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Determination of Low Concentrations of Hydrogen Chloride in Moist Air R. R. Bailey, P. E. Field, and J. P. Wightman* Department of Chemistry, Virginia Polytechnic lnstitute and State University, Blacksburg, Va. 2406 7
Hydrogen chloride, while not considered a major air pollutant, has received considerable attention recently as regards its effect on the environment (1-4). The quantitative detection of hydrogen chloride present in the atmosphere at ppm levels is important for several reasons including the assessment of environmental impact on such events as solid rocket launches ( 4 ) ,incineration of chlorinated hydrocarbons (5), and manufacture of products resulting in hydrogen chloride as a by-product or air pollutant ( 5 ) . Analysis of low concentrations of HCl is a formidable experimental problem due to the chemical reactivity of hydrogen chloride in the presence of ambient moisture. Many techniques (6, 7) and instruments (8) have been used for the analysis of hydrogen chloride including selective ion electrode (9),chemiluminescence (IO), correlation spectrophotometry and coulometric methods (11). These techniques for HCl analysis, while useful for many analysis problems are not applicable for ultra-low doses of HC1 when a large number of monitoring stations are needed. Inexpensive collection methods employing pre-evacuated grab sample containers, condensation traps, adsorption tubes and/or plastic grab bags, commonly used for atmospheric gas analysis, are not suitable to HC1 analysis because of sample loss prior to analysis (8).This paper describes an inexpensive, simple collection technique for the quantitative analysis of ppm level concentrations of hydrogen chloride in moist air. EXPERIMENTAL Instrumentation. A microcoulometer (Dohrmann Model C 200B) was used to analyze the 1-10 p1 chloride samples. The microcoulometer consisted of an electrochemical titration cell, a differential amplifier, and recorder. The titration cell contained two pairs of silver electrodes. One pair monitored the titrant ion concentration (Ag+) and the other pair replenished the silver ion lost by reaction with chloride entering the cell. The coulombs required to restore the original silver ion concentration were recorded and related to the amount of chloride introduced into the cell. A typical calibration curve obtained by the direct injection of NaCl standard solutions into the cell is shown in Figure 1. Hydrogen Chloride-Air Mixture. A Matheson hydrogen chloride (42 ppm)-nitrogen mixture was further diluted with wet air in the flow system shown schematically in Figure 2. The effluent HCl concentration was typically 4.2 ppm. Wet air was produced by passing cylinder air (Airco) through a bubbler (B) containing distilled water. The HC1 and air lines were Teflon and Tygon, respectively. The wet air flow was 11. min:' while the HCl Nz flow was 100 cm3 min-l. The bulk of this mixture was vented to the atmosphere via a Teflon tee (T).The flow lines were passivated by flowing the HC1-air mixture for a t least 30 min before a collector tube (CT) was placed in the system. The HC1 sample was introduced into the collector tube by opening valve (V) to an air diaphragm pump (Universal Electric Co.) (PI. The flow (99.3 cm3 min-l) through the collector tube was controlled by the needle valve (NV) and measured by the flow meter (FM3). The concentration of HC1 in the gas stream was calculated from the measured flow rates. After dosing, the collector tube was set aside for later analysis. Collector Tubes. The I-m Pyrex collector tubes (1.04 f 0.01 mm i.d.) were cleaned using concd " 0 3 , detergent, and then rinsed with triple deionized water. The tubes were coated by repeated wetting with M NaN03 solution, drained in a vertical position, and allowed to dry a t ambient temperature overnight. The tubes were then
placed in a 68% relative humidity (RH)chamber (maintained by a saturated KN03 solution) for a t least 24 h prior to use. HCl Analysis. The end of the HC1-dosed collector tube nearest the source was dipped into triple deionized water. The height of the water column (via capillarity) (1.5-3.5 cm) was measured precisely. The tube was alternately tilted to allow the column to transverse the tube repeatedly. A microsyringe was inserted to withdraw a measured sample which was then injected directly into the microcoulometer.
RESULTS AND DISCUSSION The results for the analysis of 5 NaNOs-coated tubes each loaded with 189 ng of chloride are shown in Table I. This loading was achieved by passing 4.2 ppm HC1 (in moist air) through a collector tube for 20 s a t a flow rate of 100 cm3 min-I. The height refers to the water column prior to analysis. The chloride concentration in nglpl was determined using the calibrated microcoulometer. The amount of chloride on the tube was taken as the product of the measured concentration and the volume of water column. The relative accuracy of the technique is 0.1 and the relative standard deviation is 3%.For comparison, 15 ng of chloride were obtained for coated tubes not dosed with HC1 which represents a minimal background contribution. A standard deviation of 8 ng was obtained for these blank tubes. Further investigations of the effects of temperature, humidity, and time were made. Eight tubes were capped with rubber septums and stored in the laboratory from 2-14 days prior to dosing with HC1 (189 ng of chloride). Subsequent analysis showed no time dependence of measured chloride on the storage time of the capped, coated tubes. An average value of 176 ng chloride with a standard deviation of 24 ng was obtained. Thirteen tubes were placed in the laboratory uncapped
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Figure 1. Calibration of microcoulometer Error lines represent deviation from theoretical peak area calculated by using [CI-] = A ((4.4 X 102)/i2)where [CI-] is the amount of chloride introducedinto the cell, A is the area of the peak produced, and R and 4.4 X lo2 are factors based on the gain setting of the instrument
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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Figure 2. Diagram of flow system
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Table I. Reproducibility of the Technique for 189 ng Chloride on Coated Tubes Tube No.
Height.
43 47 44 57 10
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Figure 3. Chloride analysis of NaN03-coatedtubes as a function of dose time to moist air containing 4.2 ppm HCI at a flow rate of 100 cm3 min-'
(f 3%)
for 12-132 h prior to dosing with HC1. Again, subsequent analysis showed no time dependence of measured chloride on the storage time of the uncapped, coated tubes. An average value of 161 ng of chloride with a standard deviation of 46 ng was obtained. A post exposure experiment was done with seven tubes dosed with HC1 and stored uncapped for 2.5 h in three different locations (67% RH, 28 "C; 70% RH, 22 "C; 87% RH, 28 "C) to determine loss of any analyzable chloride. An average value of 258 ng of chloride was obtained. The standard deviation in this experiment was significantly more than for the previous ones (50 ng). On the other hand, no definite effect of relative humidity or temperature could be ascertained. Three tubes were dosed with HCl, capped, and then stored for several days. No loss of analyzable chloride was noted. An average value of 170 ng of chloride and a standard deviation of 32 ng was obtained. All the previous experiments were performed with 20-s doses of 4.2 ppm HC1 at 100 cm3/min which corresponds to 189 ng of chloride. Studies of the ability of the coated tubes to trap smaller and larger doses of HCl were performed and the results are cjhown in Figure 3. The analysis of 10-90 s doses of 4.2 ppm HC1 a t 100 cm3 min-l can be made. The curve in Figure 3 begins to level off a t 90 s indicating that the 1-m coated tubes have a analyzable chloride concentration of 400 ng maximum. This maximum value could presumably be increased by lengthening the coated tube. The demonstrated trapping ability of the NaNO3-coated tubes for HC1 may be based on the hygroscopic nature of NaN03. The water associated with the NaN03 coating would have a larger capacity for HC1 than an uncoated capillary. Such a mechanism would also be expected to be insensitive to the time, temperature, and humidity ranges studied as shown experimentally. The results obtained with NaN03coated tubes suggest their use as inexpensive field collectors
for ultra-low doses of HC1 with later chloride analysis ( > 5 ng/pl) by a microcoulometer or other instruments.
ACKNOWLEDGMENT The authors thank C. A. Potter for his assistance in the preliminary work with the microcoulometer. The advice of B. R. Emerson, Jr., G. L. Gregory, and C. H. Hudgins concerning the operation of the microcoulometer and the glassblowing services of A. Mollick are recognized. LITERATURE CITED Chem. Eng. News, 52 (49), 16 (1974). G. Ayrey, B. C. Head, and R. C. Potter, J. Polym. Sci., Macromoi. Rev., 8, l(1974). K. L. Paciorek, R. H. Kratzer, J. Kautamn, J. Nakahara, and A. M. Hartstein, J. Appi. Polym. Sci., 18, 3723 (1974). "Environmental Statement for the Space Shuttle Program", NASA, Washington, D.C., 1972. "Disposal of Organochlorine Wastes by Incineration at Sea. US. Environmental Protection Agency", Washington, D.C., July 1975, EPA-430/ 9-75-014. M. I. Brittan, N. W. Hanf, and R. R. Liebenberg, Anal. Chem., 42, 1306 (1970). M. G. Meador and R. M. Bethea, Environ. Sci. Techno/., 4, 853 (1970). "Hydrogen Chloride: A Survey of Methods for Detecting, Measuringand/or Monitoring HCI Resulting from Combustion of Propellants", JANNAF, Jan. 75, Safety and Environmental Protection Working Group, Environmental Protection Committee, Subcommittee on Instrumentation. T. G. Lee, Anal. Chem., 41, 391 (1969). L. Gregory, 8.R. Emerson, and C. H. Hudgins, "Evaluation of a Chemiluminescent Hydrogen Chloride and a N.D.I.R. Carbon Monoxide Detector for Environmental Monitoring", JANNAF Propulsion Meeting, Oct. 1974. R. L. Miller, "Hydrogen Chloride; A Study of Methods for Detection, Measurement and/or MonltoringHCI Resulting from Combustion of Propellants", JANNAF Propulsion Meeting, Oct. 1974.
RECEIVEDfor review March 17,1976. Accepted May 27,1976. Financial support for this work under NASA Contract NAS113175-1 including a graduate research assistantship for one of us (RRB) is gratefully acknowledged.
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