Arya, Satya V.

Garg, Anil K.

Malik, Wahid U.

English

NOPR

Indian Journal of ChemistryVol. 14A, December 1976, pp. 994-996Polarographic Studies on La(III), Sm(III), Eu(III) & Yb(III)Complexes with Diethylenetriaminepentaacetic, Triethylenetetraminehexa-acetic & Tetraethylenepentamineheptaacetic AcidsANIL K. GARG· & SATYA V. ARYAtDepartment of Chemistry, D.A.V. College, Muzaffarnagar 251001andWAHID U. MALIK!Department of Chemistry, University of Roorkee, Roorkee 247667Received 28 November 1974; revised 28 Apri! 1976; accepted 12 May 1976Diethylenetriaminepentaacetic acid (DTPA). triethylenetetraminehexaacetic acid (TTHA)and tetraethylenepentamineheptaacetic acid (TPHA) form 1: 1 complexes with La(III). Sm(III).Eu(III) and Yb(III), which are quite stable in acidic medium. The stability constants of thesecomplexes have been determined by the displacement method. The effect of various support-in~ electrolytes, aqueous alcoholic media, pH and surface active substances on the polaro-graphic behaviour of these complexes have been described.IN recent years, much interest has b~en show!1in the complexes of rare earth with multi-dentate ligands, especially with aminopoly-carboxylic acidsl-6 due to their possible applicationas laser materials. Polarographic studies on rareearth complexes involve certain difficulties. Sincethe trivalent rare earths have negative reductionpotentials, similar to those of alkaline earth metals,there is possibility of secondary reactions with themedium which may mask the primary wave. Asurvey of literature reveals that extensive polaro-graphic studies have been carried out on the rareearth complexes w ith various nitrogen donorst!'.However studies on complexes with ligands contain-ing both oxygen and nitrogen donors such asaminopolycarboxylic acids are limited to aqueousmedia owing to the low solubility of the supportingelectrolytes in organic non-aqueous media.The polarographic behaviour of La(III), Sm(III),Eu(III) and Yb(III) complexes with aminopolyaceticacids in which the carboxylate groups vary fromfive to seven has been studied and the results arereported in this note. The stability constants ofthese complexes have been calculated by Schwar-zen bach's method. Solvent effect, effect of pH,and various supporting electrolytes on the polaro-graphic behaviour of rare earth complexes havealso been studied.Materials and MethodsNitrates of the rare earth metals were prepared bydissolving weighed quantities of rare earth oxides(Sigma Chemical Co., USA) in dilute nitric acid.The excess of acid was removed by evaporation*To whom all correspondence should be addressed.tPresent address: Department of Chemistry C.C.R.Degree College, Muzaffarnagar.!Present address: Vice-Chancellor, Bundelkhand Univer-sity. Jhansi.994and the solutions standardized by complexometrictitration 12, using Xylenol Orange as all indicator.Extra pure disodium salts of the ligands, viz.diethylel1etriaminepcntaacetic acid (DTPA), triethy-lenetetraminehexaacetic acid (TTHA) and tetraethy-lenepentamineheptaacetic acid (TPHA) were,obtained through the courtesy of Dojindo & Co.Ltd, j apan , and were used as such, A solution ofcadmium was prepared by dissolving AR cadmiumni tra te,Polarographic studies were carried out on amanual Toshniwal polarograph, in conjunction witha Pye-Scalamp galvanometer, using a d.rn.c. andpotentials were measured against a saturated calomelelectrode (SCE). The capillary characteristics wasm2J3 tll6 = 2·06 mg3/3 sec-1J2•The solution s \\<eH~ de aerated by bubbling purifiednitrogen gas for about 10 min before recording thepolarograms at 30'±0·1°.The liga:lC] concentration employed. was 1·0 X10-2111'. Acetic acid-sodium acetate buffer solutionwas used in all the cases except when alternate pro-cedure is described. The buffer concentration wasmaintained at O·Lv! with respect to both acid andbase form. Reagent grad(~ potassium nitrate wasadded to maintain the ionic strength and to Dringtile concentration up to 0'5111'. Distilled ethyl al-cohol was used and supporting electrolytes LiCIand (C2H5),NBr were of (BDH) AR grade. TritonX-lOO (Rohm & Haas Co., Philadelphia, USA) wasused as maxima suppressor. Doubly distilled waterwas used for the preparation of all the solutions.pH measurements were made with a Toshniwal(C. No. -42-1) pH meter using glass electrode. Allother chemicals were of extra pure grade.Results and DiscussionPolarographic study - Polarograms of La(III),Sm(III), Eu(III) and Yb(III) in the presence ofGARG et ai.: AMPEROMETRIC STUDIES OK SOME RARE EARTH COMPLEXESDTPA, TTHA and TPHA exhibit a large maximumat [rare earth ion] > 1O-3M. The maximum iseasily suppressed by the addition of surface activeagents but the half-wave potential is stronglyaffected by the presence of even small amoun ts ofsuch maxima suppressons. Hence all polarogramswere recorded by keeping the concentration of rareearths below 1O-31'\J. The nature of the polaro-graphic waves was studicd-". It was found thatreduction in all the cases was diffusion-controlledand the waves were found to be irreversible a"evinced from the log plots-". The polarographicdata are recorded ill Table 1. The method ofdetermining the stability constan t for reversible\\ avo could not, therefore, be applied.Stabilitv constants - The stabili tv constants weredctermir~d by llsing Schwarzen bach's displacementmcthodl" using cadmium a" indicator ion.(% Cd~+)2K = (100--% Cd2"'j2The stability constants were calculated fromabove cquatior, and their values arc given in Table 2.Solutions containing cadmium-T'I'H'A complex(O'OOlM), R(N03)3 (O'OOlM), sodium acetate(O·OUI-n, acetic acid (O'OlM) wr re prepared aNIpotassium ni trate was added to keep the ionicstrength constant (0'1). Triton X-lOa was addedto suppress the maximum and solutions were dilutedaccordingly. Similar solutions were also preparedat an ionic strength of 1·0. All the solutions wereplaced in the thermostat for 48 hr and werewell shaken to ensure complete reaction and thenpolarograms were recorded. The id value of eachrare earth ion was calculated, the amount of freecadmium in the above solution was calculated bytaking blank polarogram of Cd2+ in potassium nitrate.Two well-defined waves appeared, as expected, thefirst wave corresponding to Cd-TTHA complexan d the second wave to free rare earth ion.The results of the present amperometric titrationsindicate that rare earth (III) forms 1:1 complexeswith TTHA, DTPA and TPHA which are quitestable ill the pH range 4-6, 5-6 and 2-6 respectively.The effect of ionic strength OIl the stability con-stant has also been studied and the results showthat the stability constant decreases with anincrease of ionic strength.A close inspection of stability constants (Table 2)show that DTPA forms the most stable complexeswith rare earths. TTHA and dodecadentate ligandTPHA forms comparatively less stable complexes.It is supposed that coordination number may de-crease with the steric hindrance caused by the largesize of the ligand, as the rare earths (III) ionsdecreases in size.Effect of dielectric constant of the medium - Arelation has been shown to exist between varia-tion of the dielectric can stan ts of the differentaqueous-alcoholic mixtures and the half-wave poten-tial becomes more and more positive as this constantdiminishes (Table 1). The E1/2 and slope valuesfor various rare earth and their complexes ill 25and 50% ag. ethanol are given in Table 1. Theresults indicate that El!2 becomes more positive withan increase in alcohol content of the medium,whereas diffusion current (id) diminishes. Accordingto the slope of Ed.m.c. vs. log (i/id-i) plot, the reductionof these ions at d.m.e. becomes more irreversibleas the proportion of ethanol increases.Effect of surface active substances - The effectof surface active substances on the polarographicwaves of rare earths. TTHA, DTPA and TPHAcomplexes was investigated using strongly adsorbedionic and non-ionic snrfactants. It has beengenerally observed that the polarograms were------------------------------TABLE 1 -l'OLAROGRAPHIC ])ATA ON RARE EARTHS Al'iD THEIR COMPLEXES[Supporting electrolyte for rare earths, LiCl 0·1M; and for rare earth complexes (C2Hs).NDr, 0'l1le1;maximum suppressor,triton X-I00, 0·01%JRare earth EtOH (25%) EtOH (50%)or ----- ._-------rare earth pH El;2 SI, 'Pl': J pH E1I2 Slopec rcomplexes (\i) (\T)La 3·00 --1·90 0030 3·121 3·00 -1'88 0·090 2'461La* 3·00 --1-82 0·040 2·378Sm* 3·00 --1'74 0·050 2·021 3·00 -1'80 0·040 2'216Eu* 2·90 -- 0·68· 0·714 2·80 -O'72a 0·080 0·953--1'351> 1·778 -1'321- 0·035 1·834Yb* 2·80 ·1'82" 0028 1·520 2·90 -1,8010 0·030 1·886La (nn':\) 3·00 -1,98 0044 2·935 3·00 -1,97 0·040 2·782La erTHA) 3·10 --1'96 0062 2·377 3·00 -1,94 0·060 2·219La (Tl'RA) 3·20 2·02 0055 2·33fl 3·00 -2·00 0·060 2·110Sm (DTI'A) 5·20 1·8K 0032 2·937 5·00 -1,85 0·044 2·719Sm (TTHA) 5·00 -1,91 0·066 2·125 5·10 -1·88 0·056 2·091Srn (T]'Il,\) s·on - 203 0063 1·983 5·00 -2'01 oosr 2·247Vb (DTPA) 4·00 -1-87 0·080 1·762 4·00 -Hl6 0·066 1·640Vb (TTHA) 4·10 -1,93 0·065 1·450 4·10 -1'90 0·073 1·590Yh (TI'HA) 4·30 ·-2·06 0·058 1·640 4-10 -1·98 0·054 1·450(a) Half-wa ve potential of first wave.(b) Half-wave potential of second wave.(c) Slope of E,l.m.e. vs log (i/id-i) plot.·At EtOH, 75%.995INDIAN J. CHEM., VOL. 14A, DEem.IBER 1976TABLE 2 - STABILITYCONSTANTSOF RARE EARTH (III)COMPLEXESWITH DTPA, TTHA AND TPHA[Temp., 30o±1°; ionic strength = 0·1 (KN03)]Rare log kearthDTPA TTHA TPHAcomplex complex complexLa 19·32 19·52 19·10Sm 22-80 19·54 20·50Eu 22·91 19·57 20·70Yb 22'59 19·46 19·82strongly distorted in the presence of even very smallamount of non-ionic (0'005%), Triton X-lOO andanionic (0,002%) sodium dodecyl sulphate. Theelectrocapillary curve of mercury shows that theaddition of Triton X-lOO to the solution causes alarge decrease in the drop time of the capillary, theelectro capillary maximum shifts to the positiveside with increasing concentration of Triton X-lOO.These facts show that an adsorption of Triton X-100occurs on the mercury electrode; the film formationof surface-active substances can be regarded as arapid process. The inhibiting effect of the absorbedfilm of Triton X-100 on the electron transferrenders the current potential curve more irrever-sible16•Effdct of pH - As the pH of the solution increases,the Em shifts towards more negative values. Thehalf-wave potential of Eu-TTHA complex was,measured in the pH range of 3 to 10 (borax-NaOH,HaPO,-boric acid CHaCOOH-NaOH, NH,Cl-NaOHbuffer solutions). In the case of Yb-TPHA complexthe half-wave potential shifted to the more negative996values with increasing pH from 4·40 to 5'45, andbecame constant in the pH range 5·45 to 6'45, be-cause of the formation of stable complex species.The limiting current ia decreased gradually withincreasing pH and the electrode process wasirreversible.Acknowled~ementThanks are due to Dr Ramesh Bembi, Depart-ment of Chemistry, University of Roorkee, forhelpful discussion.References1. SCHWARZENBACH,G., GUT, R. & ANDEREGG, G., lIelv.chim. Acta, 37 (1954), 937.2. SCHWARZENBACH,G. & GUT, H., Helv. chim, Acta, 39(1956), 1589.3. SPEDDING, F. H., POWELL, J. E. & \VHEELWRIGHT,E. J., j. Am. chem, Soc., 78 (1956). 34.4. HARDER, R. & CHABEREK.S.• J. inorg. nucl. Chem., 11(1959). 197.5. GRENTHE. I.. J. Am. chem. Soc., 83 (1961). 360.6. MOELLER. T. & FERRUS, H., J. inorg. nucl, Chem., 20(1961). 261.7. \VHEELWRIGHT.E. J .. SPEDDING. F. H. & SCHWARZEN-BACH. G.• [: Am. chem. Soc .. 75 (1953), 419.8. :'IIoELLER,T., :\IARTIN. D. F .• THOMPSON.L. c.. FERRUS.H.. FEISTEr., G. B.. & RANDALL,W. J., Chem. Rev .• 65(1965). 1.9. FORSBERG. J. H .• Coord. Chern. Rev .• 10 (1973). 195.10. MISUMI. S. & l\IASUDA.Y.• Rev. Polarogr .• 14 (1967). 173.11. SINHA, S. P.. Complexes of the rare earths (PergamonPress. Oxford). 1966. 24.12. SCHWARZENBACH,G., Complexometric titrations (Inter-science. New York). 1957, 73.13. KOLTHOFF. L. M. & LINGANE, J. J., Polarography.Vol. 1 (Interscience, New York), 1952. 106; 203.14. MEITES. L.. Polarographic techniques (Interscience, NewYork). 1967. 232.15. IWASE, A., Bull. Yamagata Univ. natn, Sci .. 5 (1962). 659.16. HEYROVSKY. J. & KIJ A. J .. Principles of polarography(Academic Press, New York), 1966. 303.

Polarographic Studies on La(III), Sm(III), Eu(III) & Yb(III) Complexes with Diethylenetriaminepentaacetic, Triethylenetetraminehexaacetic & Tetraethylenepentamineheptaacetic Acids

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Polarographic Studies on La(III), Sm(III), Eu(III) & Yb(III) Complexes with Diethylenetriaminepentaacetic, Triethylenetetraminehexaacetic & Tetraethylenepentamineheptaacetic Acids

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