Dhuley, D. G.

Dongre, V. G.

English

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INDIAN J. CHEM., VOL. 20A, FEBRUARY 19818. VOGEL,A. I., A text book of quantitative inorganic analysis(Longmans, London), 1961,497.9. SYAMAL,A. & KALE, K. S., Indian J. Chem., 19A (1980),225.10. SINN, E. & HARRIS, c. M., Coord. chern. Rev., 4 (1969)391; TOKu, T., MUTO, Y., KATO, M. & JONASSEN,H.B., J. inorg. nucl. Chem., 34 (1972), 3377.11. SYAMAL,A. & KALE, K. S., Indian J. Chem., 16A (1978),46.12. KATO, M., Jo ASSEN,H. B. & FANNING, J. C., Chern.Rev., 64 (1964), 99.13. BLEANEY,B. & BOWERS,K. D., Proc. R. Soc., London,Ser. A., 214 (1952), 451.14. DUBICKI, L. & MARTIN, R. L., Inorg. Chem., 5 (1966),2203.15. GREGSON, A. K., MARTIN, R. L. & MITRA, S., Proc.R. Soc., London, Ser. A., 320 (1971), 473.Study of Cadmium Complexes of Amino Acids attheir Isoionic Points : Cd(U)-Amino Acid & Cd(U)-Thiourea- Amino Acid SystemsD. G. DHULEY* & V. G. DONGREDepartment of Chemistry, Marathwada University,Aurangabad 431004Received 18 February 1980; revised 9 July 1980;accepted 28 July 1980Cadmium(II)-glycine, cadmium(II)-~ -alanine, cadmium(II)-thiourea-glycine and cadmium(II)-thiourea-(3-a1aninesystems havebeen investigated polarographically at the isoionic points ofthe amino acids in aqueous medium and at 25° ± 0.5°C. Stabili-ties of Cd(II)-amino acid complexes both in simple and mixedsystems are low because of ion pair formation. The mixed ligandcomplex species have higher stabilities than predicted on statisti-cal considerations. The increased stability may be due tosimultaneous -e-bondlng between Cd(II) and both the Iigands.SIMPLE systems as well as mixed ligand systemsof glycine (gly) and ~-alanine (~-ala) with Cd(II)have been studied- at pH above their pKa. valuesbecause at pH > pKa• the NHt dissociates makingavailable a lone-pair of electrons on nitrogen ofamino group and facilitating chelation to give com-paratively more stable complex species'. No in-vestigations have been made of the systems at theisoionic points of these ligands and this promptedus to undertake the title investigation. The pKalandpKa. of gly are 2.43 and 9.80 and those of ~-ala3.52 and 10.15 respectively. The pH = (pKa +pKo.)/2 is the isoionic point. Isoionic points 1 forgly and ,8-ala are therefore 6.10 and 6.80 respectively.The pH's of all the resulting solutions were main-tained at the respective isoionic points. As thioureawas chosen as a secondary ligand for mixed ligandcomplexation, the Cd(II)-(tu) system was studiedonly to get the stabilities of Cd(II)-(tu) complexesin situ.All the reagents used were of AR grade. Solu-tions were prepared in a doubly distilled water.The ionic strength of all the experimental solutionswas kept constant at O.IM with sodium perchlorate.A Cambridge a~tomatic recording polarographwas used for recording polarograms. No maximumsuppressor was found necessary and the polarograms208(of the deaerated solutions were recorded at 25 ±O.5°C. SeE was used as reference half-cell.The dropping mercury electrode had the followingcharacteristics : t = 3.0 see in 0.5 M KCI (opencircuit); and m = 1.96 mg/sec.Cd(/I)-thiourea system - In each solution theconcentration of Cd(II) (cadmium nitrate) wasmaintained constant at ImM while the thioureaconcentration was varied from 0.04 to 0.80 M. TheEll? of Cd(II) ion was -0.587 V which increased inthe presence of thiourea to a more negative valueindicating complex formation (Table 1). The re-lationship between -(Et)c and log CL where CLis the ligand concentration was not linear but asmooth curve with four segments showing the exis-tence of 1: I, 1:2, 1:3 and 1:4 complex species. Themethod of Def'ord-Hume" gave the following valuesfor stability constants: log PlO = 1.40, log ~20 =1.70, log P30 = 1.60 and log P40 = 3.30. Thesevalues are nearly identical to those observed byLane et al:? and Migal et al», The slight deviationsmay be due to the difference in the ionic strengthand the media used. The higher value of ~4 ascompared to ~l' ~2 or ~3 may be due to the fact thatduring the formation of 1:4 complex, the coordi-nation number four of Cd(II) is satisfied.Cd(II)-glycine system ~ The concentration ofglycine was varied from O.04M to O.80M. Otherconditions were identical to those maintained in theabove system. The pH of every solution was keptconstant at pH 6.10. The shift of Et of Cd(II) tomore negative values (Table 1) by the addition ofglycine indicated complex formation. Two complexspecies 1:1 and 1:3 were detected and their stabilityconstants were log ~Ol = 0.78 and log P03 = 1.36.Cd(II)-p-alanine system ~ ~-Alanine concentra-tion was varied from 0.04M to O.80M maintainingthe other conditions same as in the previous systems.TABLE 1 - SHIFTSIN HALF-WAVEPOTENTIALOF Cd (II) IN THEMIXED LIGAND SYSTEM[Cd = 1 mM; I.L = 0.1 M (NaCIO.); -(Ell.). = 0.587 V]Ligand -e., (vs. S.C.E.)(tu)M O.OOM 0.16M O.40M O.12M 0.40MgIY/(3-ala gly gly (3-ala (3-ala0.04 0.590 0.595 0.610 0.598 0.6130.08 0.602 0.607 0.619 0.607 0.6220.12 0.606 0.612 0.625 0.615 0.6270.16 0.612 0.619 0.632 0.621 0.6330.24 0.623 0631 0.642 0.633 0.6440.32 0.631 0.639 0.651 0.639 0.6530.40 0.641 0.646 0.657 0.649 0.6600.48 0.649 0.653 0.663 0.656 0.6660.56 0.655 0.659 0.668 0.661 0.6720.64 0.661 0.663 0.673 0.667 0.6770.72 0.666 0.668 0.677 0.673 0.6820.80 0.672 0.672 0.680 0.677 0.686log A log B log C log D0.30 1.40 1.78 3.260.65 1.60 2.87 3.300.40 1.48 2.00 3.300.95 1.95 2.70 3.55The pH of each solution was maintained at 6.8.The formation of 1: 1, 1:2 and 1:3 complex specieswas observed with their stability constants log~Ol = 1.06, log ~O;l = 0.48 and log ~03=1.73.The low values of stability constants for Cd(II)-glyand Cd(II)-~-ala systems are expected because ofion-pair formation. At isoionic point the aminoacids predominantly exist as zwitterions. Underthese conditions there is probability of ion pairformation. The trend in stability constants ~i(gly)< ~i(~-ala)] was on expected lines because of higherbasicity of ~-alanine than that of glycine.Cadmiumill y-thiourea-glycine and cadmium(II)-thiourea-is-alanine systems ~ The two concentrations0.16M and 0.40M of gly and 0.12M and 0.40M of~-ala at which 1:1 and 1:2 complex species predo-minated were chosen for the mixed ligand study.The concentration of (tu) was varied. The solutionscontaining ImM Cd(II), 0.I6M gly and a requisiteamount of sodium perchlorate to maintain the ionicstrength at O.IM were used. The same procedurewas repeated for gly concentration 0.40M and fortwo concentrations 0.l2 M and 0.40M of ~-ala.In each set a linear relationship was observedbetween Ed.e. versus log (ilia-i) with slope valueslying in the range 30-33 mY. This showed that thereduction of Cd(II) ion in the absence and in thepresence of the ligand was reversible and involvedtwo electrons. The reduction was found to bediffusion controlled from the direct proportionalityof id with "li.The following observations were made :(i) At each gly and ~-ala concentration the Etof Cd(II) shifted to a more negative value withincrease of (tu) concentration (Table 1).(ii) The shift in the Et in the presence of tu-glyand tu- ~-ala was greater than in the presenceof tu alone. This proved the presence ofmixed ligand complexes.(iii) When all the gly as well as ~-ala moleculeswere replaced the shifts of the Et in thesimple and mixed systems nearly coincided(Table 1).The plot between -(Et)c and log CL for both thegly as well as ~-ala concentrations were found to besmooth curves. With the help of Schaap andMcMasters rnethod'' the values of A, B, C and D atboth the concentrations of glyas well as ~-ala wereobtained. These are presented in Table 1.From these values the stability constants werecalculated for all mixed ligand complex species exist-ing in the solutions under the present experimentalconditions. These are : (a) for Cd(II)-thiourea-glycine system: log ~12= 2.20, log ~u = 2.50 and(b) for Cd (Ilj-thiourea-p-alanine system: log ~12=2.62, log ~21= 2.84.In both the systems the absence of 1:1:1 mixedcomplex species was indicated by the negative valuesof log ~11'Log D values in the gly and ~-ala sets are respec-tively 3.26 and 3.30; and 3.3 and 3.55 which are/INOTEShigher than log ~30' the stability constant of thesimple [Cd(tu)a]2+ complex. This may probablybe due to the incomplete dissociation of [Cd(tuk(gly)]+ and [Cd(tu)2-(~-alaW complexes under thegiven conditions.Prediction of ts-values from the statistical factors -The statistical factors in the complexation constantswere evaluated following the procedure suggested byWatters",Cd(II)-thiourea-glycine system ~ The predictedvalues for complexation constants were 3 X ~%3X~~'3 for [Cd(tuk(gly)]+ and 3 X~~~3 X ~~/23 for[Cd(tu)-(gly)J which were 102.06and 10°.89respec-tively. The observed values are 102•5 an dI02.2, show-ing an increase of 10+0.44and 10+1-31respectively.Cd(II)-thiourea-~-alanine system - The predictedvalues for complexation constant were 3 X ~N3X ~1'33 for [Cd(tuk(~-ala)]+ and 3 X ~~~3X ~~'23 for [Cd(tu)-(~-ala)2] which were 102•19and 101.33 respectively. The corresponding experi-mental values are 102.84 and 102.62 showing anincrease of 10+°.65 and 101•29respectively.The authors thank Dr A. K. Sundaram of BARC,Bombay for his valuable discussions. Thanks arealso due to Prof. K. A. Thakar, and Prof. D. D.Khanolkar for their continued encouragementduring the progress of the work. One of the authors(Y. G. D.) is thankful to the CSIR, New Delhi forthe award of a junior research fellowship.References1. MARTELL, A. E., Stability constants, Special PublicationNo. 17, and Supplement No. 1 (The Chemical Society,London), 1971, 381 and 399.2. DE FORD, D. D. & HUME, D. N., J. Am. chem, Soc., 73(1951), 5321.3. LANE, T. J., RYAN, J. A. & BRITIEN, E. F., J. Am. chem,Soc., 80 (1958), 315.4. MIGAL, P. K. & TSIPLYAKOVA,V. A., Zh. neorg. Khim.,9 (1964), 601.5. SCHAAP,W. B. & McMASTERS,D. L., J. Am. chem, Soc.,83 (1961), 4699.6. WATTERS,J. I. & DEWITT,R., J. Am. chem. Soc., 82 (1960),1333.Stability Constants of Complexes of Cu(n), Ni(II),Co(II), Mn(II), Zn(II) & Cd(II) with 5-Hydroxy-2-methyl-L, 4-naphthoquinoneB. VENKATAPPARAO, DAMODERREDDY & M. L. N. REDDY·Department of Chemistry, Nizam College, Hyderabad 500001Received 28 March 1980; revised and accepted 9 July 1980Acid dissociation constant of 5-hydroxy-2-methyl-l,4-naph-thoquinone and the stability constants of its metal chelates withCu(I1), Ni(I1), Co(I1), Mn(II), Zn(II) and Cd(II) have beeudetermined in 75% (v/v) ethanol-water mixture at 30° and 0.1ionic strength by adopting the pH-titration technique of Irvingand Rossotti. The stability constants follow the Irving-Williamorder.NoT much' work is reported--" on the chelatingability of 5-hydroxy-2-methyl-I,4-naphthoqui-209

Study of Cadmium Complexes of Amino Acids at their Isoionic Points : Cd(II)-Amino Acid & Cd(II)- Thiourea- Amino Acid Systems

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Study of Cadmium Complexes of Amino Acids at their Isoionic Points : Cd(II)-Amino Acid & Cd(II)- Thiourea- Amino Acid Systems

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