Document 6530670

Indian Journal of Chemistry
.
Vo1.39A, May 2000, pp. 560-563
Trace metals analysis in molybdenite
mineral sample
Praveen Kumar Tamrakar & K S Pitre*
Department of Chemistry
Dr Hari Singh Gour University
Sagar 470 003 (M P) India
Received 12 October 1999; revised 2 February 2000
DC polarography and other related techniques, viz., DPP and
DPASV have been sucessfully used for the simultaneous
determination of trace metals in.molybdenite mineral sample. The
polarograms and voltammograms of sample solution have been
recorded in 0.1 M (NH 4 )2 tartrate supporting electrolyte at two
different pH values i.e., 2.7 ±n.1 and 9.0 ± 0.1. The results indicate
the presence of CI12~, Mot'+ ,Cd 2+Ni 2+,In)+, Fe)+ and WM metal ions in
the sample. For the determination of tungsten(VI), II M HCI has
been used as supporting electrolyte. Thngsten(VI) produces a well
defined wave/peak with E.,,/Ep=-0.42V/-O.48V vs SCE in II M HCI.
The quantitative analysis by the method of standard addition shows
the mineral sample to have the following composition, Cu 2+(14.83),
MoM (253.70), Cd 2+ (41.36), Nj2+ (16.08), In)+(3.06), Fe)+ (83.00)
and WM (4.14)mglg of the sample. Statistical treatment of the observed
voltammetric data reveals high accuracy and good precision of
determination. The observed voltammetric results are comparable
with those obtained using AAS method.
Molybdenite mineral, a source of molybdenum, is
associated with a large number of metals 1.3, such as
Si, Mn, Fe, Zn, Pb, Ni, Co, Ca, Cd, Sn, and W. Among
these trace elements the most significant is tungsten,
because it finds wide applicability for industrial
purposes . There are many spectral and
electroanalytical methods used for the determination
of tungsten and other trace metals in natural origin
samples 4 •9 • The present' note describes the trace
analysis of molybdenite mineral sample using DCP,
DPP and DPASV. Special attention has been given
for the determination of tungsten(VI) in the sample.
Voltammetric results have been compared with those
obtained using AAS method.
Experimental
Polarographic and voltammetric measurements
were made on an Elico (India) pulse polarograph
(model CL-90) coupled with a X-Y polarocard (model
LR-108). The electrode system consisted of a
dropping mercury electrode as a working electrode,
a coiled platinum wire electrode as an auxiliary
electrode and a saturated calomel electrode as a
reference electrode. The electrochemical cell used
had provision for inserting a bubbler for deaerating
the solution. A carbon fibre electrode (NFI2, Sigitii
Eletitiogitit, UK) was used for stripping voltammetric
analysis (DPASV). All pH measurements were made
on a Systronics digital pH meter (model-335).
All the chemicals used were of AnalaR I BDH
grade. Stock solutions of ammonium tartrate (I M),
Cu 2+, Mofi+,Cd 2+, Ni 2+, In 3+, Fe3+ and W 6+ all O.OIM
solutions were prepared by dissolving the required
amounts of their soluble salts in doubly distilled water.
Gelatin (0.1 %) solution was prepared in hot distilled
water.·
The molybdenite sample was collected from
Malajkhand area of Balaghat district, Madhaya
Pradesh, India.
Procedure
Finely pulverised molybdenite mineral sample (I g)
was boiled gently with 60ml of conc. HNO" until
the volume was reduced to about 15 ml. Conc. HCI
(40ml ) was added to the solution and evaporated to
a small volume. After cooling the sample solution
was transferred to a 250 ml beaker. The residue that
was not decomposed by this treatment was placed in
a platinum crucible and heated with lOml of conc .
HCI0 4 for about 10 min . and then with lOml of conc.
HF to dissolve the residue. Both the sample solutions
were mixed and total volume of the sample solution
was made up to 100 ml with doubly distilled water.
To 10 ml of the above molybdenite sample
solution, 10ml of 1M (NH4)2 tartrate and 1 ml of 0.1%
gelatin solution (maximum suppressor) were added
and the final volume was made up to 100 ml with
distilled water. The pH of the test solution was first
adjusted to 2.7 ± 0.1 and then to 9.0 ± 0.1, as the
analysis of the sample is carried out at two different
pH values. The analyte was placed in a polarographic
cell equipped with the electrode assembly specified
above. Pure nitrogen gas was passed through the test
solution for 15 min. The polarograms and
561
NOTES
\
(a)
Fe
,
Jl
(b)
F,
Ni+ln
t
r
f-
~I cr~
a:
a:
f-
~ I'~.
a:
~
Cd
.
Cd
:)
u
N
0
CU
Mo
u
I
-O.OV
200 my
I----t
-VOLTAGE_
-O.OV
200my
f----I
VOLTAGE-
Fig. I-(a) Direct current polarogram; (b) Differential pulse polarogram of the molybdenite sample
in O.IM ammonium tartarate + 0.001 % gelatin , [PH =2.7 ± 0.1]
voltammograms were then recorded with the
following instrumental parameter: sensitivity 11lA/
V, C C compensation 5, I R compensation 4, pulse
amplitude 50 mY, drop time 0.5 s, time constant 10
ms and .scan rate 12m V S-I • For DPASV the
deposition potential was fixed at -2.0 V in the case
of 0.1 M (NH 4)2 tartrate and -1.2 V in the case of 1 I
M HCI as supporting electrolyte. The deposition
time was 60 s and 10 s resting period.
Results and discussion
The direct current polarogram and differential pulse
polarogram (Fig. 1 a and b) of the sample solution in
0.1 M (NH 4)2 tartrate (pH= 2.7± 0.1) showed three
distinct polarographic waves/peaks with E,i Ep values
=-0.09V/-0.lOV, - 0.37V/-0.36V and - 0.60V/-0.68
V vs SCE, indicating the presence of Cu2+, MoM and
Cd2+, respectively. In addition to this, two more peaks
were observed with E p=-1.32V and -1.S8V vs SCE,
which by changing the pH of the test solution to 9.0
± 0.1 was split into three waves/peaks with E,iEp=1.1OV/-1.16V,-1.32V/-1.38V and -1.63V/-1.64V vs
SCE 10, corresponding to the presence of Ni 2+, In 3+
and Fe 3+ respectively in the sample.
.
The differential pulse anodic stripping
voltammograms were recorded for the sample
solution under identical experimental conditions,
using glassy carbon fibre electrode. The
voltammograms showed well defined peaks with Ep
values =-0.04V, -0.38V, -0.66V, -1.26V,-1.34V and
- 0.65V vs SCE, indicating the presence of Cu 2+,
M 0 6+'Cd 2+, Ni 2+, In 3+ and Fe 3+ respectively in the
sample.
To investigate the presence of tungsten(VI) in the
sample, 11M HCI was used as supporting electrolyte.
INDIAN J CHEM, SEC. A, MAY 2000
562
Table I ~ Results on molybdenite mineral sample analysis for metal ions (mgtl)
Metal
ions
DCP
Parameter
Added
Found
Amount
14.62
Cu 2+
R%
C.V
Amount
R%
S.D.
C.V
255.20
99.53
0.08
0.03
Amount
41.58
Cd 2+
R%
CV
Amount
R%
S.D.
CV
15.85
97.24
0.04
0.25
Amount
3.44
In 3+
R%
CV
Amount
Fe 3+
R%
S.D.
cv.
83.77
99.47
0.05
0.06
4.04
W 6+
R%
S.D.
CV
253.70
506.55
99.02
0.05
1.20
* Average of four determinations
Recovery, %
R%
=
Standard Deviation
S.D.
Coefficient of Variance , %
C.V.
=
Added
41.36
82.12
99.52
0.04
0.00
16.08
31.45
15.85
98.49
0.05
0.31
3.06
6.42
3.44
98.61
0.05
1.63
83.00
165.90
83.77
99.59
0.03
0.04
4.14
8.12
99.26
0.06
1.44
83.00
166.10
83.77
99.47
0.02
0.02
4.04
3.06
6.41
3.44
98.76
0.Q2
0.65
83.00
165.90
16.08
31.45
15.85
98.49
0.05
0.31
3.06
6.30
41.36
82.55
41.58
99.37
0.04
0.00
16.08
31.05
253.70
507.10
255.20
99.64
0:04
O.oI5
41.36
82.42
41.58
14.83
29.40
99.83
0.05
0.34
253.70
506.90
255.20
Found
14.62
99.W
0.08
0.03
4.14
8.10
Amount
Found
99.83
0.Q2
0.13
96.92
0.Q2
0.65
S.D.
DPASV
14.83
29.40
14.62
99.oI
0.03
0.07
S.D.
Ni 2+
14.82
29.38
Added
99.76
0.05
0.34
S.D.
M 0 6+
DPP
*
4.14
8.14
4.04
99.5
0.04
0.96
NOTES
The dc polarogram and differential pulse polarogram
of the sample solution in 11M HCI produced well
defined
wave/peak
with
E,;/Ep value=
-0.42V/-0.48V vs SCE 11, indicating the presence of
tungsten(VI) in the sample. The differential pulse
anodic stripping voltammogram of the sample
solution in 11 M HCl also showed a well defined
peak with Ep=-0.44V vs SCE.
Based on the presence of the said metal ions in the
sample, some synthetic samples with varying
concentrations of the said metal ions were prepared
and their polarograms and voltammograms were
recorded. The concentration of each metal ion (taken/
found) in synthetic samples using DPP method were
found to be in excellent agreement.
The minimum tried detection limits of the DCP,
DPP and DPASV techniques for the measurement of
the individual and combined metal ions were
determined. Of these, Cu 2 +, Mo6+ and Cd 2 + were
determined in one run at pH = 2.7 ± 0.1 and other
metal ions, viz., Ni 2+, In 3+ and Fe3+ at pH = 9.0 ±O.l.
while for W 6 +, 11M HCl was used as supporting
electrolyte. The detection limits were examined by
preparing synthetic samples. The data shows that the
DPASV method using glassy carbon fibre electrode
is highly sensitive in determining the reported metal
ions down to nanogram. level, i.e., for Cu 2+(6.3),
Mo6+(9.6),Cd 2+(6.0), Ni 2+(6.2)and Fe 3+(2.8)ngl· 1 in
both cases individual and combined. For In 3+ (5.8)
and W6+ (9.5)ngl- 1 in case of individual and 11.6
ngl- I and 19.0 ngl- I respectively, in case of combined
were detected.
After ascertaining the presence of the said metals,
the quantitative analysis of the sample for its metals
content was carried out using the method of standard
addition. It is quite clear from the data in Table 1
that the recovery is over 99% for most of the metal
ions, with high accuracy and precision of
determination.
563
Analysis of molybdenite mineral sample show the
presence of Cu 2 + (14.83), M0 6 + (253.70), Cd 2 +
(41.36), Nj2+ (16.08), In 3+(3.06), Fe3+ (83.00) and W6+
(4.14)mg/g in the sample. These results were
compared with those obtained using atomic
absorption spectroscopic method which revealed the
following results, i.e., Cu 2+ (14.60), Mo6+ (249.52),
Cd 2+ (40.98), Nj2+ (16.02), In3+(3.1O), Fe 3+ (82.50)
and W 6+ (4.09) mg/g in the sample. · The agreement
between two data demonstrates the utility of
voltammetric methods for such an analyses. Further,
the voltammetric techniques are simple, fast and
economic.
Acknowledgement
The authors are highly grateful to Prof S P Banerjee,
Head, Department of Chemistry, Dr. H. S. Gour
University, Sagar, for providing the necessary
laboratory facilities.
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