Pre-concentration and determination of trace elements in

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Pre-concentration and determination of trace elements in
Indian Journal of Chemistry
Vol. 40A, April2001, pp. 430-432
Pre-concentration and determination of trace
elements in natural waters by inductively
coupled plasma-atomic emission
spectrometry
A Ramesh, K Rama Mohan & K Seshaiah*
Department of Chemistry, Sri Venkateswara University,
Tirupati 517 502, India
and
J Rajasekhar
Research & Development, Amara Raja Batteries Limited,
Karakambadi, Tirupati 517 520, India
Received I May 2000; revised 25 October 2000
A method has been developed for the determination of Cd (II),
Cu (II), Mn (II), Ni (II), Pb (II) and Zn (II) in natural waters by
ind uctively coupled plasma-atomic emission spectrometry after
pre-concentration by solvent extraction with piperidine
dithiocarbamate (pipDTC). The metals are complexed with
pipDTC and the complexes extracted into isobutyl methyl ketone
and subsequently back-extracted into nitric acid. Metals in nitric
ac id have been determined by aspirating into argon plasma of
lCP-AES. The method has been applied for the dete rmination of
the tested metal ions in natural waters.
Determination of trace elements has received
increasing attention in environmental pollution
monitoring. Inductively coupled plasma-atomic
emission spectrometry which offers fast and multielemental analysis, suffers from relatively low
sensitivity and hence direct determination of the trace
elements by ICP-AES is often difficult. Therefore, a
preliminary preconcentration of trace elements is
useful for the determination by ICP-AES. Many
methods like co-precipitation t, resin-chelation 2 , solid
phase extraction 3· 5 and liquid-liquid extraction 6· 8 have
been used foe the preconcentration of trace metals.
These preconcentration methods provide low
detection limits and also helps to avoid matrix
interferences in the analysis of real samples.
Extraction of the dithiocarbamate complexes of
metals into isobutyl methyl ketone (IBMK) and
subsequent determination has been widely applied in
the determination of metals in natural waters.
However this simple solvent extraction procedure
cannot be combined with ICP-AES due to the
difficulties of introducing organic solvents into the
plasma of ICP spectrometers 9 . 10 • However, this can be
overcome by using a special accessary, nebulizer, for
organic solvents . .
Chelating
agents
like
ammonium
pyrrolidinedithiocarbamate (APDC) and . sodium
diethyldithiocarbamate (NaDDC) are generally used
for complexation and extraction into an organic
solvent. Use of APDC and NaDDC have certain
disadvantages. Many workers have reported that Mn
(II) complex of APDC in IBMK is unstable and has
poor recovery 11 • In view of this we have studied the
preconcentration of trace elements, viz., Cd (II), Cu
(II), Mn (II), Ni (II), Pb (II) and Zn (II) by
complexation
with
piperidine
dithiocarbamate
(pipDTC) and their extraction into IBMK. Since the
IBMK layer cannot be introduced into the plasma of
ICP-AES, the metal ions in the organic layer were
back-extracted into nitric acid. The nitric acid solution
was nebulized into the argon plasma of ICP-AES. The
method provides good recovery of elements from
natural water samples.
Experimental
Atomic emission spectrometer (ICP-AES, Varian,
Liberty Series II, Australia) was used. The plasma
was run at 700 V with 15 I min- 1 argon. The analyte
lines (Cd-226.502 nm, Cu-324.754 nm, Mn-257.610
nm, Ni-221.647 nm, Pb-220.383 nm and Zn-213.856
nm) were selected on the basis of net and background
intensities as well as absence of spectral overlaps.
An Elico pH'meter (model Ll-129) with combined
glass electrode was used for pH measurements.
Borosilicate glass separatory funnels (500 ml, 60 ml)
fitted with Teflon stopcocks were used for
extractions.
All the reagents used were of AR grade. Deionised,
doubly distilled water was used throughout the
experiments. A multi-element standard solution (0.1
J.lg ml- 1) was prepared by appropriate dilution of ICP
standards (Merck, Germany) of Cd (II), Cu (II), Mn
(II), Ni (II), Pb (II) and Zn (II). Nitric acid (Glaxo
ExcelaR) was used without additional purification.
Isobutyl methyl ketone (SD Fine Chern, AR) was
used and purified 12 by fractional disti!lation.
Sodium salt of piperidine dithiocarbamate was
prepared by adding carbon disulphide (80 g) slowly to
a solution of piperidine (85 g) in 25 ml of water at
5°C with constant stirring, followed by 40 g of
NOTES
sodium hydroxide dissolved in 20 ml of water. The
product was warmed to room temperature and washed
repeatedly 2-3 times with purified acetone. The
reaction product was purified by recrystallisation in
acetone. The compound thus produced had a melting
point of 280-282°C at 740 mm pressure.
Acetate buffer was prepared by dissolving 8.2 g
sodium acetate in 800 ml water. The pH was adjusted
to 4.0 with high purity glacial acetic acid. The
solution was purified by addition of Sml of 2%
solution of purified pipDTC and extraction with 25 ml
IBMK.
Procedure
The water samples (200 ml) were transferred to 500
ml separatory funnels . For each sample, the pH was
adjusted to about 4 ± 0.2 with HCl (1+1) or ammonia
solution (1: 1) and 2 ml of acetate buffer followed by
the addition of 5.0 ml of 2 % pip DTC solution. The
funnel was shaken, 15 ml of IBMK solution was
added and the mixture was shaken for 3 min. The
bottom (aqueous) layer was discarded and the
separatory funnel rinsed with a small volume of
431
deionised doubly distilled water to remove the film of
water adhering to the glass surface. The organic layer
was transferred to a 60 ml separatory funnel. HN0 3
( 10 ml, 4 N) was added and the contents were shaken
for 2 min. The acid layer was drained into
polyethylene bottles and metal ions in acid solution
were determined by aspiration into the argon plasma
ofiCP-AES .
Results and discussion
The effect of pH on the preconcentration of metals
by complexation with pipDTC was studied by taking
50 J.Lg of each element individually in 200 ml
deionised doubly distilled water and determjned by
complexing with pipDTC in the pH range of 2-6
following the procedure described above. HN0 3 (4N)
was employed for back-extraction. The data
corresponding to each element are shown in Fig. 1. At
pH 4.0 maximum recovery was obtained for all the
elements. So pH 4.0 ± 0.2 was selected for further
studies.
Table !-Recovery of trace elements from spiked water samples
Element
10
~
ao
~
~
~ 60
Cd (II)
"
1 - - . Mn
2 <>----e Cd
) - - Pb
6
3
2
:s;:~
5
s
4
pH
Cu (II)
6
Fig. !-Effect of pH on the complexation of metal with pip DTC
Mn (II)
100
Ni (II)
.. 80
!::
Pb (II)
-Mn
<>--<)
40
___.
.lo---A
D----iJ
,_.....
20
Cd
Pb
Zn
Ni
Cu
Zn (II)
o.L---~--~---L--~~--L---~----~
2·0
3·0
4 ·0
5·0
HNO:I (N)
Fig. 2-Effect of HN0 3 concentration on extraction of metals
r'
Recovery* ,
Added
Found
%
%
25
22.79
91.16
4.28
Metal ion, p.g
RSD*,
50
45.76
91.52
4.15
75
68 .43
91.24
4.20
100
91.82
91.82
4.08
25
23.49
93 .96
3.51
50
46.81
93 .62
3.71
3.61
75
70.38
93.84
100
94.26
94.26
3.44
25
22.78
91.12
4.1 8
50
45.41
90.82
4.24
75
68.49
91 .32
4.11
100
90.22
90.22
4.48
25
23.38
93.52
3.70
50
46.86
93 .72
3.61
75
70.26
93 .68
3.51
100
93 .84
93 .84
3.63
25
22.81
91.24
3.82
50
45.51
91 .02
4.00
75
68.58
91.44
3.85
100
91.08
91.08
3.93
25
24.06
96.24
3.43
50
47.90
95 .80
3.50
75
72.30
96.40
3.40
100
96.50
96.50
3.33
*n=4
INDIAN J CHEM, SEC. A, APRIL 2001
432
Table 2- Determination of trace elements in natural water
samples collected from different locations in Tirupati
Sample
Cd(II)
Metal ion*, ~g 1- 1
Cu(II) Mn(II) Ni(II) Pb(II)
Zn(II)
Sample I
21
(20)
38
(36)
29
(27)
31
(32)
38
(35)
897
(890)
Samp le 2
22
(20)
41
(42)
32
(29)
27
(28)
35
(33)
804
(80I)
Sample 3
19
(19)
34
(32)
29
(26)
29
(32)
41
(38)
855
(852)
Note: The values are corrected to I 00% recovery assuming "' 90%
or more recovery in the extraction for individual elements.
*Values in parentheses are results obtained with extraction fl ame
14
AAS •
The effect of HN0 3 concentration on the backextraction of trace metals from IBMK solution was
studied. Results show that extraction is dependent on
the concentration of HN0 3 ; in most cases highest
recoveries were obtained by using a HN0 3 (4 N)
solution. The results are shown in Fig. 2. Therefore 10
ml of HN0 3 (4 N) was selected for this study.
Though the back-extraction of the metals into an
acidic aqueous solution is not generally practised, it
has
considerable
advantages.
Back-extraction
stabilizes the metal ions in acidic solution from which
losses are minimal and there are no significant
changes in metal ion concentrations over a period of
13
several weeks . It allows direct aspiration of acidic
aqueous solution in to argon plasma of ICP-AES
whereas organic solutions such as IBMK cannot be
introduced.
The accuracy of the procedure was investigated by
determining the metal ions in spiked water samples.
Deionised doubly distilled water samples (200 ml
each) were spiked with known amounts of
1
multi-element standards (5 J..lg-20 J..lg = 25 J..lg 1- -100
J..lg 1- 1 ) and determined as described above. The
detection limits for Cd (II), Cu (II), Mn (II), Ni (II),
Pb (II) and Zn (II) were found to be 14, 17, 15, 18, 25
and 12 J..lg 1- 1 respectively. Results show sufficiently
high recoveries (> 90 %) for the tested metal ions
(Table 1) with a RSD of 3.33-4.48 %. The method
was applied to the determination of trace elements in
natural waters collected from different sources in and
around Tirupati. The accuracy of the method was
ascertained by comparing the results with extraction
14
flame AAS method • T he results are presented in
Table 2.
Acknowledgment
The authors are thankful to M/S Amara Raja
Batteries Limited, Karakambadi , Tirupati, for
providing ICP-AES facilities .
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