Study of mixed complexes by lmlarography : cadmium-glycinemethionine and cadmiem-glycine-ethylenedi~m ne complexes

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1 Proc~ Indian Acad. Sci. (Chem. Sci.), Vol. 91, Number 2, April 1982, pp S6. ~) Printed in India. Study of mixed complexes by lmlarography : cadmium-glycinemethionine and cadmiem-glycine-ethylenedi~m ne complexes L M RAMAIAHt, B G BI~T* and R SUNDARESANtt Chemistry Department, Indian Institute of Technology, Powai, Bombay , India t Present address:chemistry Department, Regional Engineering Colloge, Warangal , India tl" Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay , India MS received 24 May 1980 ; revised 23 January 1982 Abstract. The mixed complexes of cadmium with glycine and methionine and glycine and ethylenediamine have been studied by polarography and the stability constants of the various species formed have been evaluated. Keywerds. Mixed complexes; cadmium; glycine ; methionine ; cthylenediamine ; polarography. Introduction Though Schaap and McMasters (1961) pioneered the extension of the polarographic method of DeFord and Hume (1951) to the study of mixed complexes, not much work has since been reported in the literature. This paper presents a polarographie study of the mixed complexes of cadmium with glycine-metl~onine and glycineethylenediamine. Glycine (E Merck, pro analysi) and DL-methionine (E Merck, LR) were used without purification. Ethylenediamine (E Merck, L ) was standardised against hydrochloric acid using methyl orange as the indicator. A stock solution of cadmium was prepared from cadmium sulphate (E Merck, GR) and standardised with ev*a, Potassium nitrate, used as the supporting electrolyte, was of BDH AnalaR grade. The solutions were made in double distilled water and ph was measured with a Philips ph-meter (pp-,9040). Polarograms were taken on a manual set-up using a H~cell with an agar plug and a saturated calomel electrode (scl~) served as the reference electrode. Currents are reported after correcting for the residual currents. Correction for the IR drop in the potentials was not neee~mry. *'To whom correspondence should be made, 151

152 M Ramaiah, B G Bhat and R Sundaresan & Re, its stud discussion 3.1. Cad'aium-glycine-methionine complexes ' Simple' complexes of cadmium with glycine and with methionine were studied separately in 1.

2 152 M Ramaiah, B G Bhat and R Sundaresan & Re, its stud discussion 3.1. Cad'aium-glycine-methionine complexes ' Simple' complexes of cadmium with glycine and with methionine were studied separately in 1.0 M potassium nitrate at different ph. The electrode reaction was reversible in both the cases. The glycinate [G] and the methieninate [Me] concentrations were calculated from the ph of the solution and the p/~ of the ligands determined as for glycine and 9.10 for methionine. The half-wave potently.is were measured as a function of log [G] or log [Me] from which the stability constants were calculated as log fla for cadmium-glycine and log ~=3.80, log fla= 6.35 and log/is for cadmium-methionine systems by the methods of Lingane (1941) and DeFord and Hume (1951) respectively In the investigation of cadmium~glycine-methionine system three series of meamrem~ts were made under the sam~. experimental conditions as for the 'simple' system% keeping [Me] constant and varying [G]. The polarograms were well defined and the reduction was reversible. The half.wave potentials (table 1) were used to calculate a function, Foe, using the relationship (Schaap and McMasters 1961) Foe (Me, G) = antilog [ nf/rt{eliz ~,~ - Ell2 u)} (1) + log {in {.ffi~ c.>}] where the symbols have the usual meaning. This may be written, at constant methioninate concentration, as where Foo(Me, G) =.4 + BIG] + C[G] 2 + D[G] a (2) A = [I p.,,~. [Vie] +/i.,.~, [Me]~ + p.,.~ [Me]~] = [I + fl~o[me] +fleo [Me] 2 "~ pao [Mela], B = [fl.~,~, + PM.,~ [Me] + p,,,~ [Me] s] = [,8ol +/i~ [Me] + p~ [Me]~], (3) c = [p.,~. + p,,,,.o. [Me]] = [fie2 +/i~s [Me]] and o = p..o~ =B0~. ~,,,~ refer to the stability constants of the mixed complex species Cd Me, Gw. The constants A, B, Cand D, evaluated by a graphical procedure, are reported in table 1. The values of A agreed with the calculated values based on the stability constants obtained from the 'simple ' system. ]~ and/i~t were calculated from the values of B using (3) as 1"0 x 10 e and 1"26 x 10 g respectively. Similarly flee and fl~s were determined from C as 3.98 x l0 ~ and 3.16 x l0 g respectively. The average value of D corresponds to flea and agreed with that obtained from cadmiumglyeine system. The relative stability of a mixed complex over the parent birary complex and the compatibility between the ligands, indicated by the "mixing constant" ku

3 Mixed complexes of polarography 153 Table 1. Cadmiura-methionino-glyeirte system : Half-wave potential data Cd=3"8 xl0-*m; /~=l-0(klqos); pi-[=9"50 ; m2/a~.1/6 ~l.84mg 2/a see-l/t ; E1/~.(,) = -- Q" 580 V vssce ; [~(.) = 2" 35/tA ; [j(.) mean = 2"00/tA. [01 x 10 ~ [1vI1 - - VvsSCE El/~ Fm x 10 -a Fxo x 10 -e F2o x 1G -s [M,;] =3"58 X 10-'~M 1 "42 0"701 1" 25 1"74 0"66 2" 84 0" 705 1" 69 2" 43 2" 75 5"67 0"710 2"49 2" "93 0"717 4"25 3"27 1 "63 14"19 (1"725 7"84 4"82 2"24 21"29 0"733 13"94 6"08 2"08 28" 37 0"738 21" 24 7" t3 1"93 42"57 0"743 31" 15 7"08 1" 28 56"74 (1' " 1 18" A=,.I'00xI0*; B=,l'f5x10e; C=.1.55 D=2"30 xlo j. [Me] =,7"15 X 10-'M 1"42 0"725 7"74 5"25.. 2"84 0"727 9"03 7"14 3"32 5"67 0" " '61 9" " 31 10" 38 4" 21 14" "97 9" 84 2" "29 0"745 37" 25 14" " "60 15"37 3"23 42"57 0" "9 23"93 4"16 56"74 0" "7 26"91 3"65 A=,7"00 B=6"20 xl0 e; C~2"75 x108; D =*2"20 X 10'. [Me] = 0"107 M 1"42 (1'739 22"93 10" "84 0"739 22"93 5"04.. 5"67 0"743 31"16 17"04 3"93 9" " 12 17" 83 3"05 14"i9 0"749 47"48 18"31 2"47 21" 29 0" " 37 23" 89 4" 27 28"37 0" " 3 31" 30 5" 72 42" 57 0" " 1 34" 44 4" 61 56" 74 0" " 4 46" 86 5" 65 A B----I'~.8 C~3.75x10s; D=2"70 ~IG t.

4 154 M Ramaiah, B G Bhat and R Sundaresan and the enhanced (or sometimes decrease4) stability due to factors other than statistical, given by the "stabilisation constant" k, are calculated from the expressions (Marcus and Eliezer 1962) k~,,,~ = #,, 9 P..'o" "Po..", (x + y = m), (4) and log k, = log km -- log (ml/x! y!). (5) The values of ku and k0 for this system, calculatcd iv. this manner, are given in table Cadmium-.glyctne-ethylenediamine complexes A preliminary investigation of cadmium-ethylenediamine complexes in 1.0 M potassium nitrate indicated reversible reduction. The dissociation constant, pka of ethylenediamine was determined as I0-10 and the stability constant,ss was calculated fromthe half-wave potential data as 10 TM. In the study ofcadmiumglycine-ethylenediamine system, three sets of data were obtained at three corcentrations of glycine, varying that of ethylenediamine. The recktction was reversible and the half-wave potential data (not presented here for the sake of brevity) were solved for the stability constants of the various complex species as discussed earlier. These values as well as log ku and log k, are given in table 2. Table 2. Stability con~ants and log k u and log k, values. i Complex species log ~ los k H log k s Present Literature Present work work valuta Cd (Me) 3" Cd(Me), 6" 35 6" Cd(Me)a 8" 19 8' Cd(G) Cd(O), 7, Cd(O), Cd(Me) G 6" "98-1"28 Cd(Me)t O " 03 Cd(Me) G, 9.50,. +0" 50 +0"02 CA(En).. 5-6o.... Cd(En),.. I 0" Cd(Ea)l 11' 98 12" Cd(En) O o.01 -o" 31 Cd(l~n) OI q-0" 97 q-0.50 Cd(Bn)~ O 12" ' 90 -~0.42

5 Mixed complexes of polarography t! ~Me3 "" O.DSM 50 x m E o I.D ~25 2G \ tog [O] Figure 1. Distribution of cadmium as complexes. Table 3. Compl0x equilibria and equilibrium constants. i Equilibria log K C,t Mc + O.~ C6(Mc) (G) CO, +2Me -t-o ~ Cd(Mo)2 (G) Cd + Me + 2G ~ Cd(Mc) (G')z Cd(Mc) (G) + O ~ Cd(Me) (G)a Cd (Me) (O) + Me ~ Cd (Me), (O) Cd(Mo)~ (O) + O.~ Cd(Me) (O)~ + Me Cd(G), + Me.-~ Cd(Mo) ((3), Cd(Mo) + O.~ Cd(Me) (G') Cd(M~)2 + G ~ Cd(Mo) (G) Mo Cd(Me)s + G ~ Cd(Me)z (G) + Me Cd(M~) (G)~ + G" ~- Cd(O)s + Mc Cd ~ En + Q ~ Cd(En) (G) Cd +En ~- Cd(EB)(O)~ Cd + 2En + 0 ~ Cd(En)2 (O) Cd(~)(O) + En.~- Cd(E~), (0) Cd(En) (G) + O.~- Cd(EB) (O)t Cd(E~) (Oh + E~.~ Cd(EB), (O) O Cd(Ot + EB.~ Cd(EB) (O) + O Cd(G). + Ea. Cd(En) (G), + (3") Cd(ErN (O) + EB.-~ Cd (EB), + 0 6"Oe 9"10 9"50 3"50 3"10 0"40 1 "90 2" " 35 0"91 --0"10 9"11 11" 23 12"02 2" "79 1"51 1"83 --0" 04 II I r I I I III I

I56 M Ramaiah, B G Bhat and R Sundaresan It is seen from table 2 that log ku for the 1, 2 and 2, 1 complexes of both the systems are positive indicating the compatibility between the ligar.ds.

6 I56 M Ramaiah, B G Bhat and R Sundaresan It is seen from table 2 that log ku for the 1, 2 and 2, 1 complexes of both the systems are positive indicating the compatibility between the ligar.ds. Glycine is more compatible with ethylenediamine probably because the chelating power of ethylenediamine is more due to the presence of two nitrogen donors. The co-ordination unsaturated 1, 1 is ' not important' in both the systems as inferred from the negative values of log k~ and log ku. It is, therefore, apparent that the species present in solution in the eor.centration ranges studied are mostly 20, 30, 21, 12 and 03 complexes. The distribution of cadmium as these complex species at [MeJ = 0"05 M is depicted in figure 1 as a function of giycinate coreentration as an example. The equilibria between the different complex species ere given in table 3 from which the facility with which a ligand adds on to or substitutes another ligand may be deduced. References DeFord D D and Hume D N 1951 J. Am. Chem. Soc Lingane J J 1941 Chem. Re), Marcus Y and Eliezor E 1962 J. Phys. Chem Schaap W B and McMasters D L 1961 J. Am. Chem. See

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