Kinetics of the Partially Autocatalytic Hexacyanoferrate(II)−Iodate Reaction and the Effect of a Added Iodide

The oxidation of [Fe(CN)6] by IO3 in the pH 1.62 -2.70 range is autocatalytic whether the reductant or oxidant is in excess. Typical first order plots of -ln(A∞ -At) versus time are curved showing an increase in rate with the progress of the reaction. It was possible, however, to obtain the pseudo first order rate constants (ku) for the uncatalyzed [Fe(CN)6]-IO3 reaction from the linear parts of these plots. The linearity of the plots decreases with increasing concentrations of [Fe(CN)6], IO3 and H. The pseudo first order rate constant (kobs) for the whole reaction was obtained using ln[(At/A∞)/(1 At/A∞)] versus time and/or a nonlinear curve fitting for sigmoidal reactions. The rate constant, kobs obtained by both methods varies with the initial concentration of [Fe(CN)6] according to Eqn. (i), where ku is the rate constant of the direct reaction between [Fe(CN)6] and IO3 and ka is the rate constant of the autocatalytic reaction. The values of ku, at different reactions conditions, obtained from plots of kobs vs [Fe(CN)6]i are in fairly good agreement with values obtained from linear sections of first order plots. kobs = ku + ka[Fe(CN)6]i (i) Both ku and ka showed a first order dependence on [IO3]. The dependence of ku on [H] is complex as shown in Eqn. (ii). A polynomial fit of k2(1 + K1[H]) vs [H] has a zero intercept in agreement with Eqn. (iii) The catalytic rate constant varies with [H] according to Eqn. (iii) k2 = ku/[IO3] = {(k7K1 + k8K3)[H] + (k9K1K2 + K1K3)[H+]} (ii) (1 + K1[H]) k2(1 + K1[H]) = (k7K1 + k8K3)[H] + (k9K1K2 + K1K3)[H+] (iii) ka = k11[H][IO3] (iv) The overall dependence of kobs on [Fe(CN)6], [IO3] and [H] is given by Eqn. (v). kobs = {(k7K1 + k8K3)[H] + (k9K1K2 + K1K3)[H+]}[IO3] + k11[Fe(CN)6]i[IO3][H] (v) (1 + K1[H]) At pH 4.33 no reaction between [Fe(CN)6] and IO3was observed. When iodide is added to the reaction mixture at this pH, the reaction proceeded at measureable rate.

reaction from the linear parts of these plots. The linearity of the plots decreases with increasing concentrations of [Fe(CN) 6 ] 4-, IO 3 and H + . The pseudo first order rate constant (k obs ) for the whole reaction was obtained using ln[(A t /A ∞ )/(1 -A t /A ∞ )] versus time and/or a nonlinear curve fitting for sigmoidal reactions. The rate constant, k obs obtained by both methods varies with the initial concentration of [Fe(CN) 6 ] 4according to Eqn. (i), where k u is the rate constant of the direct reaction between [Fe(CN) 6 ] 4and IO 3 and k a is the rate constant of the autocatalytic reaction. The values of k u , at different reactions conditions, obtained from plots of k obs vs [Fe(CN) 6 4-] i are in fairly good agreement with values obtained from linear sections of first order plots. k obs = k u + k a [Fe(CN) 6 4-] i (i) Both k u and k a showed a first order dependence on [IO 3 -].  6 ] 4− is in vast excess has been reported previously [1]. It was observed that the reaction is autocatalytic whether IO 3 or [Fe(CN) 6 ] 4− is in vast excess. This is in agreement with an earlier report [2]. Nonetheless, rate constants were measured from the linear parts of pseudo first order plots with the reductant in vast excess over that of the oxidant. The closely related reaction between BrO 3 and [Fe(CN) 6 ] 4− has also been reported and has shown a similar complexity [3,4]. The kinetics of this reaction in the relatively high [H + ] (0.05 -0.5 M) was found to be partially autocatalytic. Autocatalysis is believed to be caused by Br 2 formed from the relatively fast reaction between Br − and BrO 3 − in the employed [H + ] range [3]. The Br 2 − [Fe(CN) 6 ] 4− reaction is shown to be very fast [3]. The kinetics of the BrO 3 − − [Fe(CN) 6 ] 4− reaction in the pH 3.6 -5.8 range is also complex. The complexity of this reaction in this [H + ] range, is claimed not to arise from the involvement of Br 2 , which is not formed in the BrO 3 --Brsystem at this pH range [4,5], but rather from the involvement of aquahexacaynoferrate(II) ([Fe(CN) 5 [4]. Autocatalysis in IO 3 --[Fe (CN) 6 ] 4− reaction most likely arises from reaction between the product iodide and iodate in acidic solutions. The lower redox potential of Ifacilitates its oxidation to I 2 and/or I 3 − by IO 3 − [7,8] at the pH values employed. The effect of deliberately added Ion the IO 3 − -[Fe(CN) 6 ] 4− reaction has been reported [9]. It was found that added I − accelerates this reaction considerably and the rate of reaction increases with increasing [I − ] even at pH values where the reaction does not take place in absence of iodide. It has known that I 2 reacts very rapidly with [Fe(CN) 6 ] 4− [10]. In recent years there has been a lot of interest and research activity in oscillating reactions. The system IO 3 − − SO 3 2− − [Fe(CN) 6 ] 4− is oscillatory. In these types of reactions it is important to know the rate constant of the likely processes. These are used to build models for oscillating reactions [11]. In this study we report the kinetics of both the uncatalized and the autocatalytic processes in the IO 3 − − [Fe(CN) 6 ] 4− reaction with iodate in large excess.

Experimental
All the chemicals were reagent grade and were used as received. Stock solutions, buffer (HSO 4 − /SO 4 2− ) and solutions of K 4 Fe(CN) 6 .3H 2 O were freshly prepared and used immediately as described earlier [1]. The absorption of the product [Fe(CN) 6 ] 3− was monitored at λ max 420 nm using a Perkin Elmer Model Lambda 25 or a Shimadzu 1800 UV/VIS absorption spectrophotometer both equipped with a thermostatted cell-holder. The absorbance-time traces were recorded on XY recorder. The pH of the reaction solutions was measured using an ELICO LI 120 pH meter. The kinetics of the reaction was investigated under constant reaction conditions with [IO 3 -] in vast excess over that of [(Fe(CN) 6 ) 4-]. Both reactants concentrations as well as the pH were varied.  6 ] 4− and [H + ]. The higher the concentration of the reactants, the shorter is the linearity of the plots. The first order rate constants for the uncatalyzed path, k u , were obtained from the linear parts of these plots. It was found that k u is fairly constant and does not depend on the initial concentration of [Fe(CN) 6 ]] 4− at fixed reaction conditions as shown in Table 1. The data of Fig. 1 is analyzed using a treatment for an autocatalytic reaction [12][13][14]. Plots of ln[

Results and Discussions
] versus time are linear as shown in Fig. 2. This treatment was applied only in the range where autocatalysis becomes dominant. In the low pH range autocatalysis starts early and a nearly S-shaped form of absorbance-time curve is observed. The values of the slopes (k obs , s -1 ) obtained from Fig. 2 and similar plots are dependent on the initial concentration of [Fe(CN) 6 ) 4− ] i . The absorbance-time data were also treated using a nonlinear curve fitting using an Origin 7.0 sigmoidal growth curve fitting program. A typical nonlinear fit for the data in Fig. 1 is shown in Figure 3. Values of k obs obtained using both methods are within 0-8% variation. Most of k obs values were obtained using the non-linear curve fitting program.   The value of k obs varies with the [Fe(CN) 6 ) 4− ] i , at fixed reaction conditions, according to Eqn. (2). The values of k u obtained from plots o Eq. (2) are in fairly good agreement with values obtained directly from linear portions of the first order plots. The rate constants k u and k a are rate constants for the uncatalyzed and the autocatalytic reactions pathways respectively. Values of k a were also calculated from Eqn. (2) using known values of k obs , k u and [Fe(CN) 6 4− ] i . k obs = k u + k a [Fe(CN) 6 4− ] i (2) Table 1 shows that k u varies linearly with [IO 3 -] at fixed reaction conditions. The results in Table 1  (3) Hexacyanoferrate(II) is known to be extensively protonated in aqueous acidic solutions [15].
The thermodynamic dissociation constants for the monoand diprotonated species ([HFe(CN) 6 ] 3− and [H 2 Fe(CN) 6 ] 2− respectively) have been determined as 5.25 x 10 -5 and 4.3 x 10 -3 at 25 ℃ respectively. At 1.0 M ionic strength and 25 ℃ the values of the dissociation constants are 4.68 x 10 -3 and 0.32 for the mono and diprotonated species respectively [16]. The iodate ion is appreciably protonated in aqueous acidic solutions and has a thermodynamic dissociation constant of 0.16 at 25 ℃ [17]. The value of the dissociation constant at I = 1.0 M, and T = 25 ℃ is reported as 0.514 M [18].
The major reaction steps for the uncatalyzed path are given by Eqns. 4 -11. [Fe(CN) 6 (14) Eqn. (13) may be rearranged to the form of Eqn. (14). A polynomial of the second degree fit of a plot of k 2 (1 + K 1 Fig. 4. From the polynomial fit (Fig.4) the value (k 7 K 1 + k 8 K 3 ) = (81.4 ± 17.2) and (k 9 K 1 K 2 + k 10 K 1 K 3 ) = (2.01 ± 0.07) x 10 4 . These values are approximately twice as much as the previously determined values [1]. It was previously reported that the rate constant, contrary to expectation, decreases with increasing ionic strength. This may be rationalized by recalling that the protonation constants K 1 Table 2 at constant [IO 3 − ], ionic strength and temperature. The dependence of k a on [H + ] (Fig. 5) shows a linear plot of k a versus [H + ] with an almost zero intercept.

[H + ]) vs [H + ] is shown in
The autocatalytic pathway is caused by a product generated by the uncatalyzed reaction. The Iproduced in the direct reaction between [Fe(CN) 6 ] 4− and IO 3 − in acidic solutions (Eqn.1) reacts with IO 3 to produce a more reactive species with [(Fe(CN) 6 )] 4than IO 3 such as those in Eqn. (15). IO 3 − + I − + 2H +  HIO 2 + HIO (15) The reaction may also proceed to produce I 2 which also reacts very rapidly with [Fe(CN) 6 Table 2 and Fig.5 show that k a varies linearly with [H + ] and is described by Eqn. (16). k a = k 11 [H + ][IO 3 -] (16) In conclusion the iodate -hexacyanoferrate(II) reaction is partially autocatalytic whether the reductant or the oxidant is in excess. Autocatalysis becomes important at high reactant concentrations and low pH. Autocatalysis may be caused by I(III), I(I) and/or I 2 that are produced during IO 3 − -I − reaction. At low reactants concentration and high pH autocatalysis only appears at late stages of the reaction. Qualitative experiments showed that the deliberate addition of I − speeds up the reaction and it proceeds at a measureable at pH values it would not usually take place.