Dov Zehavi and Joseph Rabani
1368
Pulse Radiolysis of the Aqueous Ferro-Ferricyanide System. I I. Reactions of Hydrogen Atoms and eaq- with Ferrocyanide and Ferricyanide Ions ov Zehavi and Joseph Rabani" Department o f Physical Chemistry, The Hebrew University, Jerusalem 91000, Israel
(Received November 27, 1973)
Hydrogen atoms react with both ferro- and ferricyanide. The reaction with ferrocyanide, previously unF ~ ( C N ) G ~=- )4 X lo7 M - l sec-l, independent of pH in known, proceeds with a rate constant k(H the range 1-8. This reaction is followed by two consecutive processes, of which the first one is first order, while the second one is a reaction of an intermediate with ferrocyanide ions. Aquapentacyanoferrate(I1) was identified as a final product. All the intermediates reacted with ferricyanide. The nature of the reaction products is discussed and the rates of the various processes measured as a function of pH are given. F ~ ( C N ) G ~IOOH radicals react according to reaction 1 during the electron pulse. Fe(CN)c4- and Fe(CN)s3- include also the various protonated forms of ferrocyanide and ferricyanide, OM C Fe(CN):-
-
O H
+ Fe(CN),"
H
H
+ ferrocyanide
+ ferricyanide
(process 11) XI X, (process 111)
(1)
-+
-
--+
X,
+ ferrocyanide
€3'
XI,
+ ferricyanide
XI, + ferrocyanide
+ ferricyanide
(2) (2')
(3) -+
+
Y
(3')
XI,,
(4)
Z (4') Rate Constants. Rate constants were measured from plots of log (4 - Dt)us. t (where Dt is the absorbance a t the end of process i). These plots were linear for all three processes (I, 11, and 111).From such plots and Table I, the reaction rate constant k2 = (3.9 0.6) X IO7 M - l sec-l could be calculated for the pH range 1.0-3.3. (At pH 2 ) partially suppresses all three reactions. The absorbance obtained a t the end of process I11 decays partially (-30% a t 480 nm) within a few seconds (experiments were carried out in the pH range 1.8-2.5). This is observed at wavelengths >440 nm (practically no change is observed at 420 nm), when [ferrocyanide] > 0.05 M. No systematic measurements were carried out for the investigation of this process (process IV). The chemistry of the system can schematically be represented by the following equations (process I)
--+
*
Pulse Radiolysis of :he Aqueous Ferro-Ferricyanide System
1369
TABLE I: M e a s u r e m e n t s of kr in Acidic Solutionsa
0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0 02 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.5 1.0 1.5 2 0 3.0 5 ,0 10.0 20.0
50.0 1.5 1.5 2.5 4.0 7.5 7.5 15.0 8.5
0.10
3.3 2.8 2.4 2.1 1.8 1.5 1.2 1.0 0.7 3.0 3.0 2.5 2.0 1.6 1.6 1.3 1.9
41 41 39 37 43 44 45 45 44 60 123 59 58 60 120 65 55
3.5 3.4 3.9 3.9 3.8 3.4 3.9 4.1 2.8 4.3 3.4 4.1 4.3 4.0 3.6 3.7 4.1
6.0
0.22
0.29
4.0
6.0
045
..Y
0,'s 0.83 1.32
0.0 9.76
0.00s
6.0 21.3
0.UI
..8
6.2 8.0 1.7 4.5 0.39
2 0 x I0.5 4.0
x 10-5
11.3
1.0 x 10-4
0.04
1,s
11.5 11.1 11.3
0.10
1.2
11.3 11.3
1.0 x 1.0 x
0.30
0.8
2."
11.1 11.3
1 . 0 x 1o.Q 0.002s
3.8
0.005
3.3
0.01
2.8
0.015
1.3
x 10-4
4.G x 10.' 2 . 5 x 10-4
'
3.2 l i . ~
2.0
2 . 5 x 10.4
ii.8
8.13
5.0 x 10-4
11.8
16.1 11.1
2.47
0.95
0.19
O . l i 3 . 6
0.77
0.36 0.91
1.3 158 4.3 15.8
1 . 0 x 10.4
2.0 x 10-4 5.0 2
0.055
1.0 x 10-4
The reaction rate constants ks, k r , k b and k 4 can be evaluated from Table 11, using eq I and 11. hII = k , 1- h,,[ferricyanide] (1) krII
--
h,!ferrocyanide]
+ k,.[ferricyanide]
(11)
In Figures 2 and 3 we demonstrate the dependence of kI1 and k111 on [ferricyanide] according to eq I and 11. The concentration of ferricyanide at each process was calculated with the aid of the appropriate reaction rate constants and yields, and using a values from Table II. From the slopes and intercepts of figures such as 2 and 3 it is possible to determine k ~ k:+r, , k b and k q , . Figures 4 and 5 sum-
41.6 M
O I P 0.91JO.D
0.14 0.9712.4
F E R R O C Y I N I D L 0.9s
0.16
9.0
1.21
C.56
6.21 71.0
9.4
1.43
0.97 0.08
15.0
1.92
0.31
9.4
3.01 6.74 1.31
0.56
11.6 9.1
8.18
0.85
18.1
119.8
0.87185
0.72 0.75
30.4 41.0 69.1 84.0 94.7
11-3 168.14
3.8
i.is
o . 0 ~ 30.8
1.62
0.75
1.Y
11.6 3.8 11.6
3.16
0.m
7.10
0.Sl
63.7 18.8 30.8
(S751 (5 = (I / G H ) ( l f h,[Fe(CN),4-]/k, [2-propanol]) (111) mental yield of Hz, GHz is the so called “molecular yield” of Hz, GH is the radical yield of H atoms a t neutral pH, and 126 is the reaction rate constant for reaction 5. G(Hz)
1/’(G(HJ
H 4- CHpCHOHCH3
+
Hz
+ CH&OHCHB
The Journal of Physicai Chemistry, Vol. 78, No. 14, 1974
(5)
Dependence of k4 and k4‘ on pH: cyanide; 0 , 0 ,0.1 M ferrocyanide.
Figure 5.
0,
., 0.01 M ferro-
= GHz = 0.42 f 0.02 was measured in the absence of 2propanol over a wide range of ferrocyanide concentrations (10-3 to 0.5 M ) in neutral solutions and in acid solutions
Pulse Radiolysis of the
Aqueous Ferro-Ferricyanide System
1371
7 -
b
OOIZl
'
A
0.00
0040
D Q D ia
DH)
&,ml
Figure 6. Spectra (processes I - i l l ) for 0.01 M ferrocyanide solutions at pH 1.5: (a) optical absorptions at the end of each process; (b) optical absorptions corrected by subtraction of the ferSpectrum ricyanide absorbance (see Table I I for the dose); (0) at the end of process I; ( A ) spectrum at the end of process ( I ; ( 0 )spectrum at the end of process I l l ; [XI] = 8.9 X [ X i [ ] = 7.9 X 1W6, [XIII] = 6.3 X [Y] = 1.0 X [Z] = 1.6 X 1 0 - 6 M .
( 5 x 10- M ferrocyanide in 0.15 M H2S04). A plot of 1/ (G(H2) - G&) vs. [Fe(CN)g4-]/[2-propanol] yielded a straight line as demonstrated in Figure 9. From Figure 9, k 2 = (4.4 4 0.8) x 107 M-1 sec-l can be obtained, based on k s = 7.2 x io7 M - I sec-1.8 These competition experiy source, with a dose ments were carried out using a 137Cs rate of about 2000 rads/min and a total dose up to lo5 rads. An upper limit, 100 M- cm- I, for the extinction coefficient of XI at 420 nni could be estimated from pulse radiolysis experiments. Reaction with paq- Solutions of 0.2 M ferrocyanide and 0.5 M ethanol were argon saturated and pulse irradiated a t high ionic strength (1M Na2S04). The addition of ethanol had the purpose of eliminating the ferricyanide formation by reaction 1. An upper limit, k(%,- + ferrocyanide) < 4 x 104 Ivd-l see-I could be estimated from the rate of decay of cas- (followed a t 580 nm). This is in agreement with the limit of lo5 M - I sec-l reported previou~ly.~ Reactions of Ferricyanide Ions. Reaction with Hydrogen Atoms. The rate constant for the reaction has been determined previously3 as k = (6.5 .f 0.5) x lo9 M - I sec-l in atcid (pH 2 ) solutions, where ferrocyanide was present as an OH scavenger. We determined this rate constant over a wide range of HC184 concentrations (pH 0.4-3.2) as (5.5 0.4) x lo9 M - sec- Corrections were carried out for the competing reaction 2 and recombination of H atoms. When we carried out such corrections for the previous results,3 a value of (6.0 f 0.8) x lo9 was obtained. The ~
*
300 L.nm
Figure 7. Spectra (processes I - I l l ) for 0.1 M ferrocyanide solutions at pH 2.4. Process I I is not well separated from process I I I . See Figure 6 and Table II for explanations: [ X i ] = 14.4 X l o e 6 , [Y] = 4.5 X [XI,] = 9.9 X [X~II] = 8.3 X [Z] = 1.6 X M.
mean value of all the results is (5.8 f 0.6) X lo9 M - l sec- I. Absorption measurements a t 480 nm, where aquapentacyanoferrate(II) has optical absorption ( E 200 Mcm-I 6,7 a t pH 2 ) , showed that not more than 2% of the H atoms reacted according to reaction 6 rat pH 2 M ethanol was added to eliminate OH radicals). H
+ Fe(CN);-
3 Fe(CN),H20- + HCN
(6)
In neutral solutions, about 10% of the H atoms reacted according to reaction 6. (N2O and tert-butyl alcohol were present a t sufficiently high concentrations to eliminate ea,- andOH.) Reaction with eaq-. The reaction rate constant had been determined by Gordon, et al.1° Dorfman and Mathes o d l have shown that if log k is plotted us. p1l2/(l a straight line is obtained with a slope 3 when p is low. At higher ionic strength 01 > 0.03) the slope tends to the value of 2, as ferricyanide associates with K + or Na+. We reexamined the reaction and evaluated the rate constant for KFe(CN)& from a plot according to eq IV, which was used bef0re.l
+
=2
where
a
= 10-3
0 6 ~ " /I1 '
+ Zp".,
b
=
02pl'- /(I
+ 2p"')
kexpt is the experimental value, where k o is the rate conThe Journal of Physical Chemistry, Vol. 78,No. 14, 1974
Dov Zehavi and Joseph Rabani
1372 I
I
r
b
aac
0
-0
0.5 [Fe(CNt-]/
io
l.5
[ isoproponoi]
Figure 9. Competition between ferrocyanide (03 M) and 2-propanol for H atoms
Spectra (processes 1 - 1 1 1 ) for 0.05 M ferrocyanide solutions containing 2.5 X M ferricyanide at pH 3.0. See Figure 6 and Table II for explanations: [ X i ] = 9.1 X [Xi,] Figure 8.
=z
0.6 X
0.3x 10-61W.
[ X r i i ] = 0.3 X
[Y] = 8.5 X
[Z] =
stant a t -- 0. The rate constants ha obtained were 2.4 X 109 for Fe(CNI63- (in full agreement with 2.5 X lo9 M-l sec-1 11) and 2.7 X IO9 M - l sec-l for KFe(CN)& Optical absorption measurements a t 480-550 nm in deaerated neutral solutions of 10- M ferricyanide + 10M ferrocyanide 1 M ethanol and 5 x M ferricyanide 1 M %propanol were carried out. It was found that about 12% of the eeq- reacted according to reaction 7.
+
+
e,,
+ Fe(CN)i- 3 Fe(CN),H203- + CN-
(7)
Discussion Reduction of Ferricyanide Ions by H and eag-. The reaction of H atoms with ferricyanide may well involve an addition of the H to the ligand, followed by electron transfer. Halpern12 suggested a general mechanism for reduction by H atoms Fe3+X- + R [Fe3+-.X-...H] --+ FeZ+XH (8) FezCXEI4 H20 Fe2+X- + H,O' (9) Fe2+XR I- H20 ----t Fe2+H20+ HX (9') --+
This mechanism is in good agreement with our results, assuming reaction 8 is rate determining. As the Fe-CN bond is strong, reaction 4 is more important as compared to (9'). In acid solutions, the asociation of the intermediate FeZ+XH with HsQ+ would probably strengthen the thus preFe-CN bond, as is the case in ferr~cyanide,'~ venting reaction 9' almost completely. This is in agreement with the negligible yield of aquapentacyanoferrate(I1) in acid solutions. The reaction of %q- with ferricyanide is a diffusionThe Journal of Physical Chemistry, Vol. 78, No. 14, 7974
controlled electron transfer r e a ~ t i 0 n . lAs ~ for many complexes,l* there might be tunneling of the electron from the water to a d orbital of the metal. In such a reaction an excited state, F ~ ( C N ) E ~ -may * , be formed as an intermediate,14 which by monomolecular dissociation will form to some extent the aquapentacyanoferrate(I1)complex. Reaction of H Atoms with Ferrocyanide Ions. The results presented in the previous sections show that H atoms react with ferrocyanide (reaction 2). There are reasons to propose that Fe(1) complexes are formed as a consequence of this reaction. Such a complex has been proposed previously in the electrolytic reduction of ferrocyanide, in the presence of excess of cyanide ions.15 The reaction of H atoms with ferrocyanide, with the subsequent formation of aquapentacyanoferrate(ll), has also been proposed .I6 It might be argued that H atoms oxidize ferrocyanide, either uia the Hz+ mechanism1' or via hydride formation.18 However, in such a case reaction 2 is expected to be enhanced by H+, contrary to our observations. Moreover, material balance would require that Hz and some form of ferricyanide would be produced. This is again in disagreement with the data. (a) No ferricyanide was obtained after the termination of reaction 1. (b) No absorption which could be attributed to Fe(CN)sHz02- was observed at 565 nm, which is the peak absorption of this Fe(II1) c0mp1ex.l~(c) The hydrogen yield in the absence of organic scavengers and ferricyanide was identical with the so-called "molecular yield." Moreover, the product of reaction 2 reacts with ferricyanide ions, in agreement with the assumption that it is a reduction product. Another possibility that might be argued, is the addition oi W atom to the -+N bond and subsequent oxidation of the ligand. Such reactions possibly take place when H atoms or OH radicals react with free CN- (with rate constants of about 3 x 109 M- sec- 1 2 O ) . However, product analysis showed that CN- is formed in an equivalent yield to the aquapentacyanoferrate(I1) complex,5 ruling out significant amounts of oxidation products. We propose that Xr is identical with Fe(CN)6H4-, so that reaction 2 is in fact reaction 10
Pulse Radiolysis of the Aqueous Ferro-Ferricyanide System
1-I -5 Fe11(CN),4--+ Il'e(CN),H'Reactions 3 and 3' become 11 and ll', respectively Fe(CPu'),H4-
-+
- Fe"'(CN),"----t
Fe(CK)),H4-
Fe'(CN):-HCN ,2Fe"(CN);or Fe"( CN):-
-
1373
(10) (11)
+ H+
(11')
+ Fe"( CN),*- + HCN
Reactions 4 and 4' become 1 2 and 12', respectively FeT(Ci\')l-
Fe'(CN),'-
+ Fe"(i:N),'^-
+ Fe'"(CK),'-.
-+
Fe"(CN);-
Fe"(CN),'-
+ Fe'(CN);+ Fe11(CiY),4-
(12) (12')
Reactions 10 and 11. can be explained in a way which is schematically similar to the ferricyanide reduction (reactions 8 and 9). The H atom adds to the ligand CN- (reaction 1.0) producing. the radical (NC)5Fe-&NH. We assume that the bond Fe-CNH is weak (in comparison with the F e - e N bond in the normal complexes) and it breaks according to reaction 11. Aquapentacyanoferirate(I1) is produced in reactions ll', 12, and 12', as it is reasonable to assume that Fe11(CN)53- becomes very quickly 'hydrated. AH the reaction rate constants are expected to be pH dependent, due to possible protonation of the reactants. .As the reactions 11, ll', 12, and 12' are between charged species, ionic strength effects are also expected. The rate of reaction 11 is linearly dependent of [H+]. It is perhaps t h e to a catalytic effect on the breakage of the Fe-CNH bond. We do not know the fate of F&(CN)$- formed in reaction 12. It is probably oxidized by ferricyanide or H202 to yield ferrocyanide ions, within a few seconds or even later. Due to light instability and photolysis, we were unable to investigate this possibility. Process 171, observed a t high ferrocyanide concentrations, may be the equilibrium which was proposed by Emschwiller7x2I Fe(CN),4-
+ Fe(CS),H,B"
=F=k
(NC),Fe(CN)Fe(CN),'-
+ H20
(13)
It might be argued that €3202, formed as a radiation product ( G 0.7), can react with the various Fe(1) complexes. Changing the pulse intensity (and hence [HzOz]) at various F ~ ( C N ) Gand ~ - F e ( C N ) 8 concentrations gave no evidence for such reactions as being important under our conditions. Spectra in Acid pM. According to the mechanism proposed, the spectra in Figures 6-8 must be assigned to aquapentacyanoferrate(I1) which is formed via reactions
-
Il', 12, and 12', along with other products. Optical absorbance is indeed produced in the range where the aquapentacyanoferrate(I1) is known to absorb. However, a quantitative examination shows that the aquapentacyanoferrate(I1) is not the only absorbing product. Other products may have E
