ionic strength and temperature. However, the slope at which the electrode is operating and the concentration of ammonia in the sample (within a factor of 10) must be known. As can be seen from Table VI, the two procedures gave comparable results and were interchangeable. Automation. With certain provisions, the ammonia electrode can be adapted to automatic analysis. The major drawback is that some provision must be made for the addition of sodium hydroxide just prior to presentation of the sample to the electrode. This was overcome by using the ammonia electrode in conjunction with a Beaker Butler (FWPCA, 1970). an instrument for automating ion selective electrode analyses. The Beaker Butler was modified to provide for sodium hydroxide addition just prior to electrode immersion. Using a 5-min sampling period, acceptable results were obtained with standard solutions in distilled water.
L i t e r a t u r e Cited Environmental Protection Agency. National Environmental Research Center, Analytical Quality Control Laboratory. Cincinnati, Ohio, “Methods for Chemical Analysis of Water and Wastes.” 1971. Environmental Protection Agency. National Environmental Research Center, Analytical Quality Control Laboratory, Cincinnati, Ohio. “Handbook for Analytical Quality Control in Water and Wastewater Laboratories,” March 1972. Federal Water Pollution Control Administration, Div. of Water Quality Research, Analytical Quality Control Laboratory. Cincinnati, Ohio, “Evaluation Report 1: Automatic Beaker Sample Changer, 1Y7U. Orion Research Inc.. Cambridge, Mass., Instruction Manual. Ammonia Electrode Model 95-10, 1971,
Received for recieu August 3, 1972. Accepted December 1.5, 1972. Mention of commercial products is for identification on/? and does not constitute endorsement b? the Encironmentai Protection A g e n t ) or the L.,S. Gocernment.
Collection and Determination of Sulfur Dioxide Incorporating Permeation and West-Gaeke Procedure Kenneth D. Reiszner and Philip W. West’ E n v i r o n m e n t a l S c i e n c e s I n s t i t u t e . C o a t e s C h e m i c a l L a b o r a t o r i e s , L o u i s i a n a S t a t e U n i v e r s i t y , B a t o n R o u g e , La. 70803
A method to measure average pollutant concentrations directly for a desired study period has been developed for sampling and determining sulfur dioxide in the ambient atmosphere. Sampling is accomplished by the permeation of the SO2 through a silicone membrane. The permeated pollutant is trapped and stabilized as dichlorosulfitomercurate(I1) and is subsequently determined by the WestGaeke procedure. Sampling pumps are not required because permeation is a physical phenomenon which occurs freely. Measured sample volumes are not required because rate of permeation is proportional to concentration. There are no known significant interferences; the procedures are simple and straightforward; the results are comparable with those obtained with present monitors; and the only equipment required, other than a spectrophotometer or filter photometer, is a permeation device costing less than $50. Because the analytical values relate to state or federal standards without recourse to statistical or other special mathematical study. the method is ideally suited for monitoring applications. Federal as well as most municipal and state regulations that have been promulgated to establish ambient air quality standards for sulfur dioxide stipulate average concentrations for specified intervals of time. For example, the federal code requires that the average daily concentration must not exceed 365 pg/m3 for more than one day during a year and the average concentration for that year must not exceed 80 pg/m3 (“Federal Register,” 1971). There is an obvious need for a direct, simple, and reliable method for determining average concentrations, whether it be for 12- or 24-hr periods or for a seven-day or even a 30-day average. Such a method has now been developed. 1
To whom correspondence should be addressed.
526
Environmental Science & Technology
The approach employs a sampling device based on the permeation principle and an analytical finish based on the West-Gaeke procedure for determining sulfur dioxide. Most determinations of sulfur dioxide for air pollution monitoring require the use of relatively expensive equipment which uses a colorimetric method such as the WestGaeke procedure (West and Gaeke, 1956), a coulometric measurement, or possibly a conductometric determination made on samples that are continuously pulled through the apparatus for appropriate treatment and measurement. Alternatively, a gas chromotographic or flame photometric procedure may be used. Each sampling represents an investment of thousands of dollars necessary for equipment. The final processing of data to obtain values for average concentrations of pollutant requires further investment in time and talent. A simple approach that has been often used is the deployment of lead peroxide candles or sulfation plates, which serve as low-cost substitutes for the sophisticated but expensive SO2 monitors. Unfortunately, the lead peroxide candles do not measure concentration but instead give a measure only of the relative insult due to sulfur dioxide. They represent an “effects method” giving information somewhat similar to that obtained by corrosion coupons used in establishing the relative corrosiveness of the ambient atmosphere. The lead peroxide candle method suffers further because the final determination is based on the measurement of resultant lead sulfate, and this requires a relatively tedious turbidimetric, gravimetric, or colorimetric analysis. A new method is now proposed which is simple, low in cost, essentially free of interferences. requires no complicated apparatus or availability of power, and may be employed for sampling times as short as 6 hr or as long as a week or more. Of special significance is the fact that this new method provides “integrated” values directly; therefore, average concentrations are provided for each expo-
sure period and thus relate directly to federal or state standards. The present method utilizes a permeable membrane to collect the sample a t a rate proportional to concentration. The permeated sulfur dioxide is captured and stabilized in sodium tetrachloromercurate(I1) with the subsequent determination being carried out by the West-Gaeke procedure.
1 Hole /No
8 Rubber Stopper lmm
No 10
I.D.
2 Capillary
Rubber
st o p p e l l
( length 8 c m )
R e c o m m e n d e d Procedure
Apparatus. Any suitable spectrophotometer or filter photometer for measurement of absorbance a t 575 nm may be used. Preparation and Calibration of Permeation Device. The permeation device (Figure 1) is prepared by sealing a disk of single-backed dimethyl silicone rubber polymer (General Electric Co., One River Road, Schnectady, N.Y. 12305) to an 8-cm column of 41-mm 0.d. glass tubing using silicone rubber cement. The device is then inserted in a rubber stopper as shown in the figure. The membrane backing should be on the exterior of the finished device. Place 10 ml of 1M sodium tetrachloromercurate(I1) in the permeation device. Expose to a known concentration of sulfur dioxide for an appropriate time-typically 3000 pg/m3 for 2 hr-making sure that less than 2% of the sulfur dioxide in the calibration mixture is absorbed by the permeation instruments being exposed. Larger losses of sulfur dioxide can be tolerated by sacrificing calibration accuracy. The constant, k , is then calculated from the following equation: k = --Ct W
