg. by extrapolation. Results and Discussion Measurements obtained by the observer panel using the visual color comparator were analyzed statistically, and the average value was compared to the NO? value determined spectrophotometrically (Table I). In the range of concentrations of interest, the average difference between results for these two methods is less than 17 %; an exception occurs when the concentration of NOy of the liquid sample is less than 1.0 pg. per 10 ml. as determined spectrophotometrically, in which case the average difference is 27%, the lower values being obtained by the visual comparator method. NOz values determined with the visual comparator are within *20 of the values obtained spectrophotometrically 95 of the time, except when the lowest color density filter is used. These data iridicate that the visual color comparator is most suitable for analyses of liquid samples containing more than 1 and less than 7 pg. of NOu. If the method is applied to air analysis, sampling time o r sampling rate should be gaged so that 1 to 6 pg. of NO, is collected.
Conclusions Standard colored filters prepared by the Tintometer Co. for use with the standard Lovibond Comparator are suitable for estimating atmospheric NOy concentrations if a deviation
Table I. Comparison between Visual Comparator and Spectrophotometric Methods for NO2 Suectro
Solution u ~ . : l O
NO'>.
Arithmetic- Methods.
Decision, .%b
1 2 3 4 5 6 7 8 4
0.63 1.13 1.67 2.11 3.25 4.20 5.27 6.18
0.03 0.05 0.07 0.09 0.14 0.19 0.23 0.27
For a 12-liter air sample.
0.6 1.1 1.6 2.2 3.5 5.1 6.0 7.1
0.5 1.0 1.5 2.0 3.0 5.0 6.0 7.0
0.4 1.0 1.3 ... 3.0 4.9 5.9 7.0
0.4 1.0 1.4 2.0 3.1 4.2 5.8 6.8
0.5 1.0 1.4 1.9 3.2 4.8 5.8 7.1
0.4 1.1 1.2 1.8 3.2 4.8 6.0 7.1
0.4 0.9 1.4 2.0 2.9 4.9 5.9 7.0
0.5 1.1 1.4 2.0 3.1 4.2 5.9 7.1
8 1 . 1 1 . 0 10 1 . 5 1 . 4 10 1.8 1.7 9 3 . 2 3 . 0 10 4 . 2 4 . 2 10 5 . 8 5 . 5 10 7 . 5 6 . 5 10 ...
* Given samples of size No., one expects estimated value from visual comparator method
a%
of time.
...
0.4-0.6 0.9-1.1 1.2-1.6 1.7-2.2 2.9-3.5 4.2-5.1 5.5-6.0 6.5-7.5
0.46 1.03 1.41 1.93 3.12 4.63 5.86 7.02
- 27
-9 - 16 -9 -4 +10 +11 +I4
95 >95 >95 >95 >95 >95 >95
to fall within range of spectrophotometric value %20%,
Volume 1, Number 1, January 1967
75
*ZOz
of from results obtained with the spectrophotometric method is tolerable. The visual color comparator method should be useful in preliminary field surveys over a wide area when short-term measurements of air pollution are desirable and electric power is at a premium. If air sampling is conducted with battery-powered air pumps, both sampling and analysis are independent of an outside source of power. Acknowledgment
Saltzman, B. E., Anal. Clzerz. 26, 1949 (1954). Tintometer Ltd., Salisbury, England, “Handbook of Colorimetric Chemical Analytical Methods.” U. S. Department of Health, Education, and Welfare, Public Health Service, Cincinnati, Ohio, “Selected Methods for Measurement of Air Pollutants.” 1965.
Seymour Hochheiser G. A. Rodgers
The authors express appreciation to those individuals participating as color-matchers and to James W. Richhart for performing statistical analyses of data.
Technical Assistance Branch Division of Air Pollution Robert A. Taft Sanitary Engineering Center Cincinnati, Ohio
Literature Cited Haagen-Smith, A. J., Brunelle, M. F., Intern. J . Air Pollution 1, 51 (1958).
Receired ,fbr reciew October 5 , 1966. Accepied December I , 1966. Southeastern Regional Meeting, ACS. Louisrille, K y . , October 1966.
76 Environmental Science and Technology
