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Potentiometric studies on stepwise mixed ligand complex formation Cu(II), Ni(II) or Zn(II)-nitrilotriacetic acid-hydroxy acid
J. inorg,nucl.Chem., 1970,Vol,32, pp. 1273to 1278. PergamonPress, Printedin Great Britain
POTENTIOMETRIC STUDIES ON STEPWISE MIXED LIGAND COMPLEX FORMATION Cu(II), N i ( l l ) O R Zn(II)-NITRILOTRIACETIC ACID-HYDROXY ACID G. S H A R M A and J. P. T A N D O N Chemical Laboratories, University of Rajasthan, Jaipur-4, India (First received 30June 1969; in revised form 27August 1969)
Abstraet-Potentiometric studies of the interaction between copper(IlL nickel(ll) and zinc(ll) nitrilotriacetates with certain bidentate ligands, such as salicylic acid(SA), 5-sulphosalicylic acid (SSA) and 1,2-dihydroxy benzene-3,5-disulphonic acid (Tiron) are reported. Ternary complexes having a 1 : l : l molar ratio of metal ion to nitrilotriacetic acid (NTA) to the secondary ligand are formed. Their formation constants (Log KMAB)are reported and the probable reaction mechanism is discussed. The order of stability in terms of metal ion has been found to be Cu(ll) > Zn(lI) > Ni(ll) and in terms of secondary ligand as Tiron > SA > SSA. INTRODUCTION
a/.[l,2] studied the interaction of a number of bivalent metals with NTA using potentiometric technique. It was later shown[3] that similar chelating agents occupy only four coordination positions of the metal ion, the rest being satisfied by water molecules and that at high pH one of the water molecules is replaced by a hydroxyl group'. The water molecules attached to the metal nitrilotriacetates can also be replaced by other bidentate ligand[4]. It was, therefore, considered of interest to carry out potentiometric studies of Cu(II), Ni(II) and Zn(II) nitrilotriacetates in the presence of some hydroxy acids, such as SA, SSA and Tiron and to determine the formation constants of the resulting ternary complexes. SCHWARZENBACH e t
EXPERIMENTAL Stock solutions of metal nitrates (AnalaR BDH) were prepared and standardized against disodium ethylenediaminetetraacetic acid using murexide (copper and nickel) and eriochrome black T (zinc) as indicators. Commercial SA(BDH), SSA(Fluka), Tiron(BDH) and NTA(E.Merk) were used. Their purity was checked by potentiometric titrations against 0.1M potassium hydroxide. In the case of tiron titrations were carried out under nitrogen; all ligands were used in the diprotonated form except NTA, which was used as the disodium salt due to its low solubility in water, pH titrations were carried out using a Cambridge pH meter, standardized against 0-05M solution of potassium hydrogen phthalate (AnalaR BDH) at room temperature (25 ± I°C). Ionic strength was maintained constant (p. = 0.1 ) by the addition of molar potassium nitrate solution (AnalaR BDH). Calculations. The first acid dissociation constant of tiron was determined by the method of Chaberek and Martell[5]. Other pK values were taken from the literature [6] (Table I). Formation constants (log KMAB)of the ternary complexes were calculated by the method of Thompson and Loraas [7]. 1. 2. 3. 4. 5. 6. 7.
G. Schwarzenbach and E. Freitag, Heir. chim. Acta 34, 1492 ( 1951 ). G, Schwarzenbach and J. HeUer, Helv. chim. A cta 34, 1889 ( 1951). A. E. Martell and V. L. Hughes, J. Am. chem. Soc. 78, 1319 (1956). Y.J. Israeli, Can. J. Chem. 4, 2710 (1963). S. Chaberek, Jr. and A. E. Martell, J. Am. chem. Soc. 74, 5052 (1952). G . A . L'Heureux and A. E. Martell, J. inorg, nucl. Chem. 28, 481 (1966). L. C. Thompson and Loraas, lnorg. Chem. 2, 89 (1963). 1273
1274
G. S H A R M A and J. P. T A N D O N Table 1. Ionization constants of the secondary ligands
t
Ligand
PKI
pK~
SA SSA Tiron
2.88 2.50 7.67
13'6 11'7 12"48
=
2 5 +- I°C;~ = 0.I (KNOa).
RESULTS AND DISCUSSION
When solutions containing Cu(II) and the secondary ligand in the molar ratio of l : l are titrated an inflection is observed for all the systems at a = 2 (where a = moles of base added per mole of ligand) indicating the formation of 1 : 1 metal chelates. Further addition of alkali results in the disproportionation of the 1 : 1 metal complex to give Cu(OH)2 and a metal chelate containing a 2: 1 molar ratio of the ligand to the metal ion. The nature of the precipitate, colour of the supernatant liquid and the results of earlier studies[8] confirm these observations. For the Ni(II) and Zn(II) systems containing a 1 : 1 molar ratio of metal to the secondary ligand (SA or SSA), the first inflection at a = 1 corresponds to the neutralization of the proton of the carboxylic group of SA or SSA. Beyond this point precipitation starts at pH - 7.5 and a steep inflection at a = 2 clearly indicates complete precipitation of the metal hydroxide showing either that no l : l complex formation takes place or the chelate formed is so weak that it disproportionates in this pH region. Systems containing Ni(II) or Zn(II) and Tiron(1 : l) give the first inflection at a = 2 indicating the formation of 1:1 metal chelate. The second inflection at a = 3 shows the formation o f a hydroxo complex,
soT,
so;
Mz+ +
+
H0
S0~
20H-
~
\~ ~-'~"so~ 2<0<5 II
0H-
~o-~.~so; The metal - N T A ( ! : 1) systems ~ve a sharp inflection at a = l, indicating the formation of a l : l metal chelate. In the C u ( I D - N T A system only, a further inflection at a = 2 shows the formation of a hydroxo anionic complex. Martell et
a/.[9] report similar observations.
8. A. K. Babko, J. gen. Chem. U.S.S.R. 17, 443 (1947). 9. R. C. Courtney, R. L. Gustafson, S. Chaberek, Jr. and A. E. Martell, J. Am. chem. Soc. 80, 2121 (1958).
Potentiometric studies on mixed ligand complex formation
1275
The mixed ligand titrations involving M(II)-NTA-secondary ligand ( l ' l ' l ) can be represented as follows: MA + B ~ MAB,
KMA a -~-
[MAB] [MA][BI"
Where M stands for metal ion, A for the NTA anion and B for the secondary ligand anion. Evidence for the stepwise combination of A and B with the metal ion may be obtained by the superimposable nature of the theoretical composite curve[10] (drawn by adding the horizontal distance of the secondary ligand curve to the horizontal distance of the M(II)-NTA curve at the same pH) in the lower buffer region. This indicates that complete formation of the M(II)-NTA ( l : l ) chelate takes place before the formation of the mixed ligand complex. The initially lower pH in the M(II)-NTA-SA (Curve l, Figs. 1-3) and M(II)NTA-SSA (Curve 3, Figs. 1-3) systems relative to the corresponding theoretical /,"
II---
/,
III
/,
II/
•
/'°
I0--
9--
iX~
I
/ IX
8 XI
7--
Z Q.
6
-
.j
x./ ~0 ~0
--
0
.4 x ]
i
I
2
J
J
2
3
L
4]
3
I 3
I 5
6
0
Fig, 1. Potentiometric titrations of mixed ligand chelates of Cu(ll) and N T A . All solutions 5 × 10-aM in copper nitrate and 0.10M in KNO3 at the start of the titration, a = moles of base added per mole of metal ion. l, 1 : 1 : l C u ( I I ) - N T A - S A ; 3, l : 1 : l C u ( I I ) - N T A - S S A ; 5, I : 1 : l C u ( I I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding systems. 10. G . H . Carey and A. E. Martell, J. Am. chem. Soc. 1t9, 2 8 5 9 (1967).
1276
G. SHARMA and J. P. TANDON
~"
/f
/~,
"
s Q
I,,
/ •
i
;/
/ _
( Q.
,lJ s I D~, o
0
I
I
I
0
I
2
I
I
I
I
2
3
I
2
I
3
1
3
J
4
]
5
6
Fig. 2. Potentiometric titrations of mixed ligand chelates of Ni(II) and N T A . All solutions are 5 × 10-3M in nickel nitrate and 0.10M in KNO3 at the start o f the titration. a = moles of base added per mole o f metal ion. 1, 1 : 1 : 1 N i ( I I ) - N T A - S A ; 3, N i ( I I ) - N T A - S S A ; 5, N i ( I I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding systems.
composite curves (Curves 2 and 4, Figs. 1-3) is due to the ionization of the proton of the carboxylic group of SA or SSA, respectively. In these systems, a sharp inflection (curves 1 and 3; Figs. 1-3) at a = 2 indicates that the neutralization of the proton of the carboxylic group in SA or SSA takes place prior to the addition of the secondary ligand. The lower pH relative to the theoretical composite curve (Curves 2 and 4, Figs. 1-3) between a = 2-3 shows that the ternary complex is formed in this region. In the case of the M(II)-NTA-Tiron systems (Curve 5, Figs. 1-3), sharp inflections are observed at a--- 1 and a = 3. The titration curve for the mixed ligand complex is superimposable on the M(II)-NTA (1 : 1) curve up to a = 1, indicating that in the lower pH region only the M(II)-NTA (1 : 1) complex is present. The lowering of the buffer region in comparison to the theoretical composite curve (Curve 6, Figs. 1-3) between a -- 1-3 is evidence for the formation of the ternary complex.
1277
Potentiometric studies on mixed ligand complex formation
X-" F" ,/" "*"'~, / / 7
ic
;7 ,,/.//
t
x m
4_ j sl
3
-
.
~,,(
.x
a
O
l 2 I 3 I 4
I i
o o I
i I
2 I
I
2
3
J 3 I 5
0
Fig. 3. Potentiometric titrations of m i x e d ligand chelates of Z n ( l l ) and N T A . All solutions are 5 × 10-3M in zinc nitrate and 0.10M in KNO3 at the start of the titration. a = moles of base added per mole of metal ion. 1, 1 : 1 : 1 Z n ( I I ) - N T A - S A ; 3, Z n ( I I ) - N T A - S S A ; 5, Z n ( l I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding s y s t e m s .
The sequence of the reactions involved in these systems with the salicylic acid as an example of the secondary ligand may be summarized as follows:
CHzCOOM 2+ .1,.
H+N/OH2 COO --
CHzCOO+
OH-
-
6"o'=I
~CHeCOO~ i
OOH H
N
~/~CH2COO~' " M ~+ ~CHzCOO/
+ OH-
/-,,=a~:Z
~-..~COO v OH CHzCO0-
CH~COO,-
~CH2CO0~ M N _-- 2+
\c. coo-/
÷ -OOC.,~ HO" v
-P OH-
2
\ --CH2COO--
/ \ o--'~'
1278
G. S H A R M A and J. P. T A N D O N
However, in the case of M ( I I ) - N T A - T i r o n system neutralization of both the protons of the phenolic groups and the formation of the mixed complex take place simultaneously. There is a change in the initial light blue colour of the C u ( I I ) - N T A (1 : 1) chelate to greenish blue and finally to green in the presence of the secondary ligand. Thus, in the systems discussed above, 1 : 1 metal-NTA chelates are initially formed in the lower buffer region. In the systems C u ( I I ) - N T A - S A , Cu(II)NTA-SSA, Cu(II)-NTA-Tiron, Ni(II)-NTA-SSA, Ni(II)-NTA-Tiron, Z n ( I I ) - N T A - S S A , and Zn(II)-NTA-Tiron stepwise addition of the secondary ligand also takes place resulting in the formation of the ternary complexes. However, in the N i ( I I ) - N T A - S A and Z n ( I I ) - N T A - S A systems, there is no evidence for the formation of such mixed ligand complexes. This is supported by the following facts: (1) The colour of the metal-NTA complex persists throughout the titration. (2) Salicylic acid has very little or no tendency to form even simple complexes with Ni(II) or Zn(II). (3) No lowering of pH in comparison to the theoretical composite curve is obtained in these systems in the pH range where ternary complex formation is observed in the other systems. Table 2. Formation constants of the mixed iigand chelates System
log KMAa
Cu(II)-NTA-SA Cu(I I ) - N T A - S S A Cu(II)-NTA-Tiron Ni(II)-NTA-SA Ni(II)-NTA-SSA Ni(ll)-NTA-Tiron Zn(II)-NTA-SA Zn(II)--NTA-SSA Zn(II)-NTA-Tiron
7"20-+0'06 5"62 -+ 0"05 9.51 -+0"08 3.92 _+0.06 6.76 - 0"05 4.23 - 0.09 7.07 -+ 0'07
t = 25+ I°C;/x = 0"1 (KNOa).
Formation constants of the mixed ligand chelates. A comparison of the formation constants (log KMAB) given in Table 2 indicates that the order of stability of the ternary complexes in terms of metal is Cu(II) > Zn(II) > Ni(II) and in terms of secondar ligand as Tiron > SA > SSA. This order is the same as in the case of 1 : 1 metal: secondary ligand chelates and the same as that observed for similar uranium(IV) chelates [ 10]. Acknowledgements-We thank Professor R. C. Mehrotra for the provision of facilities and CSIR for a Junior Research Scholarship (G. S.).
POTENTIOMETRIC STUDIES ON STEPWISE MIXED LIGAND COMPLEX FORMATION Cu(II), N i ( l l ) O R Zn(II)-NITRILOTRIACETIC ACID-HYDROXY ACID G. S H A R M A and J. P. T A N D O N Chemical Laboratories, University of Rajasthan, Jaipur-4, India (First received 30June 1969; in revised form 27August 1969)
Abstraet-Potentiometric studies of the interaction between copper(IlL nickel(ll) and zinc(ll) nitrilotriacetates with certain bidentate ligands, such as salicylic acid(SA), 5-sulphosalicylic acid (SSA) and 1,2-dihydroxy benzene-3,5-disulphonic acid (Tiron) are reported. Ternary complexes having a 1 : l : l molar ratio of metal ion to nitrilotriacetic acid (NTA) to the secondary ligand are formed. Their formation constants (Log KMAB)are reported and the probable reaction mechanism is discussed. The order of stability in terms of metal ion has been found to be Cu(ll) > Zn(lI) > Ni(ll) and in terms of secondary ligand as Tiron > SA > SSA. INTRODUCTION
a/.[l,2] studied the interaction of a number of bivalent metals with NTA using potentiometric technique. It was later shown[3] that similar chelating agents occupy only four coordination positions of the metal ion, the rest being satisfied by water molecules and that at high pH one of the water molecules is replaced by a hydroxyl group'. The water molecules attached to the metal nitrilotriacetates can also be replaced by other bidentate ligand[4]. It was, therefore, considered of interest to carry out potentiometric studies of Cu(II), Ni(II) and Zn(II) nitrilotriacetates in the presence of some hydroxy acids, such as SA, SSA and Tiron and to determine the formation constants of the resulting ternary complexes. SCHWARZENBACH e t
EXPERIMENTAL Stock solutions of metal nitrates (AnalaR BDH) were prepared and standardized against disodium ethylenediaminetetraacetic acid using murexide (copper and nickel) and eriochrome black T (zinc) as indicators. Commercial SA(BDH), SSA(Fluka), Tiron(BDH) and NTA(E.Merk) were used. Their purity was checked by potentiometric titrations against 0.1M potassium hydroxide. In the case of tiron titrations were carried out under nitrogen; all ligands were used in the diprotonated form except NTA, which was used as the disodium salt due to its low solubility in water, pH titrations were carried out using a Cambridge pH meter, standardized against 0-05M solution of potassium hydrogen phthalate (AnalaR BDH) at room temperature (25 ± I°C). Ionic strength was maintained constant (p. = 0.1 ) by the addition of molar potassium nitrate solution (AnalaR BDH). Calculations. The first acid dissociation constant of tiron was determined by the method of Chaberek and Martell[5]. Other pK values were taken from the literature [6] (Table I). Formation constants (log KMAB)of the ternary complexes were calculated by the method of Thompson and Loraas [7]. 1. 2. 3. 4. 5. 6. 7.
G. Schwarzenbach and E. Freitag, Heir. chim. Acta 34, 1492 ( 1951 ). G, Schwarzenbach and J. HeUer, Helv. chim. A cta 34, 1889 ( 1951). A. E. Martell and V. L. Hughes, J. Am. chem. Soc. 78, 1319 (1956). Y.J. Israeli, Can. J. Chem. 4, 2710 (1963). S. Chaberek, Jr. and A. E. Martell, J. Am. chem. Soc. 74, 5052 (1952). G . A . L'Heureux and A. E. Martell, J. inorg, nucl. Chem. 28, 481 (1966). L. C. Thompson and Loraas, lnorg. Chem. 2, 89 (1963). 1273
1274
G. S H A R M A and J. P. T A N D O N Table 1. Ionization constants of the secondary ligands
t
Ligand
PKI
pK~
SA SSA Tiron
2.88 2.50 7.67
13'6 11'7 12"48
=
2 5 +- I°C;~ = 0.I (KNOa).
RESULTS AND DISCUSSION
When solutions containing Cu(II) and the secondary ligand in the molar ratio of l : l are titrated an inflection is observed for all the systems at a = 2 (where a = moles of base added per mole of ligand) indicating the formation of 1 : 1 metal chelates. Further addition of alkali results in the disproportionation of the 1 : 1 metal complex to give Cu(OH)2 and a metal chelate containing a 2: 1 molar ratio of the ligand to the metal ion. The nature of the precipitate, colour of the supernatant liquid and the results of earlier studies[8] confirm these observations. For the Ni(II) and Zn(II) systems containing a 1 : 1 molar ratio of metal to the secondary ligand (SA or SSA), the first inflection at a = 1 corresponds to the neutralization of the proton of the carboxylic group of SA or SSA. Beyond this point precipitation starts at pH - 7.5 and a steep inflection at a = 2 clearly indicates complete precipitation of the metal hydroxide showing either that no l : l complex formation takes place or the chelate formed is so weak that it disproportionates in this pH region. Systems containing Ni(II) or Zn(II) and Tiron(1 : l) give the first inflection at a = 2 indicating the formation of 1:1 metal chelate. The second inflection at a = 3 shows the formation o f a hydroxo complex,
soT,
so;
Mz+ +
+
H0
S0~
20H-
~
\~ ~-'~"so~ 2<0<5 II
0H-
~o-~.~so; The metal - N T A ( ! : 1) systems ~ve a sharp inflection at a = l, indicating the formation of a l : l metal chelate. In the C u ( I D - N T A system only, a further inflection at a = 2 shows the formation of a hydroxo anionic complex. Martell et
a/.[9] report similar observations.
8. A. K. Babko, J. gen. Chem. U.S.S.R. 17, 443 (1947). 9. R. C. Courtney, R. L. Gustafson, S. Chaberek, Jr. and A. E. Martell, J. Am. chem. Soc. 80, 2121 (1958).
Potentiometric studies on mixed ligand complex formation
1275
The mixed ligand titrations involving M(II)-NTA-secondary ligand ( l ' l ' l ) can be represented as follows: MA + B ~ MAB,
KMA a -~-
[MAB] [MA][BI"
Where M stands for metal ion, A for the NTA anion and B for the secondary ligand anion. Evidence for the stepwise combination of A and B with the metal ion may be obtained by the superimposable nature of the theoretical composite curve[10] (drawn by adding the horizontal distance of the secondary ligand curve to the horizontal distance of the M(II)-NTA curve at the same pH) in the lower buffer region. This indicates that complete formation of the M(II)-NTA ( l : l ) chelate takes place before the formation of the mixed ligand complex. The initially lower pH in the M(II)-NTA-SA (Curve l, Figs. 1-3) and M(II)NTA-SSA (Curve 3, Figs. 1-3) systems relative to the corresponding theoretical /,"
II---
/,
III
/,
II/
•
/'°
I0--
9--
iX~
I
/ IX
8 XI
7--
Z Q.
6
-
.j
x./ ~0 ~0
--
0
.4 x ]
i
I
2
J
J
2
3
L
4]
3
I 3
I 5
6
0
Fig, 1. Potentiometric titrations of mixed ligand chelates of Cu(ll) and N T A . All solutions 5 × 10-aM in copper nitrate and 0.10M in KNO3 at the start of the titration, a = moles of base added per mole of metal ion. l, 1 : 1 : l C u ( I I ) - N T A - S A ; 3, l : 1 : l C u ( I I ) - N T A - S S A ; 5, I : 1 : l C u ( I I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding systems. 10. G . H . Carey and A. E. Martell, J. Am. chem. Soc. 1t9, 2 8 5 9 (1967).
1276
G. SHARMA and J. P. TANDON
~"
/f
/~,
"
s Q
I,,
/ •
i
;/
/ _
( Q.
,lJ s I D~, o
0
I
I
I
0
I
2
I
I
I
I
2
3
I
2
I
3
1
3
J
4
]
5
6
Fig. 2. Potentiometric titrations of mixed ligand chelates of Ni(II) and N T A . All solutions are 5 × 10-3M in nickel nitrate and 0.10M in KNO3 at the start o f the titration. a = moles of base added per mole o f metal ion. 1, 1 : 1 : 1 N i ( I I ) - N T A - S A ; 3, N i ( I I ) - N T A - S S A ; 5, N i ( I I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding systems.
composite curves (Curves 2 and 4, Figs. 1-3) is due to the ionization of the proton of the carboxylic group of SA or SSA, respectively. In these systems, a sharp inflection (curves 1 and 3; Figs. 1-3) at a = 2 indicates that the neutralization of the proton of the carboxylic group in SA or SSA takes place prior to the addition of the secondary ligand. The lower pH relative to the theoretical composite curve (Curves 2 and 4, Figs. 1-3) between a = 2-3 shows that the ternary complex is formed in this region. In the case of the M(II)-NTA-Tiron systems (Curve 5, Figs. 1-3), sharp inflections are observed at a--- 1 and a = 3. The titration curve for the mixed ligand complex is superimposable on the M(II)-NTA (1 : 1) curve up to a = 1, indicating that in the lower pH region only the M(II)-NTA (1 : 1) complex is present. The lowering of the buffer region in comparison to the theoretical composite curve (Curve 6, Figs. 1-3) between a -- 1-3 is evidence for the formation of the ternary complex.
1277
Potentiometric studies on mixed ligand complex formation
X-" F" ,/" "*"'~, / / 7
ic
;7 ,,/.//
t
x m
4_ j sl
3
-
.
~,,(
.x
a
O
l 2 I 3 I 4
I i
o o I
i I
2 I
I
2
3
J 3 I 5
0
Fig. 3. Potentiometric titrations of m i x e d ligand chelates of Z n ( l l ) and N T A . All solutions are 5 × 10-3M in zinc nitrate and 0.10M in KNO3 at the start of the titration. a = moles of base added per mole of metal ion. 1, 1 : 1 : 1 Z n ( I I ) - N T A - S A ; 3, Z n ( I I ) - N T A - S S A ; 5, Z n ( l I ) - N T A - T i r o n ; 2, 4 & 6 are the theoretical composite curves for the corresponding s y s t e m s .
The sequence of the reactions involved in these systems with the salicylic acid as an example of the secondary ligand may be summarized as follows:
CHzCOOM 2+ .1,.
H+N/OH2 COO --
CHzCOO+
OH-
-
6"o'=I
~CHeCOO~ i
OOH H
N
~/~CH2COO~' " M ~+ ~CHzCOO/
+ OH-
/-,,=a~:Z
~-..~COO v OH CHzCO0-
CH~COO,-
~CH2CO0~ M N _-- 2+
\c. coo-/
÷ -OOC.,~ HO" v
-P OH-
2
\ --CH2COO--
/ \ o--'~'
1278
G. S H A R M A and J. P. T A N D O N
However, in the case of M ( I I ) - N T A - T i r o n system neutralization of both the protons of the phenolic groups and the formation of the mixed complex take place simultaneously. There is a change in the initial light blue colour of the C u ( I I ) - N T A (1 : 1) chelate to greenish blue and finally to green in the presence of the secondary ligand. Thus, in the systems discussed above, 1 : 1 metal-NTA chelates are initially formed in the lower buffer region. In the systems C u ( I I ) - N T A - S A , Cu(II)NTA-SSA, Cu(II)-NTA-Tiron, Ni(II)-NTA-SSA, Ni(II)-NTA-Tiron, Z n ( I I ) - N T A - S S A , and Zn(II)-NTA-Tiron stepwise addition of the secondary ligand also takes place resulting in the formation of the ternary complexes. However, in the N i ( I I ) - N T A - S A and Z n ( I I ) - N T A - S A systems, there is no evidence for the formation of such mixed ligand complexes. This is supported by the following facts: (1) The colour of the metal-NTA complex persists throughout the titration. (2) Salicylic acid has very little or no tendency to form even simple complexes with Ni(II) or Zn(II). (3) No lowering of pH in comparison to the theoretical composite curve is obtained in these systems in the pH range where ternary complex formation is observed in the other systems. Table 2. Formation constants of the mixed iigand chelates System
log KMAa
Cu(II)-NTA-SA Cu(I I ) - N T A - S S A Cu(II)-NTA-Tiron Ni(II)-NTA-SA Ni(II)-NTA-SSA Ni(ll)-NTA-Tiron Zn(II)-NTA-SA Zn(II)--NTA-SSA Zn(II)-NTA-Tiron
7"20-+0'06 5"62 -+ 0"05 9.51 -+0"08 3.92 _+0.06 6.76 - 0"05 4.23 - 0.09 7.07 -+ 0'07
t = 25+ I°C;/x = 0"1 (KNOa).
Formation constants of the mixed ligand chelates. A comparison of the formation constants (log KMAB) given in Table 2 indicates that the order of stability of the ternary complexes in terms of metal is Cu(II) > Zn(II) > Ni(II) and in terms of secondar ligand as Tiron > SA > SSA. This order is the same as in the case of 1 : 1 metal: secondary ligand chelates and the same as that observed for similar uranium(IV) chelates [ 10]. Acknowledgements-We thank Professor R. C. Mehrotra for the provision of facilities and CSIR for a Junior Research Scholarship (G. S.).