JOURNAL OF THE LESS-COMMON METALS 347

POLAROGRAPHY OF LANTHANUM IN CHLORIDE MEDIA

M. L. MITTAL AND K. S. SAXENA

Department of Chemistry, Government College, Kota.

TETRAMETHY LAMMONIUM

Raj. (India)) and Department of Chemistry, Malaviya Regional Engineering College, Jaipur (India)

(Received January 5th, 1965; revised February 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+ concentra- tion indicate that the wave produced by La 3+ is not diffusion-controlled; it is neither very reproducible nor well defined.

INTRODUCTION

W. NODDACK AND A. BRUCKL~ have studied the polarography of lanthanum

without supporting electrolyte and reported that it reduces at the dropping mercury

electrode in steps, (I) Las+ + e = Las+ (E, = -1.9 V) and (2) La2+ + 2e + La (metal) (E, = 2.0 V). PURSHOTAM AND RAGHAVA RAO~ employing LiCl media also suggested a two-step reduction process for lanthanum, viz. Las+ + La2+ + La. SANCHO GOMEZ AND ALMAGRO~ 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/0 ethanol at [H+] = I x 10-3 or 2 x 10-a. AKIO IWASE~ has studied the polarographic behaviour of lanthanum in

0.1 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 worth- while to reinvestigate the nature of the reduction wave produced by Laa+ions by study- ing the effects of change of temperature, mercury pressure, pH and concentration of La3+ on the wave height.

J. Less-Common Metals, 8 (1965) 347-351

343 M. L. MXTTAL, R. S. SAXENA

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&amp Galvanometer as current recorder was employed for determining polarograms. A capillary having the following character- istics, 112 = 1.823 mg/sec, t = 4.60 set, and &!atr/a = 1.925 mg213 set-* (in 0.2 M (CH&NCl at &I.~. = -1.0 V zts. S.C.E.) was used in conjunction with saturated 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 con- centration 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,L 7.~~ 0.50 1.00 1.50 2.00 2.50

Concentration (mM/t)

Fig. I. Curve I, plot of da/c VS. concentration of La(NO&: curve 2, plot of id vs. concentration of La (NO&.

1. Less-Common Me&& 8 (1965) 347-351

POLAROGRAPHY OF TANTALUM IN TETRAMETHYLAMMONIUM CHLORIDE MEDIA 349

TABLE I

TESTFOKTHE LINEARITY OF WAVE HEIGHTWITH La(NO& CONCENTRATION

Concentration of La(A;O&

CmMlt)

ia after cowection for ir (PA)

0.500 6.92 13.840 7.19 I .ooo 13.12 13.120 6.82

1.250 ‘3.56 10.800 5.58

I.667 ‘5-56 9.334 4.85

2.000 18.40 9.200 4.79

2.500 x9.59 7.800 4.65 - ~.~ ..__^...___~

It is evident from Table I that the values of id/c and I (diffusion current con-

stant) are not constant and the curves I and 2 (Fig. I) plotted between c and id/c and

c and id are not linear.

The polarograms of solutions 2.0 mM/l with respect to La(NO& were drawn

at five different temperatures ranging from 28” to 51T. The values of temperature

coefficient of diffusion current for each temperature interval were calculated and they

are tabulated in Table II together with the values of 0.059/n, i.e. slopes of the plots of

F -d.e. vs. log(i/& - i) determined at same temperature levels.

TABLE II

EFFECT OF THE VARIATION OF TEMPERATURE ON WAVR CHARACTERISTICS ~____..__._ ._ .-^_l_

Temperature ia aftev correction for i, Temp. coefficient Slope

(“C) (PA) f %i”Cl lo.o59lnJ

28.5 19-75 0.0630 35.0 2I.00 0.974 0.0530 40.3 22.60 I.438 0.0500 46.2 24.15 1.163 0.0450 51.0 26.25 1.810 0.0392

The plot of icy vs. temperature (Fig. 2, curve I) and values of temperature

coefficients of diffusion current indicate that id does not show a linear relationship

with the change in temperature and suggest that the height of the wave is not solely

controlled by the diffusion factor.

The plot of Ed.& vs. log(i/id - i) for the wave produced by z m&f/l La(NOS)S

yield a straight line of slope 0.063 indicating that the wave is reversible and a one-

electron transfer takes pIace at the d.m.e. The values of the slopes for the waves at

different temperatures (Table II) h s ow a considerable decrease with the increase in

temperature, and suggest that at higher temperatures the nature of the wave changes

and the number of electrons taking part in reduction process increases. The shift of

.??k (-1.34 V at 28.5”C to -1.37 V at =jI.o”C) andits temperature coefficient (-1.33

mV/“C) support the above fact and suggest that the wave produced is reversible. The

j. Less-Common 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 deflec- tions 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”, 14

(1960) r-8; cf. CA, 54 (1959) wqd. 4 AKIO IWASE, Nippon Kagaku Zasshi, 80 (1959) 1133-8.

/. Less-Common Metals, 8 (1965) 347-351