Anal. Chem. 1981, 53, 1377-1380
1377
Determination of Cyanide or Thiocyanate at Trace Levels by Derivatization and Gas Chromatography with Flame Thermionic Detection Koichi Funazo, Minoru Tanaka, and Toshiyuki Shono“ Department of Applied Chemistty, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan
A new gas chromatographic method is described for the determination of cyanide or thiocyanate in aqueous matrices. Cyanide or thiocyanate is quantitatively methylated with dimethyl sulfate to form acetonitrile or methyl thiocyanate, respectively. The resulting acetonitrile or methyl thiocyanate is extracted into ethyl acetate and determined by gas chromatography with flame thermionic detection. Good linear calibration curves are obtained in the ranges of 0.05-1.00 pg CN-/mL and 0.25-10.0 pg SCN-/mL which are low enough to detect these anions in environmental and biological matrices. Thls method is not affected by inorganic anions which commonly coexist with cyanide or thiocyanate, except for ferricyanide and ferrocyanide anions which podtivety interfere wHh the cyanide analysis. Cyanide in wastewater samples was determined by both the gas chromatographic and the colorimetric methods. Good agreement was found between the methods. The recoveries of cyanide and thiocyanate added to human urine samples are over 90 % .
Cyanide (CN-) has been known for its extreme toxicity to human beings, animals, and aquatic life. On the other hand, thiocyanate (SCN-) plays a biologically important role in living organisms. Therefore, the determination of these anions in various matrices is of great importance, A large number of reports have been published for the determination of CN(1-3) and SCN- ( 4 5 ) . The recommended standard method for the determination of CN- (6) is the pyridine-pyrazolone method developed by Epstein (7), which is a modification of the pyridine-benzidine method reported by Aldridge (8). The pyridine-benzidine method can also be used for the determination of SCN-. For the determination of SCN-, several colorimetric methods have also been reported which are based on the formation of a red-brown complex with iron(II1) cation (6,9) and a green complex with copper(I1) cation and pyridine (10). Electrochemical methods based on the use of ion-selective electrodes are also used for the determination of CNor SCN-. However, they are subject to several interferences. Much of analytical work has been carried out which involves the use of analytical derivatization prior to gas chromatographic analysis-derivatization gas chromatography. Several reviews have been published for derivatization gas chromatography (11-14). We have investigated the determination of inorganic anions by using derivatization gas chromatography (15,161. Derivatization gas chromatographic methods for the determination of CN- and/or SCN- (17-20) have been reported. In these methods, CN- and/or SCN- are derivatized to cyanogen halide by bromine or chloramine T, and the resulting cyanogen halide is subsequently determined by gas chromatography with electron capture detection. Total amount of CN- plus SCN- can be determined by using this gas chromatographic method. This paper describes a new method to determine CN- or SCN- by derivatization gas chromatography. In this method, CN- or SCN- was methylated with dimethyl sulfate to form acetonitrile or methyl thiocyanate, respectively. The reactions are formulated as
+ (CH30)2SO2 SCN- + (CH30)2S0, CN-
-
-+
CH3CN
+ CHSOSOc
CH3SCN +
(1) CH30S03- (2)
The resulting acetonitrile or methyl thiocyanate was determined sensitively by gas chromatography with a flame thermionic detector (FTD). EXPERIMENTAL S E C T I O N Apparatus. A Shimadzu GC-GAM gas chromatograph equipped with a FTD (Kyoto, Japan) was used. A glass column (1 m X 3 mm i.d.) was packed with 80-100 mesh Porapak Q. Nitrogen was used as the carrier gas a t a constant flow rate of 45 mL/min. The detector and injection port temperatures were kept at 250 OC. The column temperature was maintained at 155 “C for the determination of acetonitrile derivatized from CN- and a t 205 “Cfor that of methyl thiocyanate from SCN-. The peak areas were measured by a digital integrator (Shimadzu Chromatopac-ElA, Kyoto, Japan). Reagents. All reagents were of analytical reagent grade and were used without further purification unless otherwise stated. Dimethyl sulfate used was a commercial grade reagent (98% up) purchased from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). Deionized water was distilled before use. Procedure. The recommended procedure for the determination of CN- or SCN- was as follows. Dimethyl sulfate (0.1 mL) was added to a 1.0-mL aliquot of neutral aqueous sample in a reaction vessel (ca. 10 mL) with a glass stopper. When CN- was methylated in this case, 2.0 N KOH aqueous solution (0.1 mL) was added before adding dimethyl sulfate. On the other hand, in the case of methylation of CN- in wastewater samples, dimethyl sulfate (0.1 mL) was added to a l.l-mIdaliquot of wastewater sample with no addition of 2.0 N KOH aqueous solution, because the basicity of wastewater samples was adjusted to the optimum one (see “wastewater” section). Then the vessel was shaken for 20 min in an incubator controlled at 70 “C. A t the end of the reaction period, 1.0 mL of ethyl acetate solution containing an internal standard (4.0 X 10” M) was added to the cooled reaction solution. As an internal standard, propionitrile was used in the analysis of CN-; ethyl thiocyanate was used in the analysis of SCN‘. Then the derivatized product, “acetonitrile or methyl thiocyanate”, was extracted by shaking for 10 min at room temperature of 18 “C, and the organic layer was separated from the aqueous layer. An aliquot (1.0 hL) of the organic layer was injected into the gas chromatograph and derivatized acetonitrile or methyl thiocyanate was determined with a FTD. Samples. Wastewater. The samples were collected from plating and metal refining factories. Instantly after collection, 10 mL of 2.0 N KOH aqueous solution was added to 100 mL of each water sample to avoid the evolution of hydrogen cyanide and also to obtain the optimum basicity for methylation. The samples were filtered t o remove turbidity. By both the colorimetric method (6) and this gas chromatographic method, the filtered water samples were analyzed directly and after distillation of hydrogen cyanide. The distillation was carried out according to the standard method (6) in order to reduce interferences. Human Urine. The samples were diluted 5-fold with water and were directly used for analysis, as soon as collected. RESULTS AND D I S C U S S I O N Optimum Derivatization Conditions. For the optimum derivatization conditions, the overall yields (= derivatization yield X extractability) of acetonitrile and methyl thiocyanate were examined in various cases. The overall yield of aceto-
0003-2700/81/0353-1377$01,25/0 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
r
7
1
Table I. Derivatization Yields and Extraction Loss' CN- (%) SCNoverall yield extractability of CH,CN or C H , S C N ~ ~ C corrected derivatization yield
55.5 i 1.5 58.9 i 1.9 94.5 i 2.2
(a)
99.7 i 4.2 99.5 i 2.6 100.2
i
4.4
' Mean i standard deviation of five replicate analyses is
shown. The concentration of CN- or SCN-and that of CH,CN or CH,SCN are 2.0 x M. In the extraction of CH,CN, the volumes of water and organic layers are 1.1 and 1.0 mL, respectively, whereas in the case of CH,SCN, the volumes of both layers are 1.0 mL. 5
1
2 [ ii2io4
1
0
i \ I
1
2
1
i
I
h('ll
.
1
6
'
8
1
Figure 1. Effect of normality of added acid or base on overall yields of acetonitrile (0,1.0 pg/mL) and methyl thiocyanate (0, 10.0 pg/mL).
nitrile was estimated as follows. The peak area of acetonitrile extracted into ethyl acetate from the reaction mixture was compared with that of the acetonitrile standard solution. This standard solution contains acetonitrile in ethyl acetate at just the same molar concentration as that of CN- in the aqueous solution used for derivatization. Similarly, the overall yield of methyl thiocyanate was also estimated from the peak area of methyl thiocyanate. In order to ascertain the optimum derivatization conditions, we examined the effects of pH, reaction temperature, and reaction time on the overall yield. In this series of studies to find the optimum derivatization conditions, 1.0 pg CN-/mL and 10.0 pg SCN-/mL aqueous solutions were used as the samples. The effect of pH was examined as follows. To 1.0 mL of each aqueous solution of CN- and SCN-, KOH or H 8 O 4 aqueous solution (0.1 mL) with different concentrations was added before adding dimethyl sulfate. Then the methylation reaction was carried out by shaking at 70 "C for 20 min. After extraction with ethyl acetate, the overall yield was estimated by the method mentioned above. Figure 1shows the effect of KOH and HzS04concentrations added to the samples on the overall yields of acetonitrile and methyl thiocyanate. By the addition of HzS04(0.5-3.0 N), the yield of acetonitrile was reduced down to zero, whereas the derivatization of SCNproceeded quantitatively. In the case of KOH addition, the yield of methyl thiocyanate decreases linearly with an increase in KOH concentration, while the yield of acetonitrile increases and reaches a constant value of 55% at the KOH concentration of about 1.0 N. From these results, a 2.0 N KOH solution was added to the sample before adding dimethyl sulfate in the CN- analysis. On the other hand, dimethyl sulfate was directly added to the sample in the analysis of SCN-. The effects of reaction temperature on the overall yields of acetonitrile and methyl thiocyanate were investigated. The yields do not vary between reaction temperatures of 20-70 "C. However, when methylation was carried out at lower temperatures than 50 "C, a broad peak with a longer retention time appeared on the gas chromatogram. This peak always appeared by the injection of dimethyl sulfate. To avoid troubles from dimethyl sulfate fluctuating the base line and deteriorating the column, we fixed the reaction temperature a t 70 "C. The effects of reaction time were also examined. The term "reaction time" means the period for which the reaction solution was shaken in an incubator controlled at 70 "C. In both cases, methylation completes within about 10 min. The reaction time, therefore, was fixed at 20 min. Derivatization Yields a n d Calibration Curves. Table I shows the results of derivatization of CN- and SCN- under the optimum conditions described in the Experimental Sec-
tion. The direct injection of the reaction mixtures into the gas chromatograph gave rise to the great fluctuating base line. Therefore, an appropriate pretreatment, liquid-liquid extraction, distillation, or column cleanup was inevitable. The distillation and column cleanup procedures did not bring reproducible results and were, moreover, time-consuming. It was very difficult to extract the resulting acetonitrile from the aqueous mixture into organic solvents. Of common organic solvents tested, ethyl acetate gave the highest extractability (ca. 60% ). Acetonitrile was not extracted quantitatively. However, this extraction procedure gave the reproducible results as in Table I provided that it was performed under the controlled conditions where the volumes of ethyl acetate and reaction mixture and the extraction temperature were kept constant. The corrected derivatization yields for extraction loss were calculated by using the extractability and are listed in Table I together with the overall yields. The corrected derivatization yields show that CN- and SCN- are quantitatively methylated. A calibration curve was constructed by plotting the peak area of methylated product vs. the concentration of CN- or SCN-. A good linear relationship is obtained in the concentration range of 0.05-1.00 pg CN-/mL or 0.25-10.0 pg SCN-/mL. According to the extractabilities shown in Table I, a two times better detection limit for SCN- than that for CNL should be expected. Actually, the detection limit (the concentration of anion solution which gives a signal twice that of the average background noise) is 0.01 pg CN-/mL (= 3.85 x lo-' pM)or 0.1 pg SCN-/mL (= 1.72 pM).This phenomenon can be explained as follows: the response of acetonitrile to a FTD is about 2.5 times great as that of methyl thiocyanate, and the background noise is higher in the case of methyl thiocyanate chromatographed at 205 "C than in the case of acetonitrile at 155 "C. These determination levels and detection limits are low enough to determine these anions in various environmental and biological matrices. Interference Study. The interferences of some anions on the determination of CN- and SCN- are shown in Table 11. The anions selected are those normally found in environmental and biological samples. The concentrations of anions added are much higher than those in environmental and biological samples. These anions do not appear to interfere with this analysis except for ferricyanide and ferrocyanide anions which positively interfere with the CN- analysis. Applications. A few wastewater samples were analyzed both by this gas chromatographic method and by the pyridine-pyrazolone method (6), which has been used widely as the standard method for the determination of CN-. These wastewater samples were collected from plating and metal refining factories and made alkaline (see Experimental Section). The samples were filtered and distilled before analysis. The distillation was performed to liberate CN- from tightly bonded metal cyanide complex in the sample. Therefore, total CN- in the sample was determined by using the distilled sample and free CN- was done by analyzing directly. The
ANALYTICAL CHEMISTRY, VOL. 53, NO.
9, AUGUST 1981
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Table 11. Interference Study peak areab methyl acetonitrile thiocyanate ~~
aniona standard SCN CN F-
c1-
BrICH,COONO; St NO; co,2H,PO; Fe(CN),,Fe(CN),+
added as KSCN KCN NaF NaCl NaBr NaI CH,COOK NaNO, Na,S NaNO, Na,CO, KH,PO; K,Fe(CN), K,Fe(CN),
100.0 i 3.0 101.7 t 1.8 101.7 c 99.0 c 98.5 c 101.1 i 98.9 ?: 102.4 c 102.2 c 100.7 i 101.2 t 100.8 i 114.0 i 109.9 i
5.1 4.7 2.6 2.9 2.2 2.1 3.6 4.6 3.5 2.3 2.7 1.7
~
~~
100.0 L 2.5
99.7 i 100.6 L 99.1 i 99.1 t 101.4 i 98.6 i 100.1 t 102.2 t 99.7 t 100.6 L 99.9 t 99.3 t 100.6 t
1.5 1.5 1.2 1.0
2.1 1.4 1.5 1.9 2.8 3.2 1.5 4.4 2.8
Concentration of anions added is 100 pg/mL. CNand SCN- concentrations are 1.0 pg CN-/mL and 10.0 p g SCN-/mL. Mean i standard deviation of four replicate analyses is shown. Table 111. Intercomparison Study for the Determination of CNcolorimetric GC-method methodC wastewater samplea ( p g CN -/mL) (lip CN-/mL) Ad 0.29 i 0.01 0.30 0.28 c 0.02 0.29 A Bd 0.20 0.20 i 0.01 B 0.21 0.20 i 0.02 Cd 0.05 c
