Indian Journal of ChemistryVol. 20A, July 1981, pp. 695-698

Equilibrium Studies of Mixed Ligand Complexes of Zinc Ion withCitric Acid & Some Aliphatic Dicarboxylic Acids!

B. D. MALI:!: & D. N. SEN*National Chemical Laboratory, Poona 411 008

Received 8 August 1980; revised and accepted 26 November 1980

The stability constants of binary and ternary complexes of Zn(I1) with citric acid. as primary ligand and oxalic,malonic, succinic, tartaric, malic and glutaric acids as secondary ligands have been determined potentiometricallyin aqueous medium at 30° ± 0.1 °C and IL = 0.1 M (NaClO.). It is found that Zn(II) forms 1 .Lcomplexes with all theJigands except oxalic acid where both 1:1 and 1:2 complexes are formed. The formation of 1:1:1 mixed ligandcomplexes is inferred from the simultaneous equilibria. The mixed ligand stability constant (I'>xy) is found todepend on (i) the size of the chelate ring formed by secondary Iigands, (if) the stabilities of binary complexes and(Hi) the dissociation constants of secondary Iigands. The formation of 1:1:1:1 tertiary complexes of Zn(II)-citric-malic-Z and Zn(II)-citric-tartaric-Z (where Z = oxalic, malonic and succinic acids) by simultaneous equilibriais also observed. The values of D. log K, Koxy and K.xy or K2yx for the ternary complexes are calculated. Thevalues of log KaYx vary linearly with the product of acid dissociation constants ('I,pK) of secondary ligands,

As a part of our studies on metal chelates asmicro nutrients for plants, we have investigatedthe simple and mixed ligand systems of

. hydroxycarboxylic acids (citric, tartaric and malicacids) and dicarboxylic acids (oxalic, malonic, succi-nic and glutaric acids) with Zn(H) by pH-titrationmethod. A study of the complexing abilities of theseligands with Zn(II) is thought worthwhile, since zincdeficiency in plants', such as maize, onion and peach,and plants belonging to citrus family is cured by theapplication of zinc chelates through sailor as foliarspray=". However, the availability of zinc in soil de-pends to a large extent on the metal chelate stability,higher the stability of the chelate, better it is able tosupply the metal to the plants under adverse physio-logical conditions". The quantitative approach out-lined in this paper makes it possible, by setting upsuitable models to examine the metal ion equilibriain growth media and should help in designingoptimum growth conditions for desired types oforganisms and tissues.

Except for IR spectroscopic studies on piperidinecomplexes of zinc salts of dicarboxylic acids", mixed-ligand studies of Zn-oxalatc-glycollate/Iactate bysolubility method? and Zn-oxalate-ethylenediarninecomplexes by potentiometry", quantitative data onmixed ligand chelates of Zn(II) appear to be lacking.

Materials and MethodsAll acid (AR) solutions (O.OIM) were prepared in

deionized water and standardised potentiometricallywith sodium hydroxide (0.1 M). Stock solution ofzinc perchlorate in perchloric acid (E. Merck) was

tNCL Communication No. 2614.~Present address : Regional Forensic Science Laboratory,

State of Maharashtra, Dhantoli, Nagpur 440012

prepared from granulated zinc metal (AR) and themetal and acid contents were determined volumetri-cally against EDTA9 and standard alkali.

An Elico model Ll-120 digital pH meter (accuracy± 0.0 IpH unit) in conjunction with a glass electrode(type EM-60) and a KCI saturated electrode (typeER-70) was used for pH measurement. It wascalibrated before and after each potentiometrictitration with buffer solutions of pH 4.01 and 9.14(30°C)lO.

The following solutions were titrated at 30° ±O.l °Cat,.,. = 0.1 M (NaCI04) against carbonate-free NaOHunder nitrogen atmosphere: (i) HCI04 (8.0 X 1O-3M);(ii)HCI04 (8.0 X 1O-3MH-ligand(X)(2.0 X 1O-3M)(iii) HCI04 (8.0 X 1O-3M) + ligand (X) (2.0 X IO-3M)+Zn(II) (M) (2.0 X 1O-3M); (iv) HCI04 (8.0 X 1O-3M)-l-Iigand (Y) (2.0 X 1O-3M); (v) HCI04 (8.0 X 1O-3M)+ligand (Y) (2.0 X 1O-3M)+Zn(II)(M)(2.0 X 1O-3M);(vi) HCl04 (8.0+10-3M) + ligand (X) (2.0xI0-3M)-[- ligand (Y) (2.0 X 10-3 M) + Zn (II)(M) (2.0 X 10-3M); (vii) HCl04 (8.0xlO-3M) + ligand (X) (2.0x10-3M) + ligand (Y) (2.0x 10-3M) +ligand (Z)(2.0 X 10-3 M); and (viii) HCl Oi8.0 X 10-3 M)+ ligand(X) (2.0xI0-3M) + ligand (Y) (2.0xlO-3M) +ligand (Z) (2.0 X 10-3M) + Zn (II) (M) (2.0 X 1O-3M).

Results and Discussion

Acid dissociation constants and stabilities of binaryzinc complexes - The method adopted has beendescribed earlier-'. The acid dissociation constantsof the ligand and the stability constants of the binarycomplexes were determined in duplicate, with varyingconcentrations ofligand and metal ion, by the methodof least squares'> and are presented in Table I alongwith the literature values. The agreement betweenour values and the literature values was fairly good.All the acids form 1 : 1 complexes with Zn(II) except

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INDIAN J. CHEM., VOL. 20A, JULY 1981

TABLE 1 - DISSOCIATION CONSTANTs OF LIGANDS AND STABIUTYCONSTANTS OF ZINC COMPLEXES

[Temp. 30° ± O.I°C; (.L = O.IM (NaClO.)1

Acid rx. pK2 pK3 logK1 log K26

Citric 2.86 4.29 5.66 4.79(2.79) (4.30) (5.65) (4.85) :I:

Oxalic 1.12 3.92 3.87 2.21 Co 5(1.13) (3.85) (3.88) (2.35)

Malonic" 2.70 5.30 2.97(2.97)

"Succinic" 3.87 5.24 1.60(1.60)

Malic· 3.22 4.70 2.41Tartaric" 2.85 3.99 2.64 3

Glutaric 4.14 5.12 1.60(4.14) (5.01) 0.60)

2Values in brackets are taken from ref. 14."pK values taken from ref. 13.

oxalic acid which forms both 1 : I and 1 : 2 comp-lexes.

Mixed-ligand complexes - The mixed ligand che-late is formed on simultaneous replacement of fourwater molecules from the coodination sphere of zincion by the donor atoms of the two different ligands.The qualitative evidence for the formation of mixed-ligand complexes in all the systems was obtainedfrom the shift of the precipitation point of the mixedligand complexes (MXY) from that of the corres-ponding binary (MX and MY) complexes. Furtherevidence was provided by comparing the mixed ligandtitration curves with the composite curve (C) (Fig. 1).The latter was drawn theoretically by the graphicaladdition of 1 : 1 MX curve to the titration curve ofthe free ligand Y. If there is any interaction betweenMX and Y the composite curve should be displacedfrom the experimental one. The deviation of thecomposite curve C from the mixed-ligand titrationcurve was observed in each case. All the systems werefound to be in simultaneous equilibrium accordingto the method of Carey and Martell-". The procedure

8~------------'-----~--~----~--7r.----'7

o 2 3 " 5 6m

Fig. 1- pH titration curves of {(.L=O.IM (NaCI0.) and 30°} ofsystems (i) HClO, (1). (ii) HClO. + malonic acid (2). (iii) HCIO,+ malonic acid + zinc ion (3). (iv) HCIO, + citric acid (4).(v) HCIO. + citric acid + zinc ion (5). (vi) HCIO. + citricacid + malonic acid + zinc ion (6), (vii) HCIO. + citric +malonic + tartaric acid (7). (viii) HCl o. + citric + malonic +tartaric acid + zinc ion (8). C-composite curve, m = moles ofbase added per mole of ligand.

followed to understand the complexation is discu-ssed for one representative system. While the comple-xation reactions only are given for the remainingsystems in Table 2.

In the titration of Zn(H) mixed-ligand systemsagainst NaOH, the following possible complexationequilibria, are involved (Eqs 1-7).

KMXl = K(M + X ¢ MX)KMXi? = K (MX + X ~ MX2)

KMYI = K (M + Y ~ MY)KMY2 = K(MY + Y ¢ MY:?)K:?xy = K(MX + Y ~ MXY)K:?yx = K(MY + X ~ MXY)

~xy = K (M + X + Y ~ MXY)The disproportionation constants given

(1)(2)(3)(4)(5)(6)(7)

by Eq s(8-11)

[Temp. 30° ± 0.1°, (.L

TABLE 2 - MIXED LIGAND SYSTEMS OF Zn(U) WITH COMPLEXATION EQUILIBRIA

O.IM (NaClO.)]

Ligand (Y) Precipitation point(pH)

Complexation equilibria

(I) Zn(II)-citric acid (Xl-ligand (Y) systemZn2+ + H2X2- + Y2-Zn2+ + H.X'- + HY-

OxalicMalonicSuccinicMalicTartaricGlutaric

7.807.99}7.397.377.417.50

LIGANDS (Y & Z)

Zn2+ + H2X2-Zn2+ + H2X'-ZnH + H.x.-

+ HY2-+ H.Y'-+ HY-

= ZnXY'-= ZnXY2-= Zn.XY"

+ 2H++ 3H+

+ 3H++ 4H++ 3H+

Oxalic-tartaricMalonic-tartaricSuccinic-tartaricMalic-tartaricOxalic-malicMalonic-malicSuccinic-malic

(II) Zn(II)-citric acid (X)-ligand (Yj-Iigand (Z) system

9.86 Zn'+ +H.X'- +H.Y·- + Z·- = znXYZ'- + 4H+8.90}9.83 Zn'+ + H. X·- + HsY- + Z·-= zexvz> + 5H+

10.029.91 Zn'+ + H.X·- + H.Y'- + Z·- = ZnXYZ4- + 4H+9.40}9.39 Zn'+ + H.X'- + H3 Y- + Z·-= ZnXYZ3- + 5H+

696

MALl & SEN : MIXED LIGAND COMPLEXES OF ZINC(II)

Koxy = K(MX + MY ~ MXY + M) (8)Koxy = K(MXa + MY ~ MXY + MX) (9)Ktxxs = K(MX + MYz ~ MXY + MY) (10)KDXY = K(MX2 + MY2 ~ 2MXY) (11)

for the corresponding systems have been calculated.The equilibrium constant of mixed-ligand complexes(~XY) were calculated by known procedure." and arepresented in Table 3. .

The stability of mixed-ligand complex IS conve-niently characterised in two ways-" (i) based on thedifference (6. log K) in stabilities of the binary andternary complexes and (ii) the disproportionationconstant, KDXY.

Zn(Il)-cifric acid (H4X)-malonic acid (HaY)system - The formation of mixed ligand. compIe:,(MXY) was inferred from the fact that while precr-pitation in the mixed ligand system occurred at pH7.99, precipitation in the Zn(II)-citric acid (MX) andZn(II)-malonic acid (MY) systems. ~ccurred at p~7.11 and 7.24 respectively. Both citric and malonicacids formed only 1 : I complexes with Zn(lI) inthe pH range 3.5-5.1. The deviation of the mixedligand curve from the composite curve C was betweenm = 1.5 and 5.0. The horizontal difference betweenthe mixed ligand complex curve and the compositecurve increased from pH 3.5 to 4.8. Since the mixedligand curve did not coinci?e with either of the in.di-vidual metal complex titration curves, the formationof I : I : I complex by simultaneous equilibria wasconfirmed.

The last column in Table 3 (6. log K = log K2xy-log KMYI = logK2yx -log KMXl) represents thestability of the mixed ligand complex in terms ~fwhether M + Y ~ MY or MX + Y -= MXY ISpredominant. The second equilibrium is comparableto the reaction MY + Y ~ MY 2 with respect to theavailability of coordination sites for the ligand Y inMY or MX. Generally KMY1> KMY2, because morecoordinating sites are normally available for bindingthe first ligand to a metal ion than for the second.The addition of ligand Y to MX should also showthe same trend. Evidently, KMY1> K2xyor 6. log Kis negative. 6. log K can also be calculated by theexpression

1'Jog K = log I'XY - (log KMX1 + log KMy1)

TABLE 3 - EQurLIBRIUM CONSTANTS OF TERNARY COMPLEXES OFZINC

[Temp. 30° ± 0.1 °C; 11 = O.IM (NaClO.)]

Acid system (3XY KDXY· K2xy Kvoc -fllogK

Citric-oxalic 6.49 -2.17 1.70 2.62 -2.17-O.Slb

Citric-malonic 6.80 -0.96 2.01 3.83 -0.96Citric-succinic 5.99 -0.40 1.20 4.39 -0.40Citric-malic 6.55 -0.65 1.76 4.14 -0.65Citric-tartaric 6.64 -0.79 1.85 4.00 -0.79Citric-glutaric 5.98 -0.41 1.19 4.38 -0.41

The accuracy of(3xy values was between 0.05 and O.IOlogsunits.

(a) MX + MY oF MXY + M and(b) MX + MY. ¢ MXY + MY

6.10g K is negative for all the systems under study.It means that ligand X as well as Y prefer to add onto the free aquo metal forming the binary complexes(MY or MX) in preference. to the ter.nary comp~ex:es(MXY) and this tendency IS greater In Zn(JI)-cltnc-oxalic acid system, in which 6. log K is higher(-2.17). The 6. log Kvalues go on decreasing as thechelate ring of secondary Iigands goes on increasingfrom five [Zn(II)-citric-oxalic acid] to eight-memberedring [Zn(II)-citric-glutaric acid]. The. al.most i?e.ntical6. log K values in systems Zn(JI)-cltnc-succmlc an~Zn(II)-citric-glutaric acid may be due to equal stabi-lity of their binary complexes (1.60) and nearly equalT.pK values.

The values of log KDXY represent the dispropor-tionation of equilibria (8) and (10). The 6. log Kvalues run parallel to the log Koxy values calculatedfor the equilibrium (8) in all the systems. The logKoxy values of all systems indicated that they arecomparatively less stable than those predicted onstatistical groundsl8'19 (log Kox» = 0.6) due to ~esserstability of the ternary complexes than the binaryones.

The Zn(II)-citric-oxalic acid complex is morefavoured by equilibrium (8) (log KoXY= - 2.17) thaneauilibrium (10) (log Koxy = -0.51).

'The lower stability of Zn(II)-citric-oxalic acidcomplex compared to Zn(lI)-citric-malonic acidcomplex may be due .to tl~e strain developed in fi~e-membered ring (oxalic acid) than six-membered ring(malonic acid) of secondary ligand. Such observa-tions have also been recorded by Kapoor et al.20

in the case of Zn(II)-NTA-oxalic acid chelate whi~his found to be less stable than Zntf Ij-N'I'Avmalonicand Zn(II)- NT A-succinic acid chelates.

A comparison of equilibrium constants log K2.xy·(1. 70) and log KMy2. (2.21) for Zn(II)-citric- oxalicsystem shows that the oxalic acid dianion prefers toform a binary complex (MX) rather than a ternarycomplex (MXY). A similar conclusion may be drawnfrom 6. Iocr K and log Koxy values. This shows thatthe ternary ~omplex is favoured by equilibrium (8).

The values of overall stability constants ~XY forall the systems are in the order (wi~h respe~t tosecondary ligands); malonic:::- tart~nc.>. malic>oxalic> succinic = glutaric acid, WhICh IS 111 accor-dance with the 1 : 1 binary complex stabilities'"i.e. the plot of ~XY versus log K, is linear (Fig. 2).One exception to this is oxalic ~cid, wh~re lowerstability may be due to the stenc hindrance 111 ternarycomplex.

The log K2yx values follow the order of basicitiesof the ligands. A linear relationship of log K2yX versusT. pK (pKl + pK,;.,) of sec~ndar~ li~and has beenobserved (Fig. 3) in present mvesugation ..

Tertiary complexes - The tertiary complex isformed by simultaneous replacement of six watermolecules from coordinated sphere of Zn(II) by thedonor atoms of three different ligands, since zinccan easily move from tetrahedral to octahedral geo-metry depending upon the nature of bound ligan~s22.

The analysis of the titration curves of all tertiaryacid systems against sodium hydroxide solution indi-cates the tendency of zinc ion to take up one more

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INDIAN J. CHEM., VOL. 20A, JULY 1981

7.00>--2.

4.05 IG.50' - c

'"se~6.00 <J

6

!)'50'0 2 3 4

log K1

Fig. 2 - Plot of (3XY versus log K, of secondary ligands. [(1)oxalic, (2) malonic, (3) succinic, (4) malic, (5) tartaric and

(6) glutaric acids].

S·.O ..-------------,

4'0x>-

~3'0

2'0L-_-L__ L-_-L __ L-_~4·S S·!! 6·S 7·S 8·S s-s

~PK

Fig. 3 - Plot of K.XY versus !,pK values of secondary ligands.[(1) oxalic, (2) malonic, (3) succinic, (4) malic, (5) tartaric and

(6) glutaric acids].

ligand which is different fro~ the ligand~ alre~dycoordinated to it to form a tertiary complex involvingthree different ligand molecules. The inference isagain based on the shift of precipitation pO.ints of thezinc-tertiary ligand system from that of zinc-ternaryand zinc-binary ligand systems (Table 2). A represen-tative system is discussed below.

Zn(II)-citric (H4X)-malonic (H2 y)-fartaric acid(H4Z) system - The titrati.or: cl;lrves ?f this sys~em(Fig. 1) show that the precipitation point of tertiarycomplex (pH 8.90) has shifted from ternary (pH 7.9~)and binary complexes (7.11, 7.24 and 7.09 respecti-

TABLE 4 - EQUILIBRIUMCONSTANTSOF TERTIARYCOMPLEXESOF ZINC

Acid system pH range forcalculation

Log{3XYZ

8.799.208.499.488.149.008.03

3.80 - 4.203.80 - 4.404.80 - 5.104.10 - 4.603.70 - 4.704.60 - 5.004.30 - 4.80

Citric-oxalic-tartaricCitric-malonic-tartaricCitric-succinic-tartaricCitric-malic-tartaricCitric-oxalic-malicCitric-malonic-malicCitric-succinic-malic

Accuracy of (3XYZ values was between 0.05 and 0.10 logunits.

698

vely). Furthermore, the tertiary and ternary comple-xes are formed in the pH range 3.5-4.4. Thisleads to the formation of 1 : 1 : 1 : 1 complex, inthe system Zn(II)-citric-malonic-tartaric acid bysimultaneous equilibria. The values of equilibriumconstants calculated-" for tertiary complexes aregiven in Table 4. The order of stabilities of thesecomplexes are found to be : Zn(II)-citric-malonic-tartaric> Zn(II)-citric-oxalic-tartaric > Zn(H)-citric-succinic-tartaric, which follow the stability order oftheir ternary complexes : Zn(II)-citric-malonic >Zn(II)-citric-oxalic > Zn(II)-citric-succinic.

The same conclusion can be drawn for other threesystems where tartaric acid is replaced by malic acid.This shows that in ternary and tertiary complexes thestability is influenced by steric hindrance offered bymalonic, oxalic and succinic acids.

The tertiary complexes involving tartaric acid aremore stable than the complexes involving malic acid.This may be due to the higher stability of Zn(H)-tartaric acid complex than Zn(II)-malic acid complex.However, the Zn(II)-citric-malic tartaric acid complexshows the highest stability in all tertiary complexes.

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