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Comparative studies of the determination of cyanide at low concentration levels in waste waters
755
Analytica Chimica Acta, 283 (19931755-761 Elsevier Science Publishers B.V., Amsterdam
Comparative studies of the determination of cyanide at low concentration levels in waste waters P.C. do Nascimento
1 and G. Schwedt
Institut fiir Atwrganische und Analytische Chemie, Technische Universit~t Clausthal, Paul-Ernst-Str. 4, 38678 Clausthal-Zelle$eld (Germany) (Received 13th January 1993; revised manuscript received 29th June 19931
Ah&act Different methods for the determination of cyanide in waste waters were compared. After formation of the stable tetracyanonickelate(II1 anion complex spectrophotometric, polarographic and atomic absorption spectrometric methods were used as off-line systems. Potentiometric determination with a flow-injection system was used on-line. Criteria for the comparison of methods were the limit of detection, linear range, precision, recovery and analysis time. The on-line analysis provided the simplest and most rapid method with a limit of detection of 60 pg 1-l. All the methods used showed a cyanide recovery in waste waters in the range 90-104%. The influence of some interfering ions was also tested. Keywords: Atomic absorption spectrometry; Plow injection; Polarography; Potentiometry; IN-Visible tometry; Cyanide; Waters
spectropho-
The high toxicity of cyanide and its widespread industrial applications make it necessary to determine it at very low concentration levels [ll. A number of methods for the determination of cyanide ions have been proposed for various applications, but comparative studies are relatively rare. After a distillation step, cyanide can be determined by spectrophotometry [2-61, fluorimetry [7-91 or electrochemical methods [lo-151. The highest allowed cyanide concentrations in waste waters internationally accepted [16] are in the sub-mg 1-l range. The aim of this work was to develop and to compare methods for cyanide determination in water, suitable for routine measurements.
Different methods were used for cyanide determination. For off-line methods the rapid formation of a stable nickel-cyano complex [tetracyanonickelate(I1) anion] in ammoniacal solution was used. This anion has a characteristic ultraviolet spectrum [161 and therefore cyanide could be determined spectrophotometrically. Polarographic determination was based on the fact that the nickel-cyan0 complex is active in aqueous solution at - 1350 mV (vs. Ag/AgClX After its extraction into an organic phase, indirect atomic absorption spectrometric (AAS) determination of cyanide was possible. As an on-line system, flow-. injection analysis @IA) with potentiometric detection without complex formation was used.
Correspondenceto: G. Schwedt, Institut fiir Anorganische und Analytische Chemie, Technische Universitgt Clausthal, PaulErnst-Str. 4, 38678 Claus&d-Zellerfeld (Germany). ’ Present address: UPSM (Campus), Caixa Postal 5051, Santa Maria 97119 911, RS-Brazil.
EXPERIMENTAL
0003-2670/93/$06.00
Analytical-reagent grade chemicals were used without further purification. Working standard
Q 1993 - Elsevier Science Publishers B.V. All rights reserved
756
P. C. do Nascimento and G. Schwedt /Anal
cyanide solution was prepared from a potassium cyanide stock standard solution (100 mg 1-i CNin 0.1 mol I- ’ NaOH), standardized by argentimetric titration with dithizone as indicator [17]. Solutions of 2 X low3 mol 1-i NiCl, (pH 9) and 0.2 mol 1-l NH,/NH,Cl were required for cyanide complex formation. For extraction of the nickel-cyan0 complex, a solution of 20 g l- ’ tetrabutylammonium bromide (TBAB) in butanol was required. Absorbance measurements were made with a Perkin-Elmer Lambda 5 spectrophotometer using l-cm quartz cells. A PU 9200 atomic absorption spectrometer @ye Unicam) was used at 232.0 nm (nickel lamp; lamp current 5.0 mA) in an air-acetylene flame (0.9 1 min-‘) with a spectral bandpass of 0.2 nm and deuterium compensation. Electrochemical measurements were performed with a Polarographie-Stand VA 663 and Polarecord E 506 (Metrohm) with a three-electrode arrangement (mercury dropping electrode, saturated silver/ silver chloride electrode and glassy carbon electrode). The polarographic parameters used were drop size = 2, drop time = 0.8 s per drop, scan rate = 20 mV s-l, current range = 2.5 x lo-” A mm-‘, pulse amplitude = 50 mV, purge time = 2 min, equilibrium time = 15 s and sample volume for differential-pulse polarography (DPP) = 15 ml. On-line measurements were made with a FIAstar 5020 analyser coupled with a FIAstar 5025 ISE-meter and a FIAstar 5032 detector controller (Perstorp Analytical). A cyanide-sensitive electrode (Perstorp Analytical) and a saturated silver/ silver chloride electrode were used. The flow-rates of the reagents for the on-line measurements were 1.2 ml min-’ for the carrier and electrolyte and 0.6 ml min-’ for the reference electrode.
Chim. Acta 283 (1993) 755-761
Absorbance
0.4 I_ b 0,3
olLL&L_ 240
270
330
300
Wavelength
lnml
Fig. 1. Ultraviolet spectrum of [Ni(CN),]‘- in (a) water and (b) butanol-TEMB solution recorded against the respective reagent blanks.
40 s was sufficient. Extraction with apolar solvents such as chloroform, toluene, benzene, tetrachloromethane and hexane was not possible. Absorption spectra Fig. 1 shows the absorption spectra of the nickel-cyan0 complex in water and butanolTBAB phases, recorded against the respective reagent blanks. The wavelength of maximum absorbance for both systems is 267 nm.
Absorbance 21 with complex extraction 2 without complex extraction 1,5-
. l-
RESULTS AND DISCUSSION
Quantitative extraction of the nickel-cyan0 complex into the butanolic phase was observed in the presence of TBAB, which forms an ion pair with the nickel-cyan0 complex at the required pH to ensure stability of the complex (pH > 4). For extraction of the complex, a shaking time of
0
03
1 mg/l Cyanide
185
2
Fig. 2. Calibration graphs for the spectrophotometric determination of the nickel-cyan0 complex in (1) the organic and (2) the aqueous phase.
P.C. do Nascimento and G. Schwedt/Anal
757
Chim. Acta 283 (1993) 755-761
Beer’s law for determinations in the aqueous and organic phases (Fig. 2) was obeyed over a wide range. Plot 1 (Fig. 2) shows the results obtained after the extraction procedure (butanolic phase) and represents replicate measurements (n = 51, whereas plot 2 shows the results obtained for the aqueous phase and represents single data points. The molar absorptivity of the nickel-cyan0 complex in the butanolic phase was about 16000 whereas Scoggins [ 161 obtained 11000 in aqueous phase. AAS detem’nations After the extraction of the nickel-cyan0 complex the nickel excess remained in the aqueous phase. Hence the nickel concentration in the organic phase was proportional to the total cyanide concentration and so an indirect determination of cyanide ions by means of AAS was feasible. For indirect determination by means of flame AAS, aqueous nickel solutions (without cyanide) were submitted to the extraction procedure and the butanolic phase obtained was used as a blank solution to avoid transport interferences due to the different nebulizer efficiencies in the aqueous and organic phases. The standard and blank solutions were atomized directly in the spray chamber of the spectrometer. Table 1 shows the absorbance of the cyanide solutions after extraction. The original cyanide concentration in the aqueous phase is given in the first column. The low absorbance of the blank solution [ c(CN-) = 0.0 mg l-l] confirms the slight extraction of free nickel species.
5 ANi(ions
- 600
- 800
-1200 - 1000 Polarisation [mVl
- 1400
Fig. 3. Pulse polarographic waves U-4) before and ($6) after nickel-cyan0 complex formation.
Polarographic determinations For the polarographic determination of cyanide ions, the behaviour of the nickel-ammonia complex in the presence of cyanide ions was investigated. In the absence of cyanide the reduction of nickel in ammoniacal solution occurs at -0.98 V (plots l-4, Fig. 3), whereas in the presence of cyanide this signal decreases and a polarographic wave arises at - 1.35 V (plots 5 and 6, Fig. 3). The height of this signal is proportional to the cyanide concentration and permits the determination of small amounts of cyanide in the range 100-2000 pg 1-i where the calibration graph is linear (Fig. 4). i (nA) 25r
20~pG&F
,
TABLE 1 Cyanide determination by AAS after nickel-cyano-complex extraction CW concentration (mg 1-l)
Absorbance (mean, n = 10)
0.0 0.2 0.5 1.0 2.0 4.0
0.003 f 0.002 0.044 f 0.003 0.092 f 0.002 0.177+0.002 0.338 f 0.002 0.618 f 0.003
2 mg/l Cyanide Fig. 4. Calibration graphs for the polarographic determination of cyanide after nickel-cyan0 complex formation.
758
P.C. do Nascimento and G. Schwdt/Anal Pump
Carrier
m
‘y’c”
M,x,ngcoil
w
Electrolyte
r /
Fig. 5. Flow scheme for the on-line determination of cyanide.
In Fig. 3, range 0.01 mg l-‘, an excess of cyanide (relative to the nickel content) was added and so this peak height does not correspond to a linear calibration behaviour. After extraction of the nickel-cyano complex into the butanolic phase, the the DPP determination of cyanide was not feasible. On-Line &terminations The arrangement in Fig. 5 was used for the on-line determination of cyanide with potentio-
Chizn. Acta 283 (1993) 755-761
metric detection. The electrolyte (0.01 mol 1-l NaOH-0.1 mol 1-l KNO,, pH 11) ensures a suitable ionic strength and also the pH to keep the cyanide in the CN- form. The reference electrode was in contact with a 3 mol l- ’ KCl solution, which was pumped directly onto it (walljet configuration). The ion-selective electrode in this wall-jet configuration was also continuously in contact with a narrow jet, from which the sample streams to the sensitive surface. Calibrations for the on-line determination of cyanide are shown in Fig. 6. The graphs are not linear in the concentration range studied but calibration is possible either in the linear interval or by means of a non-linear calibration procedure in the total concentration interval. The right-hand plot (Fig. 6) shows an approximately linear interval but with a preliminary addition of cyanide to the electrolyte to reduce the memory effect of the sensitive electrode. In this test 100 pg 1-l cyanide was used as the original concentration in the electrolyte. Although these electrodes respond to changes in concentration until a statistical limit of detection is reached [ 181, the measurements are usually limited to the linear region of response. The limit
a)
b)
sooPotential ImVl
300Potential ImVl
r
I 600 -
400.
,,,,,,, IOO-
0:001
0,Ol
mg/l
0.1
Cyanide
1
,,rUY 10
:,0,
, ,,,,,, 0.1
, ,,,,,,1
, ,,,, 10 u
mg/l Cyanide
Fig. 6. Calibration of the cyanide determination by means of the FL4 system with potentiometric detection. (a) Total concentration range; (b) linear range.
P.C. do Nascimento and G. Schwedt/Anal
of detection depends on different factors for different electrodes. Midgley [18] pointed out that the detection limits for most electrodes extend to far lower concentrations than those of the linear or Nemstian response limit. Nevertheless, in this work, for the sake of practical convenience, a simple practical limit of detection may be considered. This practical limit of detection may be taken as the measured species concentration at the intersection point of the extrapolated linear segments of the calibration graphs [19]. The lefthand plot in Fig. 6 shows the calibration procedure used to calculate the practical limit of detection. The extrapolated intercept of the rough linear segments indicates the practical limit of detection on the abscissa. Comparison of the methods The different methods of cyanide determination were compared by means of the usual criteria of detection limit, relative standard deviation, linear correlation coefficient in calibration, analytical range of analysis, recovery of cyanide and duration of analysis, as shown in Tables 2 and 3. The detection limits for the spectrophotometric and AA!.Sdeterminations were calculated from the standard deviations of the blanks [20] for five determinations. For the polarographic measurements [21] the detection limit was calculated from the noise of the equipment at the maximum sensitivity (1.5 X 10-r’ A mm-‘) during 3 min at - 1.35 V and, for the flow-injection measurements, by graphical calculations [19]. The range of analysis was determined relative to the lowest concentration by means of the deTABLE 2 Statistical criteria for comparing different methods of cyanide determination a (R.S.D. = relative standard deviation (n = 5-10); r = regression coefficient) Method Spectrophotometry AAS Polarography FIA-ISE ’ L, = limit of detection.
r
bg 1-V
R.S.D. (%c)
40 50 50 60
4.3-0.4 6.0-0.5 6.0-0.8 8.4-2.4
0.999 0.998 0.998 0.995 b
Ld
b See Fig. 6.
759
Chin Acta 283 (1993) 755-761 TABLE 3
Analytical criteria for comparing different methods of cyanide determination ’ Method
A* (m&I1-V
R (%I
A,
Spectrophotometry AAS Polarography FIA-ISE
0.1 -2.0 0.2 -4.0 0.1 -2.0 0.05-4.0
104 98 95 90
2.5 1.0 2.0 0.5
(min)
‘A, = Analytical range of analysis; R = recovery of cyanide; A, = analysis duration.
termination limit, L, [201, taking into account that this limit can be determined with a fmed maximum relative standard deviation. For practical purpose it was considered that L, = pbl
+
lorb,,
where pb, is the mean signal of the blank and a,, is the relative standard deviation, for the spectrophotometric, AA!J and polarographic determinations. For the flow-injection determinations the determination limit was considered as the lowest concentration close to the graphically calculated limit of detection. The upper limit of concentration was not critical and it was taken approximately as the end of the linear calibration range. The recovery of cyanide was tested for each method of analysis with waste water samples [22] after filtration with a microporous membrane (0.45 pm). Cyanide ions were determined by means of the various methods in waste water samples without cyanide addition. No signal was found and the samples, for practical purposes, were considered to be free from uncombined cyanide ions. The waste water samples were subsequently spiked with cyanide ions at levels ranging from 0.5 and 1.0 mg 1-r in the final samples. Interferences Although the determination of cyanide in waste water samples often requires a previous separation step in order to remove interfering ions that are commonly present in such samples, here the proposed methods were studied without distilation steps to establish the direct effect of some ions on the recovery of cyanide. The results ob-
760
P.C. do Nascimento a.& G. Schwdt /Anal Chim. Acta 283 (1993) 755-761
TABLE 4 Effect of diverse ions on the determination of cyanide as the nickel complex Method
Foreign ion
Foreign ion concentration (mg 1-l)
Cyanide concentration (mg l- ‘1
Recovery Mean (%I
Spectrophotometry
srs,o:SO;H,PO; NO,
10 10 10 100 10
1.0 1.0 1.0 1.0 1.0
> 150 93 90 105 97
Polarography
s*so:IBrSCNco:H,PO,-
10 25 10 10 10 10 4
0.5 0.5 0.5 0.5 0.5 0.5 0.5
82 92 96 98 95 90 106
AAS
.S*s,o:so;H,PO,NO,
10 10 10 100 10
0.5 0.5 0.5 0.5 0.5
92 100 90 92 92
tamed for cyanide determination in presence of these ions are given in Table 4. Among the tested ions only sulphide gives a serious interference. However, cyanide and sulphide do not often occur in the same samples [23]. The interference of sulphide in the determination of cyanide in waste water was tested by means of the flow-injection system with regard to the effect of sulphide on the potentiometric cyanide signal. The electrode is equally sensitive to sulphide and cyanide ions and therefore the elimination of sulphide ions before the analysis is necessary. conclusions This investigation has shown that the flow-injection system provides a very simple, rapid and sensitive method for direct cyanide determination. The indirect analysis after stabilization of cyanide as the nickel-cyano complex also showed advantages owing to the possibility of using particular analytical methods. The criteria used for the comparison of the different methods indicated a similar sensitivity and cyanide recovery for the off-line and on-line
methods, but the shorter analysis duration is an advantage of the on-line determination. Among the interfering ions tested only sulphide could not be tolerated at low levels.
REFERENCES 1 International Standards for Drinking Water, World Health Organization, Geneva, 1958. 2 N.P. Kelada, J. Water Polhtt. Control. Fed., 61(1989) 350. 3 S. Nagashima, Anal. Chem., 56 (1984) 1944. 4 G. Schiindorf and H. Engelhardt, Fresenius’ Z. Anal. Chem., 333 (1989) 719. 5 E. Figuerola, A. Florido, M. Aguilar and J. de Pablo, Fresenius’ Z. Anal. Chem., 331 (1988) 620. 6 S. Nagashima, Anal. Chhn. Acta, 91(1977) 303. 7 D. Chen, M.D. Luque de Castro and M. Val&rcel, Talanta, 37 (1990) 1049. 8 A. Sano, M. Takezawa and S. Takitani, Anal. Sci., 2 (1984) 491. 9 T. Toida, S. Tanabe and T. Imanari, Chem. Pharm. Bull., 29 (1981) 3763. 10 A.G. Fogg and R.M. Alonso, Analyst, 112 (1987) 1071. 11 U. Spohn, G. Nagy and E. Pungor, Anal. Sci., 2 (1985) 431. 12 P.W. Alexander, P.R. Haddad and M. Trajanovicx, Anal. Chem., 56 (1984) 2417. 13 R.A. Durst, Anal. Lett., 10 (1977) 961.
P.C. do Nascimento and G. Schwedt /Anal Chim. Acta 283 (1993) 755-761 14 B. Philar, L. Kosta and B. Hristowski, Talanta, 26 (1979) 805. 15 F. Gpekar, Anal. Chim. Acta, 183 (1986) 293. 16 M.W. Scoggins, Anal. Chem., 44 (1972) 1294. 17 E.E. Archer, Analyst, 83 (1958) 571. 18 D. Midgley, Ion-Sel. Electrode Rev., 3 (1981) 43. 19 IUPAC Analytical Chemistry Division, Pure Appl. Chem., 48 (1976) 129.
761
20 D.L. Massart, B.G.M. Vandeginste, S.N. Deming, Y. Mochotte and L. Kaufman, Chemometrics: a Textbook, Elsevier, Amsterdam, 1988. 21 M. Geissler, Polarographische Analyse, 1. Aufl., Verlag Chemie, Weinheim, 1981. 22 P.C. Nascimento, Dissertation, Technische Universitit Clausthal, Claus&xl-Zellerfeld, 1991. 23 G. Schwedt, Labor Praxis, (1990) 596.
Analytica Chimica Acta, 283 (19931755-761 Elsevier Science Publishers B.V., Amsterdam
Comparative studies of the determination of cyanide at low concentration levels in waste waters P.C. do Nascimento
1 and G. Schwedt
Institut fiir Atwrganische und Analytische Chemie, Technische Universit~t Clausthal, Paul-Ernst-Str. 4, 38678 Clausthal-Zelle$eld (Germany) (Received 13th January 1993; revised manuscript received 29th June 19931
Ah&act Different methods for the determination of cyanide in waste waters were compared. After formation of the stable tetracyanonickelate(II1 anion complex spectrophotometric, polarographic and atomic absorption spectrometric methods were used as off-line systems. Potentiometric determination with a flow-injection system was used on-line. Criteria for the comparison of methods were the limit of detection, linear range, precision, recovery and analysis time. The on-line analysis provided the simplest and most rapid method with a limit of detection of 60 pg 1-l. All the methods used showed a cyanide recovery in waste waters in the range 90-104%. The influence of some interfering ions was also tested. Keywords: Atomic absorption spectrometry; Plow injection; Polarography; Potentiometry; IN-Visible tometry; Cyanide; Waters
spectropho-
The high toxicity of cyanide and its widespread industrial applications make it necessary to determine it at very low concentration levels [ll. A number of methods for the determination of cyanide ions have been proposed for various applications, but comparative studies are relatively rare. After a distillation step, cyanide can be determined by spectrophotometry [2-61, fluorimetry [7-91 or electrochemical methods [lo-151. The highest allowed cyanide concentrations in waste waters internationally accepted [16] are in the sub-mg 1-l range. The aim of this work was to develop and to compare methods for cyanide determination in water, suitable for routine measurements.
Different methods were used for cyanide determination. For off-line methods the rapid formation of a stable nickel-cyano complex [tetracyanonickelate(I1) anion] in ammoniacal solution was used. This anion has a characteristic ultraviolet spectrum [161 and therefore cyanide could be determined spectrophotometrically. Polarographic determination was based on the fact that the nickel-cyan0 complex is active in aqueous solution at - 1350 mV (vs. Ag/AgClX After its extraction into an organic phase, indirect atomic absorption spectrometric (AAS) determination of cyanide was possible. As an on-line system, flow-. injection analysis @IA) with potentiometric detection without complex formation was used.
Correspondenceto: G. Schwedt, Institut fiir Anorganische und Analytische Chemie, Technische Universitgt Clausthal, PaulErnst-Str. 4, 38678 Claus&d-Zellerfeld (Germany). ’ Present address: UPSM (Campus), Caixa Postal 5051, Santa Maria 97119 911, RS-Brazil.
EXPERIMENTAL
0003-2670/93/$06.00
Analytical-reagent grade chemicals were used without further purification. Working standard
Q 1993 - Elsevier Science Publishers B.V. All rights reserved
756
P. C. do Nascimento and G. Schwedt /Anal
cyanide solution was prepared from a potassium cyanide stock standard solution (100 mg 1-i CNin 0.1 mol I- ’ NaOH), standardized by argentimetric titration with dithizone as indicator [17]. Solutions of 2 X low3 mol 1-i NiCl, (pH 9) and 0.2 mol 1-l NH,/NH,Cl were required for cyanide complex formation. For extraction of the nickel-cyan0 complex, a solution of 20 g l- ’ tetrabutylammonium bromide (TBAB) in butanol was required. Absorbance measurements were made with a Perkin-Elmer Lambda 5 spectrophotometer using l-cm quartz cells. A PU 9200 atomic absorption spectrometer @ye Unicam) was used at 232.0 nm (nickel lamp; lamp current 5.0 mA) in an air-acetylene flame (0.9 1 min-‘) with a spectral bandpass of 0.2 nm and deuterium compensation. Electrochemical measurements were performed with a Polarographie-Stand VA 663 and Polarecord E 506 (Metrohm) with a three-electrode arrangement (mercury dropping electrode, saturated silver/ silver chloride electrode and glassy carbon electrode). The polarographic parameters used were drop size = 2, drop time = 0.8 s per drop, scan rate = 20 mV s-l, current range = 2.5 x lo-” A mm-‘, pulse amplitude = 50 mV, purge time = 2 min, equilibrium time = 15 s and sample volume for differential-pulse polarography (DPP) = 15 ml. On-line measurements were made with a FIAstar 5020 analyser coupled with a FIAstar 5025 ISE-meter and a FIAstar 5032 detector controller (Perstorp Analytical). A cyanide-sensitive electrode (Perstorp Analytical) and a saturated silver/ silver chloride electrode were used. The flow-rates of the reagents for the on-line measurements were 1.2 ml min-’ for the carrier and electrolyte and 0.6 ml min-’ for the reference electrode.
Chim. Acta 283 (1993) 755-761
Absorbance
0.4 I_ b 0,3
olLL&L_ 240
270
330
300
Wavelength
lnml
Fig. 1. Ultraviolet spectrum of [Ni(CN),]‘- in (a) water and (b) butanol-TEMB solution recorded against the respective reagent blanks.
40 s was sufficient. Extraction with apolar solvents such as chloroform, toluene, benzene, tetrachloromethane and hexane was not possible. Absorption spectra Fig. 1 shows the absorption spectra of the nickel-cyan0 complex in water and butanolTBAB phases, recorded against the respective reagent blanks. The wavelength of maximum absorbance for both systems is 267 nm.
Absorbance 21 with complex extraction 2 without complex extraction 1,5-
. l-
RESULTS AND DISCUSSION
Quantitative extraction of the nickel-cyan0 complex into the butanolic phase was observed in the presence of TBAB, which forms an ion pair with the nickel-cyan0 complex at the required pH to ensure stability of the complex (pH > 4). For extraction of the complex, a shaking time of
0
03
1 mg/l Cyanide
185
2
Fig. 2. Calibration graphs for the spectrophotometric determination of the nickel-cyan0 complex in (1) the organic and (2) the aqueous phase.
P.C. do Nascimento and G. Schwedt/Anal
757
Chim. Acta 283 (1993) 755-761
Beer’s law for determinations in the aqueous and organic phases (Fig. 2) was obeyed over a wide range. Plot 1 (Fig. 2) shows the results obtained after the extraction procedure (butanolic phase) and represents replicate measurements (n = 51, whereas plot 2 shows the results obtained for the aqueous phase and represents single data points. The molar absorptivity of the nickel-cyan0 complex in the butanolic phase was about 16000 whereas Scoggins [ 161 obtained 11000 in aqueous phase. AAS detem’nations After the extraction of the nickel-cyan0 complex the nickel excess remained in the aqueous phase. Hence the nickel concentration in the organic phase was proportional to the total cyanide concentration and so an indirect determination of cyanide ions by means of AAS was feasible. For indirect determination by means of flame AAS, aqueous nickel solutions (without cyanide) were submitted to the extraction procedure and the butanolic phase obtained was used as a blank solution to avoid transport interferences due to the different nebulizer efficiencies in the aqueous and organic phases. The standard and blank solutions were atomized directly in the spray chamber of the spectrometer. Table 1 shows the absorbance of the cyanide solutions after extraction. The original cyanide concentration in the aqueous phase is given in the first column. The low absorbance of the blank solution [ c(CN-) = 0.0 mg l-l] confirms the slight extraction of free nickel species.
5 ANi(ions
- 600
- 800
-1200 - 1000 Polarisation [mVl
- 1400
Fig. 3. Pulse polarographic waves U-4) before and ($6) after nickel-cyan0 complex formation.
Polarographic determinations For the polarographic determination of cyanide ions, the behaviour of the nickel-ammonia complex in the presence of cyanide ions was investigated. In the absence of cyanide the reduction of nickel in ammoniacal solution occurs at -0.98 V (plots l-4, Fig. 3), whereas in the presence of cyanide this signal decreases and a polarographic wave arises at - 1.35 V (plots 5 and 6, Fig. 3). The height of this signal is proportional to the cyanide concentration and permits the determination of small amounts of cyanide in the range 100-2000 pg 1-i where the calibration graph is linear (Fig. 4). i (nA) 25r
20~pG&F
,
TABLE 1 Cyanide determination by AAS after nickel-cyano-complex extraction CW concentration (mg 1-l)
Absorbance (mean, n = 10)
0.0 0.2 0.5 1.0 2.0 4.0
0.003 f 0.002 0.044 f 0.003 0.092 f 0.002 0.177+0.002 0.338 f 0.002 0.618 f 0.003
2 mg/l Cyanide Fig. 4. Calibration graphs for the polarographic determination of cyanide after nickel-cyan0 complex formation.
758
P.C. do Nascimento and G. Schwdt/Anal Pump
Carrier
m
‘y’c”
M,x,ngcoil
w
Electrolyte
r /
Fig. 5. Flow scheme for the on-line determination of cyanide.
In Fig. 3, range 0.01 mg l-‘, an excess of cyanide (relative to the nickel content) was added and so this peak height does not correspond to a linear calibration behaviour. After extraction of the nickel-cyano complex into the butanolic phase, the the DPP determination of cyanide was not feasible. On-Line &terminations The arrangement in Fig. 5 was used for the on-line determination of cyanide with potentio-
Chizn. Acta 283 (1993) 755-761
metric detection. The electrolyte (0.01 mol 1-l NaOH-0.1 mol 1-l KNO,, pH 11) ensures a suitable ionic strength and also the pH to keep the cyanide in the CN- form. The reference electrode was in contact with a 3 mol l- ’ KCl solution, which was pumped directly onto it (walljet configuration). The ion-selective electrode in this wall-jet configuration was also continuously in contact with a narrow jet, from which the sample streams to the sensitive surface. Calibrations for the on-line determination of cyanide are shown in Fig. 6. The graphs are not linear in the concentration range studied but calibration is possible either in the linear interval or by means of a non-linear calibration procedure in the total concentration interval. The right-hand plot (Fig. 6) shows an approximately linear interval but with a preliminary addition of cyanide to the electrolyte to reduce the memory effect of the sensitive electrode. In this test 100 pg 1-l cyanide was used as the original concentration in the electrolyte. Although these electrodes respond to changes in concentration until a statistical limit of detection is reached [ 181, the measurements are usually limited to the linear region of response. The limit
a)
b)
sooPotential ImVl
300Potential ImVl
r
I 600 -
400.
,,,,,,, IOO-
0:001
0,Ol
mg/l
0.1
Cyanide
1
,,rUY 10
:,0,
, ,,,,,, 0.1
, ,,,,,,1
, ,,,, 10 u
mg/l Cyanide
Fig. 6. Calibration of the cyanide determination by means of the FL4 system with potentiometric detection. (a) Total concentration range; (b) linear range.
P.C. do Nascimento and G. Schwedt/Anal
of detection depends on different factors for different electrodes. Midgley [18] pointed out that the detection limits for most electrodes extend to far lower concentrations than those of the linear or Nemstian response limit. Nevertheless, in this work, for the sake of practical convenience, a simple practical limit of detection may be considered. This practical limit of detection may be taken as the measured species concentration at the intersection point of the extrapolated linear segments of the calibration graphs [19]. The lefthand plot in Fig. 6 shows the calibration procedure used to calculate the practical limit of detection. The extrapolated intercept of the rough linear segments indicates the practical limit of detection on the abscissa. Comparison of the methods The different methods of cyanide determination were compared by means of the usual criteria of detection limit, relative standard deviation, linear correlation coefficient in calibration, analytical range of analysis, recovery of cyanide and duration of analysis, as shown in Tables 2 and 3. The detection limits for the spectrophotometric and AA!.Sdeterminations were calculated from the standard deviations of the blanks [20] for five determinations. For the polarographic measurements [21] the detection limit was calculated from the noise of the equipment at the maximum sensitivity (1.5 X 10-r’ A mm-‘) during 3 min at - 1.35 V and, for the flow-injection measurements, by graphical calculations [19]. The range of analysis was determined relative to the lowest concentration by means of the deTABLE 2 Statistical criteria for comparing different methods of cyanide determination a (R.S.D. = relative standard deviation (n = 5-10); r = regression coefficient) Method Spectrophotometry AAS Polarography FIA-ISE ’ L, = limit of detection.
r
bg 1-V
R.S.D. (%c)
40 50 50 60
4.3-0.4 6.0-0.5 6.0-0.8 8.4-2.4
0.999 0.998 0.998 0.995 b
Ld
b See Fig. 6.
759
Chin Acta 283 (1993) 755-761 TABLE 3
Analytical criteria for comparing different methods of cyanide determination ’ Method
A* (m&I1-V
R (%I
A,
Spectrophotometry AAS Polarography FIA-ISE
0.1 -2.0 0.2 -4.0 0.1 -2.0 0.05-4.0
104 98 95 90
2.5 1.0 2.0 0.5
(min)
‘A, = Analytical range of analysis; R = recovery of cyanide; A, = analysis duration.
termination limit, L, [201, taking into account that this limit can be determined with a fmed maximum relative standard deviation. For practical purpose it was considered that L, = pbl
+
lorb,,
where pb, is the mean signal of the blank and a,, is the relative standard deviation, for the spectrophotometric, AA!J and polarographic determinations. For the flow-injection determinations the determination limit was considered as the lowest concentration close to the graphically calculated limit of detection. The upper limit of concentration was not critical and it was taken approximately as the end of the linear calibration range. The recovery of cyanide was tested for each method of analysis with waste water samples [22] after filtration with a microporous membrane (0.45 pm). Cyanide ions were determined by means of the various methods in waste water samples without cyanide addition. No signal was found and the samples, for practical purposes, were considered to be free from uncombined cyanide ions. The waste water samples were subsequently spiked with cyanide ions at levels ranging from 0.5 and 1.0 mg 1-r in the final samples. Interferences Although the determination of cyanide in waste water samples often requires a previous separation step in order to remove interfering ions that are commonly present in such samples, here the proposed methods were studied without distilation steps to establish the direct effect of some ions on the recovery of cyanide. The results ob-
760
P.C. do Nascimento a.& G. Schwdt /Anal Chim. Acta 283 (1993) 755-761
TABLE 4 Effect of diverse ions on the determination of cyanide as the nickel complex Method
Foreign ion
Foreign ion concentration (mg 1-l)
Cyanide concentration (mg l- ‘1
Recovery Mean (%I
Spectrophotometry
srs,o:SO;H,PO; NO,
10 10 10 100 10
1.0 1.0 1.0 1.0 1.0
> 150 93 90 105 97
Polarography
s*so:IBrSCNco:H,PO,-
10 25 10 10 10 10 4
0.5 0.5 0.5 0.5 0.5 0.5 0.5
82 92 96 98 95 90 106
AAS
.S*s,o:so;H,PO,NO,
10 10 10 100 10
0.5 0.5 0.5 0.5 0.5
92 100 90 92 92
tamed for cyanide determination in presence of these ions are given in Table 4. Among the tested ions only sulphide gives a serious interference. However, cyanide and sulphide do not often occur in the same samples [23]. The interference of sulphide in the determination of cyanide in waste water was tested by means of the flow-injection system with regard to the effect of sulphide on the potentiometric cyanide signal. The electrode is equally sensitive to sulphide and cyanide ions and therefore the elimination of sulphide ions before the analysis is necessary. conclusions This investigation has shown that the flow-injection system provides a very simple, rapid and sensitive method for direct cyanide determination. The indirect analysis after stabilization of cyanide as the nickel-cyano complex also showed advantages owing to the possibility of using particular analytical methods. The criteria used for the comparison of the different methods indicated a similar sensitivity and cyanide recovery for the off-line and on-line
methods, but the shorter analysis duration is an advantage of the on-line determination. Among the interfering ions tested only sulphide could not be tolerated at low levels.
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