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Anal. Chem. 1991, 63, 1008-1011
Trace Level Gaseous Hydrochloric Acid Measurement Standard Based on Sublimation of Ammonium Chloride Per F. Lindgren Department of Analytical Chemistry, University of Umeci, S-901 8 7 Umeci, Sweden
The subllmatlon equlllbrlum of ammonlum chloride salt has been utilized to generate trace levels of gaseous hydrochlorlc add. A perfluorinated strong catlon exchanger In the acld form was placed In the gas flow after the salt to remove the slmultaneously produced ammonia. At 308.15 K, the equlllbrlum concentration of HCI In the gas contactlng the salt Is 26.3f 0.2 ppbv. Experlmental data on the equlllbrlum concentratlon of HCI above ammonium chlorlde compare well with theoretical calculatlons at three dlfferent temperatures. The equlllbrlum concentration of HCI over the ammonlum salt Is predlcted by the exponentlal equatlon, ppbv HCI = 1.2322 X 10-14e0.1144r, In the Interval 288.15-308.15K wlth a preclslon of f0.3 ppbv. The HCI source was used In a dynamlc gas dllutlon system for generatlon of 0.10-1.0ppbv HCI wlth an estimated accuracy of f4% and a preclslon of f3% at the 0.10ppbv level. The performance of the salt subllmatlon source was tested by using a dlffuslon scrubber-Ion chromatographlc Instrument.
INTRODUCTION Studies on atmospheric pollution are today largely focused on measurement of trace levels of pollutants in pristine areas to gain understanding of natural sources, background levels, and global transport patterns. These measurements require dedicated analysis techniques with high specificity and detection limits in the low pptv range to produce useful data. A prerequisite for the successful development and application of such analysis techniques is the availability of adequate and accurate measurement standards for the species of interest. These standards are needed for characterization of new techniques with respect to detection limits and linear range and of sampling and storage procedures and for the establishment of accuracy through intercalibrations. Accurate measurement standards are also of importance in relative analysis techniques to reduce bias in results through periodic calibration of instrumentation and to ensure that results can be used for comparison decades from now. Instrument calibrations should ideally span the actual concentration range of the analyte and be performed with a matrix that mimics the actual sample to ensure that no bias is introduced through systematic errors in sampling, storage, and analysis (I). Many of the gaseous analytes of interest in atmospheric pollution studies are reactive or otherwise labile and should ideally be generated as needed from a stable source. Permeation and diffusion methods can be used to generate trace levels of gaseous analytes in dynamic gas dilution systems (2-4). Dilution of gases stored in gas cylinders at high concentrations is another commonly used method for the generation of trace levels of measurement standards (5). With gas cylinders, special precautions must be exercised to avoid the instability and adsorption problems related to static dilution systems (5, 6). Measurement standards of gaseous hydrochloric acid have been generated by dilution of certified cylinder gas (7) and with a permeation method (8). Adsorption problems and high 0003-2700/91/0363-1008$02.50/0
dilution factors make it difficult and impratical to use cylinder gas for generation of trace levels of HC1. There is also concern about the stability and accuracy of reactive gases like HC1 in gas cylinders (7,9). The permeation method shows flexibility in generated concentrations and can be used for generation of trace levels of HC1. However, unlike other permeation methods, which in favorable cases could be called primary standards, this permeation method itself requires calibration against an accurate standard (8). Ammonium chloride can be considered a potential source of both ammonia and hydrochloric acid due to the reversible equilibrium between the salt and its gaseous precursors. Despite a number of studies on the atmospheric particle-gas equilibrium ( l O - l Z ) , the salt has not yet been used under controlled conditions as a calibration standard for ammonia or hydrochloric acid. This paper reports on a novel generation source for trace levels of gaseous hydrochloric acid and its characterization with a recently developed diffusion scrubber-ion chromatography (DS-IC) instrument (13).
EXPERIMENTAL SECTION Gas Generation System. Zero air was produced by passing c o m p r e d house air through a 1.6-pm glass fiber filter (Whatman, Maidstone, England) and four columns (35 X 800 mm) filled with soda lime (BDH Chemicals Ltd., Poole, England), silica gel (Kebo, Stockholm, Sweden), Amberlyst 15 cation exchanger in the H+ form (Serva, Heidelberg, West Germany), and activated Charcoal (Kebo) in that order. The column assembly was followed by a second 1.6-pm filter to remove particles that may have been released from the columns. The relative humidity of the zero air after passing through the column train was less than 5%. The HCl gas generation source consisted of a 25-cm-long 10mm-i.d. poly(tetrafluoroethene) (PTFE) tube packed with 8.5 g of ammonium chloride (p.a.;Merck, Darmstadt, West Germany), followed by a 10-cm length of an identical PTFE tube packed with 10 g of Nafion 501 perfluorinated cation exchanger (Du Pont, Wilmington, DE) in the H+ form, Figure 1. The tubes were furnished with disks cut from 5-gm pore size PTFE membranes (Gore Svenska AB, Goteborg, Sweden) to keep the materials separated and in place. The entire tubing assembly was immersed in a cryothermoetating bath (HETO, Birkeriid, Denmark) capable of keeping the temperature within h0.05 "C of the set value. The zero air was thermostated by passage through a coiled 2-m length of '1,-in.4.d. copper tube placed in the thermostating bath and situated immediately before the packing. The flow of zero air through the generation source was controlled with a calibrated 0-1 standard liters per minute (SLPM) mass flow controller (Tylan GmbH, Eching, West Germany). Impingers constructed from fluorinated poly(ethene-copropene) (FEP) centrifuge tubes (Cole Palmer, Chicago, IL) were used for collection of the produced HCl(g). Water (30.0 mL) was used as the absorbing solution. The collection efficiency was >99.9% at the flow rates and concentrations used in the experiments, 15-682 mL/min and 2.4-27.0 ppb, respectively, as determined by using two impingers in series. The analysis of the impinger solution was performed with a QIC ionchrom analyzer (Dionex, Sunnyvale, CA) against standard solutions of chloride ion. Standards containing 2-10 pM chloride ion as NaCl were prepared weekly from a 0.100 M stock solution. Water was purified by using Milli-Q equipment (Millipore, Bedford, MA) and had a resistivity >16 MQ cm. 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
I
H
I
I c 0
Flgure 1. Schematic diagram of the sublimation source and the dynamic gas dilution system. A, zero air puffykrg cokrmns; B, mass Row controller for the generation gas flow; C, mass flow controller for the diluting gas flow; D, thermostating coil; E, sublimation source; F, cryothermostating bath: G, to sampler; H,ammonium chloride salt; I, cation exchanger; J, porous membrane disks.
After characterization, the generation source was used in a dynamic gas dilution system, Figure 1,for generation of sub-ppbv levels of hydrochloric acid. A dilution flow of 2-30 slpm zero air, controlled with a calibrated mass flow controller (Tylan), was introduced in a tee after the generation source to give the desired concentrations. All gas-contacting parts of the dynamic gas dilution system were made of PTFE. Quantification of ammonia in the zero air and in the gas produced by the generation source was performed by impinger collection in 1 mM HCl followed by derivatization with ophthalaldehyde and spectrophotometric determination (24). DS-IC Instrument. The DS-IC instrument used to test the generation source has recently been presented in a paper in this journal (23)and will not be described in detail here. The diffusion scrubber used in the present work (available through Analytek, Umeti, Sweden) was, however, modified with PTFE end fittings where the gas inlet had been slanted 45’ to minimize particle deposition.
RESULTS AND DISCUSSION Sublimation of Ammonium Chloride. At temperatures above 243 K and a t relative humidities below the deliquescence point, solid ammonium chloride exists as a crystalline phase in equilibrium with its gaseous components (10):
NH,Cl(s)
F!
1009
HCl(g) + NH&)
This sublimation reaction can be used to generate trace levels of hydrochloric acid or ammonia in a thermodynamically determined way. Equations derived from accepted thermodynamic and experimental data, relating the equilibrium concentration of ammonia and hydrochloric acid above ammonium chloride to the absolute temperature, have been presented in the literature (10, 15). Preliminary tests showed that the sublimation equilibrium had to be disturbed immediately after the generation source to prevent the ammonium salt from depositing on the inner walls of the outlet tube. A perfluorinated cation exchanger in H+form was therefore placed downstream from the ammonium salt to remove produced ammonia. Sampling the gas after the source showed that the cation exchanger was >99% efficient in removing simultaneously produced ammonia from the gas stream. This is actually much more than is required, since all it takes is to lower the ammonia concentration below the equilibrium level a t the ambient temperature after the source. If the source is operated substantially below ambient temperature, this ammonia removal step may be unnecessary, provided the presence of ammonia in the final gas mixture is acceptable. There is some controversy in the literature about the presence of undissociated ammonium chloride in the gas phase. Theoretical calculations predict the degree of disso-
200 400 600 Gas flow (mumin)
800
Flgure 2. Concentration of HCI as a function of the volumetric gas flow rate through the source. [m] 308.15 K; [O] 298.15 K.
ciation of the gas-phase molecule to be between unity (16,17) and 0.85 f 0.1 (18).A mass spectrometry study showed less than 0.2% undissociated ammonium chloride in the temperature range 335-485 K (19). No attempt was made in this study to positively identify gas-phase ammonium chloride. The sum of ammonia and any ammonium chloride present in the gas stream after the perfluorinated ammonia scrubber was
