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Determination of sub-ngml levels of mercury in water by electrolytic deposition and electrothermal atomic-absorption spectrophotometry
Talanra, Vol. 32, No. 10, pp. 1016-1018,1985 Printed in Great Britam. All rights reserved
0039-9140/85$3.00+ 0.00 Copyright 0 1985Pergamon Press Ltd
DETERMINATION OF SUB-ng/ml LEVELS OF MERCURY IN WATER BY ELECTROLYTIC DEPOSITION AND ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROPHOTOMETRY Bo-XING, Xv TONG-MING, SHEN MING-NENG and FANG Yu-ZHI of Chemistry. East China Normal University, 3663 Chung Shan Road fl), People’s Republic of China
Xv
Department
Shanghai,
(Received 31 January 1985. Accepted 3 May 1985) Summary-The determination of trace mercury in water samples by electrolytic deposition and electrothermal atomic-absorption spectrophotometry is described. Traces of mercury in water are preconcentrated by electrolytic reduction and deposition on a platinum wire cathode, which is then put into a graphite cup for direct atomization and measurement. The method is sensitive and simple, with a detection limit of 0.04 neiml. Almost all the metal ions commonly found in water samples can be tolerated, because of the selectiv;r’deposition at controlled potential.
Traces of mercury in water can be determined by calorimetry’ or cold-vapour atomic-absorption spectrophotometry, ‘J The latter approach is to be preferred on account of its sensitivity, but its precision is not always satisfactory, and large amounts of easily reducible elements must be absent from the sample solution. The aim of this work was to develop a new method which would overcome these disadvantages, minimize interferences and background absorption, and give maximum sensitivity. The use of controlledpotential electrolytic deposition as a separation and preconcentration technique for the determination of mercury in samples with complex matrices, by electrothermal atomic-absorption spectrometry (AAS), was developed several years ago,‘* but the equipment was not particularly simple and easy to operate and the method did not attract much attention for routine analysis. In this paper a simple electrolytic deposition is combined with electrothermal AAS for the determination of trace mercury in water.
,Copper
rod
-Spring
@I
a
-Plastic slide
Fig. 1. Platinum cathode (a) and holder (b). The cathode is released by moving the sleeve on the holder. General procedure
Transfer 60 ml of water sample and 0.5 ml of 12M acetic acid to a plastic electrolysis cell, submerge the three electrodes (platinum coil electrode, Ag/AgCl reference electrode and platinum foil anode) in the sample solution, start the magnetic stirrer and timer, and electrolyse at 0.0 V us. the Ag/AgCl electrode. After 300 set, transfer the platinum coil electrode from the cell into the graphite cup, then start the atomization procedure and measure the absorbance under the following conditions:
EXPERIMENTAL
Wavelength Bandpass Hg lamp current Sheathing gas (Ar) flow-rate Drying temperature and time Atomization temperature and time
Reagents
Doubly distilled demineralized water was used throughout. Acetic acid (analytical grade) was redistilled in a quartz still before use. A stock solution (1.3 x 10m3M) of mercury(II) chloride was stored in a polyethylene bottle and a working solution (1.3 x lo-‘M) was made by dilution with water just before use.
Cleaning temperature and time
Apparatus
A Hitachi 180-80 polarized Zeeman atomic-absorption spectrophotometer was used with a Hitachi model 056 recorder and control unit (including temperature programmer). A model 75-4B polarograph was used as a potentiostat. A Hitachi mercury hollow-cathode lamp was out. Acetic acid (analytical grade) was redistilled in a quartz still before use. A stock solution (1.3 x 10-3M) of mercury (II) chloride was stored in a polyethylene bottle and a working solution (1.3 x 10msM) was made by dilution with water just before use.
( b)
(a)
RESULTS AND Eflect
of cathode
2531 A 13A 6mA 200 ml/min 40-80”; 5 set 700”; 7 set (stopped Ar flow) 1000”; 3 set
DISCUSSION
potential
of the platinum coil electrode strongly affects the electrolytic deposition of mercury and other metals. Figure 2 shows that the amount of mercury deposited in a given time is constant and
1016
The
potential
1017
SHORT COMMIJNICATIONS
:
200
ly /
E
l-•-•-.-.
/* .
.I I
I
105
0
I
I
5
-10
V (YS Ag /Ag
Cl )
-0
Fig. 2. Selection of cathode potential. 0
maximal over the range from +O.l to - 1.0 V (us. Ag/AgCl electrode). To avoid interferences from other metal ions a potential of 0.0 V (US. Ag/AgCl) is used.
5
10
Time (min)
Fig. 3. Effect of electrolysis time on signal for a Cng/ml sample.
Electrolysis time
Results for the determination of mercury at 4 ng/ml concentration indicate (Fig. 3) that there is initially a linear relationship between amount of mercury deposited and deposition time. Deposition times up to 6 min are best, but if the concentration of mercury is lower than 4 ng/ml the time may be extended. In this work a 5-min deposition time was used as standard.
--q$s--
Fig. 4. Position of cathode in graphite cup. Interferences
Position of platinum wire in the graphite cup
The position of the coiled wire cathode in the graphite cup (Fig. 4) was found not to affect the repeatability of the results, because even if part of the wire is in the light-beam and scatters a little of it, there is almost the same degree of scattering of the measured beam (x) and the reference beam (a) in the polarized Zeeman AAS. Calibration graph, detection limit and precision
The calibration graph was linear from 0.08 to 5.2 ng/ml mercury concentration and the detection limit was 0.04 ng/ml. The relative standard deviation for 10 determinations at the O.Cng/ml level was 7.0%.
The determination of mercury at the O.+ng/ml level was not affected by the presence of 200~ng/ml levels of Fe*+, Cd*+, Mn*+, Cr’+, Pb’+, Cu*+, Ni*+ and Al’+ under the recommended conditions. Determination of mercury in water samples
Four water samples (60 ml) were analysed by the standard-addition method. The results are shown in Table 1; apparent recoveries of 91-104x were obtained in the analysis of four 60-ml water samples spiked with 52.1 ng of mercury. Conch4sions
A major advantage of this method is that the whole atomization procedure is conducted in an ordinary
Table 1. Determination of mercurv in 60-ml water samDles Hg added, Water sample No. 1 mineral water No. 2 mineral water Dian Shan lake water Campus river water
*Means of 3 determinations. tOf added mercury.
ng
Hg found*, ng
52.1 52.1 52.1 52.1
10.4 64.7 13.3 60.8 13.0 65.4 85.8 135.0
Recovery,? % 104 91 101 94
SHORT COMMUNICATIONS
graphite cup atomizer, without modification, so the method can conveniently be used for the routine determination of traces of mercury with any electrothermal AAS equipment, and requires in addition only a simple three-electrode system for controlling the potential of the platinum coil electrode, such as a common polarograph or other controlled-potential electrochemical instrument. Acknowledgement-We are very grateful to Professor Chung-Ging for his help in doing this work.
Xu
1018 REFERENCES
Calorimetric Determination of Elements, 1. G. Charlot, Elsevier, Amsterdam, 1964. R. A. Vitkum and Y. V. Zelyukova, 2. N. S. Poluektov, Zh. Analit. Khim., 1964, 19, 873. 3. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 4. H. Brandenberger and H. Bader, At. Abs. Newsl., 1967, 6, 101. 5. Idem, ibid., 1968, I, 53. P. Strasser. P. Woitowitz and D. 6. K. H. Schaller, Szadkowski, Z. Anal. Chem., 1971, 256, 123. 1. P. Deldime and Van Tran Trieu. Anal. Lett., 1976, 9, 169. 8. D. A. Frich and D. E. Tallman, Anal. Chem., 1982,54, 1217.
0039-9140/85$3.00+ 0.00 Copyright 0 1985Pergamon Press Ltd
DETERMINATION OF SUB-ng/ml LEVELS OF MERCURY IN WATER BY ELECTROLYTIC DEPOSITION AND ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROPHOTOMETRY Bo-XING, Xv TONG-MING, SHEN MING-NENG and FANG Yu-ZHI of Chemistry. East China Normal University, 3663 Chung Shan Road fl), People’s Republic of China
Xv
Department
Shanghai,
(Received 31 January 1985. Accepted 3 May 1985) Summary-The determination of trace mercury in water samples by electrolytic deposition and electrothermal atomic-absorption spectrophotometry is described. Traces of mercury in water are preconcentrated by electrolytic reduction and deposition on a platinum wire cathode, which is then put into a graphite cup for direct atomization and measurement. The method is sensitive and simple, with a detection limit of 0.04 neiml. Almost all the metal ions commonly found in water samples can be tolerated, because of the selectiv;r’deposition at controlled potential.
Traces of mercury in water can be determined by calorimetry’ or cold-vapour atomic-absorption spectrophotometry, ‘J The latter approach is to be preferred on account of its sensitivity, but its precision is not always satisfactory, and large amounts of easily reducible elements must be absent from the sample solution. The aim of this work was to develop a new method which would overcome these disadvantages, minimize interferences and background absorption, and give maximum sensitivity. The use of controlledpotential electrolytic deposition as a separation and preconcentration technique for the determination of mercury in samples with complex matrices, by electrothermal atomic-absorption spectrometry (AAS), was developed several years ago,‘* but the equipment was not particularly simple and easy to operate and the method did not attract much attention for routine analysis. In this paper a simple electrolytic deposition is combined with electrothermal AAS for the determination of trace mercury in water.
,Copper
rod
-Spring
@I
a
-Plastic slide
Fig. 1. Platinum cathode (a) and holder (b). The cathode is released by moving the sleeve on the holder. General procedure
Transfer 60 ml of water sample and 0.5 ml of 12M acetic acid to a plastic electrolysis cell, submerge the three electrodes (platinum coil electrode, Ag/AgCl reference electrode and platinum foil anode) in the sample solution, start the magnetic stirrer and timer, and electrolyse at 0.0 V us. the Ag/AgCl electrode. After 300 set, transfer the platinum coil electrode from the cell into the graphite cup, then start the atomization procedure and measure the absorbance under the following conditions:
EXPERIMENTAL
Wavelength Bandpass Hg lamp current Sheathing gas (Ar) flow-rate Drying temperature and time Atomization temperature and time
Reagents
Doubly distilled demineralized water was used throughout. Acetic acid (analytical grade) was redistilled in a quartz still before use. A stock solution (1.3 x 10m3M) of mercury(II) chloride was stored in a polyethylene bottle and a working solution (1.3 x lo-‘M) was made by dilution with water just before use.
Cleaning temperature and time
Apparatus
A Hitachi 180-80 polarized Zeeman atomic-absorption spectrophotometer was used with a Hitachi model 056 recorder and control unit (including temperature programmer). A model 75-4B polarograph was used as a potentiostat. A Hitachi mercury hollow-cathode lamp was out. Acetic acid (analytical grade) was redistilled in a quartz still before use. A stock solution (1.3 x 10-3M) of mercury (II) chloride was stored in a polyethylene bottle and a working solution (1.3 x 10msM) was made by dilution with water just before use.
( b)
(a)
RESULTS AND Eflect
of cathode
2531 A 13A 6mA 200 ml/min 40-80”; 5 set 700”; 7 set (stopped Ar flow) 1000”; 3 set
DISCUSSION
potential
of the platinum coil electrode strongly affects the electrolytic deposition of mercury and other metals. Figure 2 shows that the amount of mercury deposited in a given time is constant and
1016
The
potential
1017
SHORT COMMIJNICATIONS
:
200
ly /
E
l-•-•-.-.
/* .
.I I
I
105
0
I
I
5
-10
V (YS Ag /Ag
Cl )
-0
Fig. 2. Selection of cathode potential. 0
maximal over the range from +O.l to - 1.0 V (us. Ag/AgCl electrode). To avoid interferences from other metal ions a potential of 0.0 V (US. Ag/AgCl) is used.
5
10
Time (min)
Fig. 3. Effect of electrolysis time on signal for a Cng/ml sample.
Electrolysis time
Results for the determination of mercury at 4 ng/ml concentration indicate (Fig. 3) that there is initially a linear relationship between amount of mercury deposited and deposition time. Deposition times up to 6 min are best, but if the concentration of mercury is lower than 4 ng/ml the time may be extended. In this work a 5-min deposition time was used as standard.
--q$s--
Fig. 4. Position of cathode in graphite cup. Interferences
Position of platinum wire in the graphite cup
The position of the coiled wire cathode in the graphite cup (Fig. 4) was found not to affect the repeatability of the results, because even if part of the wire is in the light-beam and scatters a little of it, there is almost the same degree of scattering of the measured beam (x) and the reference beam (a) in the polarized Zeeman AAS. Calibration graph, detection limit and precision
The calibration graph was linear from 0.08 to 5.2 ng/ml mercury concentration and the detection limit was 0.04 ng/ml. The relative standard deviation for 10 determinations at the O.Cng/ml level was 7.0%.
The determination of mercury at the O.+ng/ml level was not affected by the presence of 200~ng/ml levels of Fe*+, Cd*+, Mn*+, Cr’+, Pb’+, Cu*+, Ni*+ and Al’+ under the recommended conditions. Determination of mercury in water samples
Four water samples (60 ml) were analysed by the standard-addition method. The results are shown in Table 1; apparent recoveries of 91-104x were obtained in the analysis of four 60-ml water samples spiked with 52.1 ng of mercury. Conch4sions
A major advantage of this method is that the whole atomization procedure is conducted in an ordinary
Table 1. Determination of mercurv in 60-ml water samDles Hg added, Water sample No. 1 mineral water No. 2 mineral water Dian Shan lake water Campus river water
*Means of 3 determinations. tOf added mercury.
ng
Hg found*, ng
52.1 52.1 52.1 52.1
10.4 64.7 13.3 60.8 13.0 65.4 85.8 135.0
Recovery,? % 104 91 101 94
SHORT COMMUNICATIONS
graphite cup atomizer, without modification, so the method can conveniently be used for the routine determination of traces of mercury with any electrothermal AAS equipment, and requires in addition only a simple three-electrode system for controlling the potential of the platinum coil electrode, such as a common polarograph or other controlled-potential electrochemical instrument. Acknowledgement-We are very grateful to Professor Chung-Ging for his help in doing this work.
Xu
1018 REFERENCES
Calorimetric Determination of Elements, 1. G. Charlot, Elsevier, Amsterdam, 1964. R. A. Vitkum and Y. V. Zelyukova, 2. N. S. Poluektov, Zh. Analit. Khim., 1964, 19, 873. 3. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 4. H. Brandenberger and H. Bader, At. Abs. Newsl., 1967, 6, 101. 5. Idem, ibid., 1968, I, 53. P. Strasser. P. Woitowitz and D. 6. K. H. Schaller, Szadkowski, Z. Anal. Chem., 1971, 256, 123. 1. P. Deldime and Van Tran Trieu. Anal. Lett., 1976, 9, 169. 8. D. A. Frich and D. E. Tallman, Anal. Chem., 1982,54, 1217.