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ISSN 1061-9348, Journal of Analytical Chemistry, 2006, Vol. 61, No. 11, pp. 1067-1073. ¿ Pleiades Publishing, Inc., 2006. Original Russian Text ¿ M.K. Beklemishev, E.N. Kiryushchenkov, E.K. Skosyrskaya, A.M. Petrenko, 2006, published in Zhurnal Analiticheskoi Khimii, 2006, Vol. 61, No. 11, pp. 1157-1164.

ARTICLES

Periodate Ion As an Oxidant in Indicator Reactions with Aromatic Amines
M. K. Beklemishev, E. N. Kiryushchenkov, E. K. Skosyrskaya, and A. M. Petrenko
Department of Chemistry, Moscow State University, Vorob'evy gory, Moscow, 119992 Russia
Received August 11, 2005

Abstract--The kinetics of 3,3',5,5'-tetramethylbenzidine (TMB) oxidation by sodium periodate in an aqueous solution was studied. For the auto-acceleration regime, the experimental data correspond to the kinetic equation wt = k [ P ]
1/2 t

[ IO 4 ]

- 1/2 t

[TMB]0, where wt is the accumulation rate of the meriquinoid product (P) of TMB oxida- 4 ]t

tion and [P]t and [ IO are the concentrations of product P and periodate, respectively, at time t. A radical chain mechanism was proposed; the mechanism explains the experimental kinetic equation and complies with the observed inhibiting effect of metal ions (Zn, Cd) in this reaction. DOI: 10.1134/S1061934806110049

Catalytic methods of analysis employ reactions of the oxidation of different compounds by the periodate ion. These reactions are primarily used for the determination of manganese [1-4] and noble metals [5, 6]; aromatic diamines (diphenylamine [7], 3,3',5,5'-tetramethylbenzidine (TMB) [3], and others [5]) are frequently used as reducing agents. The mechanism of these reactions is poorly understood. Attempts to describe the action of metal catalysts (manganese [8] and ruthenium [5]) as the alternating oxidation-reduction of metals were successful. However, the proposed schemes fail to explain the inhibiting effect of some doubly charged cations (Cd, Ni, Zn, Fe, and Hg) in this reaction, which was revealed later. Nevertheless, this does not preclude the use of this reaction as the indicator reaction for the determination of some of these metals [9, 10]. To get insight into the effect of metals, it is necessary first to study the mechanism of the noncatalytic reaction between periodate and arylamine. Only one work dealing with the noncatalytic oxidation of amine, namely, N,N-diethylamine (DEA), by periodate was published [11]. It was suggested that the rate-determining step is the decomposition of the complex [H4 IO 6 ž DEA ž H+] followed by several rapid steps involving the
- OH and H3 IO 5 radicals. -

.

.

free diamine: Q + TMB = P. The equilibrium of P and Q is described by the constant [13] 5 -1 [P] K J = -------------------------- = 2.8 × 10 M . [ TMB ] [ Q ] Interaction of this type is typical for aromatic amines [5, 12]. The product P, in turn, is in equilibrium with . radicals P 2S [13] and can be considered as the . dimer of radical S . The TMB radical is not protonated at pH 6.8 unlike doubly protonated diimine Q [14]. The oxidation of aromatic amines by persulfate is a typical chain radical process, which, in addition, is branched as demonstrated with the example of N,N,N',N'-tetramethyl-p-phenylenediamine (4MePPDA). The slowest step of the chain's propagation is the interaction of the relatively stable diamine radical . 2- . - 4MePPD A with S2 O 8 yielding SO 4 [12]. However, our attempts to describe the kinetics of the oxidation of TMB by periodate with a similar scheme always failed, which suggests the existence of fundamental differences between the mechanisms of oxidation by persulfate and periodate. Thus, the aim of this work was to get insight into the mechanism of the noncatalytic reaction between TMB and periodate to reveal the cause of the effect of metal ions (primarily, metal inhibitors) in this reaction. EXPERIMENTAL Solutions and reagents. Solutions were prepared using distilled water additionally purified on a Millipore setup. A 0.043 M solution of sodium metaperiodate ("for analysis," Reanal, Hungary) in water was prepared; a 0.02 M solution of TMB ("for analysis,"

Data exist on the oxidation of TMB by other oxidants: persulfate [12] and hydrogen peroxide [13]. The oxidation of TMB by hydrogen peroxide first yields the product of one-electron oxidation P of blue-green color (max = 370 and 650 nm), which is a meriquinoid complex consisting of diimine Q (orange, max = 450 nm) and diamine (TMB) (scheme 1). In the course of the oxidation of TMB, diimine rapidly reacts with

1067


1068

BEKLEMISHEV et al. Me NH2 Me Me NH Me Product P Me NH Me Me Product Q
Scheme 1. Products of the oxidation of TMB: complex P and quinonediimine Q.

Me NH2 Me Me NH Me

Me NH
+2H
+

Me
+

Me NH2
+

NH2 Me Me

Acros organics, Belgium) was prepared by the addition of 0.1 M HCl to a weighed portion of TMB (10% mole excess with respect to TMB), shaken for 15 min, and diluted with water to the required volume. Metal salts (CdCl2 ž 2.5H2O and ZnSO4 ž 7H2O, analytical grade, Reakhim-Acros) were dissolved in water when preparing solutions with metal concentrations of 1 g/L. Solutions with lower concentrations were prepared by dilution. An acetate-ammonia buffer solution was prepared by the addition of a 1.3 M solution of NH3 of chemi A1 0.30 0.25 0.20 0.15 0.10 0.05 0 6.2 6.4 3 2 6.6 6.8 7.0 pH 1

cally pure grade to 0.1 M CH3COOH of chemically pure grade to the required pH. Procedure for conducting the reaction. Kinetic measurements in the case of the noncatalytic reaction were performed at pH 6.8, at which the effects on this reaction are most pronounced (Fig. 1). To conduct the reaction, calculated aliquot portions of reagent solutions were placed with the use of Eppendorf dispensers in glass test tubes previously washed with HCl (1 : 1) and distilled water. Time reckoning started when the periodate solution was added. Then the test tube was shaken, and its contents were poured into the cell (l = 1 cm) of a Shimadzu UV-2200 spectrophotometer. The absorbance of the reaction products was measured at 15-s intervals for 90-360 s at two wavelengths corresponding to the absorption maxima of products P (650 nm) and Q (450 nm) (scheme). Processing of experimental data. According to the absorption spectra, the oxidation of TMB by periodate yields the same products P and Q as the oxidation by peroxide [13]. The concentrations of products were calculated from the absorbances at 450 and 650 nm using previously determined molar absorptivities of the prod650 650 450 ucts: P = 2.1 × 104, R = 6.6 × 102, P = 2.8 × 103, and R = 2.4 × 104. The current concentration of periodate was calculated by the material balance equation [ IO 4 ] = c
- IO
4

450

Fig. 1. Inhibiting effect of (1) cadmium, (2) nickel, and (3) zinc (1 ÷g/mL) depending on the pH of a borate buffer solution; 4.3 × 10-4 M NaIO4 , 2.5 × 10-4 M TMB (for zinc, 2.2 × 10-4 M NaIO4 , 1.25 × 10-4 M TMB); the difference of the absorbances A1 = A[without M(II)] - A[M(II)] at t = 1 min was taken as the measure of the inhibiting effect.

- [P] - [Q].

Products P and Q reversibly interact by the reaction Q + TMB = P [13]. We found that the constancy of this equilibrium constant is not fulfilled in the course of the reaction; i.e., the equilibrium between the products in
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the TMB-periodate reaction is not established. Therefore, it is convenient to write the reaction rate as the decrease in [TMB]: w = -(d[TMB]/dt). Differentiating the material balance equation cTMB = [TMB] + 2[P] + . . [Q] + [ S ] with respect to t and neglecting [ S ] in comparison with [P] and [Q], we obtain -(d[TMB]/dt) = 2d[P]/dt + d[Q]/dt. This expression was used for the calculation of the reaction rate w in the entire ascending portion of the kinetic curve when the concentration of the final product Q is small in comparison with the concentration of product P (up to t = 90 s). The rates of the variation of the concentrations of P and Q (d[P]/dt and d[Q]/dt) were calculated for each experimental point (15, 30, ..., 90 s) as ([P]t + t - [P]t )/t (where t is the interval between measurements equal to 15 s). RESULTS AND DISCUSSION Experimental relationships. We obtained time dependences of the absorbance of the products of TMB oxidation by periodate at different concentrations of the oxidant and the reducing agent. The work was performed under the concentration conditions at which the formation of product P is first autoaccelerated (Figs. 2a, 2b), then its concentration goes through a maximum, and significant concentrations of the final product Q begin to form. The autoacceleration of the reaction suggests chain branching. Branching, which complicates data interpretation, finally helped in revealing substantial features of the reaction mechanism. Compilation of the scheme of the reaction. The nature of the reaction initiation in this scheme must be insignificant due to the existence of branching, because under the conditions of the developed reaction the rate of branching is higher than the rate of initiation [15, p. 236]. Aromatic amines are oxidized by both periodate [8] and persulfate [12] with the initial loss of one electron; therefore, the initiation can be considered as - the interaction of IO 4 and TMB yielding the reducing . agent radical S and the product of the one-electron reduction of periodate I(+6): I(+6) + S . IO 4 + TMB Different ionic forms of iodine(+6) are known; at pH ~ 7, this is probably a mixture of comparable amounts of - 2- H5 IO 6 and H4 IO 6 [16]. Let us denote this active species as I(+6). Chain propagation. By analogy with [12], the following steps of chain propagation can be assumed: S + IO
-

c[P], M × 10 40

6

() 4 3 1

30

20

10 5 20 c[Q], M × 106 30 0 40 2 60 80 3 1 4 2 6 100 t, s

20

10

0

20
6

40 (b) 5

60

80

c[P], M × 10 20 16 12 8 4

100 t, s 1

6

7 2 3 4 20 40 60 80 100 t, s

.

0

.

- 4

I(+6) + Q (k1).

(1)

The product of the reduction of I(+6) will be I(+5), i.e., iodate; therefore, the second reaction of chain propagation must be written as follows: I(+6) + íåB S + IO

Fig. 2. (a) Concentrations of products P (upper) and Q (lower) in the TMB-NaIO4 reaction (2.2 × 10-4 M NaIO4) at different concentrations of TMB; cTMB, M: (1) 6.0 × 10-5, (2) 3.0 × 10-5, (3) 4.8 × 10-4, (4) 1.2 × 10-4, (5) 1.5 × 10-5, and (6) 0.75 × 10-5; (b) concentration of product P in the TMB-NaIO4 reaction (3.0 × 10-5 M TMB) at different concentrations of periodate; c IO , M: (1) 6.0 × 10-5, (2) 3.0 ×
4

.

- 3

(k2).
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10-5, (3) 1.5 × 10-5, (4) 0.75 × 10-5, (5) 4.3 × 10-4, (6) 1.1 × 10-4, and (7) 4.5 × 10-5. 2006

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BEKLEMISHEV et al.

Table 1. Linearization of the kinetic curves of the oxidation of TMB by periodate in different coordinates; the experimental data presented in Figs. 2a and 2b were used Coordinates Correlation coefficient at constant cTMB at constant c
IO
4

w-

[ P ] [ IO 4 ] 0.94 0.92

-

w - [TMB] [ P ] [ IO 4 ] 0.22 0.04

-

w - [ IO 4 ] [ P ] [ IO 4 ] 0.64 0.85

-

-

If S is more stable than I(+6), the slower step is the production of I(+6), i.e., reaction (1). In the other case, the rate-determining step is reaction (2). According to [13], an additional step of chain propagation is the formation of the complex of Q and TMB: (3) Q + TMB P (k3). Chain branching must be included into the scheme, because the reaction proceeds with autoacceleration. If the periodate ion interacts with the TMB rad. ical ( S ) at the step of chain propagation, it probably also interacts with the product of its dimerization P. Then, as a result of the transfer of one electron to the . periodate ion from one of the radicals S involved in the complex, P will be oxidized to Q, and the second radical will be released: P + IO
- 4

.

long chains, its rate is equal to the total reaction rate and, taking into account (8a), is written [15, p. 189] as k 4 [ P ] [ IO 4 ] - - . w = k1[ IO 4 ][ S ] = k1[ IO 4 ] -------------------------- . k5 According to (9a), the order must be 3/2 with periodate, 1/2 with respect to product P, and respect to TMB. Now let us consider the case when radical stable than I(+6). Analogously to (8a) and obtain the stationarity condition with respect k4[P][ IO 4 ] = k6[I(+6)]2
- -

(9)

respect to zero with S is less (9a), we . to S : (8b)

.

and the total reaction rate (as the rate of chain propagation step (2)): k 4 [ P ] [ IO 4 ] w = k2[TMB][I(+6)] = k2[TMB] -------------------------- . (9b) k6 In this case, the order must be equal to one with respect to TMB and 1/2 with respect to periodate and product P. Comparison of the obtained kinetic equations with experimental data. Experimental kinetic curves are not linearized in the w-[TMB] [ P ] [ IO 4 ] and w- [ IO 4 ] [ P ] [ IO 4 ] coordinates (Table 1), i.e., relationships (9a) and (9b) are not fulfilled. Other kinetic schemes alternative to (1)-(7) were considered; however, they also fail to match the experimental data. However, it was noticed that all experimental rates (about 50 measurements for all studied concentrations of the reactants and for any time of reaction development at which significant amounts of product Q are not formed) correlate with the corresponding values of [ P ] [ IO 4 ] (correlation coefficient 0.92-0.94, Fig. 3). Thus, the experimental kinetic equation can be represented in the form w = k[P]1/2[ IO 4 ]1/2[TMB]0.
- - - - - -

S + Q + I(+6)

.

(k4).

(4)

Reaction (4) is degenerate chain branching. Chain termination can occur by the reactions S +S I(+6) + S I(+6) + I(+6)

.

.

P
-

(k5),

(5) (6)
-

.

IO 3 + Q (k6),
-

termination (e.g., IO 3 + IO 4 ) (k7). (7)

Compilation of the kinetic equation. For the reaction with two active centers according to the Semenov semistationary concentration principle [15, p. 225], the concentration of the more active center can be assumed stationary. Then, in the case of the branched reaction, the concentration of the less active center and, hence, the reaction rate will increase with time. In the system under consideration, it is a priori unknown which of the . active centers ( S or I(+6)) is more reactive. Let us con. sider both cases. If S is more stable than I(+6), the termination will occur mostly by reaction (5) and the stationarity condition with respect to I(+6) will be written as follows (instead of the initiation rate, we use the branching rate and neglect all termination reactions except the predominant): k4[P][ IO ] = k5[ S ]2.
- 4

(10)

.

(8)

If S is more stable than I(+6), reaction (1) will be the slowest step in chain propagation. For sufficiently

.

Change in the assumed scheme of the reaction. Equation (10) does not agree with scheme (1)-(9), which includes the interaction of the oxidant with the reducing agent radical as the rate-determining step. The
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assumption that Eq. (10) is significant suggests that the rate-determining step of chain propagation is some reaction of the active center, which involves neither the oxidant nor the reducing agent. It could be the monomolecular transformation of the active center or its reaction with water, a component of a buffer solution, or hydroxonium. This reaction must yield the reactive form of the active center, which rapidly propagates the chain. Let this center be I(+6). It is known that different forms of iodine(+6) are produced in flash photolysis and pulse radiolysis, and these forms only slowly transform into each other, probably through hydration- dehydration (the rate constant of these transformations is no higher than 103-104 s-1) [16]. Therefore, it can be assumed that in the reaction under study the reduction of periodate first yields the less active form I(+6)', which slowly transforms into the form more reactive with respect to TMB (I(+6)''): I(+6)' I(+6)'' (k'). (11) Another possible reaction of I(+6) transformation is . the decomposition into iodate and the OH radical, which occurs at pH 7-10 at low concentrations of periodate (no higher than 5 × 10-4 M) [16]. In this case, the radical oxidizing TMB would be hydroxyl: I(+6)' IO 3 + OH
-

w, M min 80 70 60 50 40 30 20 10 0

-1

× 10

6

1 2

y = 74.498x - 1.8116 R2 = 0.94 y = 77.305x + 0.3391 R2 = 0.92

0.2

0.4

0.6

1.0 0.8 [P]1/ 2[IO4]1/ 2 × 10 4

Fig. 3. Linearization of the kinetic curves (Figs. 2a, 2b); the - reaction rate w and concentrations [P] and [ IO 4 ] were calculated for each instant in time: (1) 3.0 × 10-5 M TMB, the concentration of periodate varies; (2) 2.2 × 10-4 M NaIO4, the concentration of TMB varies.

.

(k').

(11b)

[I(+6)] we obtain k4[P][ IO 4 ] = k5[ S ]2. Then [ S ] = k 4 [ P ] [ IO 4 ] k 4 [ P ] [ IO 4 ] -------------------------- and w = k' -------------------------- , which comk5 k5 pletely coincides in form with both Eq. (14) and the experimental equation. Thus, our assumption of the rate-determining step of chain propagation as a (pseudo)monomolecular transformation of the product of either the one-electron reduction of periodate or the one-electron oxidation of TMB is in agreement with the experimental data. These cases are kinetically indistinguishable. Choice between two active reaction centers. To . decide the question of which of the active centers ( S or I(+6)') is more long-living, additional information is required. As found in our previous works [3, 9], the arylamine-periodate reaction is catalyzed by trace manganese(II) and inhibited by cadmium (Figs. 4a, 4b) and zinc ions and, to a smaller extent, by some other doubly charged ions (Hg(II), Ni(II), and Fe(II)). It is important that different arylamines (TMB, N,N-diethylaniline, and o-dianisidine) and phenols (hydroquinone) can act as a reducing agent, and in all cases the accelerating effect of manganese and the inhibiting effect of cadmium and zinc are observed. However, the effect of these metals is pronounced only in the reactions of oxidation by periodate. Thus, the rate of the reaction of TMB with other oxidants in nearly unaffected by manganese and cadmium (Table 2). Periodate is used as the oxidant in at least one-third of the procedures for the catalytic determination of manganese [17]. The total of
2006
- -

-

.

.

In any case, we assume that stage (11a) or (11b) is the slowest; therefore, for the total reaction rate, we have w = k'[I(+6)']. (12) Because it is assumed that I(+6)' is a relatively stable species, termination will occur predominantly by reaction (7): I(+6)' + I(+6)'. (Linear termination of I(+6)'', which is also possible [16], does not explain our data.) The stationarity condition should be written through . the concentration of the more active center ( S ) analogously to (8b): k4[P][ IO 4 ] = k6[I(+6)']2. Hence, [I(+6)'] = k 4 [ P ] [ IO 4 ] -------------------------- . k6
- - -

(8c)

(13)

Then, from (12) for the reaction rate, we obtain k 4 [ P ] [ IO 4 ] w = k' --------------------------. k6 (14)

Equation (14) coincides with the experimental kinetic - equation w = k[P]1/2[ IO 4 ]1/2 (10). Now let us assume the reverse: radical S is more stable than I(+6). Then, for the reaction rate, we have . w = k'[S ], and for the stationarity with respect to
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1072 c[P], M × 10 10 9 8 1 74 6 5 4 3 2 1
6

BEKLEMISHEV et al.

2

() 3

0 50 c[Q], M × 10 50 45 40 35 30 25 20 15 4 1 10 5 0 50

100
6

150 (b)

200

250

300

350 t, s

2

agent (~10-4 M); therefore, the presence of metal cannot significantly change their concentration. However, the formation of the complex with periodate or TMB theoretically could affect the rate of the process. Therefore, we checked the interactions. The absence of changes in the electronic spectra in the TMB-periodate system on the introduction of metal indicates that stable complex compounds are probably absent (we obtained for Mn(II) and Cd(II) at pH 6.8). The complexation of TMB with these ions was also checked by chromatography on paper treated with a solution of TMB in ethanol with the elution of metals with a buffer solution (pH 6.8). For comparison, chromatography was performed on paper treated with ethanol (without TMB). We found that the retention of metals is independent of the presence of TMB, which contradicts the assumption of the formation of stable complexes under these conditions. Thus, most probably, metal ions interact with some active center of the reaction rather than with the initial species (TMB and periodate). In line with the special role of periodate rather than amine in systems of this type, the product of one-electron reduction of the periodate ion is a candidate for the role of this center. This means that the most long-living active center of the reaction is iodine(+6), and the rate-determining step of chain propagation is reaction (11a) or (11b).

3

100

150

200

250

300

350 t, s

Fig. 4. Kinetic curves of the TMB-NaIO4 reaction in the presence of cadmium (3 × 10-5 M TMB, 1.1 × 10-4 M NaIO4 , pH 6.8); measurements (a) by product P at 650 nm and (b) by product Q; (1) 1 × 10-6 M Cd, (2) 3 × 10-6 M Cd, (3) 1 × 10-4 M Cd, and (4) without cadmium.

the data presented above suggest that the catalytic or inhibiting effect of metal ions in the TMB-periodate system is due to periodate rather than arylamine. The concentration of metal affecting the rate of the indicator reaction (~10-6-10-5 M) is substantially lower than the concentration of the oxidant and reducing

Effect of metal ions on the reaction rate. It is interesting to consider the question of how the metal ion affects the rate of the TMB-periodate reaction. The catalytic effect of Mn(II) is explained [8] by its oxidation by periodate to Mn(III) or Mn(IV), which readily oxidizes TMB. The inhibiting effect of metal ions (including Cd(II) and Zn(II)) in radical reactions of different types can be related [18] to either the electron transfer to the metal ion (this is improbable for zinc and cadmium, which have no other stable oxidation states without ligands that can stabilize it) or the transfer of the radical center to the metal-ligand bond. It is not clear what this ligand could be in our system and what the fate of the complex is to which an electron was transferred. Instead of this, it is reasonable to assume that the inhibitor metal ion accelerates the termination

Table 2. Effect of the nature of the oxidant on the catalytic (positive values) and inhibiting (negative values) effects of metals in TMB oxidation reactions Oxidant Metal ion, concentration KIO4 1 × 10-3 ÷g/mL Mn(II) 1 × 10-2 ÷g/mL Cd(II) 0.32 + 0.03 - 0.40 + 0.03 H2O2 0.05 + 0.02 - 0.01 + 0.01 K2S2O8 0.02 + 0.01 0.01 + 0.01 O2 0.01 + 0.01 0.01 + 0.01

Note: Differences of the absorbances of the reaction products at 650 nm (l = 1 cm) with and without cadmium and manganese within 3 min after the beginning of the reaction are presented (because of the complex shape of the kinetic curve and the absence of the data sufficient for the calculation of rate constants, the effect of metals was characterized by the absorbance of product P within the fixed time after the beginning of the reaction). Conditions: 2.5 × 10-4 M TMB, pH 6.8, 2.6 × 10-5 M oxidant (for Mn) and 5.2 × 10-5 M oxidant (for Cd); O2 denotes the oxidation of TMB by air oxygen. JOURNAL OF ANALYTICAL CHEMISTRY Vol. 61 No. 11 2006


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reaction. If the most stable active center in this reaction is iodine(+6), metal must accelerate reaction (7). It is known that the termination reactions are activationless except for disproportionation reaction; the latter can have a low activation energy [15, p. 214], which can be decreased by the inhibitor metal ion. In other words, reaction (7) represented in the form I(+6)' + I(+6)' IO 3 + IO
- - 4

supported by the Russian Foundation for Basic Research, project no. 04-03-33116a. REFERENCES
1. Mushtakova, S.P., Gumenyuk, A.P., and Khmelev, S.S., Zh. Anal. Khim., 1991, vol. 46, no. 3, p. 561. 2. Dolmanova, I.F., Yatsimirskaya, N.T., Poddubienko, V.P., and Peshkova, V.M., Zh. Anal. Khim., 1971, vol. 26, no. 8, p. 1540. 3. Beklemishev, M.K., Stoyan, T.A., and Dolmanova, I.F., Analyst, 1997, vol. 122, no. 10, p. 1161. 4. Dolmanova, I.F., Yatsimirskaya, N.T., and Peshkova, V.M., Zh. Anal. Khim., 1973, vol. 28, no. 1, p. 112. 5. Mushtakova, S.P., Gumenyuk, A.P., Kozhina, L.F., and Khmelev, S.S., Zh. Anal. Khim., 1996, vol. 51, no. 7, p. 768 [J. Anal. Chem. (Engl. Transl.), vol. 51, no. 7, p. 708]. 6. Khomutova, E.G., Khvorostukhina, N.A., and Moskvina, I.A., Zh. Anal. Khim., 1983, vol. 38, no. 1, p. 170. 7. Morozova, R.P., Nishenkova, L.P., and Blinova, L.P., Zh. Anal. Khim., 1981, vol. 36, no. 12, p. 2356. 8. Dolmanova, I.F., Yatsimirskaya, N.T., and Peshkova, V.M., Kinet. Katal., 1972, vol. 13, no. 3, p. 678. 9. Beklemishev, M.K., Stoyan, T.A., and Dolmanova, I.F., Fresenius' J. Anal. Chem., 2000, vol. 367, p. 17. 10. Beklemishev, M.K., Kiryushchenkov, E.N., Stoyan, T.A., and Dolmanova, I.F., Zh. Anal. Khim., 2005, vol. 60, no. 6, p. 662 [J. Anal. Chem. (Engl. Transl.), vol. 60, no. 6, p. 589]. 11. Pavlova, V.K., Savchenko, Ya.S., and Yatsimirskii, K.B., Zh. Fiz. Khim., 1970, vol. 44, no. 3, p. 658. 12. Nickel, U., Peris, C.V., and Ramminger, U., J. Phys. Chem., A, 2002, vol. 106, no. 15, p. 3773. 13. Josephy, P.D., Eling, T., and Mason, R.P., J. Biol. Chem., 1982, vol. 257, no. 7, p. 3669. 14. Misono, Y., Ohkata, Y., Morikawa, T., and Itoh, K., Electroanal. Chem. Interfacial Electrochem., 1997, vol. 436, nos. 1-2, p. 203. 15. Purmal', A.P., A, B, V ... khimicheskoi kinetiki (ABC of Chemical Kinetics), Moscow: Akademkniga, 2004. 16. Kläning, U.K., Sehested, K., and Wolff, T., J. Chem. Soc., Faraday Trans. 1, 1981, vol. 77, no. 7, p. 1707. 17. Perez-Bendito, D. and Silka, M., Kinetic Methods in Analytical Chemistry, New York: Ellis Horwood, 1988. 18. Kovtun, G.A., Koord. Khim., 1983, vol. 9, no. 9, p. 1155.

(k7)

(7)

can be accelerated by the inhibitor metal cation. (For example, the metal could simplify the approaching of two negatively charged I(+6) species because of the interaction with them. At a concentration of metal ions of ~10-6 M and a much lower concentration of iodine(+6), binding of 50% of the active centers requires that the stability constant of the metal- iodine(+6) complex has the order of 106 å-1, which cannot be considered improbable.) Thus, experimental kinetic data were explained by a scheme of a branched radical chain reaction including slow monomolecular (possibly, pseudomonomolecular) transformation of the product of the one-electron reduction of periodate ion I(+6)' into some species more reactive with respect to the reducing amine as the rate-determining step of chain propagation. In this case, the active species that is present in the largest concentration (i.e., is most stable) is I(+6)', which participates in predominant chain termination, and the inhibiting role of metal ions (Zn(II), Cd(II)) consists in the acceleration of this termination reaction. The proposed scheme must also be valid in the oxidation of other reducing agents by the periodate ion if the one-electron reduction of periodate occurs at the first step and it proceeds as a chain reaction. This means that metal ions that exhibit an inhibiting effect in the TMB-periodate reaction will also inhibit other reactions of the oxidation by periodate. Thus, the knowledge of the scheme of the indicator reaction will help in determining the range of potential analytes and interfering ions. This will offer more reasonable approaches to the selection of indicator reactions for the determination of metals by the catalytic method. ACKNOWLEDGMENTS We are grateful to A.P. Purmal' and I.F. Dolmanova for the discussion of results and support. The work was

JOURNAL OF ANALYTICAL CHEMISTRY

Vol. 61

No. 11

2006