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Anal. Chem. 1982, 5 4 , 470-473
Determination of Hydrogen Cyanide in Air by Ion Chromatography T. W. Dolzlne," G. G. Esposito, and D. S. Rinehart
U.S.Army Environmental Hygiene Agency, Aberdeen Proving Ground, Maryland 2 10 10
This paper descrlbes a new procedure for the determlnatlon of hydrogen cyanide (HCN) concentratlons In air. Airborne HCN is entralned in an lmplnger contalnlng aqueous aikall and subsequently converted to sodium formate; the latter Is quantitatively measured by ion chromatography. The coiiectlon procedure, reaction conditions, and chromatographic method are described. Interferences and the use of ion exchange reslns for sample cleanup were also investigated. The precision and accuracy of spiked and air samples are presented.
time. Another IC technique has been described by an instrument manufacturer (5) that appears to have good sensitivity and selectivity to cyanide, but it requires a special electrochemical detector which is not normally an accessory to the ion chromatograph. This paper describes a new IC procedure for HCN that is relatively uninvolved, provides good sensitivity, and is selective. The method is based on the hydrolysis reaction (6) that converts sodium cyanide to sodium formate and ammonia as shown by the reaction
NaCN Hydrogen cyanide (HCN) and cyanide salts are of considerable industrial importance and are encountered in a wide variety of industrial processes and in work areas contaminated by chemical decomposition. Because of the acute toxicity of HCN and its potential occurrence in the workplace, reliable and sensitive monitoring procedures are needed for worker protection. Bark and Higson ( I ) presented a thorough review of cyanide methods in use before 1963. A more recent compendium of sampling and analytical procedures can be found in the National Institute for Occupational Safety and Health (NIOSH) Criteria Document for hydrogen cyanide and cyanide salts (2) published in 1976. This exhaustive review provides over 75 references to articles describing the analysis of cyanides in a great variety of matrices including water, urine, blood, and air. These methods make use of titrimetry, colorimetry, atomic absorption, infrared absorption, and gas chromatography. Due to the presence of certain metals which may form complexes, other interfering anions, and a host of oxidizing and reducing agents, virtually every method proposed for cyanide analyses is subject to some interferences. Consequently, when the presence of complexing metals are known or suspected, it is generally necessary to isolate the cyanide by distillation from a strongly acidic solution where the metal complexes are disassociated to release HCN which is then absorbed in an alkaline solution. Because of ita relative ease of operation and reasonable accuracy, the ion-selective electrode (ISE) method has been widely accepted (3) and is the basis of the NIOSH sampling and analytical method for cyanides (2). In the NIOSH method, cyanide is collected in a bubbler containing 0.1 N NaOH and the sample is analyzed directly by use of an ion-selective (cyanide) electrode. As with most other cyanide methods, metals that form strong complexes with cyanide interfere with the ISE analysis, and special treatment with EDTA is required to eliminate this interference. In addition, sulfides will poison the cyanide electrodes and must be removed prior to ISE analysis. More recently, methods for cyanides utilizing ion chromatography (IC) have appeared in the literature. Pinschmidt (4) developed a procedure for weak acid ions including cyanide. This work demonstrated that these acids produce an increase in resistivity causing the formation of negative peaks which are generally in proportion to their concentration. In the case of HCN, however, some difficulties were encountered with reproducibility of both the detector response and retention
+ 2H20 2 NH, + HCOONa
Since an IC method (7) for formate had already been developed, this reaction appeared to offer good potential as a basis for providing a viable alternative to other analytical schemes for HCN analysis. Various parameters such as alkali strength, temperature, and reaction time were optimized to provide quantitative conversion of sodium cyanide to sodium formate. Tests were conducted a t concentration levels equivalent to 0.5, 1, and 2 times the threshold limit value (TLV) for a 40-L air sample. In this procedure, samples are collected in a midget impinger containing an alkaline absorbing solution where HCN is entrained as the sodium salt. The contents of the impinger are transferred into a vial with a Teflon-lined screw cap and heated overnight in an oven. By use of the prescribed test conditions, sodium cyanide is quantitatively converted to sodium formate which is subsequently measured by ion chromatography. Data reduction and statistical treatment of the results from the analytical method and the total monitoring procedure are presented. Test results were obtained from spiked laboratory samples collected from a 6920-L static test chamber. The method was validated over the range of 5-20 ppm using a 40-L air sample. EXPERIMENTAL SECTION Instrumentation. The chromatography was conducted with a Model 16 ion chromatograph (Dionex Corp., Sunnyvale, CA) equipped with a 100-pL sample loop. A concentrator/precolumn (3 mm X 150 mm) was used with a separator column (3 mm X 500 mm) containing a strong base anion exchange resin and a suppressor column (6 mm X 250 mm) containing a strong acid resin in the hydrogen form. These columns were previously described by Small et al. (8). The eluting solvent was 0.0050 M NazB4O7.Hz0with a flow rate of 138 mL/h. A sensitivity setting of 30 pmhos was routinely used, and peak height measurements were obtained to quantitate the amount of formate in the samples. Materials. Air samples were collected in midget impingers, modified with a bubbler (Ace Glass Inc.) and the reactions were carried out in culture tubes with screw caps and Teflon liners, 20 mm X 150 mm (Ace Glass Inc.). The Amberlite IR-120 strong cationic exchange resin (hydrogen form) was purchased from Mallinckrodt, Inc. It was washed in a Buchner funnel with 10 volumes of deionized water and brought to constant weight in a vacuum oven set at 90 "C and 15 in. of mercury. A DuPont portable pump, Model No. P-4000A (du Pont de Nemours, Inc.), was used to collect air samples from the 6920-L static chamber. The sodium formate, sodium cyanide, and sodium hydroxide were
This article not subject to U.S. Copyright. Published 1982 by the Arnerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
all reagent grade chemicals obtained from Fisher Scientific CO. Chamber Concentrations of HCN. Concentrations of HCN in air were established by placing the appropriateweight of sodium cyanide in an evaporating dish positioned inside the lower section of a heating mantle located inside of the 6920-L airtight chamber. After the chamber door was tightly sealed, 20 mL of 6 N sulfuric acid was added to the sodium cyanide through an access port. The chamber port was quickly sealed, a circulating fan was immediately activated to mix the chamber environment, and the evaporating dish and contents were heated to about 60 "C for 20 min. The circulatingfan was left on throughout the testing period. Procedure. Fifteen milliliters of absorbing solution (0.2 N NaOH) was added to the midget bubbler using a pipet. The outlet of the impinger was connected to the inlet of a liquid trap which was attached to a portable pump. (In the field, a cellulose ester membrane prefilter, 0.8 pm pore size, is placed immediatelybefore the impinger to remove particulates.) Most of the samples were collected at a rate of 1.5 L/min for 26.6 min. However, several samples were collected for time periods up to 55 min to determine the capacity of the collection system. After sample collection was complete, the bubbler stern was removed and gently tapped against the inside of the bubbler bottle to recover as much of the bubbler solution as possible. The contents of the impinger were transferred to a culture tube, and the bubbler and stem were rinsed with 2 mL of 0.2 N NaOH introduced with a pipet and combined with the sample solution. The culture tubes were tightly sealed with Teflon-lined screw caps. The samples were heated in an oven at 110 f 2 OC for 24 h and then removed from the oven, cooled to room temperature, and diluted to 50 mL in a volumetric flask with deionized water. This dilution resulted in a final alkali concentration of 0.068 N NaOH. The samples were analyzed by ion chromatography using the operating conditions previously described. About 2 mL of sample was flushed through the sample loop. The contents of the loop were then introduced onto the column and the chromatogram of formate was obtained. Peak height measurements of formate in the samples were compared to a calibration curve generated from standards containing 10,25, 50,75, and 100 pg/mL of formate ion in 0.068 N NaOH. The concentration of formate obtained from the calibration curve was back-calculated to the corresponding weight of HCN and the concentration of HCN in air determined. Calculations. pg of HCN in sample .- (pg/mL formate) X 0.60 X 50 where 50 = sample solution volume in milliliters
0.60 = factor for converting formate to HCN pg of HCN in samples concn of HCN in air mg/m3 = liters of air sampled
RESULTS AND DISCUSSION Initially, cyanide samples were reacted with different concentrations of NaOH to study the effect of alkali strength on the conversion of cyanide to formate. At the outset of the investigation, a problem emerged concerning the effect of alkali strength on the chromatography. As the strength of NaOH was increased, the retention time of formate decreased and spurious peaks appeared. These peaks interfered with the formate peak and were not reproducible when repetitive analyses were conducted on the same sample. In addition, the quantitative characteristics of the samples were altered by the presence of NaOH, making it virtually impossible to quantitate the formate peak when a high concentration of NaOH (above 0.2 N) was present. Various treatments were explored to circumvent problem8 associated with the presence of NaOH. Neutralization of base with acid produced little or no improvement and at the same time compounded the problem by the introduction of other anions which had to be accounted for during the IC analysis. Dilution with deionized water improved the chromatography but this treatment was not considered a viable alternative
471
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Figure 1. Chromatograms of sodium formate in different concentrations of NaOH (A) sodium formate In 0.1 N NaOH; (B) sodium formate in 0.2 N NaOH (C) sodium formate in 0.5 N NaOH; (D) sodium formate In 1.0 N NaOH.
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Flgure 2. Chromatograms showing effect of cationic resin "cleanup": (E) sodium formate in 0.5 N NaOH; (F) sodium formate in 0.5 N NaOH treated with ion exchange resin; (G) sodium formate In deionized water.
when dealing with samples containing high concentrations of base. The treatment that provided the best sample "cleanup" was the addition of strong cationic resin to the sample just prior to the chromatographic analysis. When cationic resin was added as purchased, the excess NaOH was removed but several extraneous peaks appeared having long retention times, thus, extending the analysis time. Furthermore, the sample volume increased when the wet resin was used. However, when the resin was washed and dried, it effectively removed the sodium ions thereby reducing the ionic strength of the solution without affecting the sample volume. Most important, the addition of resin provided a very much improved chromatogram free from artifact peaks and having a normal retention time for formate. Figure 1, chromatograms A-D, illustrates the effect of NaOH on the chromatographic separation of formate ion. As can be noted, NaOH produces spurious peaks and has a marked effect on the retention time and shape of the formate peak. Chromatogram F, Figure 2, was obtained after treatment of the sample shown in chromatogram E with the cationic exchange resin. The vast improvement resulting from this treatment is readily observed in chromatogram F. When the formate peak was quantitated, the results were equivalent to 98% of the formate ion in the sample. Chromatogram G was obtained from the sodium formate standard without the addition of NaOH. In order to determine the optimum NaOH concentration and reaction time for the cnversion of cyanide to formate, we heated 50 pg/mL solutions of cyanide in 0.1,0.2,0.5, and 1.0 N NaOH for 2,4, 6, and 24 h at 110 "C. This temperature was selected arbitrarily. As shown in Table I, the highest percent conversion in 6 h was 61%. For the 24-h reaction time 0.2,0.5,and 1.0 N solutions of NaOH provided quantitative
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Table I. Effect of Alkalinity and Time on Conversion of Sodium Cyanide to Sodium Formate at 110 "C NaOH % conversion to formate concn, N 2h 4h 6h 24 h 0.1 0.2'" 0.5b 1.0b
17
40 45 50 53
21 34 32
52 54 60 61
Table IV. Precision and Accuracy of Chamber Results'
80 98 101 101
a Diluted to 0.1 N NaOH and compared to standards in Treated with strong cationic resin, com0.1 N NaOH. pared to standards in deionized water.
a
% conversion to
teomp,
% conversion t o
formate'"
C
formate'"
28 90
105 110
95 100
85 95 a 0.2 N
NaOH.
concn of HCN solns tested, Pg/mL 14.6 ( 1 / 2 T L V ~ ) 29.3 ( 1 TLVb) 58.7 ( 2 TLVb)
av % std dev, recovery % 101
103 103
1.9 3.0 0.41
a Six samples at each concentration. air samples.
RSD, %
1.9 2.9 0.40
Based on 40-L
conversions to formate. Since the reaction could not be completed within an 8-h workday, intermediate reaction times were not evaluated. The 24-h reaction time was used throughout the rest of this investigation except for the study of interferences from metals. To study the effect of temperature on the reaction, we heated a known concentration of cyanide at various temperatures in the presence of 0.2 N NaOH. The results are shown in Table 11. One hundred percent conversion appears to occur between 105 and 110 "C. All additional samples presented in this paper were reacted at 110 "C. Based on the test results previously described, samples were collected in 0.2 N NaOH followed by dilution to 50 mL with dionized water, thereby precluding the need for treatment with ion exchange resin. This is the procedure recommended for routine industrial hygiene samples. However, to maximize the sensitivity of the method, if required, and to eliminate interferences that may occur in other matrices, i.e., wastewater, treatment with cationic resin prior to chromatographic analysis is suggested. In accordance with NIOSH guidelines (9) for validation of sampling and analysis methods, the analytical procedure was evaluated by spiking three sets of six samples with cyanide levels equivalent to 0.5, 1, and 2 times the current OSHA standard of 11 mg/m3 or 10 ppm. For a 40-L sample these concentrations correspond to 14.6, 29.3, and 58.7'pg/mL in 15 mL of absorbing solution (0.2 N NaOH). The spiked samples were analyzed according to the procedure previously described. As shown in Table 111,the precision and accuracy of the analytical procedure at the three concentration levels tested were exceptionally good. Table IV shows the precision and accuracy of results obtained on air samples collected from a 6920-L chamber. The percent recovery and precision at the 5-ppm level were not quite as good as the results obtained at
std dev, %
%
5 10 20
109 100 100
6.7 2.5 1.8
6.2 2.5 1.8
RSD,
Six samples at each concentration.
% conversion of cyanide to formate
a
Table 111. Precision and Accuracy of Spiked Samplesa
av % recovery
Table V. Interference from Metals
Table 11. Effect of Temperature on Conversion of Sodium Cyanide to Sodium Formate t$mp, C
concn of HCN in chamber, PPm
metal tested
24 ha
7 2 ha
144 ha
silver mercury copper cadmium iron nickel
