Determination of cyanide ion by homogeneous catalysis and gas chromatography

Chimica Acta. 135 (1982) 69-75 Ekevier Scientific Publishing Company, Amsterdam Analytica

DETERMINATION OF CYANIDE AND GAS CHROMATOGRAPHY

MAURI A. DITZLER* Department

and FRANCIS

of Chemistry.

College

Printed in The Netherlands

ION BY HOMOGENEOUS

CATALYSIS

L. KEOHAN

of the Holy

Cross.

Worcester.

MA

OI6IO

(U.S.A.)

W. F. GUTKNECHT Systems and Measurements NC 27709 (U.S.A.)

Division,

Research

Triangle

Institute,

Research

Triangle Park,

(Received 3rd August 1981)

SUMMARY A simple procedure for the determination of trace levels of cyanide ions is described

and evaluated.The procedure is based on the cyanide-catalyzed cleavage of benzil in the presence of methanol to produce benzaldehyde and methyl benzoate. The concentrations of ‘&ese products are determined by gas chromatography. Linearity between gas chromatographic response and cyanide concentration is observed from 0.05 to 3 ppm cyanide; a detection limit of 1 ppb is calculated. A series of anions is shown not to interfere_

At present, there are several widely accepted procedures for detecting trace levels of cyanide [l] _ There is, however, a continuing interest in finding new methods to improve the capability of selectively determining this ion at extremely low levels, as is illustrated by the appearance of a number of new procedures in recent years [Z-8] . Ion-selective electrodes responsive to cyanide ion [3, 51 have been used but are subject to interference by halides and sulfide. Indeed, isolation of cyanide by conversion to hydrocyanic acid in acid solution is often necessary [ 1, 4, 6, 8] ; sulfide, which interferes with most methods for cyanide determination, is however, also readily converted to a volatile, weak acid under these conditions, so it is often present in the final sample along with cyanide. A procedure that overcomes this problem of sulfide interference is described by Canterford who measured cyanide and sulfide simultaneously with direct current polarography [21This paper describes an alternate, relatively simple procedure requiring only widely available instrumentation and reagents that exhibit a low detection limit and a high degree of selectivity, especially relative to sulfide. The technique involves first carrying out a chemical reaction catalyzed by the substance of interest and then separating and measuring a product of that reaction using gas chromatography with a flame ionization detector. Under controlled

conditions,

the gas chromatographic

signal will be proportional

to

concentration of the catalyst. Because of the catalytic nature of the ion of interest, the benefits of chemical amplification will be realized.

the

0003-2670/82/0000-0000/$02_75

0 1982

Elsevier Scientific Publishing Company

70

While catalytic methods are available for quantifying many ions, including cyanide [4, 7, 91, gas chromatography has not yet been widely exploited as a means of enhancing the selectivity of catalytic schemes. The utility of this type of detection scheme has been demonstrated recently for the catalytic determination of copper(I) [lo] and iron(II1) [ll] _ Gas chromatographic methods have been used previously for determining trace levels of cyanide. They are based on the formation and detection of the volatile cyanidecontaining compounds, HCN [ 121, CNBr [ 131, or CNCl [ 141. Because these procedures do not involve chemical amplification, it is necessary to use either a more sensitive instrument or some form of sample concentration step to obtain sensitivity comparable with the procedure described in this report_ . The basis of this new procedure for the determination of cyanide is the gas chromatographic measurement of methyl benzoate, which is the product of the cyanide-catalyzed reaction between benzil and methanol in a basic, aqueous solution. The reaction is &@

+ CH30H

‘5

a-&,

+

@-“,-O-C,,

This reaction was selected for use in the determination of cyanide, partially on the basis of a study by Kwart and Baevsky [ 15]_ These authors found that the reaction was specifically catalyzed by cyanide. The reaction was also found to be unaffected by the addition of as much as 30% water to the methanol solvent. This is important because many samples of interest contain water. The reaction is easily carried out, making it attractive for the development of a straightforward experimental procedure. Furthermore, both products, benzaldehyde and methyl benzoate, are amenable to isolation and quantitation by gas chromatography_ EXPERIMENTAL Equipment

and reagents

AI1 gas chromatographic studies were performed with a Perkin-Elmer Sigma-4 gas chromatograph with a flame ionization detector. This instrument was equipped with a glass-lined injection port. A glass column (6 ft. X l/8 in. o-d.) packed with 3% OV-17 on 100-120-mesh Chromosorb GAW was used in all determin+ations. The instrumental output was recorded on a Houston Instrument Omniscribe recorder at 1-mV full scale (25 cm)_ All reactions were run in sealed 50-ml Florence flasks at 20°C. Ail reagents were analytical-reagent grade and used without further purification except benzil (Eastman), which was recrystallized from a 1:l mixture of water and methanol. All solvents were of spectral quality. Water was deionized in a Continental deionization unit. Adjustment of pH was made with Fisher 1.000 f 0.002 M NaOH. The cyanide solutions were diluted from an aqueous stock soiution of 100 ppm cyanide prepared from

71

reagent-grade sodium cyanide. Concentrations of cyanide are reported relative to the aqueous samples, not the final reaction mixture. In the interference studies, all solutions of anions were prepared from either sodium or potassium

salts and solutions

of cations were prepared from their chloride

Salts.

Optimization

procedure

The optimum experimental conditions were found by the sequential simplex optimization procedure [ 16-191. The computations for this procedure were done with aid of a computer program similar to that described by Nedler and Mead [20], with the exception that the massive contraction rule was replaced. In the revised program, if a particular vertex still had the worst response after a contraction, the second-worst vertex was rejected rather than performing a massive contraction. Four parameters, hydroxide concentration,

benzil concentration,

volume

ratio of sample

(aqueous)

to

methanol in the reaction mixture, and reaction time were allowed to vary. Analysis procedure

The following procedure produced optimum results and was used in all esperiments unless indicated otherwise_ Sufficient sodium hydroxide was added to the sample to give a concentration of 2.5 X lo-’ M. An aliquot (5 ml) of this solution was added to 6.5 ml of methanol and the solution was allowed to sit for 30 min to dissipate the heat generated as a result of mixing. Then 1 ml of 0.075 M benzil solution in methanol was added to this mixture, and the resulting solution was mixed by swirling_ After a set reaction time (either 15 or 30 min), a LO-/11portion was injected for measurement of benzaldehyde and methyl benzoate with the chromatographic system described above. The column was operated at 60°C with a nitrogen flow of 20 ml min-‘. The solvent peak exhibited substantially less tailing if the injection ports were cleaned daily. Responses were determined from peek heights. The 0.075 M benzil solution was protected from light and kept refrigerated when it was stored for long periods. RESULTS AND DISCUSSION Optimization results In determining the optimum experimental conditions, masimum permissible values for several of the variables were set. The hydroxide concentration of the reaction mixture was initially limited to values below lo-’ IV to avoid salt build-up in the injection port of the gas chromatograph. For convenience, a maximum of 30 min was set for the reaction time. The

maximum permissible benzil concentration was controlled by its solubility, which in turn was related to the ratio of water (from the aqueous sample) to methanol in the reaction mixture. The hydroxide concentration found to produce optimum results was the maximum level allowed. This suggested that a greater sensitivity could be

i2

obtained if solutions containing a higher sodium hydroxide concentration could be used. However, it was found that increasing the hydroxide concentration above lo-’ M resulted in a dramatic increase in the methyl benzoate blank. .This was attributed to a non-cyanide-catalyzed production of this product. As expected, the reaction time found to be the optimum was also the maximum permissible_ In reality, the optimum reaction time will depend on the needs of the user. As was noted above, maximum sensitivity was achieved at an arbitrarily set maximum time which was selected on the basis of the need to complete the determination in a “reasonable” amount of time. If higher sensitivity is necessary, it can be obtained through the use of longer reaction times. However, at high cyanide concentrations, a long reaction time will result in a substantial consumption of the reactants which will cause a deviation from the desired linear relationship between the concentration of cyanide and the concentration of benzaldehyde or methyl benzoate. Froduc

t detection

Figure 1 shows a chromatogram obtained from the analysis of a 1 ppm cyanide solution using the conditions found to be optimum. It can be seen that the signals from benzaldehyde and methyl benzoate are well resolved from the solvent peak and are easily measured. If the method is to be based on the methyl benzoate peak (it will be shown that this provides the best detection limit) rather than the benzaldehyde peak, adequate resolution can still be obtained at higher flow rates or temperatures. This will substantially shorten the time required_

Methyl Benzoate

Solvent

Benzaldehydr 4

lime (min) Fig. 1. A typical chromatogram representing the determination of 1 ppm cyanide_ Relative instrumental sensitivities are 1 for solvent (methanol), 625 for henza.ldehyde,a.nd 2500 for methyl benzoate. A 30-min reaction time was used.

73

Linearity,

sensitivity,

and detection

limit

Calibration curves based on the response of both benzaldehyde and methyl benzoate vs. the concentration of cyanide were prepared. For a 15min reaction, correlation coefficients of 0.998 were found for the range of O-3 ppm for both curves. (Data points were taken at 0, 0.05, 0.25, 0.50, 1.00, 1.50, 2.00 and 3.00 ppm cyanide.) Thus, under these experimental conditions, the reaction is apparently first order with respect to the cyanide catalyst and pseudo-zero order with respect to all other reactants. The sensitivity of the procedure is partially determined by the amplification factor (molecules of either benzaldehyde or methyl benzoate produced per cyanide ion)_ This factor was determined by measuring the instrumental response to dilute samples of benzaldehyde, thereby determining how much of this product was produced from a cyanide-containing sample. An amplification factor of about 70 molecules of benzaldehyde per cyanide ion was found for a 1 ppm cyanide sample with a reaction time of 15 min. An amplification factor of about 140 was found for a 30-min reaction_ It -was observed that even in the absence of an added catalyst, the reaction occurs to some extent. This is of interest because the uncertainty in this blank value will ultimately set the detection limit of the procedure_ A commonly accepted definition of the detection limit of a procedure is the concentration of analyte giving a signal that is twice the standard deviation in the blank. Using this definition, a series of determinations of the blank for a reaction time of 30 min (see Table l), and the measured sensitivity of the procedure to cyanide, a detection limit of 1 ppb was determined_ This detection limit was based on the response to methyl benzoate. The uncertainty in the blank value for benzaldehyde was much higher and thus unsuitable for the detection of low levels of cyanide. The large benzaldehyde blank was not present if the products were extracted from the reaction mixture into n-butyl benzoate prior to the chromatographic separation_ Apparently, benzaldehyde is formed in a non-cyanide-catalyzed reaction when the entire reaction mixture is injected into the hot injection port of the gas chromatograph.

Interferences The possibility of ions other than cyanide catalyzing the reaction is of considerable concern. This would result in an uncertainty in the origin of a response_ In addition to the possibility of catalysis, another concern was the extent to which any of these ions might inhibit the reaction. Thus, the effects of a variety of potentially interfering ions on both blanks and 1 ppm cyanide samples were measured. While a complete study of all possible ions was not made, several anions and cations were tested. The responses were determined from the methyl benzoate signals. The levels of interfering ions and the results of the interference tests are shown in Table 1. The signals obtained from the blank and 1 ppm cyanide for several replicate samples, without interfering ions present, are included for comparison and to demon-

'74

TABLE

1

Responsea for a blank and 1 ppm cyanide in the presence of various ions Ion tested

None BrclFPO:SOicojC,H,O; NO;

Ion tested

Cont.

Response (cm)

(ppm)

Blank

1.0 ppm CN-

151 2 9” 162 156 166 156

SFe(CN):‘ CU2, Ni”

1.65 1.90

Mn”

1.70

164 166

1.60 1.60

162 160

20 2 4 4 4 4 4 4 400

153 6.0 8.0 142 148 120 148 156

Cone.

Response (cm )

(PPm)

BhIIlk

1-O ppm CN-

1.63 +_O.OSb 1.70 1.70 1.75 1.75 1.90

40 600 40 40 40 40 40 200

Fe” Zn2+ Mg=+ Na+

-

=Response is based on methyl benzoate peak height with an instrumental attenuation of 2. Reaction time is 30 min. b Average of 5 results with standard deviation (A 1s).

strate the precision of the procedure at these levels. Except for Cu*+, Ni2+, and Zn2+, the cyanide response in the presence of the interfering ions was within two standard deviations of the cyanide response without potentially interfering ions present. Also, it can be seen that the signal from the blank was virtually unaffected by the presence of relatively high concentrations of the interfering ions. As noted previously, the response to sulfide is of particular interest because sulfide interferes with many of the present methods for cyanide detection_ Table 1 shows that 20 ppm sulfide does not give a signal significantly different than the blank. Because 1 ppb cyanide has been shown to be detectable under these conditions, the response of the system to cyanide is at least 2 X 10“ times as great as the response to sulfide. Another ion of interest is hexacyanoferrate(II1). The system did not respond to this ion which is undissociated in alkaline solutions [21] _ It can be seen that the transition metal ions copper(I1) and nickel(I1) significantly inhibit the reaction_ This inhibition can be readily explained by the formation of inactive metal-cyanide complexes_ The addition of iron(II1) ions to a 1 ppm cyanide solution has, at most, a slight inhibiting effect, apparently because of the formation of hydrated iron(II1) oxide species. The relatively large concentration of hydroxide with respect to the concentration of cyanide in the solution ensures that the iron(II1) will be masked and so will not significantly lower the cyanide concentration_ This is not in conflict with the earlier observation that hexacyanoferrate(II1) does not release cyanide CW to the solutions, because it is kinetically inert. The inability of the system to respond to metal-cyanide complexes is not a serious drawback. Frequently, only free cyanide is of interest [ 51. Several procedures for isolating the cyanide from the metal before quantitation are available if total cyanide is desired [l, 4,6,22, 23]_

75

The authors acknowledge the contributions of S. L. Morgan (University of South Carolina) for providing the computer program used in the simplex optimization procedure and C. H. Lochmiiller (Duke University) for offering helpful suggestions_ This work was supported, in part, by National Institute of Environmental Health Sciences (NIEHS) Grant 5 PO1 ES01581-02, NIEHS Environmental Trainee Grant IT32ES07031-Ol-Al, an ACS Analytical Division Fellowship (awarded to M.A.D.) sponsored by the Society for Analytical Chemists of Pittsburgh, and the Department of Chemistry, Duke University, where much of this work was performed. This work was presented, in part, at the National ACS Meeting in Washington, DC, September 1979. REFERENCES 1 American Public Health Association, American Water Works Association and Water Pollution Control Federation_ Standard Methods for the Examination of Water and Wastewater, 14th edn., American Public Health Assoc., Washington, DC, 1976, pp. 361-386. 2 D. R. Canterford, Anal. Chem., 47 (1975) 88. 3 H. Clysters, F. Adams and F. Verbeek, Anal. Chim. Acta, 83 (1976) 27. 4 M. Brebec, G. Delarue, B. Santoni and P. Sarrat, Analusis, 4 (1976) 127. 5 M. Hofton, Environ Sci. Technol., 10 (19’76) 277. 6 I. Sekerka and J. F. Lechner, Water Res., 10 (1976) 479. 7 T. Gkutani, H. Kotani and S. Utsumi, Bunseki Kagaku, 26 (1977) 166. 8 R. A. Durst, Anal. Lett., 10 (1977) 961. 9 G. G. Guilbault and D. N. Kramer, Anal. Chem., 38 (1966) 834. 10 M. A. Ditzler and W. F. Gutknecht, Anal. Lett., A(11) (1978) 611. 11 M. A. Ditzler and W. F. Gutknecht, Anal. Chem., 52 (1980) 614. 12 C. R. Schneider and H. Freund, Anal. Chem., 34 (1962) 69. 13 G. Nota and R. Palombari, J. Chromatogr., 84 (1973) 37. 14 J. C. Valentour, V. Agarwal and I. Sunshine, Anal. Chem., 46 (1974) 924. 15 H. Kwart and M. M. Baevsky, J. Am. Chem. Sot., 80 (1958) 580. 16 D. E. Long, Anal. Chim. Acta, 46 (1969) 193. 17 S. N. Deming, S. L. Morgan and M. R. Willcott, Am. Lab., 8 (1976) 13. 18 S. N. Deming and S. L. Morgan, Anal. Chem., 45 (1973) 278A. 19 W. Spendley, G. R. Hext and F. R. Himsworth, Technometrics, 4 (1962) -141. 20 J. A. Nedler and R. Mead, Comput. J., 7 (1965) 308. 21 E. de Barry Bamett and C. L. Wilson, Inorganic Chemistry, Longmans Green, London, 1953, p_ 203. 22 M. S. Frant, J. W. Ross, Jr. and J. H. Riseman, Anal. Chem., 44 (1972) 2227. 23 P. D. Goulden, B. K. Afghan and P. Brookshank, Anal. Chem., 44 (1972) 1845.