Envhon. Sd. T & d .
1891. 28, 1394-1396
Acetylene Interference with Hydrogen Cyanide Determination by Selective Ion Electrode Wen
C. Lln and Jost 0. L. Wendt'
Department of Chemical Engineering, University of Arizona. Tucson, Arizona 85721 Jerald A. Cole and Jack E. Rumbaught
Energy and Environmental Research Corporation, 18 Mason, Imine, California 92718 Introduction The determination of hydrogen cyanide in combustion processes is important for the understanding of fixed nitrogen chemistry and molecular processes involved in both the formation and the control of nitrogen oxides (1-7). During the cornbustion of coal and some oils, hydrogen cyanide is produced from fuel nitrogen compounds and later oxidized to form nitrogen oxides (6, 7). In addition, hydrogen cyanide is formed in the first step of the so-called "prompt" mechanism ( I ) ,which occurs in virtually all fossil-fuel combustion where acetylene and other hydrocarbons are present as well. During experiments with counterflowdiffusion flames, the measured concentrations of hydrogen cyanide were significantly higher than could be explained hy known chemical processes. Also, the concentration profiles of hydrogen cyanide were similar in shape to those of Cz hydrocarbons. Previous measurement of hydrogen cyanide concentrations during gas reburning studies suggested that acetylenic species might be linked to hydrogen cyanide formation (8,9). In addition, alkynes are known to form insoluble heavy metal compounds such as silver acetylide (11). All of these observations suggest that acetylene, which is soluble in neutral and basic aqueoussolutions (10, ll),produces apositive interference in the selective-ion electrode (SIE) determination of hydrogen cyanide through reaction with the silvermembrane electrode. It is of interest to note that hydrocarbons are not included among the manufacturerprovided list of species that would potentially interfere with the SIE determination of the cyanide ion (12). In order to identify the suspected source of bias in the determination of hydrogen cyanide concentration in methane-fueled diffusion flames, several simple experiments were performed. The experiments measured the interferences produced in the SIE by acetylene, ethylene, ethane, and methane. This is of considerable significance since these hydrocarbons, acetylene in particular, are ubiquitous in practical combustion of most fossil-fuels: they occur at significant concentrations under fuel-rich conditions during the combustionof pulverized coal where high hydrogen cyanide concentrations have been reported (6,7).Therefore,anyanalysisinvolvingsimilarprocedures for the determination of hydrogen cyanide in combustion systemshas the potentialriskof bias. (Forexample: South Coast Air Quality Management District Method 202.1.) Since there are apparently no literature references available regarding acetylene interference with hydrogen cyanide determination using SIE, this study is important in identifying the potential for analytical bias. Instead of examiningthe detailed chemistry behind the interference, *Current address: Q-Laboratories, 4129 E. La Palma Ave., Anaheim, CA 92807. 1394
Endmn. Scl. Technd.. Vd. 28, No. 7. 1994
U W
mA - _
Omn 901
Ion analyzer
Temperature Controller
Figure 1. Schematic of experimentalapparatus used 10determine Um interference of acetvlene wkh the cyanide-selective ion electrode.
this study was intended mainly to report the observed phenomena and to suggest a simple procedure to identify and possibly eliminate the interference. Experimental Section Figure 1shows the experimental apparatus used for the present study. Compressed nitrogen and acetylene (or other hydrocarbons) were mixed at controlled ratios in a gas-flow control system comprised of two rotameters and a 1-Lfilter flask. The majority of this mixture was vented while in most experiments 10 mL/min was metered into a gas scrubber containing 100 mL of deionized water. Sampling was conducted for 20 min. Immediately after sampling, the scrubber water was transferred to a 150-mL beaker, and 1mL of 10 N sodium hydroxide was added. The solution was stirred and maintained at 25 "C. An Orion 95-06 cyanide-selective ion electrode and 901 ion analyzer were used to determine the interference. In other experiments, the acetylene was introduced without nitrogen dilution at a rate of 5 mL/min at various sampling times. The interference was confirmed in a second laboratory following a slightlydifferent and independently-developed approach. These experiments were conducted using a gas scrubber and SIE essentially similar to those described above. However, acetylene was produced by the titration of calcium carbide in order to avoid any additional interference resulting from acetone Contamination. In these experiments, a large excess of acetylenewas supplied to saturate the solution prior to determination of the interference. Results and Discussion Acetylene produced apositiveintmferenceinthecyanide SIE. Methane, ethane, and ethylene did not. Figure 2 W13.930X194/09261394$04.50/0
0 1994 A W n Chsm(cal Soclsty
Table 1. Data Used To Calculate Interference of Dissolved Acetylene with Cyanide-Selective Ion Electrode' CzHZ(g), CzHz(aq) at equil,* apparent HCN(aq), apparent HCN(g), total CzHz delivered," dissolved CzHz at equil, 10-8mol 1o-B mol PPmv 10-5M interferenceC 104 M PPmv 0.2 20 f 0.4 0.4 0.8 1.6 f 0.0 50 2.0 1.0 120 f 2.4 2.0 1.0 9.7 f 0.2 10.2 250 1.1 11.4 11.6 f 0.2 1.0 143 f 2.9 2.3 280 1.9 1.0 234 f 4.7 3.8 460 18.8 18.9 f 0.4 1.0 342 f 6.8 5.4 2.7 660 26.9 27.6 f 0.6 990 40.4 41.5 f 0.8 1.0 515 f 10.3 8.1 4.0 1350 55.1 51.2 f 1.0 0.9 634 f 12.7 11.0 5.5 0.9 724 f 14.5 13.2 6.6 58.5 f 1.2 1620 66.1 0.8 786 f 15.7 16.7 8.3 63.5 f 1.3 2040 83.8 c
Acetylene was diluted with nitrogen. The equilibrium concentration was calculated based on Henry's law constant of 24.5 L.atm/mol. The interference was defined as "mol of HCN indicated/mol of CzHz dissolved". d Gas flow of 10 mL/min for 20 min. f
I
I
I
I
1 200
6 150 4 -
100
2 -
0
50
1 0
I
I
I
I
I
20
40
60
80
100
0
0
electrode obtained by bubbling pure acetylene into 100 mL of water at a rate of 5 mLlmin. The saturation concentration under the experimental condltions is 0.0408 mollL.
shows the results of experiments where pure acetylene was metered into the gas scrubber at a rate of 5 mL/min for a total time ranging from 15 to 75 s. Acetylene has a solubility in water of about 1 mL of gas/mL of H20 at room temperature (IO). At the maximum sampling time of 75 s, therefore, the amount of acetylene delivered to the scrubber was still well below saturation relative to a gasphase partial pressure of 1 atm. A positive response produced by acetylene was indicated as CN-, and increased, although not linearly, with sampling time or equivalently with the amount of acetylene delivered. In a separate series of tests, dilute mixtures of acetylene in nitrogen were metered into the scrubber overa 20-min period. Since the room temperature aqueous solubility of acetylene implies a Henry's law constant of about 24.5 L.atm/mol, the amount of acetylene used in each of these experiments was about twice the amount needed to saturate (with respect to the gas composition) the solution. (See the last two columns of Table 1.) An approach to relative saturation may therefore be assumed. The experimental results and estimated interference based on relative saturation assumption are shown in Table 1. Allowing for error in the estimation of the Henry's law constant, these results indicate that at low concentrations of dissolved acetylene (2.0-83.8 X lo4 MI, the interference with the selective-ion electrode is nearly 1mol of CN- measured/ mol of dissolved C2H2. Since the silver ion is present in the solution during cyanide analysis by SIE, the dissolved acetylene may well
200 300
400
500
600
700
800
Time, sec.
Sampling t i m e , s e c . Flgure 2. Interference of acetylene with the cyanide-selective ion
100
Flgure 3. Change in response of the cyanide selective ion electrode whlle purging the scrubbing solution with dry nitrogen at a rate of 500
mLImin.
form insoluble silver acetylide and interfere with normal analysis. A reasonable approach to avoid this interference is to eliminate dissolved acetylene before proceeding to the SIE analysis. Acetylene, which has a much lower solubility than hydrogen cyanide, may be stripped from solution without affecting the hydrogen cyanide concentration. An experiment was run in which a 10-foldexcess of pure acetylene was sparged through a gas scrubber. In this experiment, however, a 0.1 N sodium hydroxide solution was used as the scrubbing solution instead of water. After introduction of the acetylene, the SIE was placed in the scrubber solution, and a nitrogen purge was started at a rate of 500 mL/min. Readings from the SIE were taken every 30 s for a total of 12 min. Results of this test, shown in Figure 3, indicate that about 95% of the acetylene was stripped from the solution by this procedure.
Summary The interference of acetylene with the cyanide SIE has been demonstrated through relatively simple experiments. Formation of silver acetylide is believed to cause this interference. Recognizing this interference is significant because both hydrogen cyanide and acetylene are present in many combustion systems where the hydrogen cyanide determination is considered important. Hydrogen cyanide emissions are also regulated in many cases, and this interference may bias regulatory decisions regarding them. The interference is dependent on the acetylene concentration in the gas phase. Since acetylene has a much higher Envlron. Sci. Technol., Vol. 28, No. 7, 1994
1395
Henry’s law constant than hydrogen cyanide, it can be readily stripped from solution with dry nitrogen, producing a significant change in electrode reading in a relatively short time. In cases where acetylene is known to be present in high concentrations, alternative methods for hydrogen cyanide determination are recommended. Acknowledgments
Portions of this work have been supported by the Exploratory Research Program of the Electric Power Research Institute,Palo Alto, CA, under Contract RP800516 and by the Energy and Environmental Research Corp. Literature Cited (1) Fenimore, C. P. 13th Symposium (International) on
(2)
(3) (4) (5)
1386
Combustion; The Combustion Institute: Pittsburgh, 1971; pp 373-380. Morley, C. Combust. Flame 1976,27, 189-204. Haynes, B. S. Combust. Flame 1977,28, 113-121. Fenimore, C. P. Combust. Flame 1976,26, 249. Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sei. 1989, 15, 287-338.
Environ. Scl. Technol., Vol. 28, No. 7, 1994
(6) Bose, A. C.; Wendt, J. 0.L. 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 1127-1134. (7) Mereb, J.;Wendt, J. 0.L. 23rd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 1273-1280. (8) Cole, J. A.; Kramlich, J. C.; Lanier, W. S. Kinetic Modeling of NO, Formation and Destruction and Combustibles Burnout; Final Report, Contract EPA 68-02-4247,task 2; Energy and Environmental Research Corp.: Irvine, CA, 1989. (9) Green, S. B.; Chen, S. L.; Clark, W. D.; Heap, M. P.; Pershing, D. W.; Seeker, W. R. Bench-Scale Process Evaluation of Reburning and Sorbent Injection for In-Furnace NO,/ SO, Reduction; EPA-600/7-85-012;NTIS PB85-185890 U.S. EPA: Washington, DC, 1985. (10) The Merck Index, 10th ed.; Windholz, M., Ed.; Merck & Co.: Rahway, NJ, 1983; p 13. (11) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 3rd ed.; Allyn and Bacon: Boston, MA, 1973; Chapter 8. (12) Instruction Manual, Cyanide Ion Electrode Model 94-06; Orion Research: Cambridge, MA, 1982. Received for review February 1, 1994. Accepted March 21, 1994.
