Radiolysis of ferrocyanide solutions studied by infrared spectroscopy

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Radiation Physics and Chemistry 76 (2007) 1280–1284 www.elsevier.com/locate/radphyschem

Radiolysis of ferrocyanide solutions studied by infrared spectroscopy S. Le Cae¨r , G. Vigneron, J.P. Renault, S. Pommeret CEA/Saclay, DSM/DRECAM/SCM/URA 331 CNRS, F-91191Gif-sur-Yvette Cedex, France

Abstract The behavior of the neutral and basic aqueous ferrocyanide system under irradiation is investigated using the coupling of a LINAC with infrared spectroscopy. The comparison between the neutral and basic system evidences the formation of the hydroxopentacyanoferrate (III) ions and gives information on the reaction mechanisms. The pseudo-protective effect of the dissolved dioxygen on the ferrocyanide is explained via a mechanism implying the superoxide radical anion. r 2007 Elsevier Ltd. All rights reserved. Keywords: FT-IR spectroscopy; Radiolysis; Ferrocyanide; Ferricyanide; Superoxide.

1. Introduction Despite the fact that the ferrocyanide ions have been extensively studied under flash photolysis (Ohno and Tsuchihashi, 1965; Airey and Dainton, 1966a, b; Moggi et al., 1966; Waltz and Adamson, 1969; Shirom and Stein, 1971; Stasicka and Wasielewska, 1997; van Grieken et al., 2005), laser femtosecond pump-probe spectroscopy (Pommeret et al., 1998; Anderson et al., 2000) or under radiolysis (Rabani and Matheson, 1966; Rabani and Meyerstein, 1968; Zehavi and Rabani, 1972, 1974), little is known about its induced infrared spectroscopy. A femtosecond mid-infrared study has been performed showing that the electron detachment from FeðCNÞ4 6 was instantaneous (Anderson et al., 2000) in agreement with (Pommeret et al., 1998). Infrared spectroscopy is indeed particularly appropriate to study cyanide complexes, the C RN stretching region ð200022200 cm1 Þ being very sensitive to the oxidation state of the metal cation and to the nature of ligands present in the coordination sphere of the metal ion. In that region, water does not exhibit any particular absorption, enabling to probe the reactivity of ferrocyanide through the CN stretching (Le Cae¨r et al., 2006). In this study, we present the influence of the pH and dissolved gases on the ferrocyanide’s reactivity under Corresponding author. Fax: +33 1 69 08 34 66.

E-mail address: [email protected] (S. Le Cae¨r). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.019

radiolysis. That reactivity is probed with infrared spectroscopy coupled to our LINAC with an experimental set-up described previously (Le Cae¨r et al., 2006).

2. Experimental section The irradiations were performed using the electron pulses of a Titan Beta, Inc. linear accelerator which delivers electrons of 8, 9 or 10 MeV energy (Mialocq et al., 1999). In the present experiments, 10 ns pulses of 10 MeV electrons were used at a repetition rate of 10 Hz. As the irradiated volumes in the infrared cell are very small ðmLÞ, we used fluorescence which is a very sensitive technique to determine the dose per electron pulse. The dose (7 Gy/pulse) was thus determined using the coumarin dosimeter at a 103 mol dm3 concentration (Louit et al., 2005). Moreover, we have checked with larger optical paths that the dose determined by this method is analogous to the dose calculated using the SCN dosimeter. The coupling of the LINAC with Fourier transform infrared spectroscopy (FT-IR) has been described in details previously (Le Cae¨r et al., 2006). The spectra were averaged from 100 scans at a 4 cm1 resolution. We used bare CaF2 windows with mylar spacers of 25 mm. Spectra of irradiated solutions were recorded with reference to the same solution just before irradiation, which allowed us to detect the effects of irradiation by measuring differential absorption.

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The spectra were fitted as a sum of Gaussian and/or Lorentzian functions. The fit was optimized according to the least square method using the Levenberg–Marquardt method (Press et al., 1988–1992). More details about the fitting procedure are given in Le Cae¨r et al. (2006). The compounds (potassium ferro- and ferri-cyanide from Merck, potassium hydroxide by Sigma) were of analytical grade and used without further purification. The 102 mol dm3 of ferro- and ferri-cyanide solutions were prepared using ultra-pure water with a conductivity of 18:2 MO and a very low total organic carbon ðTOCo10 ppbÞ from a Millipore Alpha-Q apparatus. As these solutions undergo thermal and photochemical decompositions, they were protected against light using aluminum foil and stored at 5  C. Solutions were then transferred in syringes just before irradiation. The irradiation cell was protected from light. A computer-controlled flow system replaced the solution between each irradiation experiment, so that each experiment was done with a fresh solution. Solutions were used aerated or were bubbled with ultra-high purity N2 O (99.999%) and O2 (99.999%) for 1 h.

1281

two different doses. Fig. 1a evidences the decrease of the 2037 cm1 band under irradiation. This band is attributed to the ferrocyanide FeðCNÞ4 6 ions (Jones, 1963); this was checked by recording the infrared spectra of ferrocyanide ions. It is worth noting that upon irradiation, the loss of ferrocyanide with and without KOH is the same, showing the good reproducibility of the experimental set-up. Fig. 1b highlights products formed under irradiation. The most intense bands are located in the 210022130 cm1 spectral region. In this region, the vibrational signature can be seen as the sum of two bands: the first one is located at 2115 cm1 , whereas the second is around 2102 cm1 . The 2115 cm1 band is attributed to the ferri-cyanide FeðCNÞ3 6 ions (Jones, 1963) (this was checked by recording the infrared spectra of ferricyanide ions). When ferrocyanide solutions are pulse irradiated, HO radicals react according to reaction (3) during and immediately after the electron pulse: 3  HO þ FeðCNÞ4 6 !OH þ FeðCNÞ6 ,

k3 1:0  1010 mol1 dm3 s1 ðRabani and Matheson; 1966Þ.

3. Results and discussion The radiolysis of water can be expressed by the following processes:  þ  H2 O ! H2 ; H2 O2 ; H ;  OH; e aq ; HO2 ; H3 O ; OH .

(1)

In the presence of N2 O, the hydrated electron is converted into  OH according to reaction (2):   e aq þ N2 O ! N2 þ OH þ OH .

(2)

3.1. Comparison of the spectra obtained starting from neutral or basic ðpH ¼ 12Þ ferrocyanide solutions under ambient atmosphere The FT-IR spectra of a 102 mol dm3 ferrocyanide solution (at neutral pH and with 102 mol dm3 of KOH) recorded just after irradiation are represented in Fig. 1 for

We previously determined an integrated absorption coefficient of 15 800 mol1 dm3 cm2 for the FeðCNÞ3 6 ions (Le Cae¨r et al., 2006). Let us suppose that the absorption around 2102 cm1 is due to the FeðCNÞ5 ðOHÞ3 ions and that the integrated absorption coefficient of these hydroxopentacyanoferrate (III) ions is the same than the one of FeðCNÞ3 ions. We then calculated the G-values of the 6 species from the area of the two bands obtained during the fitting procedure for the different experimental conditions (see Table 1). This table clearly evidences that the G-values of the different species decrease with the dose, while the induced absorption increases (Fig. 1). This is due to the fact that the intensity of the bands does not increase linearly according to the dose. As stated earlier (Le Cae¨r et al., 2006), the linearity domain lies between 0 and 1500 Gy. For  an irradiation dose greater than 1500 Gy, e aq and H have consumed all the dissolved dioxygen and they can react 0.0020

0.002 Differential absorbance

ð3Þ

0.000

0.0015

-0.002 -0.004

0.0010

-0.006 -0.008

0.0005

-0.010 0.0000

-0.012

1980

2000

2020

2040

2080 2100 2120 2140 2160 2180 2060 Wavenumber (cm-1)

Fig. 1. Evolution of the differential absorbance (after/before irradiation) of a 102 mol dm3 potassium ferrocyanide solution: 1400 Gy, without KOH ðÞ; 1400 Gy, with KOH ð102 mol dm3 Þ ð&Þ; 4200 Gy, without KOH ð}Þ; 4200 Gy, with KOH ð102 mol dm3 Þ ðnÞ; (1a) in the 198022080 cm1 spectral domain; (1b) in the 207522185 cm1 spectral domain. The spectra have been recorded just after irradiation; the recording time is 24 s. The data are represented with the symbols and the fitted curves are the corresponding thin lines. It is worth noting that the fitted curves correspond nicely to the experimental ones.

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Table 1 Deconvolution of the vibrational signature in the 210022130 cm1 spectral region into two bands and determination of the G-values for different experimental conditions (dose, with and without KOH)

1400 Gy 1400 Gy, pH ¼ 12 4200 Gy 4200 Gy, pH ¼ 12

GðFeðCNÞ5 ðOHÞ3 Þ (molecule/100 eV)

GðFeðCNÞ3 6 Þ (molecule/100 eV)

GðFeðCNÞ5 ðOHÞ3 Þ þ GðFeðCNÞ3 6 Þ (molecule/100 eV)

GðFeðCNÞ4 6 Þ (molecule/100 eV)

1.3 2.5

3.4 2.6

4.7 5.1

4.7 4.7

0.5 1.7

1.7 0.9

2.2 2.6

2.3 2.3

The integrated absorption coefficient is assumed to be 15 800 mol1 dm3 cm2 for the two iron (III) complexes (Le Cae¨r et al., 2006). The error bars are 20% for the G-values of iron (III) complexes and 10% for the G-values of the iron (II) complex. The G-values have been determined 24 s after irradiation.

with ferricyanide, leading to the formation of ferrocyanide whose decrease is then less intense. An equilibrium involving the ferro- and ferri-cyanide complexes takes place. The strong effect of the pH on the spectral shape of the iron (III) complexes (Fig. 1b) without any effect on the iron (II) complex is well explained by Table 1, showing that our tentative assignment of the 2102 cm1 band to the FeðCNÞ5 ðOHÞ3 ion is justified. Considering the error bars on the G-values of the iron (III) complexes, our hypothesis on the value of the integrated absorption coefficient of the FeðCNÞ5 ðOHÞ3 ion within an error bar of 20% is then validated. Under irradiation, a cyanide ligand is substituted by a hydroxide ligand on the iron (III) metallic center according to reaction (4) and this reaction is favored when the pH increases: 3  FeðCNÞ3 þ CN . 6 þ OH ! FeðCNÞ5 ðOHÞ

(4)

Infrared spectroscopy enables us to detect the released CN anions: as a matter of fact, the shoulder observed in Fig. 1b at 2078 cm1 is assigned to the free CN ejected. They are only seen as a shoulder, as their extinction coefficient is much weaker than when they are bound within a complex; the integrated absorption coefficient for the CN stretching mode was determined as 840 mol1 dm3 cm2 for CN , to be compared to 15 800 mol1 dm3 cm2 1 for the FeðCNÞ3 dm3 cm2 for the 6 ions and 97 000 mol 4 FeðCNÞ6 ions. Fig. 1b evidences that more cyanide are released in solution when the pH increases (more cyanide anions are ejected when working with KOH than without), and when the dose increases. Indeed, the pH increases after irradiation: we measured a pH of 9.0 after a 200 Gy irradiation and of 10.4 after a 5000 Gy irradiation when starting with a neutral solution. This pH increase is a consequence of the cyanide ions release, through the acid-base HCN=CN couple (pKA ¼ 9:2 at 25  C (Beck, 1987)). Last, for large doses, we notice a peak at 2170 cm1 . This peak is assigned to the formation of iron(III) hexacyanoferrate(III) or Berlin green FeIII ½FeIII ðCNÞ6 . This complex

has already been observed by infrared spectroscopy in the photolysis of the FeðCNÞ5 ðNOÞ2 ion in aqueous solution (de Oliveira et al., 1995). The peak begins to be noticeable when the irradiation dose is greater than 2500 Gy. The formation of Berlin green implies that some iron(III) complexes can lose all their ligands, leading to the formation of FeIII aq . As a consequence, one would expect that it will be easier to form Berlin green for high pH values since it will be easier to release CN ions via the ligand substitution (reaction 4). This is what we observe at a 4200 Gy irradiation dose. 3.2. Influence of dissolved gases on reactivity The influence of dissolved gases (air, O2 , N2 O) on the infrared spectrum of a 102 mol dm3 ferrocyanide solution after a 1400 Gy irradiation is presented in Fig. 2. From infrared spectra, we can deduce radiolytic yields for the loss of ferrocyanide ions (Table 2). It is worth noting that the value obtained for O2 -saturated solutions is in good agreement with GðFeðCNÞ3 6 Þ ¼ 3:2 molecule=100 eV determined when studying by pulse radiolysis in the microsecond time range the formation of the ferricyanide ion in a O2 -saturated solution (Adams et al., 1965), although the two studies are performed under completely different time conditions. Table 2 demonstrates that the ferrocyanide’s radiolytic yield increases when the dioxygen concentration increases i.e. dioxygen acts as a radioprotector of the ferrocyanide. This surprising result is due to the interplay of the radiolytic species and the ferro-/ferri-cyanide redox couple. When O2 is present in solution, hydrated electrons react according to reaction (5) to form the superoxide radical ion, whereas H radicals react with oxygen to form hydroperoxyl radicals ðHO2 Þ (reaction 6). Those two radicals are involved in an acid-base equilibrium (7). As stated earlier, the pH of the solution is basic after irradiation. Thus, the acid-base equilibrium will be displaced in favor of the superoxide radical ion O 2 . The dismutation reaction of the superoxide radical ion (reaction 8) is extremely slow even within the timescale considered

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Differential absorbance

S. Le Cae¨r et al. / Radiation Physics and Chemistry 76 (2007) 1280–1284

1283 0.0015

0.000 -0.002

0.0010

-0.004 0.0005

-0.006 -0.008

0.0000

-0.010 2000

2025

2050

2080

2100

2120

Wavenumber (cm-1) Fig. 2. Evolution of the differential absorbance (after/before irradiation) of a 102 mol dm3 potassium ferrocyanide solution irradiated with a 1400 Gy dose: under ambient atmosphere ðÞ; in the case of a O2 -saturated solution ðnÞ; in the case of a N2 O-saturated solution ð&Þ; (2a) in the 200022075 cm1 spectral domain; (2b) in the 208022140 cm1 spectral domain. The data are represented with the symbols and the fitted curves are the corresponding thin lines.

Table 2 Approximate radiolytic yield for the loss of ferrocyanide ions as a function of the dissolved gases for a 1400 Gy irradiation Gas

O2

Air

N2 O

GðFeðCNÞ4 6 Þ (molecule/100 eV)

3.3

4.7

6.7

The error bar is 10% for the G-values of the ferrocyanide. The G-values have been determined 24 s after irradiation.

here (24 s) and will be neglected hereafter.  e aq þ O2 ! O2 ,

H þ O2 ! HO2 , HO2 #Hþ þ O 2 ;

ð5Þ ð6Þ pK7 ¼ 4:8,

Hþ

  O 2 þ O2 ! HO2 þ O2 ;

ð7Þ

k8 o0:35 dm3 mol1 s1

(Bielski et al., 1985).

ð8Þ

From the standard G-values (Ferradini and Jay-Ge´rin, 1999), we estimated that 3:8  104 mol dm3 e aq and 0:8  104 mol dm3 H radicals have been created for a 1400 Gy irradiation. Under ambient atmosphere, the dissolved dioxygen concentration is 2:6  104 mol dm3 at 298 K. If we consider that all the dioxygen is consumed through reactions (5) and (6), the formed O 2 concentration will 3 4 then be ½O  ¼ 2:6  10 mol dm . Under O2 atmoair 2  sphere, if we admit that all eaq and H radicals are consumed through reactions (5) and (6), the formed O 2 4 concentration will then be ½O mol dm3 . 2 O2 ¼ 4:6  10 3 It has been reported that O 2 is able to reduce FeðCNÞ6 with a low rate constant, according to the reaction: 3 4 O 2 þ FeðCNÞ6 !FeðCNÞ6 þ O2 ,

k9 ¼ 2:7  102 mol1 dm3 s1 (Zehavi and Rabani, 1972) and k9 ¼ 6:9  103 mol1 dm3 s1 (Bradic and Wilkins, 1984). ð9Þ

This reaction is complete at the times considered here (24 s are required to record the spectra, once irradiation is cut off). From the G-values, we then deduce the concentration of ferrocyanide that has been protected by the dioxygen for the O2 -saturated experiment compared to the experiment performed under air: ½FeðCNÞ4 6 O2 =air ¼ 2:0  104 mol dm3 . This concentration is consistent with the amount of superoxide radical anions formed under irradiation (see above). The behavior of the system under a N2 O atmosphere is also represented in Fig. 2. In this case, the loss of ferrocyanide is more pronounced in the presence of N2 O than under ambient atmosphere; we get GðFeðCNÞ4 6 Þ¼ 6:7 molecule=100 eV (Table 2). This value is consistent with (Schuler et al., 1980, 1981) who found GðFeðCN3 6 ÞÞ ¼ 6:1 molecule=100 eV at pH10:5 for nitrous oxide saturated solutions of 102 mol dm3 ferrocyanide. The shape of the induced absorption spectrum in the iron (III) complexes region (Fig. 2b) is slightly different from the ones measured in aerated or O2 -saturated solutions. The equilibrium between the ferricyanide and the hydroxopentacyanoferrate (III) is displaced towards the ferricyanide, indicating that more ferricyanide ions are formed directly upon reaction (3). 4. Conclusion Infrared spectroscopy is indeed a very powerful and sensitive tool to identify species. We have built a new experimental set-up, by coupling a LINAC with infrared spectroscopy, which enables us to study reaction mechanisms initiated by irradiation. We have shown that at pH ¼ 12, more FeðCNÞ5 ðOHÞ3 ions are formed together with a larger release of CN anions than when starting from a neutral solution. The formation of Berlin green under extended irradiation is also favored as compared to neutral solutions. Dioxygen plays a very important role in the redox chemistry of ferrocyanide through the formation and reactivity of the superoxide radical anion, and it displaces the ferro-/ferricyanide equilibrium towards the ferrocyanide.

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References Adams, G.E., Boag, J.W., Michael, B.D., 1965. Spectroscopic studies of reactions of the OH radical in aqueous solutions. Reaction of OH with the ferrocyanide ion. Trans. Faraday Soc. 61, 492–505. Airey, P.L., Dainton, F.S., 1966a. The photochemistry of aqueous solutions of Fe(II) I. Photoelectron detachment from ferrous and ferrocyanide ions. Proc. Roy. Soc. A 291, 340–352. Airey, P.L., Dainton, F.S., 1966b. The photochemistry of aqueous solutions of Fe(II) II. Processes in acidified solutions of potassium ferrocyanide at 25  C. Proc. Roy. Soc. A 291, 478–486. Anderson, N.A., Hang, K., Asbury, J.B., Lian, T., 2000. Ultrafast mid-IR detection of the direct precursor to the presolvated electron following electron ejection from ferrocyanide. Chem. Phys. Lett. 329, 386–392. Beck, M.T., 1987. Critical survey of stability constants of cyano complexes. Pure Appl. Chem. 59, 1703–1720. Bielski, B.H.J., Cabelli, D.E., Arudi, R.L., Ross, A.B., 1985. Reactivity of HO2 =O 2 radicals in aqueous solution. J. Phys. Chem. Ref. Data 14, 1041–1096. Bradic, Z., Wilkins, R.G., 1984. Comparative behavior in the kinetics of reduction by superoxide and dithionite ions. J. Am. Chem. Soc. 106, 2236–2239. de Oliveira, M.G., Langley, G.J., Rest, A.J., 1995. Photolysis of the ½FeðCNÞ5 ðNOÞ2 ion in water and poly(vinyl alcohol) films: evidence for cyano radical, cyanide ion and nitric oxide loss and redox pathways. J. Chem. Soc. Dalton Trans., 2013–2019. Ferradini, C., Jay-Ge´rin, J.-P., 1999. La radiolyse de l’eau et des solutions aqueuses: historique et actualite´. Can. J. Chem. 77, 1542–1575. Jones, L.H., 1963. Nature of bonding in metal cyanide complexes as related to intensity and frequency of infrared absorption spectra. Inorg. Chem. 2, 777–780. Le Cae¨r, S., Vigneron, G., Renault, J.P., Pommeret, S., 2006. First coupling between a LINAC and FT-IR spectroscopy: the aqueous ferrocyanide system. Chem. Phys. Lett. 426, 71–76. Louit, G., Foley, S., Cabillic, J., Coffigny, H., Taran, F., Valleix, A., Renault, J.P., Pin, S., 2005. The reaction of coumarin with the OH radical revisited: hydroxylation product analysis determined by fluorescence and chromatography. Radiat. Phys. Chem. 72, 119–124. Mialocq, J.-C., Hickel, B., Baldacchino, G., Juillard, M., 1999. The radiolysis project of CEA. J. Chim. Phys. 96, 35–43.

Moggi, L., Bolletta, F., Balzani, V., Scandola, F., 1966. Photochemistry of coordination compounds—XV Cyanide complexes. J. Inorg. Nucl. Chem. 28, 2589–2597. Ohno, S., Tsuchihashi, G., 1965. The photochemistry of hexacyanoferrate(II) ions in aqueous solutions. Bull. Chem. Soc. Jpn. 38, 1052–1053. Pommeret, S., Naskrecki, R., van der Meulen, P., Menard, M., Vigneron, G., Gustavsson, T., 1998. Ultrafast events in the electron photodetachment from the hexacyanoferrate (II) complex in solution. Chem. Phys. Lett. 288, 833–840. Press, W.H., Flannery, B.P., Teukolsky, S.A., Vetterling, W.T., 1988–1992. Numerical Recipes in C: The Art of Scientific Computing. Cambridge University Press, Cambridge. Rabani, J., Matheson, M.S., 1966. The pulse radiolysis of aqueous solutions of potassium ferrocyanide. J. Phys. Chem. 70, 761–769. Rabani, J., Meyerstein, D., 1968. Pulse radiolytic studies of the competition H þ H and H þ ferricyanide. The absolute rate constants. J. Phys. Chem. 72, 1599–1603. Schuler, R.H., Patterson, L.K., Janata, E., 1980. Yield for the scavenging of OH radicals in the radiolysis of N2 O-saturated aqueous solutions. J. Phys. Chem. 84, 2088–2089. Schuler, R.H., Hartzell, A.L., Behar, B., 1981. Track effects in radiation chemistry. Concentration dependence for the scavenging of OH by ferrocyanide in N2 O-saturated aqueous solutions. J. Phys. Chem. 85, 192–199. Shirom, M., Stein, G., 1971. Excited state chemistry of the ferrocyanide ion in aqueous solution. I. Formation of the hydrated electron. J. Chem. Phys. 55, 3372–3378. Stasicka, Z., Wasielewska, E., 1997. Photosubstitution and photoredox behaviour of cyanometallates: reaction models. Coord. Chem. Rev. 159, 271–294. van Grieken, R., Aguado, J., Lopez-Munoz, M.-J., Marugan, J., 2005. Photocatalytic degradation of iron-cyanocomplexes by TiO2 based catalysts. Appl. Catal. B 55, 201–211. Waltz, W.L., Adamson, A.W., 1969. Photochemistry of complex ions. VIII. Photoelectron production. J. Phys. Chem. 73, 4250–4255. Zehavi, D., Rabani, J., 1972. Pulse radiolysis of the aqueous ferro–ferricyanide system. I. The reactions of OH, HO2 and O 2 radicals. J. Phys. Chem. 76, 3703–3709. Zehavi, D., Rabani, J., 1974. Pulse radiolysis of the aqueous ferro–ferricyanide system. II. Reactions of hydrogen atoms and e aq with ferrocyanide and ferricyanide ions. J. Phys. Chem. 78, 1368–1373.