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Kinetic studies of complexation reactions of water-soluble hydrazones with nickel(II) and palladium(II) ions
[--\:t
Talanta
. ;'//
ELSEVIER
Talanta 42 (1995) 1229-1237
Kinetic studies of complexation reactions of water-soluble hydrazones with nickel(II) and palladium(II) ions Tsugikatsu Odashima *, Mitsuru Yamaguchi ', Hajime Ishii Institute [or Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai, M(vagi 980, Japan Received 1 August 1994; revised 21 February 1995; accepted 23 February 1995
Abstract
The kinetics of complexation reactions of five water-soluble heterocyclic hydrazones with nickel(II) and palladium(II) ions have been investigated by stopped-flow spectrophotometry. Rates of complexations with nickel(II) and palladium(II) in the absence of chloride ion were found to be proportional to the first order of the ligand and metal ion concentrations and to the inverse first order of the hydrogen ion concentration except for the complexation of ct-(2-benzimidazolyl)-~t-(5-nitro-2-pyridyl)hydrazono-3-toluenesulfonic acid with palladium(II). Rates of complexation with palladium(lI) in the presence of chloride ion were best described by a two-term expression, both terms being first order in the palladium ion and ligand concentrations and inverse first order in the hydrogen ion concentration. The first term has zero dependence of the chloride ion concentration, whereas the second is first order with respect to the chloride ion concentration. The rate constant for each complexation reaction was determined. The complexation of the hydrazones with nickel(II) was estimated to go according to an Eigen mechanism and that with palladium (II) according to the associative mechanism.
1. Introduction
cently found to be very useful as highly sensitive spectrophotometric reagents for metals. Among the water-soluble hydrazones which Some of them were applied to the determinaconsist of 5-nitro-2-pyridylhydrazine and hete- tion of trace amounts of cobalt [1,4] and nickel rocyclic ketones having one or two sulfo [3,5]. In addition, complexation equilibria begroups in their molecules, :t-(5-nitro-2- tween the above-mentioned five hydrazones pyridyl)hydrazono-:t-(2-pyridyl)-3-toluenesuland metal ions including cadmium(ll), fonic acid (NPHPTS) [!], ~-(5-nitro-2- cobalt(II), copper(lI), iron(lI), palladium(II) pyridyl)hydrazono-~- (2-quinolyl)-3-toluenesul- and zinc(II) were investigated previously [6]. In fonic acid (NPHQTS) [2], :t-(2-benzothiazolyl) this work kinetics of complexation reactions of - :t - (5 - nitro - 2 - pyridyi)hydrazono-3-toluenesul- these five hydrazones with nickel(lI) and pallafonic acid (BTNPHTS) [2], ~-(2-benzimida- dium(II) have been studied in detail by zolyl)-ct-(5-nitro-2-pyridyl)hydrazono-3-toluene- stopped-flow spectrophotometry in order to sulfonic acid (BINPHTS) [2] and disuifonated more fully understand these complexation reac(2-benzimidazolyl)(phenyl)methanone 5-nitro- tions. 2-pyridylhydrazone (DSBINPH) [3] were re* Corresponding author. ' Present address: Analytical Research Center, Central Research Laboratory, Research & Development Division, Japan Energy Corporation, 3-1%35. Niizo-minami, Todashi, Saitama 335. Japan.
2. Experimental
2.1. Reagents All reagents were of analytical-reagent grade
0039-9140 95509.50 '-: 1995 Elsevier Science B.V. All rights reserved SSDI (}039-9140(95J01565-5
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T. Odashima et al. / Talanta 42 (1995) 1229-1237
and all solutions were prepared with distilled, demineralized water, unless stated otherwise. Hydrazone solutions, 5.0 x 10 -4 or 1.0 x 10 -3 M, were prepared by dissolving each hydrazone, synthesized as in earlier work [1-3], in water or 0.01 M sodium hydroxide solution. These solutions were further diluted with water if necessary. Standard solutions of nickel(II) and palladium (II), about 0.5 mgcm -3, were prepared by first dissolving their nitrates in 0.01 M perchloric acid and then standardizing by EDTA titration. Working solutions were prepared by dilution of these solutions with water. Buffer solutions were prepared by mixing 0.2 M tris(hydroxymethyl)aminomethane (Tris) solution and 0.2 M nitric acid.
screen and by the recorder. The absorbance values were collected regularly and processed by linear regression using programs provided with the microcomputer for the application of first-order reaction plots (Eq. (4)) and KezdySwinbourne's plots (Eq. (6)) [7,8]. Kinetics measurements for the complexation of NPHQTS and BTNPHTS with nickel(II) were performed in 10% (v/v) 1,4-dioxane/H20 because of their low solubilities in water.
3. Results and discussion In general, since the rate of complex formation depends on the concentrations of metal ion, ligand, and hydrogen ion, the rate of formation of the metal-hydrazone complex is assumed to be
2.2. Apparatus
A JASCO KS-100M stopped-flow system was used, which consists of a stopped-flow module with a flow cell of optical path 10mm, a diode-array spectrophotometer, an oscilloscope, an X - Y plotter, a microcomputer and a thermostatic bath. This system has a minimum dead-time of 2 ms and affords, on both the oscilloscope screen and the plotter, a three-dimensional (time-resolved) adsorption spectrum (corrected by subtracting the blank signal of the reaction product over the region of + 170 nm from a fixed wavelength, and its absorbance changes with time (kinetic curves) at repeated arbitrary wavelengths. 2.3. Procedure
Kinetic runs were performed under pseudofirst-order conditions with respect to hydrazone. Each run was repeated five times, the average signal being adopted. Equal volumes of nickel(II) (or palladium(II)) and hydrazone solutions, both of which were buffered at a fixed pH between 7.0 and 8.5 (or adjusted to a fixed acidic pH value between 1.0 and 2.0 for the palladium(II) complex) with a constant ionic strength of 0.2 M (or 2.0 M for the palladium(lI) complex), were mixed in the mixing chamber of the stopped-flow apparatus. The temperature was controlled at 25 + 0.1 °C. A kinetic curve at the absorption-maximum wavelength of the complex was recorded on the oscilloscope
d[M] = k*[M]"[R]b[H +]c dt
(1)
where t is the reaction time, k* is the rate constant, M is the metal ion, R is the hydrazone and a, b and c are the reaction orders with respect to the concentrations of metal ion, hydrazone and hydrogen ion, respectively. Under a pseudo-first-order excess of the hydrazone and at a constant pH, the reaction rate can be expressed as d[M] dt = k°b~[M]~
(2)
where kob~d is the observed pseudo-first-order rate constant, assuming a = 1. By integrating Eq. (2), taking the mass balance into consideration and then applying Beer's law, Eq. (3) can be derived, which is rewritten as Eq. (4). A~ - A, = (A~ - A0) exp( - Kobsdt)
(3)
--ln(A~ -- A,) = -ln(A:~ - A o ) + k o b s d t
(4)
where Ao, A, and A~ are the absorbance of the reaction system at t = 0, t and ~ , respectively. 3.1. Kinetics o f formation o f nickel complexes 3.1.1. Reaction order with respect to nickel-ion concentrations Fig. 1 shows a kinetic curve measured in the presence of a large excess of DSBINPH and at pH 7.43 for the complex formation of DSBINPH with nickel(ll), along with a - - l n ( A . , - A , ) vs. t plot prepared from this
1231
T. Odashima et al./ Talanta 42 (1995) 1229-1237
kinetic curve. The plot gives a straight line with a slope of 0.0436, which indicates that the rate of complexation is first order with respect to the nickel(II) concentration, i.e. a = 1 in Eq. (1), and the kobsd is 4.36 x 10 - 2 S-l under the experimental conditions given in Fig. 1. In the complex formation of the other hydrazones with nickel(II), however, more than 20 h were required for attaining equilibrium. Accordingly the absorbances at infinite time (A~) were not obtained within the experimental period (about 1 h). Therefore the kinetics analysis for these complexation reactions was performed according to Kezdy-Swinbourne's method [7,8] as follows. As is clear from Eq. (3), after the elapse of a fixed time, At, from t, Eq. (5) holds A ~_ - A, + a, = (A ~ - Ao) exp[ - kob~(t + At)]
(5)
0.20 0.15 j : //.: '~" O.10 / 0.05
:: /" / /
0.00 0.O0
/ f .S
f: <:, 0.O5 O.10
0.'15
O.20
At÷At
Fig. 2. Kezdy-Swinbourne plot for the BINPHTS-nickel(ll) system. (BINPHTS, 7.5 × 10 -5 M; pH 7.52; Wavelength, 500 rim. Other conditions are the same as those in Fig. 1.)
were obtained for N P H P T S - , N P H Q T S - and BTNPHTS-nickeI(II) systems, which indicates that the rate of complexation in these systems is first order with respect to the nickel(II) concentration, i.e. a = I.
From Eqs. (3) and (5), Eq. (6) is obtained A, = A ~_[1 - exp(kob~dAt)] + A, + a, exp(kob~At)
(6) Eq. (6) implies that a plot of A, versus A,+a, gives a straight line with a slope of exp (kob~dAt). As the value of At is known, the kob,~ value can be calculated from this slope. Furthermore, A, is equal to A,+a, and also equal to A ~ at t = ~ , so an intersecting point of this line with a straight line A, = A,+a, (i.e. a line with a slope of unity passing through the origin) should give the value of A~. Fig. 2 shows a plot of A, versus A,+~, for the BINPHTS-nickeI(II) system as an example, in which the plot gives a straight line with a slope of i.92. Similar results to that shown in Fig. 2 0.3 E e-
3.1.2. Reaction order with respect to hydrazone concentration From the above results the rate equation for the formation of the nickel complexes can be written as follows d[Ni(II)] = kobsd[Ni(II)] = k0(H)[Ni(II)][R] b (7) dt kob ~ = k0iH)[R] b - ko(H)C ~
(8)
where k0(H) is the conditional second-order rate constant and CR is the total concentration of the hydrazone. The effect of the hydrazone concentration on the observed rate constant was examined in order to determine the reaction order with respect to the hydrazone concentration. Fig. 3 shows the result for the DSBINPH-nickel(II) system, in which plots of kob~l v e r s u s Ca give straight lines passing
15
} 0.2
@ G}
J lO
a
~o 0.1 5
000
5
10
15
20
25
--OO 3 6 0
Reaction time/s Fig. I. Reaction curve and plot of - I n ( A z - A , ) vs. t for the formation of DSBINPH-nickeI(]I) comp|ex. (DSBINPH, 8.3 × i 0 - S M ; Ni(ll), 2.5 × 1 0 - 6 M ; pH 7.43;
ionic strength, 0.2 M (NaCIO4); temperature, 25 + 0.l *C.)
0.0
i
i
i
0.5
1.0
1.5
,
2,0
lO'C. Fig. 3. Plots of kob,a vs. C R for DSBINPH-nickeI(II) system at pH: curve a, 7.43; curve b, 8.01; curve c, 8.28; curve d, 8.37; curve e, 8.52. Other conditions are the same as those in Fig. 1.
1232
T. Odashima et al./ Talanta 42 (1995) 1229-1237
where KN~ is the hydrolysis constant of nickel(II). For the formation of the NPHPTS, NPHQTS and BTNPHTS complexes, Eqs. (10) and (11) hold
12i
10 a
~
l
°
Ni(H20) 2+ + H L -
6
kl
k2
NiOH(H20)~- + H L -
4.0
¢.s
8'.0
8'.s
9.0
, NiL + H + + 6H20(10) , NiL + 6H20
(l l)
for the formation of the BINPHTS complex, Eqs. (12) and (13) hold
OH
kl
Fig. 4. Plotsof k~m vs. pH for (curve a) BINPHTS-nick¢1(11) and (curve b) DSBINPH-nickel01) systems. CR: curve a, 9.4 x 10-5 M; curve b, 1.0 x 10.4 M. Other conditions are the same as those in Fig. I.
Ni(H20)~ + + H2L-
through the origin. Similar results to that in Fig. 3 were obtained for the other hydrazonenickel(II) systems, which indicates that the rate of complexation in these systems in first order, dependent on the hydrazone concentration, i.e. b = 1.
and for the formation of the DSBINPH complex, Eqs. (14) and (15) hold
, Ni(HL)+ H + +6H20 (12) k2
NiOH(H20)J + H2L-
, Ni(HL) + 6H20 (13)
kl [Ni(HL)]- + Ni(H20)62+ + H2 L2- ---* NiOH(H20)~- + H2 L2-
k2
+ 6H20
The effect of pH on the observed rate constant was studied, values of ko~m ( - kob~/CR) being plotted against pH for each hydrazonenickel(II) system. According to the results, the k0~m value decreased with increasing pH in the NPHQTS-nickeI(II) system, but it increased with increasing pH in the other systems. This indicates that there are two controlling reactions; one is independent of, and the other is inverse first order, dependent on the hydrogen-ion concentration. Two examples of these plots are shown in Fig. 4.
3.1.4. Rate-determining steps and rate constants
gNi =
KNi
[NiOH÷][H +] [Ni2+]
(14)
(15)
where kl and k2 represent the rate constants of respective reaction steps. Here we consider the DSBINPH-nickel(II) system as an example. On the basis of the results described above and the assumption that the rate-determining steps are competitive reactions shown by Eqs. (14) and (15), the rate equation can be expressed as d[Ni(II)] d----~ = kl [Ni2+][H2 L2-] + k2[NiOn +][H2L2-]
(16)
From Eqs. (7), (8), (9) and (16), Eq. (17) can be derived.
Since the species of nickel in the pH range 7.0-8.5 studied in this work are Ni(H20) 2÷ and NiOH(H20)~- and those of the hydrazones are H L - for NPHPTS, NPHQTS and BTNPHTS, H2L- for BINPHTS and H2L2for DSBINPH (where L denotes the undissociable part of the hydrazone), as is evident from the values of their acid dissociation constants [6], the equilibria and the reactions for the formation of monoligated complexes are expresseed by Eqs. (9)-(15). For the hydrolysis of nickel(II) "NiOH + + H +
+ 6H20
, [Ni(HL)]-
3.1.3. Effect of hydrogen-ion concentration
Ni 2 + + H 2 0 .
H+
(9)
kl + (k2KNi/[H +]) k°~m=
1 +(KNi/[H+])
(17)
Also for the complexation reactions of the other hydrazones, Eq. (17) can be derived by similar treatment. Values of k~, k2 and KNi were calculated by the Simplex method using the data of the k~m vs. pH plot obtained already. The results are given in Table 1 and theoretical curves for the ko~m vs. pH plots are shown by solid lines in Fig. 4. The KNi values obtained in this work are nearly equal to that ((0.1-about 6.3) x 10 --~° M) reported in the literature [9]. The values of k, and k2 of each complexation reaction are also almost the same in the order, respectively, and the k2 values are
T. Odashima et al./ Talanta 42 (1995) 1229-1237
1233
15
Table I Rate constants of complexation reactions of hydrazones with nickel(ll) and hydrolysis constant of nickel(ll) at 25 + 0 . I °C and an ionic strength of 0.2 M (NaCIO4)
a
10
Hydrazone
NPHPTS NPHQTS BTNPHTS b BINPHTS DSBINPH
)
k~ (M-' s-I)
k2 (M - t s - I )
KNi (M)
3.6 -" 5.5 2.5 2.4
3.6 a 1.8 1.8 5.3
1.1 x I0 -"} _~ 1.0 x 10 -"~ 1.0 x 10 - " j 0.99 x 10 -Icj
x 102 × 10 x 102 × 10-~
x 103 × l0 ~ x 104 x l0 ~
b C
%
d
e
0
~' Not determined. Determined in aqueous 10% (v/v) 1,4-dioxane.
3.2. Kinetics of formation of palladium complexes in the absence of chloride ion 3.2.1. Reaction order with respect to palladium-ion concentration All kinetics experiments were performed in strongly acidic media because the complexation reaction between the hydrazones and palladium (II) was too fast to measure in weakly acidic media. Fig. 5 shows a - I n ( A : , - A,) vs. t plot for the formation of a BTNPHTS-palladium(II) complex measured in the presence of a large excess of DSBINPH and at an acidity of 1.16 M. The plot gives a sraight line. Similar results to those in Fig. 5 were obtained for the 3.6
3.4
Iii
3.2
3.0
2.8
0
lo
5
10
lOSCn
close to the releasing rate constant (1.5 × 104 s -~) [10] of a water molecule from the nickel aquo-ion, although the k value of the BTNPHTS-nickeI(II) system is somewhat small. This suggests that the complexation reactions between the hydrazones and nickel(lI) proceed according to an Eigen mechanism.
<
0
15
Reaction tlme/s
Fig. 5. Plot of --In(A~ - A t ) vs. t for formation of BTNPHTS-palladium(ll) complex. (BTNPHTS, 7.8 x 10-SM; Pd(ll), 2.1 x 1 0 - 6 M ; [H+], 1.16M; ionic strength, 2.0 M (NaNO~): temperature, 25 __.0.1 oC.)
Fig. 6. Plots of kob~ vs. C R for BTNPHTS-palladium(ll) system at pH: curve a, 0.77; curve b, 0.97; curve c, 1.16; curve d, 1.45; curve e, 1.93; wavelength, 590nm. Other conditions are the same as those in Fig. 5.
formation of the other hydrazone-palladium (II) complexes. These results indicate that the complexation reactions between the hydrazones and palladium(II) are all first order with respect to the palladium (II) concentration, i.e. a=l.
3.2.2. Reaction order with respect to hydrazone concentration Fig. 6 shows the effect of the concentration of BTNPHTS on the observed rate constant at various acidities. The plots of kobsd versus CR give straight lines passing through the origin. Similar results to those in Fig. 6 were obtained for the other hydrazone-palladium(II) systems, which indicates that the complexation reactions of these systems are all first order with respect to the hydrazone concentration, i.e. b = 1. 3.2.3. Reaction order with respect to hydrogen-ion concentration Curve a in Fig. 7 shows a k~H) vs. 1/[H÷] plot for the formation of the DSBINPH-palladium (II) complex. The plot gives a straight line passing through the origin. Similar results to those shown by curve a in Fig. 7 were obtained for the formation of the other hydrazone complexes except for the BTNPHTS complex, for which a k~m vs. 1/[H÷] plot shown by curve b in Fig. 7 was obtained. These results indicate that the reaction order with respect to the hydrogen-ion concentration is inverse first order (i.e. c = - 1 ) for the formation of the hydrazone complexes except for the BTNPHTS complex, and is measured as a complicated function of the hydrogen-ion concentration in
T. Odashima et al. I Talanta 42 (1995) 1229-1237
1234
the BTNPHTS system. This discrepancy seems to be influenced by an intramolecular hydrogen bond in BTNPHTS. At the present stage, however, we cannot give a clear chemical interpretation for the intramolecular hydrogen bond. 3.2.4. Rate-determining steps and rate constants
The species of palladium under the conditions studied is Pd(H20)] + and those of the hydrazones are considered to be H3L + and H2L for NPHPTS, NPHQTS and BTNPHTS, H4L+ and H3L for BINPHTS and H4L and H3L- for DSBINPH. Taking into consideration the results described above, however, only Eqs. (18)-(20) are feasible as formation reactions of monoligated complexes between the hydrazones and palladium (II). For NPHPTS and NPHQTS ki
Pd(H20)] + + H3L +
, [Pd(HzL)] 2+ +H ++4H20
(18)
for BINPHTS , [Pd(H3L)]2+
Pd(H20)~ + + HaL +
+ H + + 4H20
(19)
and for DSBINPH Pd(H20) 2+ + H4L
kl
, [Pd(H3 L)] + +H ++4H20
(20)
The rate equation can be expressed by Eq. (21) for every reaction system, except the BTNPHTS-palladium(II) system.
d[Pd(II)] = k,[pd2+] dt
CR/[H +] 1 + (K,,/IH +])
(21)
where K,j is the acid dissociation constant of 10
Table 2 Rate constants o f complexation reactions of hydrazones with palladium(ll) in the absence o f chloride ion at 25 + 0.1 *C and an ionic strength of 2.0 M (NaCIO4) Hydrazone
kI (M-t s -I)
k _ z/ K '., ( M - I s -~)
NPHPTS NPHQTS BTNPHTS BINPHTS DSBINPH
3.0 2.7 1.5 4.3 7.6
-
x x x x x
103 102 10a 102 102
7.7 × 10 - 4 -
each hydrazone. From Eqs. (2), (8) and (21) k0(H)[1 + (Kai/[H +])] = k,/[H +]
(22)
is obtained. Eq. (22) means that the plot of kO~H)[I+(KaU[H+])] versus I/[H +] gives a straight line with a slope of k, passing through the origin. Actually this plot for each hydrazone-palladium (II) system gave a straight line. The value of kt obtained for each reaction system is given in Table 2. As for the BTNPHTS-palladium(II) system, on the other hand, the reverse reaction was taken into consideration in addition to the forward one as shown in Eq. (23) because the reaction order with respect to the hydrogen-ion concentration was not inverse first order but complex, as was already shown by curve b in Fig. 7. Pd 2+ + H3L + .
kl
::" [Pd(H2L)] z+ + H +
.k_l
(23)
where k_, is the rate constant of the reverse reaction. The rate equation is expressed by Eq. (24)
d[Pd(II)] = k1[pd2+][H3L+][H+]_, dt - k_I[Pd(HzL)2+][H + ]
2
(24)
The complcxation reaction of BTNPHTS with palladium(II) is represented by Eq. (25), its equilibrium constant, Keq, being given by Eq.
a
(26) [61 Pd 2+ + H3 L+ .x'% Pd(HL) + + 2H +
(25)
[Pd(HL)+][H+]2 = 1049 [pd2+][H3L+]
(26)
K~q= O k'.
,i
I
0.0
0.5
1.0
1/fH+l
0 1.5
Fig. 7. Plots o f k~sn~ vs. I/[H +] for (curve a) D S B I N P H palladium(ll) and (curve b) B T N P H T S - p a l l a d i u m ( l i ) systems at CR: curve a, 8.2 x I0 -~ M; curve b, 7.8 x 10 -~ M. Other conditions are the same as those in Fig. 5.
The proton dissociation of the complex Pd(H2L) 2+ is represented by Eq. (27), its proton dissociation constant, K',, being given by Eq. (28). Pd(H2L) z+ . ' c " Pd(HL) + + H +
(27)
1235
T. Odashima et al. I Talanta 42 (1995) 1229-1237
Eq. (30) means that the plot of ko(n)[l + (K,~/ [H+])] versus 1/[H +] gives a straight line, the values of k t and k_ t/K'~ being calculated from the slope and the intercept of the line, respectively. Fig. 8 shows this plot for the BTNPHTS-pailadium(II) system, which gives a straight line. The values of k t and k ,/K" calculated are given in Table 2.
2.0
1.5
"; 1.0 +
j
0.5
O
3.3. Kinetics of formation of palladium complexes in the presence of chloride ion
0.0
-0.5 0.0
t 0.3
I 0.6
1 1.2
i 0.9 1/[H+]
Fig. 8. Plot of kc,H)(l +K~J[H+]) vs. I/[H +] for BTNPHTS-palladium(ll) system at C R, 7.8 × 10 -s M. Other conditions are the same as those in Fig. 5.
K~-
[Pd(HL)+][H +] [Pd(H:L)2÷ ]
(28)
From Eqs. (7), (8), (24), (26) and (28), Eq. (29) can be derived, which is rewritten as Eq. (30) d[Pd(II)] = [Pd2+]CR dt 1 + (Ka~/[H+]) × ( k , / [ H +] - K~qk_,K'~) (29)
= kO(H)[pd2+]CR
ko,H,[1 + ( K . , I [ H ÷])] = k , / [ H +1 -
K~k_, IK'= (30)
3.3.1. Reaction orders with respect to concentrations of palladium ion, hydrazone and hydrogen ion Fig. 9(A) shows an In(As - A,) vs. t plot for the complexation of DSBINPH with palladium (II) in the presence of large excesses of DSBINPH and chloride ion and at a constant acidity of 1.06 M. The plot gives a straight line. Figs. 9(B) and 9(C) show kobsd VS. C R plots at various acidities and a ko(H) vs. 1/[H +] plot, respectively, for the DSBINPH-palladium(II) system. Every plot gives a straight line passing through the origin. Similar results to those shown in Figs. 9(A)-9(C) were obtained for the other hydrazone-palladium(II) systems. These results reveal that the reaction orders with respect to the concentrations of both palladium(II) and the hydrazone are first order, (B)
15
ab
cd
~3.5
e
f
J lO -
0
2
%
,
4
Reaction timels
00 (C)
!
I
5
10
t05~
15
3,
J¢
?
j2
o
O
0 0.0
I
I
0.5
1.0
111H+1
1.5
1
0
2
4 104Cc~
6
Fig. 9. Plots o f ( A ) --In(A.L - A , ) vs. t, (B) k o ~ vs. C a, (C) kt~n) vs. I/[H +] and (D) k4~H) vs. Cc~ for D S B I N P H - p a l ladium(ll)-chloride system. DSBINPH, 8.2 x 10 -~ M; (A), (C), (D) Pd(ll), 2.1 x 10 -6 M; (B) [H+]: (curve a) 0.81, (curve b) 1.01, (curve c) 1.22, (curve d) 1.42, (curve e) 1.72, (curve f) 2.03; (A), (D) [H+], 1.06; (A), (B), (C) [CI-], 2.12 x l0 -4 M; ionic strength, 2.0 M (NaNO0; wavelength, 564 nm; temperature, 25 +0.1 °C.
T. Odashima et al./ Talanta 42 (1995) 1229-1237
1236
that with respect to the hydrogen-ion concentration being inverse first order.
3.3.2. Reaction order with respect to chloride-ion concentration Fig. 9(D) shows a ko~m vs. Co plot for the DSBINPH-palladium(II) system, which gives a straight line with an intercept. Similar results to those shown in Fig. 9(D) were obtained for N P H P T S - , NPHQTS- and BINPHTS-palladium (II) systems, but no such result was obtained for the BTNPHTS-palladium(II) system. These results indicate that the reaction order with respect to the chloride-ion concentration for the complex formation between palladium(II) and the hydrazones, except BTNPHTS, under the conditions studied is zero order and first order. 3.3.3. Rate-determining steps and rate constants The species of palladium(II) under the conditions studied are regarded as Pd(H20)~ + , PdCI(H2Oh+ and PdCI2(H20)2. On the basis of the above results, however, potential complexation reactions between the hydrazones and palladium(II) are restricted to Eqs. (31)-(36) and each pair of reactions is considered to proceed competitively and govern the complexation reaction in each hydrazone-palladium(II) system. For NPHPTS and NPHQTS kl
Pd(H,O)42+ + H3 L+
, Pd(H2L) 2+ +H ++4H20 k2
PdCI(H20)[ + H3L +
(31)
, Pd(H 2L)C1 + +H + +3H20
(32)
Table 3 Rate constants of complexation reactions of hydrazones with palladium(ll) in the presence of chloride ion at 25 + 0.1 *C and ionic strength of 2.0 M (NaCIO4) Hydrazone
k~ (M-I s -I)
k, (M-t s -I)
NPHPTS
8.0 x 10 ~
1.9 x 10 4
NPHQTS
5.6 x 10 2
7.4 x 10 ~
BTNPHTS BINPHTS
2 3.9 x 10 2
,, 8.1 x 10 3
DSBINPH
1.5 x 10 3
1.7 x 10 4
Not determined. d[Pd(II)] , [Pd(II)][L] k2[Pd(II)][L][Cl-],_ dt =r, [-H--g~ + [H+] (37) By taking into consideration the acid dissociation of the hydrazones and the mass balance of the hydrazones and palladium(II) and using the side-reaction coefficient, ~cl), of palladium(II) for chloride ion and the formation constant, fl~, of the monochloro complex of palladium(II), Eq. (38) can finally be derived from Eq. (37)
'
[k,+k2Ccl~'l~,c,, A (38)
k°m' = (1 + [Hk~+~ ])[H + ] where Ca is the total concentration of chloride ion. Eq. (38) means that the kom) vs. Ccl plot, which is equivalent to Fig. 9(D) gives a straight line; the values of k, and k2 can be calculated from the intercept and the slope of this line, respectively. The results obtained are summarized in Table 3.
for BINPHTS kl
Pd(H20)] + + H4L +
, Pd(HsL) 2 + +H + +4H20 k2
PdCI(H2Oh+ + H4L +
(33)
, Pd(H3L)CI + +H ++3H20
(34)
and for DSBINPH Pd(H20)~ + + H4L
kl
, Pd(HsL) + +H + +4H20
(35)
k-,
PdCI(H20)I + H4L
-, Pd(HaL)CI +H ÷ +3H20
(36)
where k~ and k 2 represent the rate constants. The rate equation for these complexation reactions can be expressed as
3.3.4. Mechanism of formation of palladium complexes Palladium(II) usually forms the square planar complex [11]. As is clear from Table 3, the rate constants for the complexation of palladium(lI) obtained in this work depend on the kind of hydrazone, which suggests that the complexation reaction of the hydrazones with palladium(II) goes according to the associative mechanism. The value of k2 is larger than that of kl for every hydrazone-palladium(II) system, indicating that the palladium aquo-ion is activated by the formation of monochloro ion. Also, the k, values obtained here are in accordance with those obtained in the absence of chloride ions, demonstrating the reliability of this work.
T. Odashhna et al./ Takmta 42 (1995) 1229-1237
References [I] T. Odashima, T. Kikuch, W. Ohtani and H. Ishii, Analyst, 111 (1986) 1383. [2] K. Kohata, Y. Kawamonzen, T. Odashima and H. Ishii, Bull. Chem. Soc. Jpn., 63 (1990) 3398. [3] T. Odashima, M. Yamaguchi and H. Ishii, Mikrochim. Acta. I (1991) 267. [4] H. Ishii, T. Odashima and Y. Kawamonzen, Anal. Chim. Acta, 244 (1991) 223. [5] H. Ishii, T. Odashima and Y. Kawamonzen, Bull. Chem. Soc. Jpn., 63 (1990) 3405.
1237
[6] H. Ishii, M. Yamaguchi and T. Odashima, Talanta, 39 (1992) 1181. [7] F.J. Kezdy, J. Kaz and A. Bruylants. Bull. Soc. Chim. Beiges, 67 (1958) 687. [8] E.S. Swinbourne, J. Chem. Soc., (1960) 2371. [9] L.G. Sillen and A.E. Martell, Stability Constants of Metal-Ion Complexes, Special Publication No. t7, The Chemical Society, London, 1964, p. 56. [10] M. Eigen and K. Tamm, Z. Elektrochem., 66 (1962) 107. [11] F. Basolo and R.G. Pearson, Mechanisms of Inorganic Reactions. A Study of Metal Complexes in Solution, 2nd edn., John Wiley, Ne~ York, 1967 p. 414.
Talanta
. ;'//
ELSEVIER
Talanta 42 (1995) 1229-1237
Kinetic studies of complexation reactions of water-soluble hydrazones with nickel(II) and palladium(II) ions Tsugikatsu Odashima *, Mitsuru Yamaguchi ', Hajime Ishii Institute [or Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai, M(vagi 980, Japan Received 1 August 1994; revised 21 February 1995; accepted 23 February 1995
Abstract
The kinetics of complexation reactions of five water-soluble heterocyclic hydrazones with nickel(II) and palladium(II) ions have been investigated by stopped-flow spectrophotometry. Rates of complexations with nickel(II) and palladium(II) in the absence of chloride ion were found to be proportional to the first order of the ligand and metal ion concentrations and to the inverse first order of the hydrogen ion concentration except for the complexation of ct-(2-benzimidazolyl)-~t-(5-nitro-2-pyridyl)hydrazono-3-toluenesulfonic acid with palladium(II). Rates of complexation with palladium(lI) in the presence of chloride ion were best described by a two-term expression, both terms being first order in the palladium ion and ligand concentrations and inverse first order in the hydrogen ion concentration. The first term has zero dependence of the chloride ion concentration, whereas the second is first order with respect to the chloride ion concentration. The rate constant for each complexation reaction was determined. The complexation of the hydrazones with nickel(II) was estimated to go according to an Eigen mechanism and that with palladium (II) according to the associative mechanism.
1. Introduction
cently found to be very useful as highly sensitive spectrophotometric reagents for metals. Among the water-soluble hydrazones which Some of them were applied to the determinaconsist of 5-nitro-2-pyridylhydrazine and hete- tion of trace amounts of cobalt [1,4] and nickel rocyclic ketones having one or two sulfo [3,5]. In addition, complexation equilibria begroups in their molecules, :t-(5-nitro-2- tween the above-mentioned five hydrazones pyridyl)hydrazono-:t-(2-pyridyl)-3-toluenesuland metal ions including cadmium(ll), fonic acid (NPHPTS) [!], ~-(5-nitro-2- cobalt(II), copper(lI), iron(lI), palladium(II) pyridyl)hydrazono-~- (2-quinolyl)-3-toluenesul- and zinc(II) were investigated previously [6]. In fonic acid (NPHQTS) [2], :t-(2-benzothiazolyl) this work kinetics of complexation reactions of - :t - (5 - nitro - 2 - pyridyi)hydrazono-3-toluenesul- these five hydrazones with nickel(lI) and pallafonic acid (BTNPHTS) [2], ~-(2-benzimida- dium(II) have been studied in detail by zolyl)-ct-(5-nitro-2-pyridyl)hydrazono-3-toluene- stopped-flow spectrophotometry in order to sulfonic acid (BINPHTS) [2] and disuifonated more fully understand these complexation reac(2-benzimidazolyl)(phenyl)methanone 5-nitro- tions. 2-pyridylhydrazone (DSBINPH) [3] were re* Corresponding author. ' Present address: Analytical Research Center, Central Research Laboratory, Research & Development Division, Japan Energy Corporation, 3-1%35. Niizo-minami, Todashi, Saitama 335. Japan.
2. Experimental
2.1. Reagents All reagents were of analytical-reagent grade
0039-9140 95509.50 '-: 1995 Elsevier Science B.V. All rights reserved SSDI (}039-9140(95J01565-5
1230
T. Odashima et al. / Talanta 42 (1995) 1229-1237
and all solutions were prepared with distilled, demineralized water, unless stated otherwise. Hydrazone solutions, 5.0 x 10 -4 or 1.0 x 10 -3 M, were prepared by dissolving each hydrazone, synthesized as in earlier work [1-3], in water or 0.01 M sodium hydroxide solution. These solutions were further diluted with water if necessary. Standard solutions of nickel(II) and palladium (II), about 0.5 mgcm -3, were prepared by first dissolving their nitrates in 0.01 M perchloric acid and then standardizing by EDTA titration. Working solutions were prepared by dilution of these solutions with water. Buffer solutions were prepared by mixing 0.2 M tris(hydroxymethyl)aminomethane (Tris) solution and 0.2 M nitric acid.
screen and by the recorder. The absorbance values were collected regularly and processed by linear regression using programs provided with the microcomputer for the application of first-order reaction plots (Eq. (4)) and KezdySwinbourne's plots (Eq. (6)) [7,8]. Kinetics measurements for the complexation of NPHQTS and BTNPHTS with nickel(II) were performed in 10% (v/v) 1,4-dioxane/H20 because of their low solubilities in water.
3. Results and discussion In general, since the rate of complex formation depends on the concentrations of metal ion, ligand, and hydrogen ion, the rate of formation of the metal-hydrazone complex is assumed to be
2.2. Apparatus
A JASCO KS-100M stopped-flow system was used, which consists of a stopped-flow module with a flow cell of optical path 10mm, a diode-array spectrophotometer, an oscilloscope, an X - Y plotter, a microcomputer and a thermostatic bath. This system has a minimum dead-time of 2 ms and affords, on both the oscilloscope screen and the plotter, a three-dimensional (time-resolved) adsorption spectrum (corrected by subtracting the blank signal of the reaction product over the region of + 170 nm from a fixed wavelength, and its absorbance changes with time (kinetic curves) at repeated arbitrary wavelengths. 2.3. Procedure
Kinetic runs were performed under pseudofirst-order conditions with respect to hydrazone. Each run was repeated five times, the average signal being adopted. Equal volumes of nickel(II) (or palladium(II)) and hydrazone solutions, both of which were buffered at a fixed pH between 7.0 and 8.5 (or adjusted to a fixed acidic pH value between 1.0 and 2.0 for the palladium(II) complex) with a constant ionic strength of 0.2 M (or 2.0 M for the palladium(lI) complex), were mixed in the mixing chamber of the stopped-flow apparatus. The temperature was controlled at 25 + 0.1 °C. A kinetic curve at the absorption-maximum wavelength of the complex was recorded on the oscilloscope
d[M] = k*[M]"[R]b[H +]c dt
(1)
where t is the reaction time, k* is the rate constant, M is the metal ion, R is the hydrazone and a, b and c are the reaction orders with respect to the concentrations of metal ion, hydrazone and hydrogen ion, respectively. Under a pseudo-first-order excess of the hydrazone and at a constant pH, the reaction rate can be expressed as d[M] dt = k°b~[M]~
(2)
where kob~d is the observed pseudo-first-order rate constant, assuming a = 1. By integrating Eq. (2), taking the mass balance into consideration and then applying Beer's law, Eq. (3) can be derived, which is rewritten as Eq. (4). A~ - A, = (A~ - A0) exp( - Kobsdt)
(3)
--ln(A~ -- A,) = -ln(A:~ - A o ) + k o b s d t
(4)
where Ao, A, and A~ are the absorbance of the reaction system at t = 0, t and ~ , respectively. 3.1. Kinetics o f formation o f nickel complexes 3.1.1. Reaction order with respect to nickel-ion concentrations Fig. 1 shows a kinetic curve measured in the presence of a large excess of DSBINPH and at pH 7.43 for the complex formation of DSBINPH with nickel(ll), along with a - - l n ( A . , - A , ) vs. t plot prepared from this
1231
T. Odashima et al./ Talanta 42 (1995) 1229-1237
kinetic curve. The plot gives a straight line with a slope of 0.0436, which indicates that the rate of complexation is first order with respect to the nickel(II) concentration, i.e. a = 1 in Eq. (1), and the kobsd is 4.36 x 10 - 2 S-l under the experimental conditions given in Fig. 1. In the complex formation of the other hydrazones with nickel(II), however, more than 20 h were required for attaining equilibrium. Accordingly the absorbances at infinite time (A~) were not obtained within the experimental period (about 1 h). Therefore the kinetics analysis for these complexation reactions was performed according to Kezdy-Swinbourne's method [7,8] as follows. As is clear from Eq. (3), after the elapse of a fixed time, At, from t, Eq. (5) holds A ~_ - A, + a, = (A ~ - Ao) exp[ - kob~(t + At)]
(5)
0.20 0.15 j : //.: '~" O.10 / 0.05
:: /" / /
0.00 0.O0
/ f .S
f: <:, 0.O5 O.10
0.'15
O.20
At÷At
Fig. 2. Kezdy-Swinbourne plot for the BINPHTS-nickel(ll) system. (BINPHTS, 7.5 × 10 -5 M; pH 7.52; Wavelength, 500 rim. Other conditions are the same as those in Fig. 1.)
were obtained for N P H P T S - , N P H Q T S - and BTNPHTS-nickeI(II) systems, which indicates that the rate of complexation in these systems is first order with respect to the nickel(II) concentration, i.e. a = I.
From Eqs. (3) and (5), Eq. (6) is obtained A, = A ~_[1 - exp(kob~dAt)] + A, + a, exp(kob~At)
(6) Eq. (6) implies that a plot of A, versus A,+a, gives a straight line with a slope of exp (kob~dAt). As the value of At is known, the kob,~ value can be calculated from this slope. Furthermore, A, is equal to A,+a, and also equal to A ~ at t = ~ , so an intersecting point of this line with a straight line A, = A,+a, (i.e. a line with a slope of unity passing through the origin) should give the value of A~. Fig. 2 shows a plot of A, versus A,+~, for the BINPHTS-nickeI(II) system as an example, in which the plot gives a straight line with a slope of i.92. Similar results to that shown in Fig. 2 0.3 E e-
3.1.2. Reaction order with respect to hydrazone concentration From the above results the rate equation for the formation of the nickel complexes can be written as follows d[Ni(II)] = kobsd[Ni(II)] = k0(H)[Ni(II)][R] b (7) dt kob ~ = k0iH)[R] b - ko(H)C ~
(8)
where k0(H) is the conditional second-order rate constant and CR is the total concentration of the hydrazone. The effect of the hydrazone concentration on the observed rate constant was examined in order to determine the reaction order with respect to the hydrazone concentration. Fig. 3 shows the result for the DSBINPH-nickel(II) system, in which plots of kob~l v e r s u s Ca give straight lines passing
15
} 0.2
@ G}
J lO
a
~o 0.1 5
000
5
10
15
20
25
--OO 3 6 0
Reaction time/s Fig. I. Reaction curve and plot of - I n ( A z - A , ) vs. t for the formation of DSBINPH-nickeI(]I) comp|ex. (DSBINPH, 8.3 × i 0 - S M ; Ni(ll), 2.5 × 1 0 - 6 M ; pH 7.43;
ionic strength, 0.2 M (NaCIO4); temperature, 25 + 0.l *C.)
0.0
i
i
i
0.5
1.0
1.5
,
2,0
lO'C. Fig. 3. Plots of kob,a vs. C R for DSBINPH-nickeI(II) system at pH: curve a, 7.43; curve b, 8.01; curve c, 8.28; curve d, 8.37; curve e, 8.52. Other conditions are the same as those in Fig. 1.
1232
T. Odashima et al./ Talanta 42 (1995) 1229-1237
where KN~ is the hydrolysis constant of nickel(II). For the formation of the NPHPTS, NPHQTS and BTNPHTS complexes, Eqs. (10) and (11) hold
12i
10 a
~
l
°
Ni(H20) 2+ + H L -
6
kl
k2
NiOH(H20)~- + H L -
4.0
¢.s
8'.0
8'.s
9.0
, NiL + H + + 6H20(10) , NiL + 6H20
(l l)
for the formation of the BINPHTS complex, Eqs. (12) and (13) hold
OH
kl
Fig. 4. Plotsof k~m vs. pH for (curve a) BINPHTS-nick¢1(11) and (curve b) DSBINPH-nickel01) systems. CR: curve a, 9.4 x 10-5 M; curve b, 1.0 x 10.4 M. Other conditions are the same as those in Fig. I.
Ni(H20)~ + + H2L-
through the origin. Similar results to that in Fig. 3 were obtained for the other hydrazonenickel(II) systems, which indicates that the rate of complexation in these systems in first order, dependent on the hydrazone concentration, i.e. b = 1.
and for the formation of the DSBINPH complex, Eqs. (14) and (15) hold
, Ni(HL)+ H + +6H20 (12) k2
NiOH(H20)J + H2L-
, Ni(HL) + 6H20 (13)
kl [Ni(HL)]- + Ni(H20)62+ + H2 L2- ---* NiOH(H20)~- + H2 L2-
k2
+ 6H20
The effect of pH on the observed rate constant was studied, values of ko~m ( - kob~/CR) being plotted against pH for each hydrazonenickel(II) system. According to the results, the k0~m value decreased with increasing pH in the NPHQTS-nickeI(II) system, but it increased with increasing pH in the other systems. This indicates that there are two controlling reactions; one is independent of, and the other is inverse first order, dependent on the hydrogen-ion concentration. Two examples of these plots are shown in Fig. 4.
3.1.4. Rate-determining steps and rate constants
gNi =
KNi
[NiOH÷][H +] [Ni2+]
(14)
(15)
where kl and k2 represent the rate constants of respective reaction steps. Here we consider the DSBINPH-nickel(II) system as an example. On the basis of the results described above and the assumption that the rate-determining steps are competitive reactions shown by Eqs. (14) and (15), the rate equation can be expressed as d[Ni(II)] d----~ = kl [Ni2+][H2 L2-] + k2[NiOn +][H2L2-]
(16)
From Eqs. (7), (8), (9) and (16), Eq. (17) can be derived.
Since the species of nickel in the pH range 7.0-8.5 studied in this work are Ni(H20) 2÷ and NiOH(H20)~- and those of the hydrazones are H L - for NPHPTS, NPHQTS and BTNPHTS, H2L- for BINPHTS and H2L2for DSBINPH (where L denotes the undissociable part of the hydrazone), as is evident from the values of their acid dissociation constants [6], the equilibria and the reactions for the formation of monoligated complexes are expresseed by Eqs. (9)-(15). For the hydrolysis of nickel(II) "NiOH + + H +
+ 6H20
, [Ni(HL)]-
3.1.3. Effect of hydrogen-ion concentration
Ni 2 + + H 2 0 .
H+
(9)
kl + (k2KNi/[H +]) k°~m=
1 +(KNi/[H+])
(17)
Also for the complexation reactions of the other hydrazones, Eq. (17) can be derived by similar treatment. Values of k~, k2 and KNi were calculated by the Simplex method using the data of the k~m vs. pH plot obtained already. The results are given in Table 1 and theoretical curves for the ko~m vs. pH plots are shown by solid lines in Fig. 4. The KNi values obtained in this work are nearly equal to that ((0.1-about 6.3) x 10 --~° M) reported in the literature [9]. The values of k, and k2 of each complexation reaction are also almost the same in the order, respectively, and the k2 values are
T. Odashima et al./ Talanta 42 (1995) 1229-1237
1233
15
Table I Rate constants of complexation reactions of hydrazones with nickel(ll) and hydrolysis constant of nickel(ll) at 25 + 0 . I °C and an ionic strength of 0.2 M (NaCIO4)
a
10
Hydrazone
NPHPTS NPHQTS BTNPHTS b BINPHTS DSBINPH
)
k~ (M-' s-I)
k2 (M - t s - I )
KNi (M)
3.6 -" 5.5 2.5 2.4
3.6 a 1.8 1.8 5.3
1.1 x I0 -"} _~ 1.0 x 10 -"~ 1.0 x 10 - " j 0.99 x 10 -Icj
x 102 × 10 x 102 × 10-~
x 103 × l0 ~ x 104 x l0 ~
b C
%
d
e
0
~' Not determined. Determined in aqueous 10% (v/v) 1,4-dioxane.
3.2. Kinetics of formation of palladium complexes in the absence of chloride ion 3.2.1. Reaction order with respect to palladium-ion concentration All kinetics experiments were performed in strongly acidic media because the complexation reaction between the hydrazones and palladium (II) was too fast to measure in weakly acidic media. Fig. 5 shows a - I n ( A : , - A,) vs. t plot for the formation of a BTNPHTS-palladium(II) complex measured in the presence of a large excess of DSBINPH and at an acidity of 1.16 M. The plot gives a sraight line. Similar results to those in Fig. 5 were obtained for the 3.6
3.4
Iii
3.2
3.0
2.8
0
lo
5
10
lOSCn
close to the releasing rate constant (1.5 × 104 s -~) [10] of a water molecule from the nickel aquo-ion, although the k value of the BTNPHTS-nickeI(II) system is somewhat small. This suggests that the complexation reactions between the hydrazones and nickel(lI) proceed according to an Eigen mechanism.
<
0
15
Reaction tlme/s
Fig. 5. Plot of --In(A~ - A t ) vs. t for formation of BTNPHTS-palladium(ll) complex. (BTNPHTS, 7.8 x 10-SM; Pd(ll), 2.1 x 1 0 - 6 M ; [H+], 1.16M; ionic strength, 2.0 M (NaNO~): temperature, 25 __.0.1 oC.)
Fig. 6. Plots of kob~ vs. C R for BTNPHTS-palladium(ll) system at pH: curve a, 0.77; curve b, 0.97; curve c, 1.16; curve d, 1.45; curve e, 1.93; wavelength, 590nm. Other conditions are the same as those in Fig. 5.
formation of the other hydrazone-palladium (II) complexes. These results indicate that the complexation reactions between the hydrazones and palladium(II) are all first order with respect to the palladium (II) concentration, i.e. a=l.
3.2.2. Reaction order with respect to hydrazone concentration Fig. 6 shows the effect of the concentration of BTNPHTS on the observed rate constant at various acidities. The plots of kobsd versus CR give straight lines passing through the origin. Similar results to those in Fig. 6 were obtained for the other hydrazone-palladium(II) systems, which indicates that the complexation reactions of these systems are all first order with respect to the hydrazone concentration, i.e. b = 1. 3.2.3. Reaction order with respect to hydrogen-ion concentration Curve a in Fig. 7 shows a k~H) vs. 1/[H÷] plot for the formation of the DSBINPH-palladium (II) complex. The plot gives a straight line passing through the origin. Similar results to those shown by curve a in Fig. 7 were obtained for the formation of the other hydrazone complexes except for the BTNPHTS complex, for which a k~m vs. 1/[H÷] plot shown by curve b in Fig. 7 was obtained. These results indicate that the reaction order with respect to the hydrogen-ion concentration is inverse first order (i.e. c = - 1 ) for the formation of the hydrazone complexes except for the BTNPHTS complex, and is measured as a complicated function of the hydrogen-ion concentration in
T. Odashima et al. I Talanta 42 (1995) 1229-1237
1234
the BTNPHTS system. This discrepancy seems to be influenced by an intramolecular hydrogen bond in BTNPHTS. At the present stage, however, we cannot give a clear chemical interpretation for the intramolecular hydrogen bond. 3.2.4. Rate-determining steps and rate constants
The species of palladium under the conditions studied is Pd(H20)] + and those of the hydrazones are considered to be H3L + and H2L for NPHPTS, NPHQTS and BTNPHTS, H4L+ and H3L for BINPHTS and H4L and H3L- for DSBINPH. Taking into consideration the results described above, however, only Eqs. (18)-(20) are feasible as formation reactions of monoligated complexes between the hydrazones and palladium (II). For NPHPTS and NPHQTS ki
Pd(H20)] + + H3L +
, [Pd(HzL)] 2+ +H ++4H20
(18)
for BINPHTS , [Pd(H3L)]2+
Pd(H20)~ + + HaL +
+ H + + 4H20
(19)
and for DSBINPH Pd(H20) 2+ + H4L
kl
, [Pd(H3 L)] + +H ++4H20
(20)
The rate equation can be expressed by Eq. (21) for every reaction system, except the BTNPHTS-palladium(II) system.
d[Pd(II)] = k,[pd2+] dt
CR/[H +] 1 + (K,,/IH +])
(21)
where K,j is the acid dissociation constant of 10
Table 2 Rate constants o f complexation reactions of hydrazones with palladium(ll) in the absence o f chloride ion at 25 + 0.1 *C and an ionic strength of 2.0 M (NaCIO4) Hydrazone
kI (M-t s -I)
k _ z/ K '., ( M - I s -~)
NPHPTS NPHQTS BTNPHTS BINPHTS DSBINPH
3.0 2.7 1.5 4.3 7.6
-
x x x x x
103 102 10a 102 102
7.7 × 10 - 4 -
each hydrazone. From Eqs. (2), (8) and (21) k0(H)[1 + (Kai/[H +])] = k,/[H +]
(22)
is obtained. Eq. (22) means that the plot of kO~H)[I+(KaU[H+])] versus I/[H +] gives a straight line with a slope of k, passing through the origin. Actually this plot for each hydrazone-palladium (II) system gave a straight line. The value of kt obtained for each reaction system is given in Table 2. As for the BTNPHTS-palladium(II) system, on the other hand, the reverse reaction was taken into consideration in addition to the forward one as shown in Eq. (23) because the reaction order with respect to the hydrogen-ion concentration was not inverse first order but complex, as was already shown by curve b in Fig. 7. Pd 2+ + H3L + .
kl
::" [Pd(H2L)] z+ + H +
.k_l
(23)
where k_, is the rate constant of the reverse reaction. The rate equation is expressed by Eq. (24)
d[Pd(II)] = k1[pd2+][H3L+][H+]_, dt - k_I[Pd(HzL)2+][H + ]
2
(24)
The complcxation reaction of BTNPHTS with palladium(II) is represented by Eq. (25), its equilibrium constant, Keq, being given by Eq.
a
(26) [61 Pd 2+ + H3 L+ .x'% Pd(HL) + + 2H +
(25)
[Pd(HL)+][H+]2 = 1049 [pd2+][H3L+]
(26)
K~q= O k'.
,i
I
0.0
0.5
1.0
1/fH+l
0 1.5
Fig. 7. Plots o f k~sn~ vs. I/[H +] for (curve a) D S B I N P H palladium(ll) and (curve b) B T N P H T S - p a l l a d i u m ( l i ) systems at CR: curve a, 8.2 x I0 -~ M; curve b, 7.8 x 10 -~ M. Other conditions are the same as those in Fig. 5.
The proton dissociation of the complex Pd(H2L) 2+ is represented by Eq. (27), its proton dissociation constant, K',, being given by Eq. (28). Pd(H2L) z+ . ' c " Pd(HL) + + H +
(27)
1235
T. Odashima et al. I Talanta 42 (1995) 1229-1237
Eq. (30) means that the plot of ko(n)[l + (K,~/ [H+])] versus 1/[H +] gives a straight line, the values of k t and k_ t/K'~ being calculated from the slope and the intercept of the line, respectively. Fig. 8 shows this plot for the BTNPHTS-pailadium(II) system, which gives a straight line. The values of k t and k ,/K" calculated are given in Table 2.
2.0
1.5
"; 1.0 +
j
0.5
O
3.3. Kinetics of formation of palladium complexes in the presence of chloride ion
0.0
-0.5 0.0
t 0.3
I 0.6
1 1.2
i 0.9 1/[H+]
Fig. 8. Plot of kc,H)(l +K~J[H+]) vs. I/[H +] for BTNPHTS-palladium(ll) system at C R, 7.8 × 10 -s M. Other conditions are the same as those in Fig. 5.
K~-
[Pd(HL)+][H +] [Pd(H:L)2÷ ]
(28)
From Eqs. (7), (8), (24), (26) and (28), Eq. (29) can be derived, which is rewritten as Eq. (30) d[Pd(II)] = [Pd2+]CR dt 1 + (Ka~/[H+]) × ( k , / [ H +] - K~qk_,K'~) (29)
= kO(H)[pd2+]CR
ko,H,[1 + ( K . , I [ H ÷])] = k , / [ H +1 -
K~k_, IK'= (30)
3.3.1. Reaction orders with respect to concentrations of palladium ion, hydrazone and hydrogen ion Fig. 9(A) shows an In(As - A,) vs. t plot for the complexation of DSBINPH with palladium (II) in the presence of large excesses of DSBINPH and chloride ion and at a constant acidity of 1.06 M. The plot gives a straight line. Figs. 9(B) and 9(C) show kobsd VS. C R plots at various acidities and a ko(H) vs. 1/[H +] plot, respectively, for the DSBINPH-palladium(II) system. Every plot gives a straight line passing through the origin. Similar results to those shown in Figs. 9(A)-9(C) were obtained for the other hydrazone-palladium(II) systems. These results reveal that the reaction orders with respect to the concentrations of both palladium(II) and the hydrazone are first order, (B)
15
ab
cd
~3.5
e
f
J lO -
0
2
%
,
4
Reaction timels
00 (C)
!
I
5
10
t05~
15
3,
J¢
?
j2
o
O
0 0.0
I
I
0.5
1.0
111H+1
1.5
1
0
2
4 104Cc~
6
Fig. 9. Plots o f ( A ) --In(A.L - A , ) vs. t, (B) k o ~ vs. C a, (C) kt~n) vs. I/[H +] and (D) k4~H) vs. Cc~ for D S B I N P H - p a l ladium(ll)-chloride system. DSBINPH, 8.2 x 10 -~ M; (A), (C), (D) Pd(ll), 2.1 x 10 -6 M; (B) [H+]: (curve a) 0.81, (curve b) 1.01, (curve c) 1.22, (curve d) 1.42, (curve e) 1.72, (curve f) 2.03; (A), (D) [H+], 1.06; (A), (B), (C) [CI-], 2.12 x l0 -4 M; ionic strength, 2.0 M (NaNO0; wavelength, 564 nm; temperature, 25 +0.1 °C.
T. Odashima et al./ Talanta 42 (1995) 1229-1237
1236
that with respect to the hydrogen-ion concentration being inverse first order.
3.3.2. Reaction order with respect to chloride-ion concentration Fig. 9(D) shows a ko~m vs. Co plot for the DSBINPH-palladium(II) system, which gives a straight line with an intercept. Similar results to those shown in Fig. 9(D) were obtained for N P H P T S - , NPHQTS- and BINPHTS-palladium (II) systems, but no such result was obtained for the BTNPHTS-palladium(II) system. These results indicate that the reaction order with respect to the chloride-ion concentration for the complex formation between palladium(II) and the hydrazones, except BTNPHTS, under the conditions studied is zero order and first order. 3.3.3. Rate-determining steps and rate constants The species of palladium(II) under the conditions studied are regarded as Pd(H20)~ + , PdCI(H2Oh+ and PdCI2(H20)2. On the basis of the above results, however, potential complexation reactions between the hydrazones and palladium(II) are restricted to Eqs. (31)-(36) and each pair of reactions is considered to proceed competitively and govern the complexation reaction in each hydrazone-palladium(II) system. For NPHPTS and NPHQTS kl
Pd(H,O)42+ + H3 L+
, Pd(H2L) 2+ +H ++4H20 k2
PdCI(H20)[ + H3L +
(31)
, Pd(H 2L)C1 + +H + +3H20
(32)
Table 3 Rate constants of complexation reactions of hydrazones with palladium(ll) in the presence of chloride ion at 25 + 0.1 *C and ionic strength of 2.0 M (NaCIO4) Hydrazone
k~ (M-I s -I)
k, (M-t s -I)
NPHPTS
8.0 x 10 ~
1.9 x 10 4
NPHQTS
5.6 x 10 2
7.4 x 10 ~
BTNPHTS BINPHTS
2 3.9 x 10 2
,, 8.1 x 10 3
DSBINPH
1.5 x 10 3
1.7 x 10 4
Not determined. d[Pd(II)] , [Pd(II)][L] k2[Pd(II)][L][Cl-],_ dt =r, [-H--g~ + [H+] (37) By taking into consideration the acid dissociation of the hydrazones and the mass balance of the hydrazones and palladium(II) and using the side-reaction coefficient, ~cl), of palladium(II) for chloride ion and the formation constant, fl~, of the monochloro complex of palladium(II), Eq. (38) can finally be derived from Eq. (37)
'
[k,+k2Ccl~'l~,c,, A (38)
k°m' = (1 + [Hk~+~ ])[H + ] where Ca is the total concentration of chloride ion. Eq. (38) means that the kom) vs. Ccl plot, which is equivalent to Fig. 9(D) gives a straight line; the values of k, and k2 can be calculated from the intercept and the slope of this line, respectively. The results obtained are summarized in Table 3.
for BINPHTS kl
Pd(H20)] + + H4L +
, Pd(HsL) 2 + +H + +4H20 k2
PdCI(H2Oh+ + H4L +
(33)
, Pd(H3L)CI + +H ++3H20
(34)
and for DSBINPH Pd(H20)~ + + H4L
kl
, Pd(HsL) + +H + +4H20
(35)
k-,
PdCI(H20)I + H4L
-, Pd(HaL)CI +H ÷ +3H20
(36)
where k~ and k 2 represent the rate constants. The rate equation for these complexation reactions can be expressed as
3.3.4. Mechanism of formation of palladium complexes Palladium(II) usually forms the square planar complex [11]. As is clear from Table 3, the rate constants for the complexation of palladium(lI) obtained in this work depend on the kind of hydrazone, which suggests that the complexation reaction of the hydrazones with palladium(II) goes according to the associative mechanism. The value of k2 is larger than that of kl for every hydrazone-palladium(II) system, indicating that the palladium aquo-ion is activated by the formation of monochloro ion. Also, the k, values obtained here are in accordance with those obtained in the absence of chloride ions, demonstrating the reliability of this work.
T. Odashhna et al./ Takmta 42 (1995) 1229-1237
References [I] T. Odashima, T. Kikuch, W. Ohtani and H. Ishii, Analyst, 111 (1986) 1383. [2] K. Kohata, Y. Kawamonzen, T. Odashima and H. Ishii, Bull. Chem. Soc. Jpn., 63 (1990) 3398. [3] T. Odashima, M. Yamaguchi and H. Ishii, Mikrochim. Acta. I (1991) 267. [4] H. Ishii, T. Odashima and Y. Kawamonzen, Anal. Chim. Acta, 244 (1991) 223. [5] H. Ishii, T. Odashima and Y. Kawamonzen, Bull. Chem. Soc. Jpn., 63 (1990) 3405.
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[6] H. Ishii, M. Yamaguchi and T. Odashima, Talanta, 39 (1992) 1181. [7] F.J. Kezdy, J. Kaz and A. Bruylants. Bull. Soc. Chim. Beiges, 67 (1958) 687. [8] E.S. Swinbourne, J. Chem. Soc., (1960) 2371. [9] L.G. Sillen and A.E. Martell, Stability Constants of Metal-Ion Complexes, Special Publication No. t7, The Chemical Society, London, 1964, p. 56. [10] M. Eigen and K. Tamm, Z. Elektrochem., 66 (1962) 107. [11] F. Basolo and R.G. Pearson, Mechanisms of Inorganic Reactions. A Study of Metal Complexes in Solution, 2nd edn., John Wiley, Ne~ York, 1967 p. 414.