The results obtained during the determination of stability...

Result and Discussion

Page 93

The results obtained during the determination of stability constant using

potentiometer are analyzed by the computer programme and the stability constant

values are calculated. Graph of An vs. pH for proton ligand system was plotted and in

most of the cases found to extend between 0-2 indicates that ligand has two

replaceable protons. Ligand titration curve had a lower pH value than acid titration

curve. Displacement of ligand titration curve along volume axis with respect to acid

titration curve is indication of proton dissociation. Values of An obtained are within

range 0.2- 0.8 and 1.2-1.8 indicating formation of 1:1 and 1:2 complexes. Deviation

of A+L curves from A+L+M curves indicates formation of complex.

The Metal-ligand stability constants of binary complexes are evaluated

assuming that the formation of hydrolyzed products, polynuclear complexes,

hydrogen and hydrogen bearing complexes are absent. An examination of titration

curves indicates that complex formation has taken place in the solution on the

following grounds.

1) Maximum value of An is 2 indicating the formation of 1:1 and 1:2 complexes only.

2) The metal ion solution used in the study are very dilute, hence there is no

possibility of formation of polynuclear complexes.

3) The hydrolysis of the metal ions was suppressed due to complex formation,

precipitation did not appear during the titration.

4) The metal titration curves are displaced to the right hand side of the ligand titration

curves along the volume axis, indicating proton release upon complex formation of

the metal ion with the ligand.

5) The large decrease in pH for the metal titration curves relative to the ligand

titration curves points to the formation metal complexes.


Page 94

6) In most cases, the colour of the solution after complex formation observed to be

different from the colour of the ligand at the same pH.

Higher is the pKa value, lesser is the tendency to ionize or in other words lesser

acidic character. Some variations may be due to different experimental condition.The

addition of metal ions to free ligand solutions lowered pH value .This shows that the

complexation reactions proceed through release of protons.

Farooqui et al [1] reported that a low value of stability constant suggests that ligand is

suitable for formation of complex at optimum physiological condition. This may

favour the binding of ligand with nucleic acid of cell through transition metal and

helps in transportation of drugs to the site of its physiological action.

In the present investigation values of logK1 found to be more than logK2 which

is in agreement with literature. [2-7]. The results shows that the ratio of logK1/logK2 is

positive in most of the cases which indicate that there is a little or no steric hinderance

to the addition of secondary ligands molecules which in agreement work reported by

Deosarkar and Narwade.[8]. Higher logK1 may be due to weaker interaction of second

ligand than first one. In few cases reported the logK2 values are higher than logK1,

this is in agreement with some earlier researchers. [9-11]. LogK1 for some ligands are

higher than logK2, this is due to fact that the interaction of second bulky ligand is

weaker than the first ligand i.e.1:2 species is not formed until complete formation of

1:1 species. This can be ascribed to i) increase in the Lewis acidity of free metal ion

as compared to 1:1 chelated ion ii) the steric hinderance caused by second bulky

ligand molecule. Higher value of logK2 than logK1 indicates trans effect for second

coordination.

Dissociation process is nonspontaneous process, endothermic and entropically

unfavourable. The formation of metal complexes has been found to be spontaneous,


Page 95

endothermic and entropically favoured. In the following section, a detail discussion

regarding potentiometric evaluation of stability constant is given. These values are

represented in the table 4.1(a) & b for 1:1 and 1:2 complexes.

4.1 Complexes of Nicotinic Acid

The complexation of Nicotinic acid with transition metal ions has been studied under

the experimental condition as described in Chapter II. These are presented in tabular

form (table 4.1a & b) for 1:1 complexes & 1:2 complexes. The titration curves were

plotted for ligand, and ligand + metal system. The values were analysed to get

protonation constant and metal –ligand stability constant.

The value of An lies in the range of 0-1 indicating that ligand has only one replaceable

proton. The pKH value obtained are 5.425 and 5.200 considering the pointwise and

half integral method. In case of 1:2 ratio of metal and ligand, Co(II) complexes are

exceptionally more stable than Cu(II) complexes. Iztok Turel et al [12] have studied

complexation of Ciprofloxacin in aqueous solution at 250 C; They reported two pKa

values, 6.17 for 3-carboxylic group and 8.54 for nitrogen of piperazine group. They

also reported formation of 1:1 complexes. The complexation of Nicotinic acid with

metal ions under study gives metal ligand stability constants which are shown in the

table 4.1 a and 4.1 b

Table 4.1a Proton Ligand Constant and Stability constants of Nicotinic acid (1:1 ratio)

Proton-Ligandstability constant

Metal- ligand stability constant

Metal Log K1

Half integralpK1 = 5.200

Point wisepK1 = 5.4254

Cu (II) 3.72205

Zn (II) 3.65391

Ni (II) 3.61710

Co(II) 3.6623

Cd (II) 3.5100


Page 96

Table 4.1b Metal Ligand Stability Constants for Nicotinic acid (1:2)

Tuncer Degim et al [13] compared stabilities of naproxen and salicylic acid and

reported value 6.512 and 4.325 for carboxyl group in naproxen and salicylic acid.

M.Khalil et al [14] studied complexes of dipicolinic acid and amino acids with

bivalent metal ions in aqueous medium, evaluated pKa 4.53 and 2.32 for dipicolinic

acid. They also reported high stabilities for Copper and Nickel complexes. Patil et al

[15] studied interaction of transition metal ions with Ibuprofen and Paracetamol. They

assigned pKa value for –COOH group in Ibuprofen is 5.32. They also reported

formation of 1:1 complexes and high value of stabilities for Copper and Zinc

complexes. Sekhon et al [16] reported pKa value 5.709 (carboxyl group) and 8.044

(piperazinyl group) for Ciprofloxacin. Gamerio P. et al [17] reported pKa values 6.25,

8.44 and 6.10, 8.60 for norfloxacin and ofloxacin. Patil [18] reported two pKa values

for NA as 9.05 and 4.95. He also reported high value of stability constant for

complexes of Copper (II) and Zinc ion (II), Hence the order of stability is in

agreement with our results. Patil et al [19] studied interaction of transition metal ions

with NA and reported pKa value 4.74 for NA. They have found that the sequence of

stability complexes with respect to metal ions is due to decreasing atomic radius and

increasing second ionization potential. Janrao et al [20] have recently observed 4.90

pKa value for the NA and 6.4790 for metal ligand stability constant of Zinc complex.

They also reported formation of 1:1 complexes. Zaid et al [21] studied stabilities of

Metal ionMetal Ligand Stability Constants

Log K1 Log K2 Log β

Cu (II) 3.1144 3.1032 6.2176

Zn (II) 3.1080 3.0983 6.2064

Ni (II) 3.1099 3.1491 6.2591

Fe (III) 4.1094 3.8639 7.9733

Co (II) 3.1029 3.1311 6.2341

Cd (II) 3.1057 3.1027 6.2084


Page 97

ciprofloxacin moiety and observed high values of stability for Copper complexes.

Farooqui et al [22] evaluated two pKa values for picolinic acid as 10.95 and 8.83 in

aqueous medium using potentiometry. They also reported high values of stability

constants for Cobalt than Nickel complex. Nair et al [23] observed two stability

constants for NA as 4.69 and 7.02 at 370 C. They reported formation of 1:1 and 1:2

complexes. They also reported NA as a bidentate ligand and binding of ligand to

metal ion through N-pyridine and O of carboxalato atoms like that of pyridine-2-

carboxylic acid.

Anita Gupta [24] studied stability of ampicillin trihydrate in water ethanol

medium 50% V/V maintaining ionic strength at 0.1M KNO3 with bivalent metal ions.

She reported pKa value of 6.98 and formation of 1:1 complexes. For drugs to remain

in biologically active form, the stability constants should be in the range of 3-5. The

stability values are found to be in biologically active range and highest value was

found for Copper (II) metal ion. These stability values may be informative for

biochemist during drug design or drug discovery.

Erzalina Hernowo [25] reported two values of pKa as 2.22 and 4.59 for carboxylic

acid group and pyridine nitrogen, also compared stability constant of NA with GA

and proved that NA is weaker ligand.

For the present investigation, the order of stability may be assigned as follows

Cu(II) > Zn(II) > Ni(II) > Co(II) > Cd(II) for 1:1 complexes and

Fe(III) > Zn(II) > Co(II) > Cu(II) > Cd(II) > Zn (II) for 1:2 complexes.

Rasheed H. A. and Maqsood Z.T. [26] studied solid state complexes of NA with Cu

(II), reported that NA complexes are water insoluble, octahedral in nature and

formation of 1:1 and 1:2 complexes is detected. Cu (II) nicotinate complexes reported

to exert diverse bioactivities. Monodentate nature of NA and possibility of bonding


Page 98

through Nitrogen was observed. Low values of stability of NA-Cu (II) complexes are

due to weak nature of NA ligand. Kayande et al [27] also studied the complexation of

NA with Cu (II), observed pKa 3.48. Interactions of insulin –mimetic zinc complexes

of 2-picolinic acid was studied by E. Anna Ebedy et al [28], they also reported

complexation through pyridine nitrogen and carboxylate oxygen for pKa 1.00 and

5.19. Solution behaviour of enrofloxacin (erf) complexes with transition metal ions in

the presence of 1, 10 phenanthroline was investigated by R. Saraiva et al [29]. The

results obtained show that at physiological condition only Copper form stable

complexes, they reported pKa values 6.17 and 9.34 for -COOH group and piperazine

in erf respectively.


Page 99

Fig. 4.1 pH metric titration curve for Cu( II) + Nicotinic Acid (1:1 ratio)

Fig. 4.2 pH metric titration curve for Ni (II) + Nicotinic Acid (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

Nicotinic Acid

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 100

Fig. 4.3 pH metric titration curve for Zn (II) + Nicotinic Acid (1:1 ratio)

Fig. 4.4 pH metric titration curve for Fe (III) + Nicotinic Acid (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 101

Fig. 4.5 pH metric titration curve for Co (II) + Nicotinic Acid (1:1 ratio)

Fig. 4.6 pH metric titration curve for Cd (II) + Nicotinic Acid (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


2

Fig. 4.7 nA vs pH for Nicotinic Acid

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

1.2

1.3

1.4

1.5

1.6

1.7

1.8

na

pH

Nicotinic acid


Page 103

4.2 Complexes of Mandelic Acid

The Mandelic acid has two ionizable groups –OH and –COOH. The ionization of

strong carboxylic group occur at lower pH, due to the high stability of the carboxylate

ion by resonance and hydroxylic group occur at higher pH. For the present work the

protonation constant obtained are 2.944 and 9.425. These value are in agreement with

pKa values with earlier work in which ligand possess carboxylic and hydroxyl group.

[30-32]. Jain et al [33] studied complexation of Eu(III) with humic acid and its model

compounds. They reported formation of 1:1 and 1:2 complexes and pKa value 3.19

for Mandelic Acid. Calderia et al [34] has reported high stability values for tungeston

complexes of the MA. The complexation of Mandelic Acid with metal ions under

study gives metal ligand stability constants which are shown in the table 4.2 a and

4.2b.

Table 4.2a Proton ligand and metal ligand stability constants of Mandelic acid (1:1)

Table 4.2b Metal Ligand Stability Constants for Mandelic Acid (1:2)

Metal ionMetal Ligand stability constants


Cu(II) 7.5067 7.1232 14.6300

Zn(II) 6.3857 6.0603 12.4461

Ni(II) 7.1127 5.5842 12.6969

Fe(III) - - -

Co(II) 5.8731 5.3747 11.2478

Cd(II) 5.5844 - 5.5844

Proton LigandStability constant

Metal ligand stability constantMetal ion Log K1 Log K2 Log β

Half integralpK1 = 2.9444pK2 = 9.9425

Point wisepK1 = 2.6200pK2 = 9.9852

Fe (III) 10.9051

3.8164 14.7215

Cu (II) 6.4714 5.9449 12.4163

Zn (II) - 5.7148 5.7148

Co (II) - 4.8507 4.8507

Ni (II) - 4.8751 4.8751

Cd (II) - 5.5833 5.5833


Page 104

When the stabilities of various complexes are compared on the basis of stability

constant following sequence obtained is

for 1:1 complexes is Fe(III) > Cu(II) > Zn II) > Cd (II) > Ni (II) > Co (II)

& 1:2 complexes is Cu (II) > Ni (II) > Zn(II) > Co(II) > Cd(II).

Since the size of Ni (II) is smaller than Co(II), Ni(II) forms stronger bond with α-

hydroxy group of mandelate ligand than Co(II), similar results were reported by Ki

Young et al. [35]. The order of stability for 1:1 complexes in agreement with Irving –

Williams order. [36]. Agrawal et al [37] studied interaction of transition metal ions

with benzoic acid at 250C and evaluated 4.19 value of pKa for –COOH group of

benzoic acid, according to them Copper and Nickel complexes shows high stability

during formation of 1:1 complexes. The order of stability towards complexation with

benzoic acid observed by them was Cu (II) > Zn (II) > Mn (II).

R. Sundersanan [38] studied Indium metal complexes of glycollic acid,

mandelic acid and lactic acid polarographically and reported pKa values as 3.3,3.00

and 3.86, the order of stability of complexes as mandelate < lactate < Glycollate.

According to them increased substitution leads to lowering stability due to presence of

hindering groups. Formation of 1:1 and 1:2 complexes for mandelic acid was

predicted. The pKa value of mandelic acid is less than benzoic acid, this may be due

to substituents present. Hydroxyl carboxylic acid contain two donor groups, the

carboxyl and hydroxyl group therefore act as bidentate ligand. The proton and metal

ion complexation constants of these ligands strongly depend upon the relative position

of the two donor groups. The 2-hydoxy acids form stronger metal complexes than

simple carboxylic acids. Hydroxyl group has marked effect on dissociation behavior

of carboxylic group. The electron withdrawing effect of hydroxyl group induce an

increase in acid strength of the carboxylic group.e.g. 2- hydroxy acetic acid is 10


Page 105

times stronger than acetic acid .[39]. Similar effect is observed in case MA ligand, it

has pKa value as 2.9444 which correspond to –COOH group (our results) and phenyl

acetic acid has value of 4.28

Mehta B.H. [40] studied the complexes of various carboxylic acids with

transition metals and reported values of pKa for malic acid as 8.8828, 11.873,

assigned higher pKa for –OH group. Order of stability as Mn(II) > Fe(III) < Ni(II) <

Cu(II) < UO2(II). Basavraj et al [41] studied complexation of Schiffs bases (SB) with

transition metal ions in water ethanol medium, reported pKa for SB as 12.85 and 4.05

corresponding to –OH and –COOH group respectively. They have reported

coordination of metal ions through oxygen of –OH group and carboxylate of –COOH

group. Order of stability shown by them was

Zn (II) < Cu(II) > Ni (II) > Co(II) = Cd(II) > Mg(II)


Page 106

Figure 4.8 pH metric titration curve for Cu (II) + Mandelic Acid (1:1 ratio)

Fig. 4.9 pH metric titration curve for Zn (II) + Mandelic Acid in (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 107

Graph 4.10 pH metric titration curve for Co ( II) + Mandelic Acid in 1:1 ratio

Graph 4.11 pH metric titration curve for Fe (III) + Mandelic Acid (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 108

Graph 4.12 pH metric titration curve for Cd (II) + Mandelic Acid (1:1 ratio)

Graph 4.13 pH metric titration curve for Ni (II) + Mandelic Acid (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 109

Fig. 4.14 nA vs pH for Mandelic Acid

2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

nA

pH

Mandelic Acid


Page 110

4.3 Complexes of Gallic Acid

When the potentiometric study of bivalent complexes with GA is carried out ,two

protonation constant were obtained 5.30 and 9.2949. These are assigned to carboxylic

acid and acidic –OH group respectively. The metal ligand stability constants of GA

are given in the table 4.3a & 4.3b.

Table 4.3a Proton Ligand Constant and Metal ligand Stability Constants For Gallic

acid (1:1)

Table 4.3b Metal Ligand Stability Constants for Gallic Acid (1:2)



Cu (II) 9.7236 9.1933 18.9169

Zn (II) 8.0345 7.2774 15.3119

Ni (II) 7.0369 6.7424 13.7793

Fe (III) - 10.979 10.9790

Co (II) 7.5724 7.1308 14.7033

Cd (II) 7.1482 6.6143 13.7625

Ahmed Fizary et al [42] studied Iron(III) complexes of GA by potentiometry reported

pKa for gallic acid as 4.10 and 8.38 ,14.73(logK1) 11.93(logK2) , formation of 1:1 and

1:2 complexes. The complexes formation was also confirmed by UV-visible

spectroscopy. Fattahpour Sedeh et al [43] studied complexation of silicic acid and

phenolic acids in aqueous medium, pKa values reported are 4.141 and 8.41.

M. Saqib and Arif Kazmi [44] have reported strong chelating capacity for the GA and

formation of highly colored complexes with metals which are in agreement with our

Proton-Ligandstability Constant

Metal ligand Stability Constants

Metal ion Log K1 Log K2 Log β


Fe(III) 11.6545 10.8250 22.4795

Cu(II) 7.9150 7.0946 15.0096

Zn (II) 6.2561 6.0034 12.2595

Point wisepK1 = 5.2723pK2 = 9.2915

Ni (II) 5.6428 5.2030 10.8458

Co (II) 5.8957 5.3244 11.2201

Cd (II) 6.2115 4.7593 10.9770


Page 111

studies. Gallic acid has three –OH groups and one -COOH group, out of which two –

OH groups may be engaged in complexation. The remaining –OH group may be

engaged in hydrogen bonding with –COO- of the other ligand present on the same

molecule forming cage like structure. Such intramolecular hydrogen bonding may be

responsible for the high stability constants. In our case formation of 1:1 and 1:2 has

been reported. It is found that binary complexes of M: L ratio 1:2 are more stable than

1:1, these results are in agreement with the earlier researcher. Higher value of stability

constant indicates the possibility of covalent interaction and the low value gives idea

about ionic interaction. [45]

The order of stability for this investigation may be presented as

Fe (III) > Cu(II) > Zn(II) > Co(II) > Ni(II) > Cd(II) for 1:1 complexes

and for 1:2 complexes is Cu(II) > Zn(II) > Co (II) > Ni (II) > Cd (II) > Fe(III).

The order is in agreement with the Irving Williams order and reported by many

workers [46, 47]. The stability constant value varies with the ionic size of metal ion.

The smaller the ionic radius of the central atom, more stable is the complex formed. It

also depends upon experimental conditions used.

Nasreen Fatima and Zahida Maqsood [48] studied vanadium complexes of

catecholates, they also reported high values of stability for gallic acid with vanadium

(II) and reported formation of 1:1 and 1:2 complexes. Strong chelating capacity for

the ligand is reported. It forms highly colored complexes with metals. The pKa values

reported by them are 4.341, 9.0892 and 11.9210. The complexes of GA are more

stable than other phenolic acids. There is formation of 1:1,1:2 and 1:3 complexes.

Mahmoud Hasan M.[49] investigated interaction of Mercury (II) ions with GA

potentiometric and by optical means, they reported two pKa values as 8.51 and 10.70.

B. J. Sandmann et al [50] studied stability of divalent metal ions with GA and


Page 112

observed four pKa values as 4.4,8.6,11.2,11.38, also reported metal –ligand stability

constant for Zn as 11.38 which is in agreement with our results.

Beltran et al [51] studied pKa values of polyphenolic acids by potentiometry in water

and MeCN-water media, The pKa values obtained are 5.18, 9.31 and 11.04.

Erzalina Hernowo studied interaction of transition metal ions with GA in aqueous

medium, reported four protonation sites for GA having pKa as 4.25 (-COOH) group

and the other hydroxyl groups as 8.61, 11.03 and 12.71. He compared stability of GA

with NA and norleucine and proved that GA a strong ligand than others. The high

value of stability towards Iron is observed. The values obtained for stabilities are in

agreement with our results eg. 22.97 for Fe(III) (22.47),19.01(15.00) for Cu(II)

,10.31(10.84) for Ni(II) and for Co(II) 9.18(11.22), sequence of stability observed is

Fe (III) > Cu(II) > Ni (II) > Co (II).


Page 113

Fig. 4.15 pH metric titration curve for Cu (II) + Gallic Acid ( 1:1 ratio)

Fig. 4.16 pH metric titration curve for Co (II) + Gallic Acid (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 114

Fig. 4.17 pH metric titration curve for Zn (II) + Gallic Acid (1:1 ratio)

Fig. 4.18 pH metric titration curve for Cd(II) + Gallic Acid (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 115

Fig. 4.19 pH metric titration curve for Fe (III) + Gallic Acid (1:1 ratio)

Fig. 4.20 pH metric titration curve for Ni (II) + Gallic Acid (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 116

Fig. 4.21 nA vs. pH for Gallic Acid

3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

nA

pH

Gallic acid


Page 117

4.4 Complexes of 3, 5-Dinitro Salicylic acid

DNS is a bidentate ligand, it has two binding sites phenolic –OH which has higher

pKa and –COOH having lower pKa. Under the present experimental condition, the

protonation constant observed is 2.944 and 9.888. The value of 2.944 is due to –

COOH group. This value is low as compared to the carboxylate group (4.2) in the

normal cases, the decrease in pKa value may be due to electron withdrawing effect

exerted by two nitro groups .The Metal-ligand stability constant for M+DNS system

is shown in the table 4.4a and 4.4b

Table 4.4a Proton Ligand Constants and Metal –Ligand Stability Constants for 3,5 –Dinitro Salicylic acid (1:1)

Table 4.4b Metal Ligand Stability Constants for 3, 5 Dinitro Salicylic acid (1:2)

Merce et al [52] compared stabilities of salicylic acid with its nitro derivatives. They

reported 3,5 DNS more effective chelating agent than salicylic itself, the ligands are

not bulky and can provide sight to the complexing ability of the nitro humic

substances. The pKa values obtained are 7.08 and 9.83 for 3,5 DNS and 5 Nitro

Salicylic Acid respectively. logK for Copper complexes of 3,5DNS as 7.2,4.2. Dube

Proton-Ligandstability constant

Metal –Ligand Stability Constants

Metalion

Log K1 Log K2 log β


Point wisepK1 = 2.627pK2 = 9.888

Cu(II) 11.2239 7.56880 18.7927

Fe(III) 7.4033 6.7540 14.1573

Ni (II) 7.1129 5.1789 12.2918

Co (II) 7.3656 5.2001 12.5658

Zn (II) 8.3801 - 8.3801

Cd (II) - 5.9025 5.9025



Cu (II) 5.8084 3.8419 9.6503

Zn (II) - - -

Ni (II) 4.2998 3.8440 8.1439

Fe (III) 4.1094 3.8639 7.9733

Co (II) 3.4469 - 3.4469

Cd (II) 3.7524 - 3.7524


Page 118

and Dhindsa [53] worked on complexes of 3,5 DNS with different metal ions and

reported formation of 1:1 complexes, the order of stability reported by them is Cu(II)

> Ni(II) > Co(II) > Zn(II) > Mn(II). Salicylic acid (SA) has pKa values as 2.84 and

13.66. Abd Gaber et al [54] studied the complexes of SA with nontransition metal

ions in aqueous medium. The pKa values found are 1.31,7.00 and 2.49 , 12.00 for 3,5

DNS and 5-sulpho salicylic acid respectively. Decrease in pKa may be due to

attachment of two electron withdrawing nitro groups in case of 3,5 DNS. The

complexes of salicylic acid derivatives are studied the high stabilities for Copper and

Nickel complexes are observed. pKa value for benzoic acid 4.20 and phenol 10.00

are reported in literature. The values of Proton ligand constant (pKa) obtained are in

agreement with literature values. [55]. Some variations may be due to different

experimental condition. Stability values reported are in agreement with the literature

reported values. Among ligands high values of stability for the ligand 3,5 DNS with

lanthanides has been found. Shelke and Jahagirdar [56] worked on Neodymium

complexes with stability constant 3.33, 14.20 for 4-hydroxy salicylic acid and

formation of 1:1 complexes was observed. Faraji et al [57] determined pKa values of

salicylic acid and 5-nitro salicylic acid in ethanolic medium as 2.916 and 2.017

respectively. For the present investigation the order of stability found to be

Cu(II) > Zn(II) > Ni(II) > Cd(II) > Co(II) > Fe(III) for 1:1 complexes

and Cu(II) > Ni(II) > Fe(III) > Cd(II) > Co (II) for 1:2 complexes.


Page 119

Fig. 4.22 pH metric titration curve for Cu (II) + DNS (1:1 ratio)

Fig. 4.23 pH metric titration curve for Fe (III) + DNS (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 120

Fig. 4.24 pH metric titration curve for Co (II) + DNS (1:1 ratio)

Fig. 4.25 pH metric titration curve for Ni (II) + DNS (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 121

Fig. 4.26 pH metric titration curve for Zn (II) + DNS (1:1 ratio)

Fig. 4.27 pH metric titration curve for Cd (II) + DNS (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 122

Fig. 4.28 nA vs. pH for DNS

2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6n

A

pH

3,5 Dintro Salicylic Acid


Page 123

4.5 Complexes of 4-Amino-Benzoic Acid

The ligand 4-Amino Benzoic acid has two ionisable groups –COOH and –NH2. In the

present investigation the observed value of pKH for PABA are 9.1654 & 4.6449. The

value of protonation constant 4.6449 may be due to ionization of –COOH group

observed in many acids benzoic acid salicylic acid and the protonation constant

9.1614 may be due to ionisation of –NH2 group observed in case of amines. The

observed value of pKa for PABA are in agreement with literature [58].

The order of stability towards complexation with PABA is

Fe(III) > Ni(II) > Cu(II) > Cd(II) > Zn(II) > Co(II) for 1:1 complexes and

Co(II) > Cu(II) > Fe(III) > Ni(II) > Zn(II) > Cd (II) for 1:2 complexes.

The order is in agreement with Irving Williams natural order. The metal ligand

stability constants of PABA with metal ions are given in the table 4.5a & 4.5b.

Table 4.5a Proton Ligand Constant and Metal Ligand Stability Constants for 4-AminoBenzoic Acid (1:1)

Table 4.5b Metal-ligand Stability constants of 4-aminobenzoic acid (1:2)

Proton LigandConstant

Metal Ligand Stability Constants


Half integralpK1 = 4.6485pK2 = 9.1620Point wise

pK1 = 4.776pK2 = 8.880

Cu (II) 6.7243 6.4397 13.2129

Zn (II) 5.9446 5.5661 11.5107

Ni (II) 13.0085 4.3775 17.3166

Fe (III) 11.4881 10.5396 22.0277

Co (II) 4.5264 4.3369 8.8634

Cd (II) 6.8998 4.9594 11.8811

Metal ionsMetal-ligand Stability constants


Cu (II) 3.7159 3.2199 6.9359

Zn (II) 3.1246 3.1213 6.2460

Ni (II) 3.0666 3.1184 6.2690

Fe (III) 3.1574 3.1443 6.3017

Co (II) 3.2331 3.8327 7.0658

Cd (II) 3.1201 3.1121 6.2323


Page 124

F.B.Ansari et al [59] studied stability complexes of PABA with transition metals and

reported only one value of pKa 5.913 and they observed high value of stability for

Cobalt complexes than Nickel complexes, more stability for 1:2 complexes of Nickel

was observed. Abd-El Wahed et al [60] studied thermodynamic and electrical

properties of aminophenol and anthranillic acid (ANA), the complexes of ANA more

stable than aminophenol. The order of stability towards

ANA is Mn(II) < Fe(II) < Ni(II) < Cu(II).

The two pKa values for ANA as 8.37 and 4.45. Erzalina Hernowo reported two values

of pKa for norleucine having two protonation sites i.e.-COOH and –NH2 as 2.35 and

9.35. H. N. Aliyu and J. Naaliya [61] studied stability constants of eleven amino acids

with manganese ions. The high values of stability constants indicating formation of

stable complexes. The values of stepwise stability constants decrease in the order as

log K1 > log K2 > log K3.

Thakur S.V. et al [62] worked on complexes of adenosine (AD), determined

protonation constant as 3.292 for –OH group and 11.659 for –NH2 group. They found

the formation of 1:1 and 1:2 complexes. Shashi Agrawal and Satyendra singh [63]

studied metal complexes of glycil glycine, reported pKa as 3.1 and 8.1. Kayande et al

[64] studied complexation of amino acids and reported two pKa for glycine (2.34,

9.60) and penicillamine (1.8, 10.7) and valine (2.32, 9.62), lower value may be due

to dissociation of –COOH group and higher due to –NH2 group. Vighe et al [65]

studied effect of ionic strength on stability constant of Co (II) prolyl alanine

complexes, pKa found to be 6.80 and 10.67. They observed formation of 1:1, 1:2

complexes with high value of stabilities. Fatma Hassan [66] studied mixed ligand

complexes of metals with Schiffs bases and cysteine and determined the two pKa

values for cysteine as 7.7 (-COOH) and 8.8 (-NH2 group). Highest value of stability


Page 125

observed for iron (III) complexes which in agreement with literature. [67].The

transition metal ions because of availability of d-orbital give rise to a large number of

stable complexes.

Fig. 4.29 Potentiometric titration curve for Zn (II) + PABA (1:1 ratio)

Fig. 4.30 Potentiometric titration curve for Cd (II) + PABA (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 126

Fig. 4.31 Potentiometric titration curve for Cu (II) + PABA (1:1 ratio)

Fig. 4.32 Potentiometric titration curve for Fe (III) + PABA (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 127

Fig. 4.33 Potentiometric titration curve for Co (II) + PABA (1:1 ratio)

Fig. 4.34 Potentiometric titration curve for Ni (II) + PABA (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 128

Fig. 4.35 nA vs. pH for PABA

4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

nA

pH

4-AMINO BENZOIC ACID


Page 129

4.6 Complexes of Quinol

Table 4.6a Proton Ligand Constant and Metal Ligand Stability Constants for Quinol(1:1)

Table 4.6b Metal Ligand Stability Constants for Quinol(1:2)

Quinol is a bidentate ligand having two –OH groups attached to benzene ring in para

position. -OH group ionizes at higher pH (9-11). The protonation constant values

obtained are 9.68 and 9.55 by half integral and pointwise method. Phenols are weaker

acids than carboxylic acids so higher pKa values are observed. There is formation of

1:1 and 1:2 complexes observed. The complexes formed are coloured. Quinol found

to have good chelation power. The metal –Ligand stability constants of Quinol with

metal ions are shown in table 4.6a and 4.6b.

The pKa value for phenol is 10.00, whereas pKa values for pyrocatechol

9.314 and 12.6001. Similarly pKa values for catechol and resorcinol have are 9.25,13

and 9.2,10.90. Mariose et al [68] have evaluated two pKa values for substituted quinol

8.47 and 7.26. In agreement with above results we obtained pKa values 9.22 and

Proton Ligand stabilityConstant


Metal ion Log K1 Log K2 log β



Cu (II) 10.5993 8.6141 19.2134

Zn (II) 11.4705 4.03030 15.5008

Ni (II) 10.0944 4.44738 14.5682

Fe(III) - 4.0080 4.0080

Co(II) - 4.4951 4.4951

Cd (II) 9.12883 4.0458 13.17463

Metal ionMetal ligand stability constants


Fe (III) 12.9018 - 12.9018

Cu (II) 10.2150 9.45665 19.6717

Zn (II) 7.7827 7.1333 14.8709

Ni (II) 5.7613 5.290167 11.0515

Co (II) 5.8957 5.3244 11.2201

Cd (II) 5.8833 4.8951 10.7784


Page 130

9.680 for quinol molecule. Nasreen Fatima and Zahida T.Maqsood have reported high

stability values for colored complexes of catechol, gallic acid and gallic ester with

vanadium. High value of stabilities indicates strong chelation power of the ligand.

These High value of stability for Cu(II) may be due Jahn-Teller effect.

The order of stability observed is

Cu(II) > Zn(II) > Fe(III) > Co(II) > Ni(II) > Cd(II) for 1:1 complexes

and 1:2 complexes to be in Fe(III) > Cu(II) > Zn(II) > Co(II) > Ni(II) > Cd(II).

The order is in agreement with the Irving Williams order and reported by many

workers [69, 70]. The stability constant value varies with the ionic size of metal ion.

The smaller the ionic radius of the central atom, more stable is the complex formed. It

also depends upon experimental conditions used.

Nina S. et al [71] studied chemical properties of catechol, reported formation

of stable complexes of catechol with divalent and trivalent ions. They observed more

stabilty of Fe (III) complexes than Cu(II). The Log K1 > log K2 indicates a decrease in

bond strength with successive attachment of ligand molecules, and the observed small

differences between two values suggest the trans configuration of the chelates, such

effect is in agreement with studies of P. Venugopal and Krishnamurty K.[72]

Catechol having nearly identical pK1 and pK2 and involve complexation through –OH

group and formation of coloured complexes. The pKa values of catechol are 9.334

and 12.60. Formation of stable complexes with transition metals specially with Fe

(III) has been reported. Balraj Reddy et al [73] studied complexation of quinolinol and

various ligands with Nickel (II) in dioxane water medium at 300C, reported two pKa

values for catechol 9.43, 11.54. The Potentiometric, thermodynamic studies of

vanillin and transition metal complexes were carried out in 10% ethanol-water

mixture at three temperatures by Mubarak et al [74]. They reported pKa value 8.75


Page 131

(250C), 8.59 (350C), 8.42 (450C) due to ionization of –OH group, the stability constant

found to increase in the order Mn (II) < Co (II) < Ni (II) < Zn (II) < Cu (II). This

order largely reflects that the stability of Cu2+ complexes is larger than those of other

metals of the 3d series. Under the influence of the both the polarizing power of the

metal ion and the ligand field copper receive some extra stabilization due to tetragonal

distortion of octahedral symmetry in its complexes.

Potentiometric study of Desferrioxamine B (DFO) derivatives and their iron

complexes, carried by Houn et al [75], reported formation of 1:1 complexes. These

derivatives found to be powerful chelators at pH 7.4, the relative acidity of the ligand

functional groups; hydroxypyridinoate > hydroxamate > Catecholate, pKa values

reported are 9.70, 9.03 ,8.39, high stability values were reported by them. Beda and

Helmut [76] studied ternary complexes of pyrocatechol and BPD like ligands with

transition metal ions, determined two pKa for pyr 9.32, 11.32. The order of stability

reported by them was Cu (II) > Zn (II) > Co (II) > Ni (II).

Rahmiye Aydin and Duygu Inc [77] carried out potentiometric studies of

complexation of Lantanum (III) with catecholamines adrenaline, noradernaline,

dopamine in an aqueous media, reported formation of 1:1 and 1:2 complexes. High

stability of the complexes reported coordination through phenolates sites, pKa values

as 12.93, 9.53 and 8.58.They found that in case of adrenaline, noradernaline,

dopamine the first proton is released from phenolic hydroxyl followed by the

ammonium proton,-NH3+. The catecholamines are tridentate ligands. The protonation

of catecholamine involve the first protonation of the phenolic –OH group followed by

the second and third protonation of amino and second phenolate of caecholate ring.

Potentiometric studies of 3-hydoxy flavones in water-dioxane are investigated by

Montana et al [78], reported two pKa values 8.882 and 11.10 due to dissociation of –


Page 132

OH groups. They also found formation of coloured complexes. E. Farkas and H.

Csoka [79] studied complexes of Fe (III), Al (III) with 2, 3-dihydroxy-phenylalanine-

hydroxamic acid (Doapha), 1,2-dihydoxy -3,5 benzene disulphonate (Tiron), amine

type coordination mode is not detectable with these metal ions. The completely

protonated Doapha has four dissociable proton, out of these only three of them,

hydroxamic acid, amino and one of the phenolic moieties, dissociate in measurable

pH range. The pKa values observed them were 13.4, 6.88, 8.75 and 9.63, tiron 11.96

and 19.42. High stability values were determined by them.


Page 133

Fig. 4.36 pH metric titration curve for Co (II) + Qu (1:1 ratio)

Fig. 4.37 pH metric titration curve for Ni (II) + Qu (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 134

Fig. 4.38 pH metric titration curve for Cu (II) + Qu (1:1 ratio)

Fig. 4.39 pH metric titration curve for Fe (III) + Qu (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 135

Fig. 4.40 pH metric titration curve for Zn (II) + Qu (1:1 ratio)

Fig. 4.41 pH metric titration curve for Cd (II) + Qu (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 136

Fig. 4.42 nA vs. pH for Quinol

9.50 9.55 9.60 9.65 9.70 9.75 9.80

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

nA

pH

Quinol


Page 137

4.7 Complexes of 3-Amino Phenol

The values of An suggests that ligand behaves as monoprotic acid due to

deprotonation of –OH group. The higher pKa value indicate weak deprotonation of

the ligand same as in the case of 2-hydroxy acetophenone and phenol. [80]. From

protonation constant values it appears that AMP dissociates at higher pH in the range

of 9.00. Higher pKa value may be due to presence of aromatic ring. In our studies

pKa value for –NH2 group in AMP is undetectable this is in agreement with earlier

studies. [81]. The basicities of the –NH2 groups in amino pyridines are too weak to be

detected and certainly had no effect on the hydrogen ion concentration in a

moderately acidic range. They observed no pKa value for –NH2 group in 2-amino

pyridine, it has shown only one pKa 6.14 due to protonation of the N of pyridine ring.

The proton ligand stability constant calculated are in agreement with the reported

value. [82]. The pKa value for phenol is 10.00 and that of 3-aminophenol is 9.87. The

decrease in pKa may be due to electron release by amino group. The –NH2 group is an

electron releasing group it weakens the phenol when present in the para position but

strengthens it when in meta position. Binary complexes formed by M : L ratio 1:1 are

more stable than 1:2 ratio.

The order of stability

for 1:1 complexes is Fe(III) > Ni(II) > Cu(II) > Cd(II) > Zn(II) > Co(II) and

for 1:2 complexes is Cu(II) > Co(II) > Ni(II) > Zn(II) > Fe(III) > Cd(II).

The metal–Ligand stability constants of 3-Amino Phenol with metal ions are shown in

table 4.7a and 4.7b.


Page 138

Table 4.7a Proton Ligand Constant and Metal Ligand Stability Constants for 3-Aminophenol (1:1)

Table 4.7b Metal Ligand Stability Constants for 3-Amino phenol (1:2)

The literature values of pKa for aminophenol are 8.18 and 6.82. Deosarkar and

Narwade studied interaction of transition metal ions with 4-amino napthol sulphonic

acid and 6-amino napthol sulphonic acid investigated potentiometrically. There is

formation of 1:1, 1:2 complexes, and two pKa values as 2.61(-NH2), 9.27 (-OH) and

3.26 (-NH2), 10.47(-OH). The order of stability reported by them was Cu (II) > Co

(II) > Ni (II). Arbad et al [83] studied complex formation of methyl paraben (MP) and

phenarbitone (PB) with transition metals in 50 % ethanol water. They reported

formation of 1:1 complexes, with pKa 9.21 for –OH group in MP and 8.72 for –OH

group in PB. The stability order for above ligands Mn(II) < Co(II) < Ni(II) < Cu(II)

< Zn(II) was found by them. Atkas et al [84] studied dissociation of ten phenolic

compounds in 10% acetonitrile –water mixture and reported pKa 9.79 for 4-

fluorophenol.






Cu (II) 6.7243 6.4397 13.2129

Zn (II) 5.9446 5.5661 11.5107

Ni (II) 13.0085 4.3775 17.3166

Fe (III) 11.4881 10.5396 22.0277

Co (II) 4.5264 4.3369 8.8634

Cd (II) 6.8998 4.9594 11.8811



Cu (II) 3.798823 3.230197 7.029021

Co (II) 3.740288 3.239687 6.979975

Ni (II) 3.737082 3.233206 6.970288

Zn (II) 3.72125 3.23214 6.953403

Fe (III) 3.706572 - 3.706572

Cd (II) - 3.22753 3.22753


Page 139

Wankhede et al [85] studied chelation tendancy of flavones in 70% acetone :water

media and observed formation of 1:1 and 1:2 complexes. They reported higher pKa

10.60-11.00 for –OH group. The more stability of copper complexes than Nickel

complexes is observed.

Mhaske and Patil studied complexes of Paracetamol and determined pKa

value 9.67 for –OH group in paracetamol. The formation of 1:1 and 1:2 complexes

reported and observed the order of stability for metal chelate as Mn (II) < Co (II) < Ni

(II) < Zn (II). Shaleva et al [86] carried out measurement of pKa values of 105

organic compounds by multiplexed capillary electrophoresis technique, found 9.41

and 9.62 value for paracetamol, alprenol. P. J. Parmar [87] studied pKa values of

pyrazoline (PYZ) and found that mono basic nature of PYZ due to presence of –OH

group. He reported pKa values in the range of 8.00- 9.00.

Rathod S.P. et al [88] synthesized substituted thiazine and studied their

interaction with transition metal ions. These ligands are monobasic have –OH group

and reported pKa value as 9.9 and 11.7. They also found that Cu (II) complexes are

more stable than Co (II) complexes. Naik A.B. et al [89] studied stability of trivalent

metal ions with substituted pyrazolines, reported pKa value as 11.80, 11.58 due to

dissociation of –OH group. J.P.Nehte et al [90] studied interaction of Ribavirin with

metal ions, reported pKa as 9.70 due to dissociation of –OH group. Mittal et al [91]

studied complexation of Schiffs bases of o-amino phenol in ethanolic medium with

transition metal ions; pKa reported were in the range 7.40-7.72 due to ionization of –

OH group.


Page 140

Fig. 4.43 pH metric titration curve for Fe (III) + AMP (1:1 ratio)

Fig. 4.44 pH metric titration curve Ni (II) + AMP (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 141

Fig. 4.45 pH metric titration curve Co (II) + AMP (1:1 ratio)

Fig. 4.46 pH metric titration curve for Cu (II) + AMP (1:1 ratio)

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 142

Fig. 4.47 pH metric titration curve Zn (II) + AMP (1:1 ratio)

Fig. 4.48 pH metric titration curve for Cd (II) + AMP (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

12

pH

Vol.of NaOH


Page 143

Fig. 4.49 nA vs. pH for AMP

9.4 9.6 9.8 10.0 10.2 10.4

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

nA

pH

3-Aminophenol


Page 144

4.8 Complexes of 2, 2 Bipyridyl

2,2 Bipyridyl is used as ligand for the present study. The protonation constant for bpd

observed are 4.300 and 9.264. The ligand used is a bidentate. It provides two aromatic

nitrogen whose unshared electron pairs are properly placed to act cooperatively in

binding metal ions. This ligand is electron defficient they are excellent electron

acceptors. Order of stability constant for present study is

Co (II) > Fe (III) > Cu(II) > Ni(II) > Zn(II) > Cd(II) for 1:1 complexes and

Ni(II) > Cu(II) > Zn(II) > Co(II) > Cd(II) > Fe(III) for 1:2 complexes.

The complexation of 2,2 Bipyridyl with metal under study gives metal ligand stability

constants which are shown in table 4.8a and 4.8b. Kapinos and Sigel [92] studied

properties of substituted pyridine molecules with metal ions in aqueous solution

potentiometrically, they have got high stabilities for Copper and Zinc complexes, they

have reported pKa value 3.91 for 4-Bromo pyridine and 6.24 for 2, 3-dimethyl

pyridine. The pKa values obtained in agreement with reported values. Pyridine and 2-

methyl pyridine has pKa values of 5.17 and 5.97 due to presence of basic nitrogen.

High stabilities have been reported for binary complexes of 2, 2 bipyridyl which is in

agreement with literature. [93]. High values of stability for Iron (III) and Cu (II)

complexes reported in our results in agreement with Guillermo et al.[94]. The stability

values for binary complex of iron are in agreement with the value determined by

Mukherjee and Das. [95]. They have reported two pKa values for bipyridyl and 1, 10

phenanthroline as 4.23, 1.32 and 4.86, 1.90. The reported value for stability constant

of Iron complex as 9.13 which in agreement with our results.

Usha Nakara and Gupta O.D. [96] have studied polarographically complexes

of Cadmium (II) and Lead (II) with BPD in aqueous medium and reported high

stability for Cadmium (II) complexes. Ranjana et al [97] studied complexation of


Page 145

Copper (II) with purines in the aqueous system, observed formation of coloured

complexes, reported pKa values for adenine as 4.45 , 9.4 and 6.77 (logK1) and 5.022

(logK2) for Copper complexes .

Table 4.8a Proton Ligand Constant and Metal Ligand Stability Constants For 2,2Bipyridyl (1:1)

Table 4.8b Metal Ligand Stability Constants for 2,2 Biprydyl (1:2)



Cu (II) 3.2510 3.7118 6.9629

Zn (II) 3.2411 3.7059 6.9471

Ni (II) 3.2270 3.7605 6.9876

Fe (III) 3.7164 - 3.7164

Co (II) 3.1029 3.1311 6.2341

Cd (II) 3.1057 3.1027 6.2084

Balraj Reddy et al studied complexation of quinolinol and various ligands with Nickel

(II) in dioxane water medium at 300C, they reported two pKa values for bpd 2.98 and

9.82 and formation of stable complexes with BPD.

The ligand may bound to metal ion by N-M sigma bond, beside there is also

M-N pi bond formation by back donation of electrons from metal dπ orbitals to vacant

delocalized pπ orbitals over the ligands. This dπ –pπ interaction does not allow the

concentration of the electrons on the metal ions to increase significantly the positive

charges on the metal ions is almost same as in M 2+. This leads to formation of more

stable complexes. Kumar et al [98] prepared mixed ligand chelates of Fe (III) and Co



Metalion



Point wisepK1 = 4.265pK2 = 9.2312

Cu (II) 5.6942 3.9872 9.6849

Zn (II) 3.4258 3.4168 6.8426

Ni (II) 4.2895 3.3521 7.6417

Fe (III) - 9.6659 9.6659

Co (II) 11.3245 3.4273 14.7518

Cd (II) - 3.4150 3.4150


Page 146

(III) diphenates with 5-methyl -1, 10-phenanthroline (Ph) and 4-methyl -2,21-Dipridyl

(4BDP), reported octahedral geometry. They also reported neutral nature of Ph, 4BDP

and the co-ordination through tertiary nitrogen. Better chelation tendancy of the

ligand was mentioned. Thakur et al [99] studied interaction of the isoniazid with

alkaline earth metals, reported two pKa values as 3.192 due to dissociation of nitrogen

of pyridine ring and 10.66 due to dissociation of primary amino group and reported

formation of 1:1 and 1:2 complexes.

Shrama et al [100] studied interaction of lanthanones with 2-amino pyridine

and 4-amino pyridine. They reported two protonation constant as 6.91, 11.32 and

9.25, 11.47 respectively. Mohan et al [101] studied binary and mixed ligand

complexes of various ligands with phosphonoformic acid, observed two pKa values

for bpd and one for phen as 1.52, 5.88 and 4.87, they also reported formation of 1:1

and 1:2 complexes. Beda and Helmut studied ternary complexes of pyr and bpd like

ligands with transition metal ions, in aqueous solution, high stability values were

found. They reported that the stability of the ternary complexes depends on the π –

accepting qualities of the heteroaromatic N base, enhanced stability is lost if

heteroaromatic N base replaced by an aliphatic amine. The pKa values determined by

them are 7.14(2,2 bipyridyl amine), 2.69,5.18 (2,2’dipyridyl methane), 3.06 (2,2’

dipyridyl ketone). They observed high stability for Copper complexes. The Order

stability found by them was Cu (II) > Ni (II) > Co (II) > Zn (II).

M.M.Khalil et al [102] studied complexation of divalent metal ions with some

Zwitter ionic buffers and triazoles at room temperature in aqueous medium, they

reported complex formation occurred in stepwise manner and formation of 1:1 and

1:2 complexes. They observed two pKa values for 3-amino-1,2,4 triazole ( TRZAM )

4.17 and 10.82 and observed that the stability constants of different 1:2 metal ligand


Page 147

complexes are lower than corresponding 1:1 system. The sequence of stability for bdp

complexes with respect metal ions follows the order Zn (II) > Cu (II) < Ni (II) < Co

(II) which in agreement with Irving William order.


Page 148

Fig. 4.50 pH metric titration curve for Co (II) + BPD (1:1 ratio)

Fig. 4.51 pH metric titration curve for Ni (II) +BPD (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 149

Fig. 4.52 pH metric titration curve for Cu (II) + BPD (1:1 ratio)

Fig. 4.53 pH metric titration curve for Fe (III) +BPD (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 150

Fig. 4.54 pH metric titration curve for Zn (II) + BPD (1:1 ratio)

Fig. 4.55 pH metric titration curve for Cd (II) + BPD (1:1 ratio)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

2

4

6

8

10

12

pH

Vol.of NaOH

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

2

4

6

8

10

12

pH

Vol.of NaOH


Page 151

Fig. 4.56 nA vs. pH for BPD

3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

nA

pH

2,2,Bipyridyl


Page 152

Regression Analysis

A linear regression analysis of stability constant of complexes against physical

properties of metal ions has been carried out considering the equation y = Bx + A.

The regression coefficient r is calculated using computer software Origin 6.0. The

values of A (intercept to y axis) and B (slope) is also evaluated .These are shown in

tables 4.10. The physical properties which are considered in the present study is given

in the table 4.9. It was observed that none of the physical property of the metal ion

shows above 0.9 regression coefficient for stability constant of metal ligand

complexes. But there are some ligands which shows regression coefficient greater

than 0.7, which can be considered that good co-relation. These include, while

considering atomic number of metal ions, NA, BPD. These shows negative co-

relation with atomic number.

Ionization potential (I.P.) is one of the important properties of metal ions. For

present work second I.P. was considered. It was observed that in case of Metal –

nicotinic acid and metal 3,5 Dinitro salicylic acid complexes, a positive co-relation

exist, remaining Metal –ligand complexes does not have significantly good co-

relation coefficient. The ionic radius of metal ions used in the present investigation

shows good negative co-relation with Metal –Nicotinic acid complexes. Again the

complexes of Nicotinic acid with metal ions show good positive co-relation with

electronegativity of metal ions. Considering the atomic mass shows good negative

correlation with nicotinic acid complexes. Similarly atomic radii of metal ions with

metal Nicotinic acid complexes shows good co-relation in other cases poor co-

relation is observed.


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Table 4.9 Physical properties of metal ions

Table 4.10: Correlation coefficient for stability constant vs. physical properties of

metal

Ligand Atomicnumber

Atomicmass

I.P.KJ/mol

Atomic radius(ppm)

Ionic radius(ppm)

Electronegativity

NA -0.764786 -0.763148 0.887634 -0.763148 -0.747816 0.842044

MA -0.327734 -0.309631 0.096551 0.076273 -0.384331 0.14392

GA 0.157793 -0.372816 -0.318857 0.157793 -0.466792 0.153631

DNS 0.256216 0.519281 0.774399 0.519281 0.028713 0.418349

PABA -0.313572 -0.326524 -0.249864 0.223088 -0.374507 0.111284

Qu -0.683481 -0.274155 0.432872 -0.642499 -0.249131 0.432872

AMP -0.527229 -0.326531 -0.24987 -0.308721 -0.374514 0.111288

BPD -0.741036 -0.549131 0.069011 -0.14303 -0.377986 0.470664

Metalion

Atomicnumber

Atomicmass

Atomicradius ppm

Ionic radiusppm

I.P.KJ/mol

Electronegativity

Fe 26 55.85 126 64 1561 1.64

Co 27 58.93 125 74 1648 1.70

Ni 28 58.71 128 72 1753 1.75

Cu 29 63.54 128 69 1958 1.75

Zn 30 65.38 138 74 1733 1.66

Cd 48 112.4 154 109 1631 1.46

The results obtained during the determination of stability...

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