ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
8. Analytical Methods for Hydrogen Cyanide
8.1 Several analytical approaches have been used to measure hydrogen cyanide (HCN). These may be generally categorized as electrochemical, spectroscopic (infrared), colorimetric, and gas chromatographic. A review of many of these methods was recently published (56).

8.2 Electrochemical Methods:
8.2.1 General Description - Electrochemical techniques that have been used to measure HCN in fire gases include an amperometric method, two potentiometric techniques (including ion-selective electrodes), and differential pulse polarography. All of these techniques entail measurement of cyanide ion in solution.

8.2.2 Apparatus and Procedures:
8.2.2.1 In the amperometric method for HCN (12), the current flow between two electrodes is measured while HCN is being absorbed into the electrolyte solution.

8.2.2.2 The cyanide-ion selective electrode has been used widely for HCN measurement (12, 57-60). A silver/sulfide electrode, which measures cyanide ion indirectly, can be used. A "specific-ion meter" or expanded-scale pH meter is necessary for these electrodes.

8.2.2.3 Ion-selective electrodes have generally been used for intermittent analysis, where sample gas is bubbled into the impinger for a specified time (for example, 5 min). However, continuous analysis setups have been explored.

8.2.2.4 Standard titrimetric techniques (61, 62) can also be used for determination of hydrogen cyanide. In the absence of halide ions, cyanide can be titrated potentiometrically using (AgNo3).

8.2.2.5 Differential pulse polarography (63) can be used for time-integrated or grab sampling of cyanide.

8.2.3 Advantages and Disadvantages:
8.2.3.1 The amperometric technique permits continuous determination of gaseous HCN in the range from 0.1 ppm to greater than 100 ppm. Only H2S exhibits a major interference using this technique; however, this can be eliminated by using a solid lead carbonate scrubber.

8.2.3.2 The ion selective electrode methods are very sensitive; however, response is slow at low solution concentrations. Sulfide, iodide, bromide, and chloride interfere with the measurement of cyanide, when using the cyanide ion electrode (64). Certain interferences can often be eliminated by a change of the type of electrode used or by the addition of a masking agent to the solution.

8.2.3.3 Titrimetric procedures remain among the simplest techniques for a time-integrated analysis of cyanide in the absence of interfering ions. Interferences in differential pulse polarography include oxygen and compounds containing carbonyl groups.

8.3 Infrared and Colorimetric Methods:
8.3.1 Apparatus and Procedure:
8.3.1.1 Dispersive and Fourier transform (65) infrared spectroscopy have been used to measure HCN gas directly. Standard gas cells have been used; however, longer path length, nondispersive infrared instruments are better suited for low concentrations. Gas filter correlation techniques have also been proven useful for HCN analysis (see 7.4.2). A commercial analyzer is available.

8.3.1.2 Colorimetry has been successfully used in measuring low concentrations of HCN. The standard colorimetric procedures for cyanide are the picrate procedure (66), and the pyridine-pyrazolone method (58). A simple spectrophotometer is suitable for the colorimetric methods. Instruments have been modified to accommodate flow-through cells for continuous analysis (66).

8.3.2 Advantages and Disadvantages:
8.3.2.1 Potential interferences to infrared determination of HCN are acetylene, propane, and water vapor. This technique offers a means for continuous analysis of HCN in the gas phase, if interferences can be accounted for or eliminated.

8.3.2.2 Colorimetry is limited to low concentrations and therefore may require dilution of sample solutions. Colorimetric methods are generally time-consuming. Acetaldehyde, acrolein, acetone, and sulfur dioxide have exhibited interferences in the picrate procedure.

8.4 Gas Chromatographic Methods:
8.4.1 General Description - Gas chromatography can be used effectively to measure HCN in combustion products. Several different columns and types of detectors have been used to give the desired specificity and sensitivity.

8.4.2 Apparatus and Procedures:
8.4.2.1 A gas chromatographic method using a thermal conductivity detector (67) was developed for measuring high concentrations of HCN. However, this method had a lower limit of detectability of 0.3 volume %, which would require concentrating HCN for most fire test studies. A concentrator with a thermistor detector has been used (68, 69) to lower the detectable limit of HCN to 10 to 15 ppm per 2-mL injection volume.

8.4.2.2 A flame ionization detector (FID) is more sensitive and specific than either the thermal conductivity (TC) or thermistor type and has been used to measure HCN in a simulated hydrocarbon combustion atmosphere from 10 ppm to several hundred parts per million (70). Hydrogen cyanide in the pyrolysis products of nitrogen-containing fibers has also been measured (71).

8.4.2.3 A nitrogen-specific modification to an alkali-flame (or thermionic) detector gives greatly enhanced sensitivity for nitrogen-containing compounds. It has been used to monitor HCN in an animal exposure chamber containing CO and air (72). It was also used successfully for measurement of HCN during thermal degradation of nitrogen-containing polymeric materials (73). This detector has also been used to measure HCN trapped from combustion products using glass-lined stainless-steel tubes packed with p-2,6-diphenylphenylene oxide, a porous polymer that withstands high temperatures. For the analysis, the HCN is thermally released from the porous polymer (42).

8.4.2.4 In other studies on the analysis of HCN in the decomposition products of rigid polyurethane (35), the alkali flame detector was tuned with a sample containing normal hydrocarbons (methane through n-pentane up to 20 000 ppm) in order to achieve minimal interference from these species.

8.4.2.5 Another gas-chromatographic approach to HCN analysis has been to chemically react the HCN (74, 75) and then measure the resultant product with an electron-capture detector (76).

8.4.3 Advantages and Disadvantages:
8.4.3.1 An advantage of gas chromatography, for most combustion gas studies, is that it requires a small gas sample size. However, analyses are intermittent, rather than continuous, and sample elution times are often several minutes.

8.4.3.2 The flame ionization detector suffers from interference from low molecular weight organic species; and its response is affected by small changes in detector geometry, gas flow rates, etc. The thermionic detector minimizes the hydrocarbon and water interference problems. With a thermal-conductivity detector, water, CO, and CO2 can interfere with analysis of HCN (depending on the column used).

8.4.3.3 Hydrogen cyanide must be trapped in an impinger, for analysis by the electron-capture detector. Total treatment and analysis time is about 30 min.