Eo in the Case of Europium-Crotonic Acid System, a Curvature

NOTES

5. LAL, S., Bull. chem. Soc. Japan, 46 (1973), 2232. eo 6. LAL, S., Aust. J. Chem., 25 (1972), 1571. 7. HOLLECK, L., Z. Naturforsch., 26 (1947), 81. 8. GELLINGS, P. J., Z. Elektrochem., 66 (1962), 477; 67 (1963), 60 167 ..!! 9. DEFORD, D. D. & HUME, D. N., J. Am. chem. Soc., 73 o (1951), 5321. c 10. CRow, D. R., Polarography of metal complexes (Academic o .. 40 Press, London), 1969. 5. 'iii o 0. ~ 20 CJ Formation Constants of Dioxouranium (VI), Cu (11), M Ni(II), Zn (II), Co(II) & Mn(1I) Complexes of O~---L----~--~~--~ 2-0 '·5 ',0 0'50 o Peonoloxime -log ex V. SURESH BABU, D. UGANDHAR R.Aru & R. RAGHAVA NAIDU· Fig. 1- Distribution diagram for Eu(IlI)-crotonic acid system Department of Chemistry, Sri Venkateswara University. Tirupati 517502 In the case of europium-crotonic acid system, a Received 22 August 1978; accepted 27 January 1979 curvature obtained in the plot of Ell 2 versus -log Cx showed the formation of successive complexes. The The metal-ligand stability constants log K, and log K. and the Deford and Hume treatment as modified by Irving proton-ligand stability constant log KL have been determined in was applied and Eu(lIl) was found to form 1: 1 and 50% (vrv) dloxan-water, acetone-water. and 2-ethoxyethanol- 1:2 complexes with crotonic acid with stability cons- water mixtures using the Bjerrum-Calvin pH titration technique tant values of 60 and 4.2 x 10+2 respectively. From the as modified by Irving and Rossotti. The stability constants are known stability constant data, values of the degree in fair agreement with Irving-Williams rule. The results obtained of formation (Fig. 1) for each complex in a system are compared with the data available in the literature for struc- have been calculated using Eq.2 (ref. 10). turally similar ligand complexes. Amongst the solvents used acetone-water shows abnormal behaviour • ... (2) N view of the analytical applications- of peono- I Ioxime (2-hydroxy-4-methoxyacetophenoneoxime), Stability constant data show that the complexes of it is of interest to know the physico-chemical Eu(lIl) with crotonic acid are more stable than that properties such as absorption spectral characteristics, with acrylic acid. thermal properties, magnetic susceptibility and stabi- The reduction equilibrium for the formation of lities of its metal complexes. Hence the authors successive complexes reported above may be repre- have undertaken these studies and the present investi- sented by Eqs 3 and 4 : gation deals with the determination of the stability 2 Eu3+ HX ~ [Eu(X)]2+ H+ (3) constants of the complexes of U022i', Cu +, Ni2i', + + Zn2+, C02+ and Mn2+ with peonoloxime by Calvin- [EuX]2+ + HX ~ [EU(X)2]++ H+ (4) Bjerrum titration techniques+ as modified by The percentage distribution of various complex Irving and Rossetti", Since the chelates were inso- species as a function of ligand concentration has been luble in water (except cobalt, zinc, and uranium) calculated in the case of Eu(III)-crotonic acid system. the study was carried out at 28° in 50% (v/v) Based on this a polarographic method for the deter- dioxane-water, acetone-water and 2-ethoxyethanol- mination of microamounts of Eu(lll) has been water mixtures. developed. Under optimum conditions Eu(lll) in Peonoloxime was prepared according to literature the range 4x 10-4-2 x 1O-3M can be successfully methods" and its solution (O.IM) was prepared in determined in presence of different ions. The the respective organic solvents. The metal nitrate average percentage recovery of Eu(llI) in the presence solutions were prepared in doubly distilled water of various ions has been found to be 99.99 % with a and star.dardised .. Dioxan", acetone" and 2-ethoxy- standard deviation of 0.192. ethanol'? were purified l::efore use. Metal ions like Ag(I), Au(lIl) and Te(IV) are preci- Elico (Ll-IOA) pH meter was used for pH measure- pitated. Metal ions like Se(VI), W(VI), Se(IV), ments. Ti(IV) and Zn(lV) do not undergo reduction. Te(VI), Procedure - The experimental procedure involved Cr(VI), Sb(lll) Cr(III), As(III), Pb(II), Tl(I), U(VI), the titration of the following carl::onate-free solutions Cu(II), Yb(III), Mn(II), Co(II), Ni(IJ) and Zn(IJ) (total volume 50 mI) against standard sodium hy- undergo reduction at different potentials than that droxide (0.1025M) : of Eu(III). Only Mo(VI), V(V), Cd(lI) and In(III) (a) 5 mI of(O.OIM) nitric acid + 25 ml of organic interfere in the determination of Eu(IIJ). solvent + 20 ml of water. (b) 1 mI of (O.IM) ligand in pure organic solvent References + 5 mI of (O.OIM) nitric acid + 24 mI of 1. MACERO, D. J., ANDERSON, L. B. & MACHESKY, P., J. electro- organic solvent + 20 mI of water. onalyt . Chem., ]0 (1965), 76. (c) 0.5 mI of (O.OIM) metal solution + 1 mI of 2. MACERO, D. J., HERMAN, H. R. & DUKAT, A. J., Analyt. (O.IM) ligand + 5 mI of (O.OIM) nitric acid Chem., 37 (1965), 675. 3. ZUTSHI, K., Res. Polaroqr. Japan., ]4 (1967), 294. + 24 mI of organic solvent + 19.5 ml of 4. IDE, Y., Bull. chem. Soc. Japan, 40 (1967), 2981. water.

( \ INDIAN J. CHEM., VOL. 18A. JULY 1979

The pH meter reading (B) was corrected for the hanced stability of the metal chelates in acetone-water nonaqueous medium following the procedure of may be due to the increased coordination of the Van Uitert and Hass" (-log (H+] = B + log chelate with acetone. A plot of log ~2 versus ioni- UH) and others12,lS. sation potentials shows that the stabilities of the Under our conditions, the following log UH values complexes can be better correlated with the second were obtained in different media under considera- ionisation potential" rather than the first. tion : 0.10 (50% aq. acetone), 0.15 (50% aq. dio- A large difference between log K\ and log K2 xane) and 0.10 (50% aq. 2-ethoxyethanoI). values would beexpected due to possible steric hin- The dissociation constant (Kd of the ligand is drance because of the linking of the second ligand obtained from the formation curve of the proton-" molecule to the metal ion. The data presented in ligand system. The formation curve is constructed Table 1 show that the difference between log Kl by plotting nA values (the number of protons attach- and log K2 values are small and ratio of log KIIK2 ed to the ligand) against pH. From the titration is positive in all the cases. Separation factors bet- curves using the solutions (a) and (b) nA values were ween first and second formation constants are well calculated" at various pH values. For a ligand within the expected range and the absence of high with one dissociable proton log KL becomes equal values indicates that there is little or no steric hin- drance to the addition of second ligand molecule. to the pH corresponding to nA =:I 0.5. Proton- ligand formation constants of the ligand in 50% It is also evident from the values of log K; and log aq. 2-ethoxyethanol, 50% aq. acetone and 50% aq. K2 that there is almost equal tendency for the forma- dioxan are 10.20, 11.00 and 11.20 respectively. tion of neutral complex species ML2 as for species From the titration curves of solutions of (b) and ML+. The values of overall stability constants (log ~2) (c), ;; and pL values were calculated=!". Metal- of peonoloxime complexes show that these chelates ligand formation constants of the complexes were are more stable than the complexes of salicyladehydew, calculated applying least square method" to the ii resacetophenoneoxime'" and o-aminophenol'" but pL data (Table 1). less stable than those of S-hydroxyquinolinew com- The oxime behaves as a monobasic acid in the plexes. Calvin and Melchior" suggested that the media studied. The acid dissociation constant of the replacement of a chelated oxygen atom by a more ligand was found to increase in different media in basic nitrogen atom enhances the tendency of the the order 2-ethoxyethanol > acetone > dioxan. A coordination electron pairs to enter the d-orbital. plot of pKL against liD is linear (where D is the Eventhough the structures of salicylaldehyde and dielectric constant). However, in acetone-water peonoloxime complexes are similar, .he greater stab"- solvent the dissociation constant of the ligand is lity of the latter chelates can be attributed to a less than the expected value and it may be attributed hardness-softness factor. Metal ions with greater to the non-ideal behaviour of acetone-water solvent. number of d-electrons, preferably bind in the order The order of stability of the metal chelates observed N>023. The higher stability of peonoloxime in all the three mixed solvents studied is Zn2+ < Cu2+ chelates over o-aminophenol= complexes may be > U022+ > Ni2+ > C02+ > Mn2+. This order ascribed to the presence of a conjugated six-membered of stability follows the Irving-Williams rule14• This ring in oxime complexes as compared to a five- indicates that the nature of chelation and the type membered chelate ring in o-aminophenol complexes. of bonding in the chelate may be similar in all the From the data presented in Table 1 in respect of metal ions studied. The order of stability of the complexes of peonoloxime and that of resacetophe- complexes with respect to the solvent is dioxan > noneoxime reported in the literature" it appears acetone > 2-ethoxyethanol The dielectric cons- that the introduction of -OCHs group in place of tants for the pure as well as mixed solvents are in the -OH group tends to increase the stability of the order acetone > 2-ethoxyethanol > dioxan. This chelates. This in turn suggests that the increase of also shows the abnormal behaviour of acetone-water electron density at the reactive centre increases the mixture. Similar abnormalities were reported in stability of the chelates. The greater stability of the literature15,16. This suggests that besides die- 8-hydroxyquino1ine complexes= can be accounted lectric constant of the medium. many other pro- for by the presence of an additional aromatic ring perties17-111 such as functional group present in the and consequent higher electron density at the reactive solvent may also play a significant role. The en- centre.

TABLE 1- METAL-LIGAND FORMATION CONSTANTS IN DIFFUERENT SOLVENT MEDlA AT 28°C

SO% (v/v) aq. 2-ethoxyethanol SO% (v/v) aq. acetone SO% (v/v) aq. dioxane Metal log Kl log ~. log K. log ~I log K. log ~. logK.

Cu(ID 9.29±0.OS 18.06±0.06 8.77±0.08 9.63±0.04 18.84±0.03 9.21±0.OS 9.92±0.04 19.34±0.04 9.42±0.06 UO~+ 9.02±0.04 16.19±0.07 7.18±008 9.16±0.03 17.62±0.04 8.46±0.OS 9.8S±0.OS 19.02±0.04 9.17±0.06 Ni (II) 6.18±0.03 11.74±0.03 S.S6±0.04 7.S7±0.03 14.4S±0.04 6.88±0.OS 8.8S±0.03 lS.94±0.08 7.09±0.09 Zn (11) S.83±O.03 1O.9S±0.04 5.l2±O.OS 6.68±0.03 12.66±0.04 S.98±0.OS 6.68±0.03 12.6S±0.OS S.97±0.06 Co (II) S.63±O.03 10.64±0.03 S.OI±0.04 6.47±O.03 12.26±0.04 S.79±0.OS 6.78±0.03 12.87±0.OS 6.09±0.06 Mn(II) 4.42±O.04 8.47±0.OS 4.0S±0.06 S.28±0.04 10.13±0.OS 4.8S±0.06 S.67±0.06 10.7S±0.06 S.08±O.08

, ( NOTES

The authors are thankful to the CSIR, New Delhi, haloamines, dichloramine-T" (DCT) and dibroma- for the award of junior research fellowships to two mino-T" (DBT) as redox titrants in non-aqueous of them (V.S.B. & D.U.R.). media. Dibromamine-T (N,N' -dibromo-p-toluene- sulphonamide) has been employed in our Iabora- References tories'"! for the estimation of thiocyanate, cyanide, 1. RAJA REDDY, G., Oximes as inorganic analytical reagents, thiosemicarbazide alone or in their metal complexes, . Ph.D. thesis, Sri Venkateswara University, 1970. in the presence ofKBr and sodium acetate. The present .2. CALVIN, M. & WILSON, K. W., J. Am. chem. Soc., 67 (1945), paper reports the extension of these studies to the 2003. 3. CALVIN, M. & MELCHIOR, N. C., J. Am. chem. Sac, 70 estimation of thiourea (tu) alone or in its metal (1948), 3270. . complexes: An eight-electron change per ligand 4. BJERRUM, J., Metalammine formation in aqueous solution molecule was noticed in a direct titration of the latter (P. Hause and Sons, Copenhagen), 1941. with DBT, when visual and potentiometric methods 5. IRVING, H. M. & ROSSOTTI, H. S., J. chem. Soc., (1954), 2904; (1953), 3'397. of detecting the end point were employed. Addition 16. ADAMS, J. Am. chem. Soc., 41 (1919), 260. of a little KBr was found essential for smooth oxida- 7. VOGEL, A. I., A text book of practical organic chemistry tion of the reductant, while it served as an indicator (English Language Book Society. London), 1968. 171. to detect the end point in an Andrews type of titration :8. NAIDU, P. R. & KRISHNAN, V. R., Trans. Faraday. Soc., 61 (1965), 1494. using an organic liquid such as CCI, or CHela' The 9. VOGEL, A. I., A text book of practical organic chemistry method is rapid and is helpful for computing the (English Language Book Society, London), 1968, 721. number of ligand molecules in a thiourea complex. :10. CHU, Ytr, K. & THOMPSON, A. R., J. chem. engg. Data., 12 Thiourea is a well-known metal complexing agent (1967),333. ill. VAN UITERT, L. G. & HASS, C. A., J. Am. chem. (Soc., 7S and is used as a. stabiliser and anti-oxidant for a (1953), 451, 458, 76 (1954), 375. number of compounds such as vitamins, alkaloids, 12. CHAWLA, I. D. & SPITTERT, C. R., J. inorg, nucl. Chem., 30 fats and polymers. Several reagents such as hypoio- (1968), 2117. dite", bromine in acid medium", Nvbromosuccinimidet, 13. RAo, B. & MATHUR, H. B., J. inorg. nucl. Chem., 33 (1969), lead tetracetate" and iodine trichloride" have been 2919. 14. IRVING, H. M. & WILLIAMS, R. J. P., Nature, 162 (1948), employed for determining thiourea and substituted 746. thioureas in solution. A direct titration between 15. GRIGORIPOPA, CONSTANTINE LUCA & VASILE MAGE., Chem, thiourea and chloramine- T has been reported by Abstr., 63 (1965), 2438. Singh et al.1o while a rapid back titration procedure 16. SESHAGIRI, V. & BRAHMAJI RAo, S., Aust. J. Chem., 20 (1967), 2783-4. has been developed by Aravamudan and Rao-'. 17. International critical tables (McGraw Hill, New York), The latter method is useful for estimating thiourea (1929),22. in metal thiourea complexes'> in solution by chlor- 18. EDULJEE, H. E., KUMARAKRISHNA RAO, V. N. & NARASING amine-To RAO, M., Chem. engg. sa., 3 (1958), 44. 19. DRINKARD, W. & KIVELSON, D., J. phys. Chem., 67 (1965), Thiourea (AR, E. Merck) was recrystallized from 1494. aqueous solution and its purity checked. Metal '20. MELLOR, D. C. & MALLEY, L., Nature, 159 (1947),370. complexes, Zn tU2C12, Zn tu2(CH3COO)2' Zn tu3SO" '21. CHARLES, R. G. & FREISER, H., J. Am. chem. Soc., 74 (1952), Zn tU4C1 , Cd tU2 Cl , Cd tU2 (CH3 COO)2' Cd tU 1385. 2 2 2 22. JOHNSTAN & FREISER, H., J. Am. chem. Soc., 74 (1952), (HCOO)2' Cd tU2(NCS)2' Pb tU8(CI04)2 and Hg tU3 5239. C12were prepared by methods reported in literature. \ :23. SIGEL, H. & MCCORMICK, D. B., Ace. chem. Res., 3 (1970), The purity of complexes was checked by elemental 201. analyses. All other reagents were of accepted grades of purity. Aqueous solutions (,...,5 mg/ml) of thiourea and of its metal complexes were prepared [,...,1 mg/ml in Dlbromamine-T as an Analytical Reagent : Estimation the case of Cd tU2(NCSh and the mercury complex]. of Thiourea Alone or in Its Metal Complexes Preparation of DBT and standardization of its solution in glacial acetic acid were carried out by the M. S. AHMED & D. S. MAHADEVAPPA* method of Nair and Indrasenan". Department of Post-graduate Studies and Research in Chemistry, Recommended procedure: (a) Visual end point - To University of Mysore, Manasa Gangothri, Mysore 570006 an aliquot of thiourea or its complex solution were Received 22 November 1978; accepted 22 December 1978 added 1M KBr solution (0.5 ml), eC1, (1 ml) and enough water to retain the organic layer. The solu- A simple and rapid method for the estimation of thiourea tion was titrated against DBT to the appearance of a (to) alone or in its metal complexes, [M ton]Xy where M ••• Zn, faint yellow colour in the organic layer. Blank cor- Cd or Hg with dlbromamine- T has been developed. Both visual rection was 0.05 ml of O.IN DBT. .and potentiometric methods of detecting the end point, with an (b) Potentiometric end point - A potentiometric Andrews type of titration in the former, are proposed. Oxidation titration, using a potentiometer with Pt-SCE elec- ,involves an eight-electron change per thiourea molecule. The trode assembly of thiourea and its metal complexes products of oxidation have been identified. The proposed method with DBT was practicable in the presence of KBr is useful for computing the number of ligand molecules in a (,...,0.5 ml of 1M solution). A large potential jump nnetal-thlourea complex. of ,...,200-400 mV was noticed for the addition of 0.1 m1 of O.IN DBT at the equivalence point. The reaction mixture was generally stirred for 5 see and the readings 'THE number of suitable oxidants available in non- were recorded after about 2 min. aqueous and partially aqueous redox titrimetry Some typical results of analyses, presented in IS limited. Jacob and Nair introduced the organic Table I, show that every g-atom of sulphur present

/ (