- Email: [email protected]
Chemiluminescence flow system for the monitoring of chromium(VI) in water
ANALmcA CHIMICA
ACTA ELSEVIER
Analytica
Chin&a Acta 318 (1995) 71-76
Chemiluminescence flow system for the monitoring of chromium( VI) in water Zhujun Zhang a**, Wei Qin a, Shuna Liu b a Department of Chemistry, Shaanri Normal University, Xi’an 710062, China b Chemistry Sectio& Zhengzhou Animal Husbandry Engineering College, Zhengzhou 450045, China Received 20 April 1995; revised 30 June 1995; accepted 12 August 1995
Abstract A novel chemiluminescence(CL) system has been developed for determining chromium(VI) in water. The analytical reagents involved in the CL reaction, including luminol and hexacyanoferrate(I1) were both immobilized on an anion exchange resin column. While a volume of sodium phosphate was passed through the column, these two reagents were eluted from the resins and then mixed with a chromium(W) stream containing hydrochloric acid. By the fast oxidation reaction between chromium(W) and hexacyanoferrate(II), hexacyanoferrate(II1) was generated, which then reacted with luminol in alkaline aqueous solution to produce CL. The CL emission intensity was correlated with the chromium(W) concentration in the range 4 X lo-* to 1 X lop6 g ml-‘, and the detection limit was 1.4 X lo-* g ml-’ chromium(VI). Interfering metal ions present in water were effectively separated by a pre-column cation exchanger. A complete analysis, including sampling and washing, could be performed in 1 min with a relative standard deviation of less than 5%. The system was stable for over 200 analyses and has been applied successfully to the determination of chromium(VI) in water samples. Keywords:
Chemiluminescence;
Flow system; Chromium(VI)
1. Introduction Chromium(Vl) in waste water of leather tanning, ore dressing, metal plating and printing and dying industry is an important source of water pollution. For its measurement, many methods have been developed including spectrophotometry [l-3], fluorimetry [4,5], X-ray fluorescence spectroscopy [6], inductively-coupled plasma mass spectrometry [7], atomic absorption spectrometry [g-lo], atomic emission spectrometry [l 11, chromatography [ 12-141,
l
Corresponding
author.
Elsevier Science B.V. SSDZOOO3-2670(95)00415-7
neutron-activation analysis [15], electrochemical [ 16-181 and chemiluminescence (CL) [ 19-221 analyses. Nowadays chemical systems with the ability of continuously sensing of analytes attract considerable attention and such systems for chromium(W) have been extensively investigated, most of them being electrochemical ones. Fang et al. [23] reported an ion sensitive field effect transistor based on integrated circuit technology using Si,N,-SiO, as the insulator film and a poly(vinylchloride) matrix membrane. Ge et al. [24] proposed a sensitive chemically modified electrode for chromium(VI), which was prepared by coating an Au electrode with an over-oxidized pyr-
72
Z. Zhang et al./Analytica Chimica Acta 318 (1995) 71-76
role film and used for the determination of chromium(W) by differential voltammetry. Several ion selective electrodes exhibiting near-Nernstian response to chromium(W) were also established, based on electroactive materials such as hexadecyltriphenylphosphonium-CrO,F[25], triheptyldodecylammonium-CrO,Cl[26], and diphenylguanidinium-HCrO; [27]. In this paper, a flow-injection CL system for the determination of chromium(W) is presented. It was prepared by immobilizing luminol and hexacyanoferrate(R) on an anion exchange column, which could be eluted from the resins by the injection of sodium phosphate through the column. Chromium(W) was sensed by the CL light signal produced from the following coupled reactions: 14H+ + Cr,O;+ 6Fe( CN)iFe( CN)i-
+ 6Fe( CN)z+ 2Cr3++
+ luminol
+
7H 2 0
3-aminophthalate
reagent streams and a carrier of water at a flow rate (per tube) of 2 ml min -l. The other delivered a sample stream at a relatively low flow rate of 1.0 ml min-l for efficient removal of interfering metal ions in the cation exchange column. PTFE tubing (0.8 mm i.d.1 was used to connect all components in the flow system. 200 ~1 sodium phosphate solution was injected by a six-way injection valve through the anion exchange column. The eluted luminol and hexacyanoferrate(I1) merged and reacted with the sample stream containing 6 X lop3 M HCl in a mixing tubing of 20 cm in length to generate hexacyanoferrate(II1) and then the resulting stream merged with a 0.6 M NaOH stream just prior to reaching a spiral flow cell (200 ~1). The CL was transduced to an electric signal by an R456 photomultiplier tube placed close to the flow cell and was recorded with an XWT-204 recorder. Absorbance monitoring was done using a Model 752 UV spectrophotometer.
+ hu
OH-
Compared with the electrochemical systems, the system described offers advantages of simplicity, rapidity as well as high sensitivity for the determination of chromium(V1).
2. Experimental
2.1. Apparatus The flow system used in this work (Fig. 1) consisted of two peristaltic pumps. One delivered all
Anion lbaa!lge
2.2. Reagents All reagents were of analytical grade; doubly distilled water was used for the preparation of solutions. Stock solution of chromium(W) (1.0 X 10e4 g ml-’ in water) was prepared by dissolving 0.1414 g of K,Cr,O, in 500 ml bidistilled water. Testing standard solutions were prepared by appropriate dilution of the stock solution. A 0.25 M luminol solution was prepared by dissolving 44.3 g of luminol in 1 1 of 0.5 M NaOH solution. Amberlyst A-27 anion exchange resin purchased from Rohm & Haas was
ColLlmn
Flow cell
&&
Detector
Recorder
--cl
1 _
Catian
ErChSlg~
Column
Mixing wing
Fig. 1. Schematic
I Waste
pump2 diagram of the flow system for chromium(V1)
determination.
Z. Zhang et al. /Analytica
used for reagent immobilization. 732 sodium type cation exchange resin was obtained from Shanghai Resin Co. and converted to the hydrogen type by treating with 1 M HCI for sample separation. 2.3. Preparation
of immobilized
reagent column
0.5 g Amberlyst A-27 was stirred with 25 ml of 0.25 M luminol or 0.2 g ml-’ potassium hexacyanoferrate(I1) for 12 h, then the resin was filtered, washed with bidistilled water and dried for storage. The most convenient method to determine the amounts of luminol and hexacyanoferrate(I1) immobilized is to measure the change of their concentrations at 360 nm for luminol and at 400 mu for hkxacyanoferrate(I1). The amounts of luminol and hexacyanoferrate(I1) immobilized were 1.64 mmol and 1.75 mm01 per gram of resin, respectively. To prepare a column with immobilized reagents, resins containing immobilized 0.12 g of luminol and 0.06 g of hexacyanoferrate(I1) were mixed together and packed into a glass column with an internal diameter of 3 mm and a total volume of about 0.6 ml, and equipped with glass wool at both ends to prevent loss of the resins.
Chimica Acta 318 (1995) 71-76
73
3. Results and discussion
3.1. Selection of the eluant Anions with different eluting abilities were injected through the resin column and released different amounts of luminol and hexacyanoferrate(II), thus affecting the CL intensity. The results are shown in Table 1. It was found that sodium phosphate was the best eluant with the highest CL intensity. Therefore, sodium phosphate was chosen for subsequent work. 3.2. Effect of mixing ratio between resins with immobilized luminol and hexacyanoferrate(ZZ) To examine the influence of the mixing ratio, 0.18 g of resins with different mixing ratios were packed into the glass column. By injection of sodium phosphate at a fixed concentration of 6 X 10m4 M, different amounts of luminol and hexacyanoferrate(I1) were eluted from the resins and caused an effect on CL intensity, which is shown in Table 2. The mixing ratio of 2:l between the amount of luminol resin and that of hexacyanoferrate(I1) resin was selected in the present work for the highest CL light signal and relatively low noise.
2.4. Procedures 3.3. Effect of eluant concentration Flow lines were inserted into NaOH solution, eluant solution of sodium phosphate, water carrier and sample solution, respectively. The pumps were started to wash the whole flow system until a stable baseline was recorded. Then 200 ~1 of eluant containing 6 X 10e4 M sodium phosphate was injected into the carrier stream and luminol and hexacyanoferrate(I1) were released quantitatively. The concentration of chromium(W) was quantified by the CL intensity.
Table 1 Characteristics
of eluaots for chromium(VI)
The release of luminol and hexacyanoferrate(I1) was determined by the concentration of sodium phosphate. Various concentrations of sodium phosphate were injected through the anion exchange resin column with immobilized luminol and hexacyanoferrate@) and the downstream solutions were collected. The amounts of luminol and hexacyanoferrate(I1) released were measured by UV-visible absorbance and the CL method. The results are shown in Fig. 2.
determination
Eluant
NaCl
NaAc
NaOH
NaNO,
Na,SO,
Na,CO,
Na,PO,
Relative CL intensity
28
15
12
55
80
67
100
The concentration
of each eluant was 1
X
lo-’
M. The relative CL intensity corresponds
to the normalized
maximum
light intensity.
Z. Zhang et al. /Analytica
74 Table 2 Effect of mixing ratio between and hexacyanoferrate(I1)
Chimica Acta 318 (1995) 71-76
25. resins with immobilized
luminol
Mixing mass ratio
Relative CL intensity
(luminol to hexacyanoferrate(I1))
Signal
Noise
41 21 1:l 1:2
28 35 25 20
13 12 12 9
P s’” x
15 -
b
10
0
0
1
X
L CL conditions: 2.5 X 10m7 g ml-’ chromium(V1) 10m4 M sodium phosphate; 0.6 M NaOH.
standard;
6X
E
5-
0 x ::
0*
It can be seen that the amounts of luminol and hexacyanoferrate(I1) released both increased linearly with the concentration of sodium phosphate injected. Naturally, it would be predicted that increasing the eluant concentration would give increasing CL intensity. This situation is also illustrated in Fig. 2. To obtain long lifetime and high CL intensity, 6 X 10e4 M sodium phosphate was used for the present system. In this case, the column with immobilized CL reagents could be used 200 times. 3.4. Effect of sample acidity Since chromium oxidizes hexacyanoferrate(I1) to hexacyanoferrate(II1) under acid conditions, the effect of the HCI concentration in samples was examined and the results are illustrated in Fig. 3. It can be seen that changes in sample acidity had a great effect on CL intensity and higher HCI concentrations resulted in higher response signals, but also higher backgrounds. To maximize the signal-to-noise ratio, a HCI concentration of 6 X 10e3 M was chosen as optimum.
.
B
l
’
I
I
02
0.4
08
t 0
.
0.8
~~~phosphate@W Fig. 2. Effects of eluant concentration on (0) amount of luminol released (n = - 81, (0) amount of hexacyanoferrate(I1) released (n = - 10) and (XI CL intensity in the presence of 1 X 10e7 g ml-’ chromium(V1).
3.6. Effect of sodium hydroxide concentration Luminol reacts with hexacyanoferrate(II1) in basic solution. Therefore, sodium hydroxide was added in the flow line to improve the sensitivity of the system. Since the concentration of sodium hydroxide versus
25
3.5. Effect of length of mixing tubing To ensure the efficient reaction between chromium(VI) and hexacyanoferrate(II1, a mixing tubing was used in this system and its length was tested from 5 to 60 cm. It was found that a suitable length with a high CL intensity was 20 cm, shorter or longer than this length would cause the decrease of CL intensity because of the deficient reaction or a considerable diluting effect.
-I
0
1.2
24
3.8
4.8
8
72
Coneentratlon of HCl(mM) Fig. 3. Effect of sample acidity on CL intensity: signal of 2.5 X 10m7 g ml- ’ chromium(VI1.
(01
noise; (0)
2. Zhang et al./Analytica Chimica Acta 318 (1995) 71-76
t
:
0
-20
d
0
% %
0
10 -
0
$8
01 0
02
OA
0.6
0
0.0
on CL intensity of 1 X lo-’
CL intensity plot reached a maximum with sodium hydroxide around 0.6 M, this concentration was used in subsequent experiments (Fig. 4). 3.7. Performance measurements
of the system for chromium&I)
While sodium phosphate concentration was selected to be 6 X lop4 M, response to chromium(V1) concentration was linear in the range of 4 X lo-* to 1 X lop6 g ml-’ with a correlation coefficient of 0.9992 and a detection limit of 1.4 X lo-* g ml-’ (3~). Fig. 5 shows a typical calibration graph. The determination of chromium(V1) could be performed in 1 min including sampling and washing, giving a throughput of about 60 h-l. The relative standard deviation was less than 5% for 1 X lo-’ g ml-’ (n = 7). 3.8. Interference
’
’
0.2
’
’
’
’
a4
0.6
0.8
1
Concentration d ctwolMm(lo-sg
Con-OfNaoHo Fig. 4. Effect of NaOH concentration g ml- ’ chromium(V1).
.
Fig. 5. Relative CL intensity vs. chromium(VI)
.
19
ml-q concentration.
Cl- and HCO;, 500 for HPOi-, 200 for Ca*+ and Mg*+, 100 for A13+, Fe3+ and Ba*+, 50 for Zn*+, F-, Pd*+, Cd*+ and Hg*+, 5 for Co*+, Sn*+, Fe*+, Mn*+ Ni*+, and Cr*+, respectively. Equal amounts of cu;+ and V(V) interfered with the determination of chromium(V1). In the presence of the cation exchanger column(l50 x 5 mm i.d.1, none of these cation ions at a concentration of 1 X 10m4 g ml-’ showed any effect on CL signal compared to the 2 X lo-’ g ml- ’ chromium(V1) standard, thus confirming the efficacy of the cation exchanger in removing the metal ion interferences. As reported by the manufacturer, the theoretical capacity of this cation exchanger is 4.5 mequiv. g-’ resin. In this case, it could be estimated that the cation exchange column could be reused over 200 analyses for most water samples. It should be noted that the interference of V(V), which exists in the anion form VO;, could not be eliminated by inclusion of the cation
study
Without the cation exchange column, the effect of foreign ions was tested by analysing a standard solution of chromium(VI) (2 X lo-’ g ml-‘> to which increasing amounts of interfering ions were added. The tolerable concentration ratios with respect to 2 X lo-’ g ml- ’ chromium(VI) were more than 1000 for Na+, K+, Br-, NO;, acetate, SO:-,
Table 3 Results of the analysis of chromium(V1) Sample Waste water I Waste water II Waste water III Average
in water samples
Proposed method
Recovery
CL method
(mg/ml)
(%)
k/ml)
8.5X 10-S 2.2x 10-7 6.4X 10m7
10s 92 98
8.8X1O-5 2.0x 10-7 6.6 x lo-’
of four measurements.
Z. Zhang et al./Analytica
76
exchanger. However, it is fortunate that most water samples do not contain considerable amounts of V(V) and hence this interference will not pose a problem in practical assays. 3.9. Analysis of water samples The water samples from different electroplating workshops were filtered and analyzed for chromium(V1) by both the present CL system and a CL reference method [22]. The results are given in Table 3. It is shown that the results obtained by the system agreed well with those obtained by the CL method.
Acknowledgements This study was supported by the National Science Foundation of China.
Natural
References 111T.A. Maxcy, G.P. Willhite, D.W. Green and K.B. Merte, Appl. Spectrosc.,
45 (1991) 1036. K. Seshaiah, P.U.U.P. Rao and G.R.K. Naidn, Curr. Sci., 55 (1986) 655. I31 CL. Nazario and E.E. Menden, .I. Am. Leather Chem. Assoc., 85 (1990) 212. [41 N. Jie and W. Yu, Yingyong Huaxue, 7 (1990) 71.
121P.R. Deri, C.N. Hariprasad,
Chimica Acta 318 (1995) 71-76
151X. Guo, J. Xu and K. Shen, Chem. J. Chin. Univ., 12 (1991) 454.
1611. Watanabe, Bunseki Kagaku, 40 (1991) 25. [71 Y. Inoue and Y. Date, Kogyo Yosui, 422 (1993) 26.
181T. Shim&
A. Suzuki, A. Nitta and Y. Shijo, Nippon Kagaku Kaishi, 5 (1991) 380. L91W. Qi, S. Chen and S. Lin, Guang Puxue Yu Guangpu Fen Xi, 10 (1990) 46. [lOI P.J. Deng, Fenxi Ceshi Tongbao, 7 (1988) 35. IllI I. Yu. Andreera, E.K. Bocharora, E. Ya. Danilova and A.I. Gunchenko, Zh. Anal. Khim., 44 (1989) 67. I121 R. Roehl and M.M. AIforque, At. Spetrosc., 11 (1990) 210. I131 P. Janos, Fresenius’ J. Anal. Chem., 342 (1992) 195. [141 R. Nilacic, J. Stupar, N. Kozuh and J. Korosin, Analyst, 117 (1992) 125. [151 C.R. Lan, C.L. Tseng, M.H. Ynag and LB. Alfassi, Analyst, 116 (1991) 35. b51 C. Hua, KA. Sugar, K. Mclaughlin, M. Jorge, M.P. Meaneg and M.R. Smyth, Analyst, 166 (1991) 1117. D71 K. Saraswathi, K. Yamuna and K.A. Emmamel, J. Electrochem. Sot. India, 39 (1990) 122. k31 C. Locatelli and F. Fagioli, Mikrochim. Acta, 5-6 (1986) 269. 1191D.F. Marino and J.D. IngIe, Anal. Chem., 53 (1981) 294. DO1 F. Zhang, Y.H. Mei and H. Chen, Zhong guo Huanjing Kexue, 9 (1989) 75. ml Z.J. Zhang and J.R. Lu, Fenxi Ceshi Tongbao, 2 (1983) 16. La J.R. Lu, X.R. Zhang, B.H. Zhang, W. Qin and Z.J. Zhang, Chem. J. Chin. Univ., 14 (1993) 771. [231 P. Fang, J. Huang and D. Feng, Yingyong Kexue Xuebao, 8 (1990) 271. [241 H. Ge, J. Zhang and G.G. Wallace, Anal. Lett., 25 (1992) 429. [251 K. Wang and R. Yu, Fenxi Huaxue, 14 (1986) 599. [261 D. Guo, C. Ji, P. He and H. Gao, Anal. Proc., 24 (1987) 343. [271 A.V. Grandjean and A.K. Charykov, Zh. Anal. Khim., 46 (1991) 2370.
ACTA ELSEVIER
Analytica
Chin&a Acta 318 (1995) 71-76
Chemiluminescence flow system for the monitoring of chromium( VI) in water Zhujun Zhang a**, Wei Qin a, Shuna Liu b a Department of Chemistry, Shaanri Normal University, Xi’an 710062, China b Chemistry Sectio& Zhengzhou Animal Husbandry Engineering College, Zhengzhou 450045, China Received 20 April 1995; revised 30 June 1995; accepted 12 August 1995
Abstract A novel chemiluminescence(CL) system has been developed for determining chromium(VI) in water. The analytical reagents involved in the CL reaction, including luminol and hexacyanoferrate(I1) were both immobilized on an anion exchange resin column. While a volume of sodium phosphate was passed through the column, these two reagents were eluted from the resins and then mixed with a chromium(W) stream containing hydrochloric acid. By the fast oxidation reaction between chromium(W) and hexacyanoferrate(II), hexacyanoferrate(II1) was generated, which then reacted with luminol in alkaline aqueous solution to produce CL. The CL emission intensity was correlated with the chromium(W) concentration in the range 4 X lo-* to 1 X lop6 g ml-‘, and the detection limit was 1.4 X lo-* g ml-’ chromium(VI). Interfering metal ions present in water were effectively separated by a pre-column cation exchanger. A complete analysis, including sampling and washing, could be performed in 1 min with a relative standard deviation of less than 5%. The system was stable for over 200 analyses and has been applied successfully to the determination of chromium(VI) in water samples. Keywords:
Chemiluminescence;
Flow system; Chromium(VI)
1. Introduction Chromium(Vl) in waste water of leather tanning, ore dressing, metal plating and printing and dying industry is an important source of water pollution. For its measurement, many methods have been developed including spectrophotometry [l-3], fluorimetry [4,5], X-ray fluorescence spectroscopy [6], inductively-coupled plasma mass spectrometry [7], atomic absorption spectrometry [g-lo], atomic emission spectrometry [l 11, chromatography [ 12-141,
l
Corresponding
author.
Elsevier Science B.V. SSDZOOO3-2670(95)00415-7
neutron-activation analysis [15], electrochemical [ 16-181 and chemiluminescence (CL) [ 19-221 analyses. Nowadays chemical systems with the ability of continuously sensing of analytes attract considerable attention and such systems for chromium(W) have been extensively investigated, most of them being electrochemical ones. Fang et al. [23] reported an ion sensitive field effect transistor based on integrated circuit technology using Si,N,-SiO, as the insulator film and a poly(vinylchloride) matrix membrane. Ge et al. [24] proposed a sensitive chemically modified electrode for chromium(VI), which was prepared by coating an Au electrode with an over-oxidized pyr-
72
Z. Zhang et al./Analytica Chimica Acta 318 (1995) 71-76
role film and used for the determination of chromium(W) by differential voltammetry. Several ion selective electrodes exhibiting near-Nernstian response to chromium(W) were also established, based on electroactive materials such as hexadecyltriphenylphosphonium-CrO,F[25], triheptyldodecylammonium-CrO,Cl[26], and diphenylguanidinium-HCrO; [27]. In this paper, a flow-injection CL system for the determination of chromium(W) is presented. It was prepared by immobilizing luminol and hexacyanoferrate(R) on an anion exchange column, which could be eluted from the resins by the injection of sodium phosphate through the column. Chromium(W) was sensed by the CL light signal produced from the following coupled reactions: 14H+ + Cr,O;+ 6Fe( CN)iFe( CN)i-
+ 6Fe( CN)z+ 2Cr3++
+ luminol
+
7H 2 0
3-aminophthalate
reagent streams and a carrier of water at a flow rate (per tube) of 2 ml min -l. The other delivered a sample stream at a relatively low flow rate of 1.0 ml min-l for efficient removal of interfering metal ions in the cation exchange column. PTFE tubing (0.8 mm i.d.1 was used to connect all components in the flow system. 200 ~1 sodium phosphate solution was injected by a six-way injection valve through the anion exchange column. The eluted luminol and hexacyanoferrate(I1) merged and reacted with the sample stream containing 6 X lop3 M HCl in a mixing tubing of 20 cm in length to generate hexacyanoferrate(II1) and then the resulting stream merged with a 0.6 M NaOH stream just prior to reaching a spiral flow cell (200 ~1). The CL was transduced to an electric signal by an R456 photomultiplier tube placed close to the flow cell and was recorded with an XWT-204 recorder. Absorbance monitoring was done using a Model 752 UV spectrophotometer.
+ hu
OH-
Compared with the electrochemical systems, the system described offers advantages of simplicity, rapidity as well as high sensitivity for the determination of chromium(V1).
2. Experimental
2.1. Apparatus The flow system used in this work (Fig. 1) consisted of two peristaltic pumps. One delivered all
Anion lbaa!lge
2.2. Reagents All reagents were of analytical grade; doubly distilled water was used for the preparation of solutions. Stock solution of chromium(W) (1.0 X 10e4 g ml-’ in water) was prepared by dissolving 0.1414 g of K,Cr,O, in 500 ml bidistilled water. Testing standard solutions were prepared by appropriate dilution of the stock solution. A 0.25 M luminol solution was prepared by dissolving 44.3 g of luminol in 1 1 of 0.5 M NaOH solution. Amberlyst A-27 anion exchange resin purchased from Rohm & Haas was
ColLlmn
Flow cell
&&
Detector
Recorder
--cl
1 _
Catian
ErChSlg~
Column
Mixing wing
Fig. 1. Schematic
I Waste
pump2 diagram of the flow system for chromium(V1)
determination.
Z. Zhang et al. /Analytica
used for reagent immobilization. 732 sodium type cation exchange resin was obtained from Shanghai Resin Co. and converted to the hydrogen type by treating with 1 M HCI for sample separation. 2.3. Preparation
of immobilized
reagent column
0.5 g Amberlyst A-27 was stirred with 25 ml of 0.25 M luminol or 0.2 g ml-’ potassium hexacyanoferrate(I1) for 12 h, then the resin was filtered, washed with bidistilled water and dried for storage. The most convenient method to determine the amounts of luminol and hexacyanoferrate(I1) immobilized is to measure the change of their concentrations at 360 nm for luminol and at 400 mu for hkxacyanoferrate(I1). The amounts of luminol and hexacyanoferrate(I1) immobilized were 1.64 mmol and 1.75 mm01 per gram of resin, respectively. To prepare a column with immobilized reagents, resins containing immobilized 0.12 g of luminol and 0.06 g of hexacyanoferrate(I1) were mixed together and packed into a glass column with an internal diameter of 3 mm and a total volume of about 0.6 ml, and equipped with glass wool at both ends to prevent loss of the resins.
Chimica Acta 318 (1995) 71-76
73
3. Results and discussion
3.1. Selection of the eluant Anions with different eluting abilities were injected through the resin column and released different amounts of luminol and hexacyanoferrate(II), thus affecting the CL intensity. The results are shown in Table 1. It was found that sodium phosphate was the best eluant with the highest CL intensity. Therefore, sodium phosphate was chosen for subsequent work. 3.2. Effect of mixing ratio between resins with immobilized luminol and hexacyanoferrate(ZZ) To examine the influence of the mixing ratio, 0.18 g of resins with different mixing ratios were packed into the glass column. By injection of sodium phosphate at a fixed concentration of 6 X 10m4 M, different amounts of luminol and hexacyanoferrate(I1) were eluted from the resins and caused an effect on CL intensity, which is shown in Table 2. The mixing ratio of 2:l between the amount of luminol resin and that of hexacyanoferrate(I1) resin was selected in the present work for the highest CL light signal and relatively low noise.
2.4. Procedures 3.3. Effect of eluant concentration Flow lines were inserted into NaOH solution, eluant solution of sodium phosphate, water carrier and sample solution, respectively. The pumps were started to wash the whole flow system until a stable baseline was recorded. Then 200 ~1 of eluant containing 6 X 10e4 M sodium phosphate was injected into the carrier stream and luminol and hexacyanoferrate(I1) were released quantitatively. The concentration of chromium(W) was quantified by the CL intensity.
Table 1 Characteristics
of eluaots for chromium(VI)
The release of luminol and hexacyanoferrate(I1) was determined by the concentration of sodium phosphate. Various concentrations of sodium phosphate were injected through the anion exchange resin column with immobilized luminol and hexacyanoferrate@) and the downstream solutions were collected. The amounts of luminol and hexacyanoferrate(I1) released were measured by UV-visible absorbance and the CL method. The results are shown in Fig. 2.
determination
Eluant
NaCl
NaAc
NaOH
NaNO,
Na,SO,
Na,CO,
Na,PO,
Relative CL intensity
28
15
12
55
80
67
100
The concentration
of each eluant was 1
X
lo-’
M. The relative CL intensity corresponds
to the normalized
maximum
light intensity.
Z. Zhang et al. /Analytica
74 Table 2 Effect of mixing ratio between and hexacyanoferrate(I1)
Chimica Acta 318 (1995) 71-76
25. resins with immobilized
luminol
Mixing mass ratio
Relative CL intensity
(luminol to hexacyanoferrate(I1))
Signal
Noise
41 21 1:l 1:2
28 35 25 20
13 12 12 9
P s’” x
15 -
b
10
0
0
1
X
L CL conditions: 2.5 X 10m7 g ml-’ chromium(V1) 10m4 M sodium phosphate; 0.6 M NaOH.
standard;
6X
E
5-
0 x ::
0*
It can be seen that the amounts of luminol and hexacyanoferrate(I1) released both increased linearly with the concentration of sodium phosphate injected. Naturally, it would be predicted that increasing the eluant concentration would give increasing CL intensity. This situation is also illustrated in Fig. 2. To obtain long lifetime and high CL intensity, 6 X 10e4 M sodium phosphate was used for the present system. In this case, the column with immobilized CL reagents could be used 200 times. 3.4. Effect of sample acidity Since chromium oxidizes hexacyanoferrate(I1) to hexacyanoferrate(II1) under acid conditions, the effect of the HCI concentration in samples was examined and the results are illustrated in Fig. 3. It can be seen that changes in sample acidity had a great effect on CL intensity and higher HCI concentrations resulted in higher response signals, but also higher backgrounds. To maximize the signal-to-noise ratio, a HCI concentration of 6 X 10e3 M was chosen as optimum.
.
B
l
’
I
I
02
0.4
08
t 0
.
0.8
~~~phosphate@W Fig. 2. Effects of eluant concentration on (0) amount of luminol released (n = - 81, (0) amount of hexacyanoferrate(I1) released (n = - 10) and (XI CL intensity in the presence of 1 X 10e7 g ml-’ chromium(V1).
3.6. Effect of sodium hydroxide concentration Luminol reacts with hexacyanoferrate(II1) in basic solution. Therefore, sodium hydroxide was added in the flow line to improve the sensitivity of the system. Since the concentration of sodium hydroxide versus
25
3.5. Effect of length of mixing tubing To ensure the efficient reaction between chromium(VI) and hexacyanoferrate(II1, a mixing tubing was used in this system and its length was tested from 5 to 60 cm. It was found that a suitable length with a high CL intensity was 20 cm, shorter or longer than this length would cause the decrease of CL intensity because of the deficient reaction or a considerable diluting effect.
-I
0
1.2
24
3.8
4.8
8
72
Coneentratlon of HCl(mM) Fig. 3. Effect of sample acidity on CL intensity: signal of 2.5 X 10m7 g ml- ’ chromium(VI1.
(01
noise; (0)
2. Zhang et al./Analytica Chimica Acta 318 (1995) 71-76
t
:
0
-20
d
0
% %
0
10 -
0
$8
01 0
02
OA
0.6
0
0.0
on CL intensity of 1 X lo-’
CL intensity plot reached a maximum with sodium hydroxide around 0.6 M, this concentration was used in subsequent experiments (Fig. 4). 3.7. Performance measurements
of the system for chromium&I)
While sodium phosphate concentration was selected to be 6 X lop4 M, response to chromium(V1) concentration was linear in the range of 4 X lo-* to 1 X lop6 g ml-’ with a correlation coefficient of 0.9992 and a detection limit of 1.4 X lo-* g ml-’ (3~). Fig. 5 shows a typical calibration graph. The determination of chromium(V1) could be performed in 1 min including sampling and washing, giving a throughput of about 60 h-l. The relative standard deviation was less than 5% for 1 X lo-’ g ml-’ (n = 7). 3.8. Interference
’
’
0.2
’
’
’
’
a4
0.6
0.8
1
Concentration d ctwolMm(lo-sg
Con-OfNaoHo Fig. 4. Effect of NaOH concentration g ml- ’ chromium(V1).
.
Fig. 5. Relative CL intensity vs. chromium(VI)
.
19
ml-q concentration.
Cl- and HCO;, 500 for HPOi-, 200 for Ca*+ and Mg*+, 100 for A13+, Fe3+ and Ba*+, 50 for Zn*+, F-, Pd*+, Cd*+ and Hg*+, 5 for Co*+, Sn*+, Fe*+, Mn*+ Ni*+, and Cr*+, respectively. Equal amounts of cu;+ and V(V) interfered with the determination of chromium(V1). In the presence of the cation exchanger column(l50 x 5 mm i.d.1, none of these cation ions at a concentration of 1 X 10m4 g ml-’ showed any effect on CL signal compared to the 2 X lo-’ g ml- ’ chromium(V1) standard, thus confirming the efficacy of the cation exchanger in removing the metal ion interferences. As reported by the manufacturer, the theoretical capacity of this cation exchanger is 4.5 mequiv. g-’ resin. In this case, it could be estimated that the cation exchange column could be reused over 200 analyses for most water samples. It should be noted that the interference of V(V), which exists in the anion form VO;, could not be eliminated by inclusion of the cation
study
Without the cation exchange column, the effect of foreign ions was tested by analysing a standard solution of chromium(VI) (2 X lo-’ g ml-‘> to which increasing amounts of interfering ions were added. The tolerable concentration ratios with respect to 2 X lo-’ g ml- ’ chromium(VI) were more than 1000 for Na+, K+, Br-, NO;, acetate, SO:-,
Table 3 Results of the analysis of chromium(V1) Sample Waste water I Waste water II Waste water III Average
in water samples
Proposed method
Recovery
CL method
(mg/ml)
(%)
k/ml)
8.5X 10-S 2.2x 10-7 6.4X 10m7
10s 92 98
8.8X1O-5 2.0x 10-7 6.6 x lo-’
of four measurements.
Z. Zhang et al./Analytica
76
exchanger. However, it is fortunate that most water samples do not contain considerable amounts of V(V) and hence this interference will not pose a problem in practical assays. 3.9. Analysis of water samples The water samples from different electroplating workshops were filtered and analyzed for chromium(V1) by both the present CL system and a CL reference method [22]. The results are given in Table 3. It is shown that the results obtained by the system agreed well with those obtained by the CL method.
Acknowledgements This study was supported by the National Science Foundation of China.
Natural
References 111T.A. Maxcy, G.P. Willhite, D.W. Green and K.B. Merte, Appl. Spectrosc.,
45 (1991) 1036. K. Seshaiah, P.U.U.P. Rao and G.R.K. Naidn, Curr. Sci., 55 (1986) 655. I31 CL. Nazario and E.E. Menden, .I. Am. Leather Chem. Assoc., 85 (1990) 212. [41 N. Jie and W. Yu, Yingyong Huaxue, 7 (1990) 71.
121P.R. Deri, C.N. Hariprasad,
Chimica Acta 318 (1995) 71-76
151X. Guo, J. Xu and K. Shen, Chem. J. Chin. Univ., 12 (1991) 454.
1611. Watanabe, Bunseki Kagaku, 40 (1991) 25. [71 Y. Inoue and Y. Date, Kogyo Yosui, 422 (1993) 26.
181T. Shim&
A. Suzuki, A. Nitta and Y. Shijo, Nippon Kagaku Kaishi, 5 (1991) 380. L91W. Qi, S. Chen and S. Lin, Guang Puxue Yu Guangpu Fen Xi, 10 (1990) 46. [lOI P.J. Deng, Fenxi Ceshi Tongbao, 7 (1988) 35. IllI I. Yu. Andreera, E.K. Bocharora, E. Ya. Danilova and A.I. Gunchenko, Zh. Anal. Khim., 44 (1989) 67. I121 R. Roehl and M.M. AIforque, At. Spetrosc., 11 (1990) 210. I131 P. Janos, Fresenius’ J. Anal. Chem., 342 (1992) 195. [141 R. Nilacic, J. Stupar, N. Kozuh and J. Korosin, Analyst, 117 (1992) 125. [151 C.R. Lan, C.L. Tseng, M.H. Ynag and LB. Alfassi, Analyst, 116 (1991) 35. b51 C. Hua, KA. Sugar, K. Mclaughlin, M. Jorge, M.P. Meaneg and M.R. Smyth, Analyst, 166 (1991) 1117. D71 K. Saraswathi, K. Yamuna and K.A. Emmamel, J. Electrochem. Sot. India, 39 (1990) 122. k31 C. Locatelli and F. Fagioli, Mikrochim. Acta, 5-6 (1986) 269. 1191D.F. Marino and J.D. IngIe, Anal. Chem., 53 (1981) 294. DO1 F. Zhang, Y.H. Mei and H. Chen, Zhong guo Huanjing Kexue, 9 (1989) 75. ml Z.J. Zhang and J.R. Lu, Fenxi Ceshi Tongbao, 2 (1983) 16. La J.R. Lu, X.R. Zhang, B.H. Zhang, W. Qin and Z.J. Zhang, Chem. J. Chin. Univ., 14 (1993) 771. [231 P. Fang, J. Huang and D. Feng, Yingyong Kexue Xuebao, 8 (1990) 271. [241 H. Ge, J. Zhang and G.G. Wallace, Anal. Lett., 25 (1992) 429. [251 K. Wang and R. Yu, Fenxi Huaxue, 14 (1986) 599. [261 D. Guo, C. Ji, P. He and H. Gao, Anal. Proc., 24 (1987) 343. [271 A.V. Grandjean and A.K. Charykov, Zh. Anal. Khim., 46 (1991) 2370.