ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
7. Analytical Methods for Hydrogen Halides
7.1 General Comments:
7.1.1 The analysis of the hydrogen halide gases (hydrogen fluoride (HF), hydrogen chloride (HCl), and hydrogen bromide (HBr)) in combustion atmospheres has always been considered difficult, due primarily to the highly reactive nature of these species. The gases must be analyzed immediately or converted to a stable form to be analyzed at a later time (for example, dissolved aqueous solution in an impinger). The reactivity of these gases has led most workers to limit the length of sampling lines and to ensure that these lines are both heated and prepared from an inert material such as PTFE or glass (33, 34), as described in 5.7. Instead of in-line pumps, gas samples are generally pulled into the analytical device using a vacuum source (33, 35).
7.1.2 The techniques used for the quantitative detection of hydrogen halides (HX) can be classified into three broad categories: (1) "proton-detection devices", in which the HX is dissociated in solution and the activity of the hydrated proton is analyzed (for example, pH, conductometric); (2) "anion detector devices", in which the HX is dissociated in solution and the anion is analyzed (for example, ion-selective electrode, titrimetry, and ion chromatography); and (3) "hydrogen halide detection devices", in which the intact molecule is analyzed (for example, infrared and gas chromatography). These will be discussed in the following sections.
7.2 Proton Detection Devices:
7.2.1 General Description - One of the simplest ways to measure the concentration of acid gases in a combustion environment is to draw a portion of the gases into an aqueous solvent and measure the pH of the resulting solution, using a conventional pH electrode. This technique is not specific to any particular species (see below). Another approach involves the measurement in the change of conductance of a solution in which sample gases have been dissolved.
7.2.2 Apparatus and Procedures:
7.2.2.1 Two approaches have been described (12, 35) which use a microelectrolytic conductivity detector originally developed for use in gas chromatography (36). In the approach described by Herrington (12), filtered gases were continually pumped into the conductance cell and continuously monitored. Hileman (35) sampled gases through an 8-port gas-sampling valve, followed by discrete analysis using the conductance cell. The analysis time for a given sample was approximately 30 s.
7.2.2.2 For pH measurement, a research-quality pH meter should be employed.
7.2.3 Advantages and Disadvantages:
7.2.3.1 Simple pH measurement is prone to interferences from any other gases that can generate or remove protons on dissolving in water (that is, CO2, SO2, SO3, HCN, NO2). Thus, the pH electrode is best used to obtain a value of total acid gas concentration.
7.2.3.2 Many of the interference problems encountered in using pH electrodes are eliminated by using a nonaqueous electrolyte in the conductometric procedures. The conductometric apparatus can be used as a continuous monitor of the hydrogen halide gas; however, frequent calibration is recommended.
7.3 Anion Detection Devices:
7.3.1 General Description - The use of ion-selective electrodes (for fluoride, chloride, or bromide) has become increasingly popular for the analysis of hydrogen halides. These types of analyses can be conducted in either a continuous mode (33, 35, 37) or a batch mode (38, 39). Ion chromatography and titration procedure are also available for halide ion analysis.
7.3.2 Apparatus and Procedures:
7.3.2.1 Combustion gases may be continuously bubbled into a solution containing an ion-selective electrode and the anion concentration measured while it is constantly increasing (33). The rate of production of hydrogen halide is determined by differentiating the concentration-versus-time curve. A batch analysis may involve obtaining a gas sample in a syringe containing the dissolving solution (38, 39) or a single time-integrated sample in an impinger solution.
7.3.2.2 Recent advances in ion chromatographic methods have permitted separation of anions (40, 41) with subsequent conductivity measurement of the eluted species. The carbonate anion and various organic acids are interferences for chloride with the conductivity detector if they are not well separated chromatographically. The silver/silver chloride (Ag/AgCl) detector is specific for chloride and bromide with a very low sensitivity for carbonate and other anions. The fluoride detector is a specific detector for fluoride in combustion gas samples (42, 43). The ultraviolet (UV) detector for ion chromatography is sensitive to all anions. With indirect photometric chromatography light-absorbing eluent anions enable the sample anions to appear as negative peaks in the absorbance record (44).
7.3.2.3 A variety of methods involving titration of the hydrogen halides in municipal drinking water (43, 45) have been developed but have not been extensively applied to the analysis of combustion gases (46).
7.3.2.4 Collection tubes containing dry soda lime have proven to be useful for sampling HCl from combustion atmospheres (10). The test atmosphere is sampled over a time period, such as 3 or 5 min, but the interval can be shortened if the concentration of HCl is high. Consecutive samples can be obtained in order to provide a concentration/time plot. The chloride is extracted from the soda lime by water and is analyzed by titration.
7.3.2.5 A continuous analyzer for HCl has been described (47). It employs readily available commercial "stat" titration equipment. The method monitors HCl concentration by continuously titrating chloride ion in an impinger with silver nitrate (AgNO3).
7.3.3 Advantages and Disadvantages:
7.3.3.1 Ion selective electrodes avoid many of the problems encountered in other HX analyses, since they are ion specific. Anion interferences such as cyanide and sulfide can be minimized with proper consideration of the interfering species. Cyanide and sulfide degrade the electrode membrane, slowing the response and reducing the accuracy. Slight corrosion effects on the electrode can be removed by repolishing. Bromide interferes with the chloride ion electrode; however, chloride does not interfere with the bromide ion electrode, except at very high concentrations of chloride.
7.3.3.2 Ion chromatography may be particularly useful when strong interference from anions causes other techniques to fail. However, this technique has the disadvantage of being noncontinuous.
7.3.3.3 Collection tubes containing dry soda lime can often be used to sample from locations which would be difficult to sample from using solution absorbers or other techniques. They are compact and easy to handle and have high absorption efficiency. Care must be taken to avoid breakthrough due to too high gas-flow rate or high HCl concentration or the tendency to plug up in extremely smokey atmospheres.
7.3.3.4 There are insufficient data yet to accurately describe the advantages and disadvantages of the "stat" titration method; however, it has the potential to be a versatile continuous method for HCl with few problems from smoke particulates or liquids and requiring no calibration gases. Its disadvantages include interferences from other halide and cyanide gases and efficiency of absorption of the HCl.
7.4 Hydrogen Halide Detection Devices:
7.4.1 Hydrogen halides can be analyzed with gas chromatography, however, this is not commonly used because of difficulties with corrosion and poor analysis caused by problems with poorly formed peaks (48).
7.4.2 The gas filter-correlation analysis technique has been developed for a number of gases (49-52). Commercial instruments for HCl are available. The technique also lends itself to in situ measurements, specifically, a beam passed across an exhaust stack section (53, 54). A commercial instrument suitable for full-scale stack measurements is available. Gas-filter correlation analyzers can be designed to minimize the problem of instrument drift. Care must be taken to avoid precipitating the HCl as an aerosol; limited measurements indicate that this is unlikely if the relative humidity in the measuring system is kept below 70 to 80 % (55).
