A System for Continuously Monitoring Hydrogen Chloride Concentrations in Gaseous Mixtures Using a Chloride Ion-Selective Electrode T. G . Lee National Bureau of Standards, Washington, D.C. A METHOD has been developed for rapid and continuous measurement of hydrogen chloride concentration in gas and aerosol mixtures. The technique resulted from a study on the generation and loss of volatile decomposition products from polyvinyl chloride (PVC) and other commonly used chlorocarbon polymers under simulated fire conditions. In this study, it was necessary to monitor the HCl concentration during its generation within a closed chamber. Because the change of aerosol HCI concentration is rapid during combustion, periodic sample collection and subsequent gravimetric analysis were not convenient. Gunther, Miller, and Jenkin ( I ) used a continuous potentiometric method based on the analysis of chloride ions from absorbed HCl in solution by means of a silver-silver chloride electrode. The more rapid method reported here also takes advantage of the high solubility of HCl in water and in addition utilizes the sensitivity and range of the recently available chloride ionselective electrode. In this study, measurement of chloride ions in solution may be considered equivalent to measurement of hydrogen chloride for it is the only significant watersoluble chloride in the decomposition products of PVC-type polymers (2). (1) F. A. Gunther, T. A. Miller, and T. E. Jenkin, ANAL.CHEM., 37, 1386 (1965). (2) Samuel Madorsky, “Thermal Degradation of Organic Polymers,’’ Interscience, New York, N. Y . , 1964, p 160.
EXPERIMENTAL
Apparatus. Figure 1 is a schematic diagram of the apparatus. The detector cell consists of a standard calomelpotassium chloride reference electrode with fiber junction and a chloride ion-selective electrode of the liquid-liquid membrane (ion exchange) type. To avoid possible contamination of chloride ions from the reference electrode, a salt bridge using a 1.OM potassium fluoride solution connects the cell and the reference electrode. The cell is about ten times less sensitive to the fluoride than to the chloride ions. The salt bridge junction is a plug of porous glass 2-mm 0.d. and 4 mm long (3). Any leakage of the junction electrolyte is negligible relative to the liquid flow rate in the cell. A two-barrel push-pull syringe pump, driven by a synchronous motor and gear train, withdraws the solution from the bottom of the detector cell and also delivers distilled water to the top of the scrubber. The gas scrubber is a bead column consisting of a 13-mm i.d. glass tube packed to a height of about 30 mm with 2-mm glass beads whose surfaces have been roughened by hydrofluoric acid treatment. The sample inlet tube, located 25 mm below the top of the packing in the scrubber, has a 1-mm i.d. tip. A needle valve, placed between a vacuum pump and a rotameter, is used to control the rate of gas sampling through the scrubber. Following the scrubber are a plenum, filter, drier, and rotameter, which stabilize and monitor the sample flow rate. (3) R. A. Durst,J. Cliern. Educ., 43,437 (1966).
Figure 1. Continuous recording aqueous chloride ion analyzer for monitoring HCI concentration in a gaseous mixture
VOL. 41, NO. 2, FEBRUARY 1969
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Figure 2. Time response of the system to a step change in HCI concentration in gas sample measured as absorbed chloride ion in solution (a) 0 to 1100 ppm, (6) 1100 to 0 ppm Reagents. Standardized aqueous HC1 solutions were prepared by simple dilution using a gravimetrically standardized stock solution. Gaseous HC1 mixtures were prepared dynamically by metering HCI gas from a n airtight syringe pump into a dried air stream of measured flow rate. Procedure. In this method a pump draws the gas sample (including the aerosol) at a controlled rate through the gas scrubber, where HCI is absorbed by a counter-current stream of water. The resulting solution then flows to the detector cell. The potentiometric output of the cell, which is a function of HCI concentration in the sample, is recorded continuously. Water is injected into a bead column as rapidly falling, but separate, droplets. As a result of equal delivery and withdrawal rate, the volume of water retained in the scrubber for absorption remains constant during the run. This volume is adjusted to provide optimum absorption efficiency and response time, and to avoid entrainment of water by the gas sample beyond the packing. As the solution being analyzed must pass through the scrubber and a considerable length of tubing, the cell is maintained essentially at room temperature. The total liquid volume retained in the system (within the scrubber, 25 cm of connecting tubing, and the cell) is normally only 0.8 cm3. Pressure drop from the sample inlet to the syringe pump is about 2.5 cm of water. RESULTS AND DISCUSSION
The cell was calibrated by replacing the water in the system with standardized HC1 solution of known concentrations under normal liquid and gas sample flow rates of 1.1 cm3/min and 100 cm3/min, respectively. Air is substituted for the sample. This procedure automatically compensates for errors resulting from streaming potential and pressure effects. Equivalent gas-phase HC1 concentrations corresponding t o the standardized HCl solutions used in the calibration were calculated using the normal flow rates and assuming total absorption. The calibration curve derived from the calculated gas-phase HCI concentrations between 20 and 6000 ppm and the measured potentiometric outputs of the cell showed the usual Nernst straight line on a semilogarithmic plot. Additional calibrations using mixture of air with known HC1 gas concentrations provided data on the absorption efficiency of the scrubber and the time response of the system. Deviation of these data from the curve were within 575 of the readings, over the same range of 20 to 6000 ppm HCI. 392
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Figure 3. The generation and dropout of smoke and HCI (measured as chloride ion) when a PVC-vinyl acetate specimen is thermally decomposed in a closed chamber The detectable limit of the system is on the order of 20 ppm HC1 in the gas phase using the given flow rates. The senhitivity is such that a tenfold change in HCl concentration resulted in a potential change of about 60 mV. The electrode is not responsive to the undissociated and complexed chloride, but it shows a relatively high sensitivity to chloride ions and some interfering ions (such as perchlorate, iodide, nitrate, and bromide). The choice of this electrode over a solid-state one was governed by the type of interfering ions in the aerosol sample. Figure 2 show the time-response curves for the complete system to a step change in HCl concentration from 0 to 1100 ppm (curve a) and from 1100 to 0 ppm (curve b). The system takes about 1.5 minutes to reach 99% of its equilibrium value. Figure 3 is a typical example of the time dependence of the generation and dropout of smoke (measured by optical density) and HCI gas during the nonflaming decomposition of a vinyl chloride-vinyl acetate copolymer sheet. The 7.6 X 7.6-cm specimen, weighing 5.0 grams, was subjected to a thermal irradiance of 2.5 W/cm2, in a 0.51 m 3 closed chamber (4). This simple, rapid method, using an ion-selective electrode detector and a gas scrubber, appears to have suitable time response and dynamic range to follow the change of HCI concentration continuously under the conditions of the experiment. It seems likely that other types of ion-selective electrodes, such as those for bromide, cyanide, etc., could be used in a similar way for analysis of water-soluble components in gas or aerosol samples. In other applications, however, interfering anions and the total ionic strength of the sample solution should be considered when using the electrode. RECEIVED for review September 16, 1968. Accepted November 4, 1968. (4) D. Gross, J. J. Loftus, and A. F. Robertson, “Fire Test Methods-Restraint and Smoke 1966,” STP 422, American Society for Testing and Materials, Philadelphia, Pa., 1967, p 166.
