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Effects of auxiliary complex-forming agents on the rate of metallochromic indicator colour change—II
EFFECTS OF AUXILIARY COMPLEX-FORMING AGENTS ON THE RATE OF METALLOCHROMIC INDICATOR COLOUR CHANGE-II* MECHANISM
OF THE COLOUR CHANGE COPPER-EDTA TITRATIONS
GENKICHI
NAKAGAWA
OF PAN IN
and HIROKO WADA
Laboratory of Analytical Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya, Japan
(Received 12 December 1974. Accepted 29 December 1974) Summary-The rate of the ligand substitution reaction of copper (II)-PAN (CUR) with EDTA (Y) has been determined spectrophotometrically m 5% v/v dioxan over the pH range 5G6.3 at p = 0.1 (NaClO,) and at 25”. In the absence of l,lO-phenanthroline the rate law is expressed as -d[CuR+]/ dt = lo3 ’ [CUR’] Ty’], and the release of PAN from the reaction intermediate CuRY is the ratedeterminmg step. In the presence of l,lO-phenanthroline (X), however, copper forms a stable mixedligand complex (CuRX+), and the rate of substitution with EDTA is expressed as -d[CuRX+]/dt = (lo6 2[H’] + 10“ *[Xl) [CuRX+]. The release of PAN from the mixed-ligand complex by H+ and X ISpossibly the rate-determining step, with the copper-phenanthroline complexes produced undergoing fast exchange with EDTA. The stability constant of CuRX+ has been determined spectrophotometritally in 5% v/v dioxan at p = 0.1, and at 25” as [CuRX+]/[Cu’+] [R-l [X] = 1021’2.The acceleration of the rate of substitution of copper (IIbPAN chelate may be explained by the fact that the Cu-PAN bond in the distorted octahedral mixed-ligand complex CuRX is weaker than in the reaction interme-
diate CuRY. In chelatometric titrations with metallochromic indicators, the colour change at the equivalence point occurs by the substitution of the titrant for the .indicator in the metal-indicator complex. For a sharp end-point the rate of substitution, as well as the equilibrium condition is extremely important. Some studies of the kinetics of ligand substitution reactions of metal-indicator chelates with EDTA have been reported.Ie3 In our previous paper4 it was found that l,lOphenanthroline accelerates the rate of colour change of 1-(2-pyridylazo)-2-naphthol (PAN) in the copper(IIFEDTA titration, by forming the mixed-ligand complex Cu-PAN-phenanthroline near the equivalence point. In the present paper the rate of substitution reaction of Cu-PAN and Cu-PAN-phen with EDTA have been studied, and the mechanism of the colour change of PAN at the equivalence point is discussed. EXPERIMENTAL. Reagents Copper solution. Metallic copper (99.99% purity) was dissolved in nitric acid and the solution was diluted suitably. PAN solution. 1-(2-Pyrldylazo)-2-naphthol was purified by vacuum sublimation and dissolved in dioxan. EDTA. EDTA solution was standardized against a standard copper solution. with 4-(2-thiazolylazo) resorcinol as indicator. IJO-Phenanchrolme solution in dloxan. Sodium perchlorate solution. Prepared by dissolution of sodium carbonate in perchloric acid.
* Part I: see reference 4.
Dioxan. Commercial reagent-grade dioxan was purified by distillation after treatment with potassium iodide and sulphuric acid and refluxing with sodium for more than 20 hr. Apparatus
A Hitachi Rapid Scan Spectrophotometer type RSP-2, Hitachi Spectrophotometer type 124, Hirama Spectrophotometer type 6, and a Radiometer pH meter type PHM 26c were used. Procedure for the measurement of the rate of the substitution reaction In the absence of IJO-phenanthroline. The solutions A and B were mixed, and the absorbance at 550 nm was recorded as a function of the reaction time. The dead time of mixing was about 20 msec. Solution A: Cu = 1.01 x 10m5M; PAN = lGl&3.02 x 10-‘M; MES buffer solution4 = 0.02M, pH 5.0-6.3; dioxan = 5% v/v; ionic strength 0.1 (NaClOd. Solution B: EDTA = 1.24-3.72 x 10e4M; MES buffer solution =
002M, pH 5Q-6.3; dioxan = 5% v/v; ionic strength: 0.1 (NaClO,).
In the presence of IJO-phenanthroline. l,lO-Phenanthroline (1.04-2.52 x 10e5M) was added to solution A, and the absorbance at 545 nm was measured (in the pH range 5.8-6.5) as a function of the reaction time.
RESULTS AND DISCUSSION The formation constant of Cu-PAN-phen
complex
Figure 1 shows the spectra of the Cu-PAN chelate (Curve 1) and Cu-PAN-phen complex (Curves 2-4). The A,, values were 550 and 540 mn, respectively, and the isosbestic point was at 545 nm.
563
GENKICHI NAKAGAWAand HIROKO WADA
564
Wovelength,
The experiments were carried out under the conditions where Cu = 2.74 x 10°bM, PAN = 2.02 x IO-‘M, I,lO-phenanthroline = 5.00 x 10-5-4Q0 x lo-‘MM, at pH 9.22-11.28, p = 0.1 (KNOJ, and dioxan = 5% v/v. A plot of log &c&) vs. log[X] gave a straight line with a slope of 1. Therefore, in the concentration range of l,lO-phenanthroline between 10m4 and lo-‘M one molecule of l,lO-phenanthroline co-ordinates with the Cu-PAN chelate. The formation constant of the mixed-ligand complex was obtained from the value of log&R,, at log[X] = 0:
nm
Fig. 1. Spectra of Cu-PAN-phen. Cc.: 2.7 x 10m6M; CpAN: 2.0 X IO-6M. Cphe”: (I) 0 (pH 6); (2) 6.8 x 10-6M; (3) 1.1 x 10M4M; (4) 2.7 x 10-4M; (5) PAN blank. pH 9,
5 cm cell, 5% v/v dioxan.
and the conditional
=
constant
K
CCUI CR1
1 +
=
f
n= 1
u
CUR CUR(X)
(2)
~CURX.[~I"~
(3)
As the total concentration of copper-PAN complexes [(CUR)‘] can be obtained from the absorbances at 545 nm, i.e., A,,, and A in Fig. 1, by equation (4): [(CUR)‘] = fi
CR,
_ WuRl ___ dt
of copper-
where R and X represent PAN and l,lO-phenanthroline, respectively, and k&(x) is a side-reaction COefficient taking into account the formation of mixedligand complexes: ’ c(,-~~,~)
= lo21 2
with
Under the present experimental conditions, the substitution reaction of the Cu-PAN chelate goes to completion and the rate-law can be expressed by
(1)
PAN complex is given by
CWVI &CUR), = ---=
cRI cxl
Cu-PAN + EDTA = Cu-EDTA + PAN
CCuRXnI [CuRICXI”
formation
[CuRX]
CuRX = ccul
The rate of the substitution reaction of &-PAN EDTA
When copper-PAN complex forms mixed-ligand complexes with l,lO-phenanthroline, the formation constant of mixed-ligand complexes is given by (charges are omitted for simplicity)
P&RX,
KR.X
(4)
max
[Cu] and [R] can be calculated by means of equations (5) and (6):
= kO(R,y,H) [CuRI
rate-constant’ where -%I(R,Y,H) is the conditional which may depend on the concentrations of PAN, EDTA and hydrogen ion. By representing the absorbances of the reaction system at t = 0, t and x, as AO, A, and A,, respectively, we obtain equation (8) from equation (7):. log(A, - A,)
= - k=
t + lo&A,
- A,).
(8)
The plots of log(A, - A,) us. t were linear for at least 90% of the reaction, and k, (R,Y,H)was obtamed from the slopes of the straight lines. The values of of hydrogen ion, k OcR,Y,H)at various concentrations PAN and EDTA are given in Table 1. The data indicate that k0 (R,Y,H)does not depend on the hydrogen ion and PAN concentrations, but is linearly related to the EDTA concentration. Thus _ WuRl ___ = kp’] [CUR] dt
and k was determined as 1.7 x lo3 l.mole-‘. set-‘. Although in the pH range 5.M.3 EDTA is present ccul = Cc, - CO-WI (5) as HY3- and H2Y2-, no variation of the rate with Q”(X) varying proportion of these species was observed. The rate of the substitution reaction of copper (II)CR - CWVI 4-(2-pyridylazo) resorcinol (PAR) chelate with (ethyl(6) eneglycol) bis (2-aminoethylether)-N,N,N’.N’-tetra%W acetic acid (EGTA) was studied by Funahashi et al.* where CR and Cc, express the initial concentrations of PAN and copper, respectively. CQ~, and tLR(H) are Under their experimental conditions, i.e., pH 9-10, copper forms a 1 :2 complex with PAR, and the folside-reaction coefficients for the formation of copperphenanthroline complexes and the protonation of lowing mechanism has been proposed: PAN, respectively.* Substitution of the values of CUR:- + H + = CUR + HR[(CUR)‘], [Cu] and CR] into equation (2) gives the CUR + Y’+ RCuY-% CuY2- + HRvalues of &R)’ under the given experimental con(10) ditions. k = 8.8 x 1021.mole-‘. set-‘. [R,
=
*For the calculation of the a values, /I&, = 1020.56, K “x = 104E6 and K,,R = 10” .55 (5% v/v dioxan, p = 0.1) were used.
At pH 5-6.3 copper forms a 1:l complex with PAN and when this is taken into account there is
Colour
change
of PAN
Table 1. First-order conditional rate constants k. ,R,Y,H, 25”C, p = 01, dioxan 5% v/v, Cu: 503 x 10m6M 10’ x Cma,
6.22 6.22 6.22 9.33 9.33 9.33 9.33 12.4 12.4 12.4 12.4 12.4 12.4 15.6 15.6 15.6 18.6 18.6 18.6
M
pH
4.99 9.97 9.91 4.99 9.97 9.97 9.97 9.97 15.1 19.1 9.97 9.91 9.97 9.91 9.91 9.97 9.91 5.03 5.03
0.105 0.103 0.104 0.152 0,151 0.151 0.158 0.204 0.197 0.209 0,205 0.207 0.198 0.260 0.260 0,263 0.324 0.303 0.306
good agreement in the value of k. Thus, mechanism of substitution may be the same.
5.77
5.95
6.08
6.45
+ EDTA + Cu-EDTA + PAN + phen
log(A, - A,) = - ‘w
conditional
lo6 x C,,,,en, M 5.06 I.59 10.1 10.1 10.1 l@l 10.1 10.1 12.6 5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6
5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6 5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6
rate constants lo6 x C,,,, 8.48
8.48 636 8.48 8.48 8.48 8.48 10.6 8.48 8.48 8.48 6.36 8.48 8.48 8.48 12.6 8.48 8.48 8.48 8 48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 10.6 8.48
(11)
(121
t + log(A,, - A,). (13)
The plots of log(A, - A,) us. t were straight fines for at least 90% of the reaction. The values of k O(R,Y,HXjdetermined under the various conditions are shown in Table 2. No dependence of kO (R,Y,H,Xj on the concentrations of PAN and EDTA was found, but a linear relation to the hydrogen ion and l,lOphenanthroline concentrations was observed. From these linear relationships the equation the
As a mixed-ligand complex Cu-PAN-phen forms completely under the present experimental conditions,
PH
Cu-PAN-phen
_ d[CuRX] = k,,~,v,n,xj DW dt
The rate of the substitution reaction of Cu-PAN-phen
2. First-order
reaction may be written as;
The rate law is expressed by equation (12) or (13).
with EDTA
Table
the substitution
lo6 x CPAN.M k, (R.Y.Hjr set- ’
4.94 4.94 5.35 4.94 4.94 5.36 5.55 4.94 4.92 4.94 5.35 5.55 5.78 5.35 5.55 5.78 4.95 6.14 6.28
565
in copper-EDTA trtrations
kO(R.v,u.x,= k, B+ ] + k, [Xl
was deduced, where [X’] is the total concentration of l,lO-phenanthroline not combined with copper. From the slopes and the intercepts of the straight fines in plots of kO(R,Y,H,Xjvs. [H’] or w], k, and k2 were calculated. These values are shown in Table 3.
k. (R.Y,H,Xj25”C, p = 0.1, dioxan
M
(14)
10’ x CEDTA,M 622 6.22 6.22 9.33 12.4 15.6 18.6 6.22 6.22 6.22 6.22 6.22 6.22 9.33 12.4 6.22 6.22 6.22 622 6.22 9.33 12.4 15.6 18.6 6.22 6.22 6.22 6.22 12.4 15.6 18.6 6.22 6.22
5% v/v, ‘Cu: 5.03 x 10e6M
k0 (R.Y,H.x). set-’ 251 2.84 3.02 3.06 2.99 2.91 2.90 2.96 3.06 1.62 I .99 2.09 2.13 2.13 2.13 2.13 2.37 1.35 1.56 1.73 1.75 1.73 1.I6 1.74 1.82 0583 0824 0.951 @930 0.930 0944 0.926 1.01
GENKICHINAKAGAWAand HIROKOWADA
566
Table 3. Evaluation of kr and k, 10’ x [H+],M
lo6 x [X], A4
Intercept, set- ’
IO-“ x k2,
10m6x kl, 1. mole-‘. see-’
1.mole- I WC-1
0
0
2.56 5.07 7.59
0.19 0.34 0.52 2.59 1.80 1.32 0.56 average
16.6 11.2 8.31 3.55
1.6
6.9
* From slope. tFrom intercept. Thus the reaction mechanism equation (15) and/or (16):
CuRX+ + H+ -&
can be expressed
f&l? = m
CuX’+ + HR (15)
cux*+
+ y’ S
CuRX+ + X’k
[CuRI _ 1o~s6.
as
cuYZ- + X CuX;+ + R(16) ~
-
The value for K& is from reference 5. Usually copper is titrated at pH 5-6 and the addition of a small amount of l,lO-phenanthroline, i.e. 10-5-10-6M, is enough to accelerate the rate of colour change of PAN at the equivalence point4
I
cux:+
+ Y’ G CuYZ- f 2X’
The rate-determining steps are the release of PAN from Cu-PAN-phen by attack by hydrogen ion or l,lO-phenanthroline. Then the Cu-phen complex produced undergoes the fast substitution with EDTA. Comparison of k, k, and k2 shows the rate of substitution of Cu-PAN-phen with EDTA is much faster than that of Cu-PAN. This is probably because copper-PAN bonds in the mixed-ligand complex are weaker than in the Cu-PAN-Y reaction intermediate because of Jahn-Teller distortions. This is suggested by the reduced stability of CuRX compared with CuR (with respect to loss of R): KuRXI = 1012.2 KR CuRX = [CuX] [R]
Acknowledgement-The financial support from the Ministry of Education (Japan) is gratefully acknowledged.
REFERENCES I. M. Tanaka, S. Funahashi Chem., 1968, 7, 573. 2. S. Funahashi. S. Yamada and Sot. Japan, 1970, 43, 769. 3. S. Funahashi. M. Tabata and 44, 1586. 4. G. Nakagawa and H. Wada, 5. H. Wada and G. Nakagawa, 1964. 85, 549.
and
K. Shnai,
Inorg.
M. Tanaka, Bull. Chem. M. Tanaka, ibrd.. 1971, Talanta, 1973, 20, 829. N~ppor~ Kagaku Zasshi,
OF THE COLOUR CHANGE COPPER-EDTA TITRATIONS
GENKICHI
NAKAGAWA
OF PAN IN
and HIROKO WADA
Laboratory of Analytical Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya, Japan
(Received 12 December 1974. Accepted 29 December 1974) Summary-The rate of the ligand substitution reaction of copper (II)-PAN (CUR) with EDTA (Y) has been determined spectrophotometrically m 5% v/v dioxan over the pH range 5G6.3 at p = 0.1 (NaClO,) and at 25”. In the absence of l,lO-phenanthroline the rate law is expressed as -d[CuR+]/ dt = lo3 ’ [CUR’] Ty’], and the release of PAN from the reaction intermediate CuRY is the ratedeterminmg step. In the presence of l,lO-phenanthroline (X), however, copper forms a stable mixedligand complex (CuRX+), and the rate of substitution with EDTA is expressed as -d[CuRX+]/dt = (lo6 2[H’] + 10“ *[Xl) [CuRX+]. The release of PAN from the mixed-ligand complex by H+ and X ISpossibly the rate-determining step, with the copper-phenanthroline complexes produced undergoing fast exchange with EDTA. The stability constant of CuRX+ has been determined spectrophotometritally in 5% v/v dioxan at p = 0.1, and at 25” as [CuRX+]/[Cu’+] [R-l [X] = 1021’2.The acceleration of the rate of substitution of copper (IIbPAN chelate may be explained by the fact that the Cu-PAN bond in the distorted octahedral mixed-ligand complex CuRX is weaker than in the reaction interme-
diate CuRY. In chelatometric titrations with metallochromic indicators, the colour change at the equivalence point occurs by the substitution of the titrant for the .indicator in the metal-indicator complex. For a sharp end-point the rate of substitution, as well as the equilibrium condition is extremely important. Some studies of the kinetics of ligand substitution reactions of metal-indicator chelates with EDTA have been reported.Ie3 In our previous paper4 it was found that l,lOphenanthroline accelerates the rate of colour change of 1-(2-pyridylazo)-2-naphthol (PAN) in the copper(IIFEDTA titration, by forming the mixed-ligand complex Cu-PAN-phenanthroline near the equivalence point. In the present paper the rate of substitution reaction of Cu-PAN and Cu-PAN-phen with EDTA have been studied, and the mechanism of the colour change of PAN at the equivalence point is discussed. EXPERIMENTAL. Reagents Copper solution. Metallic copper (99.99% purity) was dissolved in nitric acid and the solution was diluted suitably. PAN solution. 1-(2-Pyrldylazo)-2-naphthol was purified by vacuum sublimation and dissolved in dioxan. EDTA. EDTA solution was standardized against a standard copper solution. with 4-(2-thiazolylazo) resorcinol as indicator. IJO-Phenanchrolme solution in dloxan. Sodium perchlorate solution. Prepared by dissolution of sodium carbonate in perchloric acid.
* Part I: see reference 4.
Dioxan. Commercial reagent-grade dioxan was purified by distillation after treatment with potassium iodide and sulphuric acid and refluxing with sodium for more than 20 hr. Apparatus
A Hitachi Rapid Scan Spectrophotometer type RSP-2, Hitachi Spectrophotometer type 124, Hirama Spectrophotometer type 6, and a Radiometer pH meter type PHM 26c were used. Procedure for the measurement of the rate of the substitution reaction In the absence of IJO-phenanthroline. The solutions A and B were mixed, and the absorbance at 550 nm was recorded as a function of the reaction time. The dead time of mixing was about 20 msec. Solution A: Cu = 1.01 x 10m5M; PAN = lGl&3.02 x 10-‘M; MES buffer solution4 = 0.02M, pH 5.0-6.3; dioxan = 5% v/v; ionic strength 0.1 (NaClOd. Solution B: EDTA = 1.24-3.72 x 10e4M; MES buffer solution =
002M, pH 5Q-6.3; dioxan = 5% v/v; ionic strength: 0.1 (NaClO,).
In the presence of IJO-phenanthroline. l,lO-Phenanthroline (1.04-2.52 x 10e5M) was added to solution A, and the absorbance at 545 nm was measured (in the pH range 5.8-6.5) as a function of the reaction time.
RESULTS AND DISCUSSION The formation constant of Cu-PAN-phen
complex
Figure 1 shows the spectra of the Cu-PAN chelate (Curve 1) and Cu-PAN-phen complex (Curves 2-4). The A,, values were 550 and 540 mn, respectively, and the isosbestic point was at 545 nm.
563
GENKICHI NAKAGAWAand HIROKO WADA
564
Wovelength,
The experiments were carried out under the conditions where Cu = 2.74 x 10°bM, PAN = 2.02 x IO-‘M, I,lO-phenanthroline = 5.00 x 10-5-4Q0 x lo-‘MM, at pH 9.22-11.28, p = 0.1 (KNOJ, and dioxan = 5% v/v. A plot of log &c&) vs. log[X] gave a straight line with a slope of 1. Therefore, in the concentration range of l,lO-phenanthroline between 10m4 and lo-‘M one molecule of l,lO-phenanthroline co-ordinates with the Cu-PAN chelate. The formation constant of the mixed-ligand complex was obtained from the value of log&R,, at log[X] = 0:
nm
Fig. 1. Spectra of Cu-PAN-phen. Cc.: 2.7 x 10m6M; CpAN: 2.0 X IO-6M. Cphe”: (I) 0 (pH 6); (2) 6.8 x 10-6M; (3) 1.1 x 10M4M; (4) 2.7 x 10-4M; (5) PAN blank. pH 9,
5 cm cell, 5% v/v dioxan.
and the conditional
=
constant
K
CCUI CR1
1 +
=
f
n= 1
u
CUR CUR(X)
(2)
~CURX.[~I"~
(3)
As the total concentration of copper-PAN complexes [(CUR)‘] can be obtained from the absorbances at 545 nm, i.e., A,,, and A in Fig. 1, by equation (4): [(CUR)‘] = fi
CR,
_ WuRl ___ dt
of copper-
where R and X represent PAN and l,lO-phenanthroline, respectively, and k&(x) is a side-reaction COefficient taking into account the formation of mixedligand complexes: ’ c(,-~~,~)
= lo21 2
with
Under the present experimental conditions, the substitution reaction of the Cu-PAN chelate goes to completion and the rate-law can be expressed by
(1)
PAN complex is given by
CWVI &CUR), = ---=
cRI cxl
Cu-PAN + EDTA = Cu-EDTA + PAN
CCuRXnI [CuRICXI”
formation
[CuRX]
CuRX = ccul
The rate of the substitution reaction of &-PAN EDTA
When copper-PAN complex forms mixed-ligand complexes with l,lO-phenanthroline, the formation constant of mixed-ligand complexes is given by (charges are omitted for simplicity)
P&RX,
KR.X
(4)
max
[Cu] and [R] can be calculated by means of equations (5) and (6):
= kO(R,y,H) [CuRI
rate-constant’ where -%I(R,Y,H) is the conditional which may depend on the concentrations of PAN, EDTA and hydrogen ion. By representing the absorbances of the reaction system at t = 0, t and x, as AO, A, and A,, respectively, we obtain equation (8) from equation (7):. log(A, - A,)
= - k=
t + lo&A,
- A,).
(8)
The plots of log(A, - A,) us. t were linear for at least 90% of the reaction, and k, (R,Y,H)was obtamed from the slopes of the straight lines. The values of of hydrogen ion, k OcR,Y,H)at various concentrations PAN and EDTA are given in Table 1. The data indicate that k0 (R,Y,H)does not depend on the hydrogen ion and PAN concentrations, but is linearly related to the EDTA concentration. Thus _ WuRl ___ = kp’] [CUR] dt
and k was determined as 1.7 x lo3 l.mole-‘. set-‘. Although in the pH range 5.M.3 EDTA is present ccul = Cc, - CO-WI (5) as HY3- and H2Y2-, no variation of the rate with Q”(X) varying proportion of these species was observed. The rate of the substitution reaction of copper (II)CR - CWVI 4-(2-pyridylazo) resorcinol (PAR) chelate with (ethyl(6) eneglycol) bis (2-aminoethylether)-N,N,N’.N’-tetra%W acetic acid (EGTA) was studied by Funahashi et al.* where CR and Cc, express the initial concentrations of PAN and copper, respectively. CQ~, and tLR(H) are Under their experimental conditions, i.e., pH 9-10, copper forms a 1 :2 complex with PAR, and the folside-reaction coefficients for the formation of copperphenanthroline complexes and the protonation of lowing mechanism has been proposed: PAN, respectively.* Substitution of the values of CUR:- + H + = CUR + HR[(CUR)‘], [Cu] and CR] into equation (2) gives the CUR + Y’+ RCuY-% CuY2- + HRvalues of &R)’ under the given experimental con(10) ditions. k = 8.8 x 1021.mole-‘. set-‘. [R,
=
*For the calculation of the a values, /I&, = 1020.56, K “x = 104E6 and K,,R = 10” .55 (5% v/v dioxan, p = 0.1) were used.
At pH 5-6.3 copper forms a 1:l complex with PAN and when this is taken into account there is
Colour
change
of PAN
Table 1. First-order conditional rate constants k. ,R,Y,H, 25”C, p = 01, dioxan 5% v/v, Cu: 503 x 10m6M 10’ x Cma,
6.22 6.22 6.22 9.33 9.33 9.33 9.33 12.4 12.4 12.4 12.4 12.4 12.4 15.6 15.6 15.6 18.6 18.6 18.6
M
pH
4.99 9.97 9.91 4.99 9.97 9.97 9.97 9.97 15.1 19.1 9.97 9.91 9.97 9.91 9.91 9.97 9.91 5.03 5.03
0.105 0.103 0.104 0.152 0,151 0.151 0.158 0.204 0.197 0.209 0,205 0.207 0.198 0.260 0.260 0,263 0.324 0.303 0.306
good agreement in the value of k. Thus, mechanism of substitution may be the same.
5.77
5.95
6.08
6.45
+ EDTA + Cu-EDTA + PAN + phen
log(A, - A,) = - ‘w
conditional
lo6 x C,,,,en, M 5.06 I.59 10.1 10.1 10.1 l@l 10.1 10.1 12.6 5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6
5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6 5.06 7.59 10.1 10.1 10.1 10.1 10.1 12.6
rate constants lo6 x C,,,, 8.48
8.48 636 8.48 8.48 8.48 8.48 10.6 8.48 8.48 8.48 6.36 8.48 8.48 8.48 12.6 8.48 8.48 8.48 8 48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 8.48 10.6 8.48
(11)
(121
t + log(A,, - A,). (13)
The plots of log(A, - A,) us. t were straight fines for at least 90% of the reaction. The values of k O(R,Y,HXjdetermined under the various conditions are shown in Table 2. No dependence of kO (R,Y,H,Xj on the concentrations of PAN and EDTA was found, but a linear relation to the hydrogen ion and l,lOphenanthroline concentrations was observed. From these linear relationships the equation the
As a mixed-ligand complex Cu-PAN-phen forms completely under the present experimental conditions,
PH
Cu-PAN-phen
_ d[CuRX] = k,,~,v,n,xj DW dt
The rate of the substitution reaction of Cu-PAN-phen
2. First-order
reaction may be written as;
The rate law is expressed by equation (12) or (13).
with EDTA
Table
the substitution
lo6 x CPAN.M k, (R.Y.Hjr set- ’
4.94 4.94 5.35 4.94 4.94 5.36 5.55 4.94 4.92 4.94 5.35 5.55 5.78 5.35 5.55 5.78 4.95 6.14 6.28
565
in copper-EDTA trtrations
kO(R.v,u.x,= k, B+ ] + k, [Xl
was deduced, where [X’] is the total concentration of l,lO-phenanthroline not combined with copper. From the slopes and the intercepts of the straight fines in plots of kO(R,Y,H,Xjvs. [H’] or w], k, and k2 were calculated. These values are shown in Table 3.
k. (R.Y,H,Xj25”C, p = 0.1, dioxan
M
(14)
10’ x CEDTA,M 622 6.22 6.22 9.33 12.4 15.6 18.6 6.22 6.22 6.22 6.22 6.22 6.22 9.33 12.4 6.22 6.22 6.22 622 6.22 9.33 12.4 15.6 18.6 6.22 6.22 6.22 6.22 12.4 15.6 18.6 6.22 6.22
5% v/v, ‘Cu: 5.03 x 10e6M
k0 (R.Y,H.x). set-’ 251 2.84 3.02 3.06 2.99 2.91 2.90 2.96 3.06 1.62 I .99 2.09 2.13 2.13 2.13 2.13 2.37 1.35 1.56 1.73 1.75 1.73 1.I6 1.74 1.82 0583 0824 0.951 @930 0.930 0944 0.926 1.01
GENKICHINAKAGAWAand HIROKOWADA
566
Table 3. Evaluation of kr and k, 10’ x [H+],M
lo6 x [X], A4
Intercept, set- ’
IO-“ x k2,
10m6x kl, 1. mole-‘. see-’
1.mole- I WC-1
0
0
2.56 5.07 7.59
0.19 0.34 0.52 2.59 1.80 1.32 0.56 average
16.6 11.2 8.31 3.55
1.6
6.9
* From slope. tFrom intercept. Thus the reaction mechanism equation (15) and/or (16):
CuRX+ + H+ -&
can be expressed
f&l? = m
CuX’+ + HR (15)
cux*+
+ y’ S
CuRX+ + X’k
[CuRI _ 1o~s6.
as
cuYZ- + X CuX;+ + R(16) ~
-
The value for K& is from reference 5. Usually copper is titrated at pH 5-6 and the addition of a small amount of l,lO-phenanthroline, i.e. 10-5-10-6M, is enough to accelerate the rate of colour change of PAN at the equivalence point4
I
cux:+
+ Y’ G CuYZ- f 2X’
The rate-determining steps are the release of PAN from Cu-PAN-phen by attack by hydrogen ion or l,lO-phenanthroline. Then the Cu-phen complex produced undergoes the fast substitution with EDTA. Comparison of k, k, and k2 shows the rate of substitution of Cu-PAN-phen with EDTA is much faster than that of Cu-PAN. This is probably because copper-PAN bonds in the mixed-ligand complex are weaker than in the Cu-PAN-Y reaction intermediate because of Jahn-Teller distortions. This is suggested by the reduced stability of CuRX compared with CuR (with respect to loss of R): KuRXI = 1012.2 KR CuRX = [CuX] [R]
Acknowledgement-The financial support from the Ministry of Education (Japan) is gratefully acknowledged.
REFERENCES I. M. Tanaka, S. Funahashi Chem., 1968, 7, 573. 2. S. Funahashi. S. Yamada and Sot. Japan, 1970, 43, 769. 3. S. Funahashi. M. Tabata and 44, 1586. 4. G. Nakagawa and H. Wada, 5. H. Wada and G. Nakagawa, 1964. 85, 549.
and
K. Shnai,
Inorg.
M. Tanaka, Bull. Chem. M. Tanaka, ibrd.. 1971, Talanta, 1973, 20, 829. N~ppor~ Kagaku Zasshi,