R.WOODS
1446
tube decomposition of fluoroform, proposes the reaction sequence
CF2
+ CFaH
--+
r"'"+
CzFbH*
HF
IGFsH
and %,FJRC~F,H > 50. No quantitative kinetic data are presently available on the pyrolysis of alkyl fluorides,21 although the
elimination of H F from CF3CH3* has been observed in the flow pyrolysis of CF3N=NCH3 at 560°.26
Acknowledgments. We are indebted to a referee for some particularly helpful comments, to Drs. 0. P. Strausz and G. Haugen for sending prepublication copies of their manuscripts, and to Dr. J. Heicklen for a helpful discussion. (25) A. H. Dinwoodie and (1965).
R. N. Hasseldine, J . Chem. soc., 2266
Arsenic(1V) as an Intermediate in the Photochemical Oxidation of Ferrous Sulfate in the Presence of Arsenic Acid
by R. Woods Chemistry Department, University of Melbourne, Parkville N . B, Vidorh, Amtralia
(Recdved Odober 4, 1966)
The photochemical oxidation of ferrous sulfate in the presence of arsenic acid yields arsenic(111) in addition to iron(II1). A complex of iron(I1) and arsenic acid is postulated to be the photoactive species leading to arsenic(V) reduction. The stability constant and quantum yields of iron(II1) and arsenic(II1) of this complex were found to be 1.9, 1.8, and 0.9, respectively. The effect of oxygen on the quantum yields gives evidence to show that arsenic(1V) is formed as an intermediate in the photolysis.
The intermediate formation of the 4+ oxidation state of arsenic, produced by the oxidation of arsenic(111) by hydroxyl- or sulfate-free radicals has been postulated for a number of chemically and photochemically induced reactions.'-' No studies are found in the literature on the reduction of arsenic(V) t o arsenic(1V).
Experimental Section Materials. Ferrous sulfate was prepared by adding an excess of iron wire (99.9% Fe) to sulfuric acid solution under an atmosphere of nitrogen. The solution was filtered, acidified with sulfuric acid, and stored under nitrogen, The iron(II1) in the ferrous sulfate The Journal of Physieal Chemistry
of the total iron conwas always less than 5 X centration. Arsenic acid was prepared by the prolonged boiling (1) M. Daniels and J. Weiss, J . Chem. soc., 2467 (1958). ( 2 ) M. Daniels, J . Phys. Chem., 66, 1473, 1475 (1962). (3) L. J. Csanyi, Discussions Faraday soc., 2 9 , 146 (1960). (4) R. Woods, I. M. Kolthoff, and E. J. Meehan, J. Am. Chem. soc., 85, 2385 (1963). (5) R. Woods, I. M. Kolthoff, and E. J. Meehan, ibid., 8 5 , 3334 (1963). (6) R. Woods, I. M. Kolthoff, and E. J. Meehan, ibid., 8 6 , 1698 (1964). (7) R. Woods, I. M. Kolthoff, and E. J. Meehan, Inorg. Chem., 4, 697 (1965).
As(1V) AS INTERMEDIATE IN OXIDATION OF FeSOl IN ARSENICACID
of arsenic trioxide with aqua regia. The aqua regia solution was faken to dryness. The solution was again taken to dryness with nitric acid and twice with water, and the residue was dissolved in water. The arsenic(II1) in the arsenic acid was always less than 2X of the total arsenic concentration. Nitrogen used for deoxygenating the cell was purified from traces of oxygen by passage over a column of finely divided copper deposited on infusorial earth and heated at 200°.8 The oxygen content was thereby reduced to the order of 4 X lo+%. Photolysis. The reaction vessel consisted of a 5cm cell fitted with polished silica windows. A side arm containing a Teflon stopcock was used for adding and removing the reaction mixture. Nitrogen or oxygen was bubbled through the solution in the cell by Of a fine glass insertedthrough the stopcock. The cell was thermostated at 25" in a water bath and irradiated with a low-pressure mercury lamp a t constant intensity. A 0.01 N uranyl oxalate actinometer solution, irradiated in the same cell, was used to measure the light intensity, the quantum yield at 2537 A being taken as 0.60e9The intensity was determined to be 2.33 X loT5einstein 1.-1 min-1. Iron(''') was determined spectrophotoas the sulfate at 3O3 mp with a Shimadzu QR50 spectrophotometer. The extinction coefficient in 1 M sulfuric acid was determined as 2190 M-' cm-1. All solutions were adjusted to 1 M in sulfuric acid before determining the iron(II1). The spectrum of iron(II1) is affected by the presence of arsenic acid, the absorption peak at 303 mp disappearing at the higher arsenic acid concentrations (Figure 1). However, the extinction coefficient at 303 mp is unaffected if the arsenic acid concentration is 60.1 M (Figure 1) and the higher arsenic acid solutions were diluted before analysis to give a solution containing 0.1 M arsenic acid. Arsenic(II1) was determined polarographically using a dropping-mercury electrode.1° The diffusion current was found to be dependent on the sulfuric acid concentration, and all solutions were adjusted to 1M in sulfuric acid before analysis. Gelatin in a concentration of 0.01% was used as a maximum suppressor. The current was measured at -0.75 v US. sce and was shown to be proportional to arsenic(II1) concentration in the M . Iron(II1) is also rerange 2 X 10-5 to 2 X duced at the dme at this potential. However, it is duced a t much more positive potentials, before the arsenic(II1) wave appears, and its diffusion current was determined and subtracted from the total current at -0.75 v.
1447
0.8-
.-*
h ul C
0' - 0.,4-
.-v 0"
0.
Y
0.0 22 0
I
1
300
380
Wavelength
mp
Figure 1. Ultraviolet spectrum of 1.5 x 10-4 M iron(II1) in 1 M H,SO~in the presence of (1) 0, (2) 0.1, and (3) 1.0 M arsenic(V).
Results Arsenic(II1) is formed during the photolysis of ferrous sulfate in the presence of arsenic acid. Prolonged irradiation of arsenic acid alone, or in the presence of the free-radical-capture agent, acrylonitrile, does not yield arsenic(III) and therefore the primary photochemical process resulting in the formation of arsenic(111) involves iron(I1). Iron(II1) is also a photolysis product. The molar extinction coefficient of iron(II1) is on the order of 200 times that of iron(I1) and therefore acts as an inner filter. Jortner and Stein" conclude that, in the photooxidation of ferrous sulfate a t high acidity, the role of iron(II1) can be fully accounted for by its inner filter effect and derived an equation relating the initial rate of formation of iron(III) to the experimental rates, for complete light absorption [FeS+]= (2AIo/B)t/[Fe3+]- 2/B where lo is the light intensity, A the initial quantum the iron('I1) formed in time t, and yield, [#$' +I
B
=
EF~S+/~F~P+[F~~+]
eFeS+ and €Fez+ being the molar extinction coefficients of iron(II1) iron(II), respectively. A plot of [Fe3+] against t/[Fe3+] extrapolated to zero [Fea+] gives the
(8) F. R. Meyer and G . Ronge, Z . Angew. Chem., 5 2 , 637 (1939). (9) E. J. Bowen, "Chemical Aspects of Light," Oxford University lg4% p 283* (10) J. J. Lingane, I d . Eng. Chem., Anal. Ed., 15,583 (1943). (11) J. J. Jortner and G. Stein, J. Phye. C h m . , 6 6 , 1258, 1264 (1962).
Volume 70,Number 6 May 1966
R. WOODS
1448
value of l / A l o from which the initial quantum yield, A , can be calculated. All initial quantum yields were determined in this manner. The ratio of the rates of formation of iron(II1) and arsenic(II1) were found to be independent of time in all cases. The initial quantum yields of arsenic(III), A’, were determined from the initial iron(II1) quantum yields and this ratio. The effect of arsenic acid concentration, a t constant iron(I1) and sulfuric acid, on the initial quantum yields was determined (Table I).
Table I: Effect of Arsenio Acid Concentration (1.25 x 10-1 M FeSO4; 1 M HzSO~) [As(V)I, M
0
0.02 0.05 0.10
0.20 0.30 0.40 1.00
-Quantum Iron(III), A
0.223 0.288 0.370 0.490 0.72 0.86 0.98 1.29
yields-Arsenic(III), A’
0.0
0.039 0.090 0.175 0.30 0.36 0.45 0.64
The effect of iron(I1) concentration on the ratio of iron(II1) to arsenic(II1) produced was determined (Table 11). I n this case the initial quantum yields were not derived as the purpose of this investigation was to ascertain whether the effect of arsenic acid tabulated above was due to competition between iron(I1) and arsenic(V).
Table 111: Effect of Sulfuric Acid Concentration (1.25 X 10-1 M FeSOd) [A@C I, M
[H&o,], M
$/Fe(III) I@ d[As(III)]/dt
0.02
0.3 1 .o 3.0 0.3 1.0 3.0
6.6 6.7 6.6
1 .oo
A
A’
1.24 1.29 1.35
0.62 0.64 0.67
of iron(II1) to arsenic(II1) produced and on the initial quantum yields was investigated (Table 111). There is no significant change in quantum yield or in the ratio of iron(II1) to arsenic(II1) formed with sulfuric acid concentration in the range 0.3 to 3.0 M . The effect of oxygen on the quantum yield was determined (Table IV). The solution was saturated with oxygen a t 1 atm, the concentration of oxygen in the sulfuric acid solution being calculated from the data given by Seidell.12 The effect of acidity on the rate constant ratio h a / klz was investigated (Table V). The molar extinction coefficient of ferrous sulfate in the presence of arsenic acid was determined a t 2537 A (Table VI). The effect of arsenic acid on the iron(I1) spectrum in 1 M sulfuric acid is shown in Figure 2. 1.21
Table 11: Ef’fect of Iron(I1) Concentration (1 M HZSO~) [As(V)l, M
[Fez+], M
d_-____ [Fe(III)]/dt d[As(III)]/dt
0.02
1 . 2 5 X lo-’ 1.25 X 1.25 X 1.25 X lo-’ 1.25 X 1.25 X lo-’
6.7 6.3 6.3 4.0 3.9 3.9
0.05
-
Changing the iron(I1) concentration 100-fold does not affect the ratio of iron(II1) to arsenic(II1) produced, and therefore the increase in quantum yield with increase in arsenic acid concentration is not due to competition between iron(I1) and arsenicw). The effect of sulfuric acid concentration on the rate The J
O U T ~of
Physical C h m k t r y
220
260 Wavelength
300
mp
Figure 2. Ultraviolet spectrum of 4 X lo-* M iron(I1) in 1 M HzSO4 in the presence of (1) 0, (2) 0.5, and (3) 1.0 M arsenic(V). (12) A. Seidell, “Solubilities,” D. Van Nostrand Co., Ino., New York, N. Y., 1955, p 1355.
As(1V)
AS
INTERMEDIATE IN OXIDATION OF FeS04 IN ARSENIC ACID
1449
Table IV: Effect of Oxygen Concentration (1 M H2SO4; Oxygen-Saturated Solutions 1.0 X lo-* M [As(V)l, M
[Fen+], M
1.25 x 1.25 x 3.75 x 7.5 x 1.25 x 1.25 x
0.2
0.05 kll/kll
--ANi
-A'----? N¶
01
02)
Oa
lo-*
AOa/ANa
0
2.0 1.8
10-2 10-2
10-2 10-l 10-1
0.64 0.72 0.72 0.37
1.07 1.07 1.02 0.60
0.27 0.30 0.30 0.090
kla/kita
40 44 38 40
0.13 0.19 0.23 0.068
is the ratio of the rate constants for the reactions of arsenic(1V) with oxygen and iron(I1) (see Discussion).
Table V: Effect of Sulfuric Acid Concentration on kla/klp (0.2 M As(V)) -A Nl
IF@+], M
[HaSO*I, M
% Oa
3.75
x
10-2
7.5
x
10-2
3.0 1.0 0.3
0 . 7 x 10-8 1 . 0 x 10-3 1 . 2 x 10-5
Table VI: Effect of Arsenic Acid Concentration on the Molar Extinction Coefficient of Iron(I1) at 2537 A in 1 M Sulfuric Acid
-A'------. Oa
Nt
Oa
kdkir
0.73
1.03
0.31
0.21
0.56
1.07
0.26
0.15
26 40 46
H
+
0 2
+HO2
(8)
the subsequent reactions being
+ HO2 + H + +Fe3+ + H202 Fe2+ + H202+Fe3+ + OH- + OH. Fe2+ + OH. +Fe3+ + OHFe2+
t,
M-1 om-1
0 0.25 0.50 0.75 1.00
15.0 15.6 16.1 16.7 16.9
(9)
(10) (1 1)
The ratio of the rate constants for reactions 8 and 6, k&, was found to be lo3. I n the presence of sufficient oxygen for reaction 8 to be very much faster than reaction 6, therefore, the initial quantum yield of iron(II1) formation is twice that in the absence of Discussion oxygen. The photooxidation of ferrous sulfate has been The ultraviolet irradiation, a t 2537 A, of ferrous extensively investigated by a number of ~ o r k e r s . ~ ~ J ~ -sulfate ~6 in the presence of arsenic acid yields arsenicJortner and Stein" give evidence to show that, at high (111). Arsenic acid is not photochemically reduced acidity in the absence of oxygen, the photolysis is in the absence of iron(I1) under the experimental accounted for by the mechanism conditions, and therefore the primary photolytic process involves iron(I1). Fe2+ (as) -% Fe2+* (1) The difference between the quantum yields in oxygen-free and oxygen-saturated solution can be inFez+* (as) + [Fe3+OH-HI solvent cage (2) terpreted by the intermediate formation of arsenic[Fe3+OH-HI ----f Fe2+ (3) (IV) in the over-all mechanism, as follows, the reactions leading to arsenic(1V) formation being repre[Fe3+OH-H] +FeOH2+ H (4) sented by the stoichiometric relationship [Fe3+OH-HI H + + [Fe3+0H-H+] H ( 5 ) Fe2+ As(V) h', Fe(II1) As(1V) (12)
+
+
+
I n the presence of oxygen, the intermediate gen atom can react to form hydroperoxo radical
+
+
(13) E. Hayon and J. Weiss, J. Chem. Soc., 3866 (1960). (14) M. Lefort and P. Dousou, J. Chim. Phys., 5 3 , 536 (1956). (15) L. J. Heidt, M. G. Mullin, W. B. Martin, Jr., and A. M. J. Beatty, J. Phys. Chem., 66, 336 (1962).
Volume 70,Number 6
May 1966
R. WOODS
1450
Previous investigation^^^^^^ have shown that arsenic(IV) will oxidize iron(I1) Fez+
-+ As(1V) +Fe(II1) + As(II1)
(13)
and, in the absence of oxygen, this will be the only significant reaction involving arsenic(1V). Arsenic(IV) can also reduce ir0n(II1),~-’ but this reaction will be insignificant in these experiments owing to the excess of iron(I1) over iron(III), present in the reaction mixture. Arsenic(1V) also reacts with dissolved oxygen to give hydroperoxo radical (or As(IV)02) which has similar properties to hydroperoxo radical) .637
As(1V)
+ 02 +As(V) + HOz
(14)
If the oxygen to iron(I1) concentration ratio is sufficient for reaction 14 to be very much faster than reaction 13, arsenic(1V) will be quantitatively converted to hydroperoxo radical which will oxidize three ferrous ions. In this case the initial quantum yield of iron(II1) will be twice that in the absence of oxygen, reactions 12 14 9 10 11, compared with 12 13, respectively, in accordance with the experimental results (Table IV). This result also shows that oxygen does not effect the primary photolytic process. It should be noted that oxidation of arsenic(II1) by hydroxyl-free r a d i ~ a l ’ * ~ *formed ~ - ’ by reaction 10 will not compete with reaction 11 as therefs a large excess of iron(I1) over arsenic(II1) in the reaction mixture. At lower oxygen to iron(I1) concentration ratios, there will be competition between iron(I1) and oxygen for arsenic(1V). The initial quantum yield of arsenic(111) is a measure of the arsenic(1V) reacting with iron(11), as arsenic(1V) reacting with oxygen is oxidized to arsenic(V), while the difference between the initial yields of arsenic(II1) in the presence and absence of oxygen is a measure of the arsenic(1V) reacting with oxygen.
+ + + +
+
As(1V) reacting with 0 2 - k14[Ozl - A’N, - A’o, As(1V) reacting with Fez+ k13[FeZ+] -4’0, Therefore
The ratio k14/k13 is found to be independent of the ratio [Fez+]/[Oz] (Table IV) in accordance with the proposed mechanism, the ratio in 1 M sulfuric acid being 40. The ratio kll/kI) is independent of arsenic acid concentration, and therefore the postulated complex of iron(I1) with arsenic acid (uide infra) reacts with arsenic(1V) a t approximately the same rate as the uncomplexed ferrous ion. The rate constant ratio is dependent on sulfuric acid concentration (Table The Journal of Physical Chemistry
V), suggesting that some complexing of iron(I1) with sulfate occurs a t the higher acidity, the reactivity of the complex with arsenic(1V) being greater than for the uncomplexed species. The mechanism of reaction 12, resulting in arsenic(IV) formation, could be accounted for by postulating that either of the intermediates H or FeH2+,formed by the photooxidation of ferrous sulfate (reactions 4 and 6) reduces arsenic(V) toarsenic (IV). However, these reactions anticipate competition either between arsenic(V) and iron(I1) for H or between arsenic(V) and hydrogen ion for FeH2+,respectively, contrary to the experimental results (Tables I1 and 111). Furthermore, either of these reactions giving arsenic(1V) followed by reaction 13 does not explain the experimental increase in initial quantum yield of iron(II1) when arsenic(V) is present. A chain mechanism would be necessary to account for this increase, but such a chain reaction is not apparent from previous investigations of reactions involving arsenic(1V).4,6,7 It could also be postulated that the hydrogen atom formed in the solvent cage before dissociation into the bulk solution is scavenged by arsenic acid [Fe2+OH-HI
+ As(V) +FeOH2+ + As(1V)
(15)
This reaction explains, qualitatively, the increase in initial quantum yield of iron(II1) and the absence of competition between arsenic(V) and iron(I1) or acidity. However, this reaction would be competition with secondary recombination. The kinetics of such competition have been investigated by Noyes,16 who derived a relationship between the increase in quantum yield and the square root of the scavenger concentration. This relationship was confirmed for scavenging of this same intermediate by hydrogen ion.’’ However, the increase in quantum yield found here is approximately linearly related to the arsenic acid concentration at low arsenic acid concentrations (Figure 3) and not to its square root. The photolysis can be explained by the formation of a photoactive complex of iron(I1) with arsenic acid, the photochemical process represented by reaction 12 being
+ As(V)
Fe2+As(V)
(16)
Fe2+As(V) h’, Fe2+As(V)*
(17)
FeZ+As(V)*+Fe2+As(V) FeZ+As(V)* +Fe3+ As(1V)
(18)
Fez+
+
(19)
(16) R. M. Noyes, J . Am. Chem. SOC.,77, 2042 (1955); 78, 5846 (1956).
As(1V)
AS
INTERMEDIATE IN OXIDATION OF FeSOl
IN
ARSENICACID
1451
Ai
A =
1
+ zel 2[A~(v) ~ 1
+
A ~ % M V 1) €1
1
+ ?K [ A ~ ( v1) el
(1)
Therefore the quantum yield is independent of total iron(I1) concentration and acidity. ~t lbw arsenic(V) Concentration eq I simplifies to
A = A1
+ AZ?K[As(V)] 6
anticipating the linear relationship between A and arsenic acid concentration shown in Figure 3. At the higher arsenic acid concentrations the denominator term in eq I becomes significant, and the relationship departs from linearity. Rearranging eq I gives
0.2
04
0.4
0.3
0.5
Figure 3. Plot of initial quantum yield in oxygen-free solution of (1) iron(II1) and (2) arsenic(II1) against arsenic(V) concentration.
A plot of ( A - Al)/[As(V)] against A is shown in Figure 4. A linear plot is obtained in accordance with this relationship. From the slope and intercept the values AB= 1.8and (ez/el) K = 2.14 are obtained. The same treatment can be made for the initial quantum yield of arsenic(III), A’; in this case AI’ = 0.
I n this case, the light absorbed by Fe2+, considering initial values and therefore the absence of iron(III), will be given by
1
€1
In
In
+ %[As(V)]
Rearranging eq I11 gives where el and t2 are the molar extinction coefficients of Fez+ and Fe2+As(V),respectively, and K is the stability constant of Fe2+As(V)
4-07
Also the light absorbed by Fe2+As(V) is T
IFe2 +As(V)
=
10
el
[Fez+] [Fez+As(V)]
-
IO K[As(V)] €1
1
+ 2K[As(V)] €1
The initial rate of formation of iron(III), Alo, will be given by
AI0 = AdFel+
A2IFez+As(V)
where A1 and A2 are the initial quantum yields of iron(II1) of Fez+and Fe2+As(V), respectively. Thus
-
Figure 4. Plot of ( A Al)/[As(V)] (ordinate) against A, curve 1, and A’/[As(V)] (ordinate) against A’, curve 2.
Volume 70,Number 6 May 1986
R. WOODS
1452
A plot of A ’/ [As(V) ] against A’ is also shown in Figure 4, the slope and intercept giving the values A’a = 0.9 and (E~/EI)K = 2.23. The ratio A z / Al z is 2, as the arsenic(1V) formed oxidizes a second iron(I1); the values of (ez/el)K are in good agreement. The extinction coefficient ratio e Z / e l can be obtained from the experimental molar extinction coefficient, e, a t 2537 A.
+ e~[Fez+As(V)]
e[Fe(II)] = al[Fe2+]
where [Fe(II)] is the total iron(I1) concentration in the solution. Therefore
+ d[As(V)l 1 + K[As(V) 1
€1 -
(V)
The variation of the extinction coefficient with arsenic acid concentration is too small to estimate K from this equation. However, substituting the value of (e*/ el)K = 2.2 determined above in eq V gives
K = {?(1 e
+ 2.2[As(V)]) - l}/[As(V)]
(VI)
Values of K derived from eq VI are 1.96, 1.92, 1.84, and 1.84 at arsenic acid concentrations of 0.25, 0.50,
The Journal of Phy&
Chemistry
0.75, and 1.00 M , respectively, giving an average value of K = 1.9. The postulation of a photoactive complex between iron(I1) and arsenic acid interprets, both qualitatively and quantitatively, the photochemical oxidation of ferrous sulfate in the presence of arsenic acid. The quantum yields are little affected by acidity in the range 0.3 to 3.0 M sulfuric acid (Table 111),suggesting that a complex of undissociated arsenic acid, rather than an arsenate ion, is involved. Complexes of undissociated phosphoric acid with thorium (Th4+H3P04)17and with cobalt pentaammine (Co(NH&H3P02+)l8 have been characterized. The initial quantum yields for the iron(I1) arsenic acid complex were determined to be 1.8 and 0.9 for iron(II1) and arsenic(III), respectively. The fraction of excited ions that dissociate, k l e / k l s , is therefore 0.9; this fraction is much greater than for the aquoferrous ion.
Acknowledgment. The author is indebted to Professor A. s. Buchanan for advice during the course of this work and in the preparation of the manuscript. (17) E. L. Zebroski, H. W. Alter, and F. K. Heumann, J. Am. Chem. soc., 7 3 , 5646 (1951). (18)W.Schmidt and H. Taube, Znorg. Chem., 2, 698 (1963).
