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Polarography of lanthanum in tetramethylammonium chloride media
347
JOURNAL OF THE LESS-COMMON METALS
IN TETRAMETHY
POLAROGRAPHY OF LANTHANUM CHLORIDE MEDIA
M. L. MITTAL
AND K. S. SAXENA
Department of Chemistry, Government College, Kota. Raj. (India)) Malaviya Regional Engineering College, Jaipur (India) (Received
LAMMONIUM
January 5th, 1965; revised February
and Department
of Chemistry,
15th. 1965)
SUMMARY
The polarographic behaviour of La a+ at a dropping mercury electrode has been investigated in the presence of 0.2 M tetramethylammonium chloride and O.OOI~/~ gelatin. At a temperature of 28 *_ o.05”C and pH range 4.8-6.0, a reversible wave having a half-wave potential of about -1.34 V (‘us. S.C.E.) with probable reaction mechanism Las+ + e = Las+ and n = I was obtained. The effects of change of temperature, height of the Hg column, concentration and pH have been studied. The values of temperature coefficients of id at different temperature levels, which were found to increase with increasing temperature (0.974% per degree at 28.5”C and 1.81% per degree at 51.0X), and non-linearity of wave height with Las+ concentration indicate that the wave produced by La 3+ is not diffusion-controlled; it is neither very reproducible nor well defined.
INTRODUCTION W. without
NODDACK
supporting
AND A. BRUCKL~ have electrolyte
and reported
studied
the polarography
of lanthanum
that it reduces at the dropping
mercury
Las+ + e = Las+ (E, = -1.9 V) and (2) La2+ + 2e + La (metal) (E, = 2.0 V). PURSHOTAMAND RAGHAVARAO~ employing LiCl media also suggested a two-step reduction process for lanthanum, viz. Las+ + La2+ + La. SANCHOGOMEZANDALMAGRO~ obtained a single-step wave, perfectly separated from the H wave, for lanthanum with a probable reaction mechanism Las+ --f Lao in 0.1 M LiCl in a mixture of water and 50 or 80 vol. o/0ethanol at [H+] = I x 10-3 or electrode
in steps,
2 x 10-a.
AKIO
(I)
IWASE~
has studied
the polarographic
behaviour
of lanthanum
in
M solutions of (CH&NI, (CH&NB r and LiCl and obtained a single step wave in a pH range 2-6. At pH less than 3, a three-electron reduction process was suggested. In view of the conflicting details of the reports of previous workers concerning the polarographic behaviour of lanthanum at the d.m.e., it was considered worthwhile to reinvestigate the nature of the reduction wave produced by Laa+ions by studying the effects of change of temperature, mercury pressure, pH and concentration of La3+ on the wave height. 0.1
J. Less-Common
Metals, 8 (1965) 347-351
M. L. MXTTAL, R. S. SAXENA
343 EXPERIMENTAL
Anal. R. (RDH) reagents, La(NO&, (CH&NCi and gelatin were used and their solutions prepared in air-free conductivity water. A manual polarograph with S& Galvanometer as current recorder was employed for determining polarograms. A capillary having the following characteristics, 112= 1.823mg/sec, t = 4.60 set, and &!atr/a = 1.925 mg213 set-* (in 0.2 M with saturated (CH&NCl at &I.~. = -1.0 V zts. S.C.E.) was used in conjunction calomel electrode connected to the cell by a low resistance salt bridge. The cell was kept immersed in an electrically maintained thermostat and the solution was swept for x0-15 min with pure hydrogen before determining each polarogram. The necessary correction was made for residual current in determining all diffusion current data. o.o010/ gelatin was used as maximum suppressor and 0.2 lkl (CH&NCl as supporting electrofyte. RESULTS AND DISCUSSION
Effect of the variatiovL of La(iVO& concentration OFZ wave height Polarograms of solutions containing different concentrations of lanth~um nitrate (0.5-2.5 m&f/l) were drawn and the value of diffusion current at each concentration was determined; the values of &,lc and diffusion current constant were calculated (Table I) and iti and i& were plotted against the concentrations of La(NO~)~ employed (Fig. I, curves I and 2).
5,L7.~~ 0.50
1.00 1.50 Concentration
Fig. I. Curve I, plot of da/c VS. concentration La (NO&. 1. Less-Common Me&&
8 (1965)
347-351
2.50
2.00 (mM/t)
of La(NO&:
curve 2, plot of id vs. concentration
of
POLAROGRAPHY
TABLE
OF TANTALUM
IN TETRAMETHYLAMMONIUM
CHLORIDE
MEDIA
I
TESTFOKTHE
OF WAVE
LINEARITY
Concentration CmMlt)
La(NO&
HEIGHTWITH
of La(A;O&
CONCENTRATION
ia after cowection for ir (PA)
0.500
6.92 13.12
13.840 13.120
7.19 6.82
1.250
‘3.56 ‘5-56 18.40 x9.59
10.800 9.334 9.200 7.800
5.58 4.85 4.79 4.65
I .ooo I.667
2.000 2.500
-
~.~..__^...___~
It is evident
from
stant)
are not constant
c and
id are not
The at five
polarograms
I that
the values
the curves
I and
of solutions
temperatures
of diffusion
are tabulated
of id/c and
2 (Fig.
-
2.0 mM/l
ranging
current
in Table
vs. log(i/&
TABLE
Table and
I
(diffusion
I) plotted
current
between
con-
c and id/c and
linear.
different
coefficient
F-d.e.
349
from
with
respect
51T.
28” to
for each temperature
II together
with
i) determined
interval
the values
at same
to La(NO& The
were
of 0.059/n,
temperature
values
were
drawn
of temperature
calculated
i.e. slopes
and they
of the plots
of
levels.
II
EFFECT OF THE VARIATION OF TEMPERATURE ON WAVR CHARACTERISTICS ._.-^_l_ ~____..__._
Temperature (“C)
ia aftev correction for i, (PA)
22.60 24.15
51.0
26.25
0.974 I.438 1.163 1.810
2I.00
The coefficients
plot
of icy vs. temperature
of diffusion
the change
controlled
by
The yield
different
the diffusion
line
and
support
id does that
I)
and
not
values
show
the height
(Table
II)
that
of electrons
i) for the wave
indicating
at the
d.m.e.
s h ow
at higher taking
to -1.37
the above
2, curve
that
suggest
-
0.063
pIace
suggest
V at 28.5”C
and
0.0392
of
temperature
a linear
of the wave
relationship is not solely
factor.
of slope
takes
(Fig.
indicate
log(i/id
vs.
temperatures
the number
mV/“C)
in temperature
transfer
temperature,
.??k (-1.34
current
plot of Ed.&
a straight
electron
and
0.0630 0.0530 0.0500 0.0450
19-75
28.5 35.0 40.3 46.2
with
Slope lo.o59lnJ
Temp. coefficient f %i”Cl
fact
that
The
values
a considerable temperatures
part
in reduction
V at =jI.o”C) and suggest
andits
that
produced
the wave
by
z m&f/l
is reversible
of the slopes decrease
process
the wave
increases.
at
increase
The
coefficient
produced
j. Less-Common
the
of the wave
temperature
a one-
for the waves
with
the nature
La(NOS)S and
in
changes shift
of
(-1.33
is reversible.
The
Metals, 8 (1965) 347-351
M. I,. MITTAL,
R. S. SAXENA
Fig. 2. Plot of id us. temperature.
catalytic effect on the wave becomes more pronounced at higher temperature, as is clear from the increased value of temperature coefficient of diffusion current at 51.o”C. Effect of variation of height of Hg column and $H A series of polarograms of I mM/l lanthanum nitrate solution were drawn at different heights of the mercury column. At high mercury pressure irregular deflections in the galvanometer were observed. The approximate values of id for waves obtained at different mercury pressures do not show linear relationship with the height of mercury column and, moreover, the values of id/h* were not found to be constant. These observations also provide evidence for the catalysed reduction wave. Polarograms of I mM/l La(NO& were also recorded at different pH levels. The height of the wave was found to be quite independent of pH, and E, remains almost unaffected in a pH range 4.8-6.0, but at higher and lower pH levels than this range, E, shifts to the negative side. The slope of the plot of Ed.e. vs. log(i/id - i), i.e. the number of electrons taking part in the reduction process also remains unaffected. These observations suggest that the wave produced due to La(NO& in the presence of 0.2 M tetramethylammonium chloride and 0.001% gelatin is reversible and not diffusion-controlled; it is neither very reproducible nor well defined and the current is partially dependent on the rate of chemical reaction occurring in the diffusion layer. The effect becomes more pronounced at higher temperature. Further work is in progress. J. Less-Common
Metals,
8 (1965) 347-351
POLAROGRAPHY
OF TANTALUM
IN TETRAMETHYLAMMONIUM
CHLORIDE
MEDIA
351
ACKNOWLEDGEMENT
The authors are indebted to the Council of Scientific and Industrial Research, New Delhi (India) for the award of a fellowship to one of them (M.L.M.). REFERENCES I W. NODDACK AND A. BRUCKL, Angew. Chem., 50 (1937) 392. 2 A. PURSHOTAM AND BH. S. V. RAGHVA RAO, Anal. Chim. Acta., 17. (1955) 589. 3 J, SANCHO GOMEZ AND V. ALMAGRO, Publs. Inst. Fis. “Antonio de Gregorio Rocasolano”, (1960) r-8; cf. CA, 54 (1959) wqd. 4 AKIO IWASE, Nippon Kagaku Zasshi, 80 (1959) 1133-8.
/. Less-Common
Metals,
14
8 (1965) 347-351
JOURNAL OF THE LESS-COMMON METALS
IN TETRAMETHY
POLAROGRAPHY OF LANTHANUM CHLORIDE MEDIA
M. L. MITTAL
AND K. S. SAXENA
Department of Chemistry, Government College, Kota. Raj. (India)) Malaviya Regional Engineering College, Jaipur (India) (Received
LAMMONIUM
January 5th, 1965; revised February
and Department
of Chemistry,
15th. 1965)
SUMMARY
The polarographic behaviour of La a+ at a dropping mercury electrode has been investigated in the presence of 0.2 M tetramethylammonium chloride and O.OOI~/~ gelatin. At a temperature of 28 *_ o.05”C and pH range 4.8-6.0, a reversible wave having a half-wave potential of about -1.34 V (‘us. S.C.E.) with probable reaction mechanism Las+ + e = Las+ and n = I was obtained. The effects of change of temperature, height of the Hg column, concentration and pH have been studied. The values of temperature coefficients of id at different temperature levels, which were found to increase with increasing temperature (0.974% per degree at 28.5”C and 1.81% per degree at 51.0X), and non-linearity of wave height with Las+ concentration indicate that the wave produced by La 3+ is not diffusion-controlled; it is neither very reproducible nor well defined.
INTRODUCTION W. without
NODDACK
supporting
AND A. BRUCKL~ have electrolyte
and reported
studied
the polarography
of lanthanum
that it reduces at the dropping
mercury
Las+ + e = Las+ (E, = -1.9 V) and (2) La2+ + 2e + La (metal) (E, = 2.0 V). PURSHOTAMAND RAGHAVARAO~ employing LiCl media also suggested a two-step reduction process for lanthanum, viz. Las+ + La2+ + La. SANCHOGOMEZANDALMAGRO~ obtained a single-step wave, perfectly separated from the H wave, for lanthanum with a probable reaction mechanism Las+ --f Lao in 0.1 M LiCl in a mixture of water and 50 or 80 vol. o/0ethanol at [H+] = I x 10-3 or electrode
in steps,
2 x 10-a.
AKIO
(I)
IWASE~
has studied
the polarographic
behaviour
of lanthanum
in
M solutions of (CH&NI, (CH&NB r and LiCl and obtained a single step wave in a pH range 2-6. At pH less than 3, a three-electron reduction process was suggested. In view of the conflicting details of the reports of previous workers concerning the polarographic behaviour of lanthanum at the d.m.e., it was considered worthwhile to reinvestigate the nature of the reduction wave produced by Laa+ions by studying the effects of change of temperature, mercury pressure, pH and concentration of La3+ on the wave height. 0.1
J. Less-Common
Metals, 8 (1965) 347-351
M. L. MXTTAL, R. S. SAXENA
343 EXPERIMENTAL
Anal. R. (RDH) reagents, La(NO&, (CH&NCi and gelatin were used and their solutions prepared in air-free conductivity water. A manual polarograph with S& Galvanometer as current recorder was employed for determining polarograms. A capillary having the following characteristics, 112= 1.823mg/sec, t = 4.60 set, and &!atr/a = 1.925 mg213 set-* (in 0.2 M with saturated (CH&NCl at &I.~. = -1.0 V zts. S.C.E.) was used in conjunction calomel electrode connected to the cell by a low resistance salt bridge. The cell was kept immersed in an electrically maintained thermostat and the solution was swept for x0-15 min with pure hydrogen before determining each polarogram. The necessary correction was made for residual current in determining all diffusion current data. o.o010/ gelatin was used as maximum suppressor and 0.2 lkl (CH&NCl as supporting electrofyte. RESULTS AND DISCUSSION
Effect of the variatiovL of La(iVO& concentration OFZ wave height Polarograms of solutions containing different concentrations of lanth~um nitrate (0.5-2.5 m&f/l) were drawn and the value of diffusion current at each concentration was determined; the values of &,lc and diffusion current constant were calculated (Table I) and iti and i& were plotted against the concentrations of La(NO~)~ employed (Fig. I, curves I and 2).
5,L7.~~ 0.50
1.00 1.50 Concentration
Fig. I. Curve I, plot of da/c VS. concentration La (NO&. 1. Less-Common Me&&
8 (1965)
347-351
2.50
2.00 (mM/t)
of La(NO&:
curve 2, plot of id vs. concentration
of
POLAROGRAPHY
TABLE
OF TANTALUM
IN TETRAMETHYLAMMONIUM
CHLORIDE
MEDIA
I
TESTFOKTHE
OF WAVE
LINEARITY
Concentration CmMlt)
La(NO&
HEIGHTWITH
of La(A;O&
CONCENTRATION
ia after cowection for ir (PA)
0.500
6.92 13.12
13.840 13.120
7.19 6.82
1.250
‘3.56 ‘5-56 18.40 x9.59
10.800 9.334 9.200 7.800
5.58 4.85 4.79 4.65
I .ooo I.667
2.000 2.500
-
~.~..__^...___~
It is evident
from
stant)
are not constant
c and
id are not
The at five
polarograms
I that
the values
the curves
I and
of solutions
temperatures
of diffusion
are tabulated
of id/c and
2 (Fig.
-
2.0 mM/l
ranging
current
in Table
vs. log(i/&
TABLE
Table and
I
(diffusion
I) plotted
current
between
con-
c and id/c and
linear.
different
coefficient
F-d.e.
349
from
with
respect
51T.
28” to
for each temperature
II together
with
i) determined
interval
the values
at same
to La(NO& The
were
of 0.059/n,
temperature
values
were
drawn
of temperature
calculated
i.e. slopes
and they
of the plots
of
levels.
II
EFFECT OF THE VARIATION OF TEMPERATURE ON WAVR CHARACTERISTICS ._.-^_l_ ~____..__._
Temperature (“C)
ia aftev correction for i, (PA)
22.60 24.15
51.0
26.25
0.974 I.438 1.163 1.810
2I.00
The coefficients
plot
of icy vs. temperature
of diffusion
the change
controlled
by
The yield
different
the diffusion
line
and
support
id does that
I)
and
not
values
show
the height
(Table
II)
that
of electrons
i) for the wave
indicating
at the
d.m.e.
s h ow
at higher taking
to -1.37
the above
2, curve
that
suggest
-
0.063
pIace
suggest
V at 28.5”C
and
0.0392
of
temperature
a linear
of the wave
relationship is not solely
factor.
of slope
takes
(Fig.
indicate
log(i/id
vs.
temperatures
the number
mV/“C)
in temperature
transfer
temperature,
.??k (-1.34
current
plot of Ed.&
a straight
electron
and
0.0630 0.0530 0.0500 0.0450
19-75
28.5 35.0 40.3 46.2
with
Slope lo.o59lnJ
Temp. coefficient f %i”Cl
fact
that
The
values
a considerable temperatures
part
in reduction
V at =jI.o”C) and suggest
andits
that
produced
the wave
by
z m&f/l
is reversible
of the slopes decrease
process
the wave
increases.
at
increase
The
coefficient
produced
j. Less-Common
the
of the wave
temperature
a one-
for the waves
with
the nature
La(NOS)S and
in
changes shift
of
(-1.33
is reversible.
The
Metals, 8 (1965) 347-351
M. I,. MITTAL,
R. S. SAXENA
Fig. 2. Plot of id us. temperature.
catalytic effect on the wave becomes more pronounced at higher temperature, as is clear from the increased value of temperature coefficient of diffusion current at 51.o”C. Effect of variation of height of Hg column and $H A series of polarograms of I mM/l lanthanum nitrate solution were drawn at different heights of the mercury column. At high mercury pressure irregular deflections in the galvanometer were observed. The approximate values of id for waves obtained at different mercury pressures do not show linear relationship with the height of mercury column and, moreover, the values of id/h* were not found to be constant. These observations also provide evidence for the catalysed reduction wave. Polarograms of I mM/l La(NO& were also recorded at different pH levels. The height of the wave was found to be quite independent of pH, and E, remains almost unaffected in a pH range 4.8-6.0, but at higher and lower pH levels than this range, E, shifts to the negative side. The slope of the plot of Ed.e. vs. log(i/id - i), i.e. the number of electrons taking part in the reduction process also remains unaffected. These observations suggest that the wave produced due to La(NO& in the presence of 0.2 M tetramethylammonium chloride and 0.001% gelatin is reversible and not diffusion-controlled; it is neither very reproducible nor well defined and the current is partially dependent on the rate of chemical reaction occurring in the diffusion layer. The effect becomes more pronounced at higher temperature. Further work is in progress. J. Less-Common
Metals,
8 (1965) 347-351
POLAROGRAPHY
OF TANTALUM
IN TETRAMETHYLAMMONIUM
CHLORIDE
MEDIA
351
ACKNOWLEDGEMENT
The authors are indebted to the Council of Scientific and Industrial Research, New Delhi (India) for the award of a fellowship to one of them (M.L.M.). REFERENCES I W. NODDACK AND A. BRUCKL, Angew. Chem., 50 (1937) 392. 2 A. PURSHOTAM AND BH. S. V. RAGHVA RAO, Anal. Chim. Acta., 17. (1955) 589. 3 J, SANCHO GOMEZ AND V. ALMAGRO, Publs. Inst. Fis. “Antonio de Gregorio Rocasolano”, (1960) r-8; cf. CA, 54 (1959) wqd. 4 AKIO IWASE, Nippon Kagaku Zasshi, 80 (1959) 1133-8.
/. Less-Common
Metals,
14
8 (1965) 347-351