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GIDEONCZAPSKIAND GABRIELSTEIN
Vol. 63
some covalent bonding between silver and oxy- direction cosines 4, y5 and w , corresponding to the gen,lQo20in agreement with the cream color of the cell edges a, b and c, which were assumed to be orcomplex. The usual valuesz1for the ionic and co- thogonal, are given in Table IV for each principal valent sums of radii are 2.66 and 2.19 A,, respec- axis, along with the isotropic temperature factor tively. The shortest silver-carbon bonds and the equivalent B = 8?r2ji2,and the mean square dissilver-oxygen bonds lie roughly in a plane. The placement, The two sets of atoms C1C20103' COT molecule is tub-shaped with D z d symmetry, in and C1C2C5Csare each approximately coplanar, agreement with previous X-ray22,23and electron with the silver atom lying in the plane of the first diffraction studies,24 with an average C=C dis- set and somewhat below the plane of the second. tance of 1.37 A. and c-C distance of 1.46 A. The The direction of greatest vibrational motion, R, is variations among the double bond distances and roughly 20" from the normal to the plane defined the single bond distances (0.09.A.at most) are well by C1C20103' and approximately parallel to the within three times the C-C standard deviation and other plane. This motion bends all bonds to the are probably not to be considered as significant. silver atom except the relatively long bonds from Ag The COT bond angles have a standard deviation of to the COT in the adjacent cell. The motion with approximately 1 5 " . smallest amplitude has a direction, P, roughly 20" The anisotropic thermal parameters were ana- from normal to the plane of C1CzC5Ceand approxilyzed by methods described elsewherez5to deter- mates a stretching of the bonds to the silver atom. mine the directions of the three principal axes P, Q Two of these three directions of motion, shown in and R of the assumed ellipsoid of vibration. The Fig. 4,are approximately in the plane of this pro(19) J. Donohue and L. Helmholz, ibid., 66, 295 (1944). jection, while the third direction is approximately (20) J. Donohue and W . Shand, ibid., 69, 222 (1947). perpendicular to this plane. (21) L. Pauling, "Nature of the Chemical Bond," Cornel1 University Press, Ithaca, N. Y.,Second Edition, 1948, pp. 164. 179, 346. Acknowledgment.-We wish to thank Professor (22) H. S. Kaufman, H. Mark and I. Fankuchen, Nature, 161, 165 S. W. Fenton for his suggestion that we undertake (1948). this study, the Office of Naval Research and the (23) J. Bregman, private communication. (24) 0. Bastiansen, L. Hedberg and K. Hedberg, J . Chem. Phys., 27, National Institutes of Health for financial support, 1311 (1957). and the Minneapolis Honeywell Company for a (25) M. G. Rossmann and W. N. Lipscomb, Tetrahedron, 4, 275 fellowship to F.S.M. (1958).
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THE OXIDATION OF FERROUS IONS I N AQUEOUS SOLUTION BY ATOMIC HYDROGEN BY GIDEONCZAPSKIAND GABRIELSTEIN Department of Physical Chenhtry, Hebrew University, Jerusalem, Israel ,
Received February 6 , 1060
Atomic hydrogen-produced in an electrodeless high frequency discharge-oxidizes ferrous ions in aqueous solution. This reaction and the reverse process of reduction of ferric ions were investigated in the presence of H2SOl in the pH range of 0,43.0, and the equilibrium ratios established. From the dependence of the rate of oxidation on the concentration of Fee+ the limiting values of the rate constant of the reaction between Fez+ and H atoms in the presence of H + ions are calculated. The possible mechanisms are discussed.
By virtue of the high energy of formation (-104 account for these results, the formation of Hzfaq kcal./mole) of the H-H bond, hydrogen atoms are ions from H atoms and H f ions was postulated, capable of acting as dehydrogenating, and thus as the species responsible for the oxidation of oxidizing, agents. They dehydrogenate many or- Fez+.'S6 It appeared desirable to investigate these ganic substances, where the C-H bond energy is processes under conditions where H atoms as such only of the order of 80-90 kcal./mole. In aqueous are formed first and then introduced into the syssolutions containing ferrous ions, and irradiated tem, We produced H atoms in an electrodeless with ionizing radiations experimental results were high frequency discharge and observed the oxidaobtained1 which were interpreted as indicating that tion of Fez+ions by the H atoms which were introH atoms are formed in these systems and t,hat these duced into the aqueous solution.6 At the same time are capable in acid solution of oxidizing ferrous to Davis, Gordon and Hart' presented similar, more ferric ions. These results were c o d ~ r m e d . ~detailed, ~~ results. The present paper contains our Similar results also were obtained when such solu- detailed results. tions were irradiated with ultraviolet light.4 To Experimental (1) T. Rigg, G. Stein and J. Weiss, Proc. R o y . Soe. (London), A211, 375 (1962). (2) N. F. Barr and C. G. King, J. A m . Chem. Soc., 7 6 , 5565 (1954). (3) A. 0. Allen and W. G. Rothschild, Radiation Res., 7, 591 (1957); 8, 101 (1958). (4) T. Rigg and J. Weiss, J. Chem. Phys., 20, 1194, (1952); CJ also. J . Chem. Soc., 4198 (1952).
The Production of Atomic Hydrogen.-We have selected the method of electrodeless high frequency discharge. The (5) J. Weiss, Nature, 166,728 (1950). (6) G. Czapski and G. Stein, ibzd., 182, 598 (1958). (7) T. W. Davis, S. Gordon and E. J. Hart, J. A m . Chem. SOL, 80, 4487 (1958).
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OXIDATIONOF FERROUS ION IN AQUEOUS SOLUTION BY ATOMIC HYDROGEN
elimination of inner electrodes enables one to avoid the difficulties arising from atomic recombination and hence bad reproducibility, caused by the metal evaporated from the electrodes. We could thus also dispense with the coating of the walls with, e.g., phosphoric acid, and the clearing of the entire system after every run.718 Our reproduclbillty was acceptable compared with the results of others.?** The frequency chosen for the discharge was 27-30 Mc. We found as did Jennings and Linnettg that operatmg at lower frequencies good yields of H atoms were obtained. Tho total possible power input was 2000 watt d.c., with an R.F. power of 1400 watt max. in the R.F. discharge. We usually utilized only 600-700 watts R.F. The discharge tube was made of quartz. It was cleaned with CrOrHzSO,, followed by HNOl and then 10% H F and finally several rinses in triple distilled water. Tens of experiments could then be run without dismantling and cleaning, reproducibility being a t best &lo% and usually 2k200/,. Our results are given with the appropriate standard deviations. The hydrogen used was Matheson electfolytic prepurified and was passed through a furnace containing palladized asbestos at 400” and followed by a trap cooled with liquid aw. The gas was stored in a large reservoir. This maintained a constant pressure of gas which was supplied to the discharge tube through a reducing valve. The gas was not recirculated but pumped through a t a velocity of 50 l./min. a t 20-30 mm. The experiments reported here were carried out at this pressure of pure Ht. Other experiments included the use of Ha or Hz-He mixtures at total pressures of up to 80 mm., up to which the discharge could be efficiently maintained. The discharge tube was cooled with an air blower and the HF coil was a 7 mm. 0.d. copper tube through which cooling water was circulated. The temperature of the tube during discharge exceeded the softening point of Pyrex, but not of the quartz which was used. In view of the high temperature metaphos horic acid coating was not used, as it reacted and gave and phosphorus under these conditions. Between the reaction vessel and the discharge tube there was a 90” bend in the vacuum line and light shields prevented ultraviolet light from the discharge reaching the reaction vessel. The reaction vessel was fitted with a detachable side arm, in which the solution was placed. The system including the solution was fully evacuated, the solution (25 ml.) tipped into the reaction vessel, Ha passed for another 15minutes and only then was the discharge switched on. Evaporation during this procedure was allowed for. Blank experiments with Hs passing: but without discharge were run with Fez+ and Fea+solutions. The temperature in the reaction vessel was maintained at about 5” throughout. Control experiments were run with AgNOa solutions to determine the amount of H atoms available. Materials.-The ferrous ammonium sulfate and sulfuric acid used were “Analar,” the ferric sulfate C.P. Triple distilled water was prepared by redistilling glass distilled water from alkaline permanganate, then from phosphoric acid. Analysis.-Using a Hilger Uvispek Spectrophotometer and 1 cm. cells ferric ion was determined as the sulfate at 305 m p using E 2180 at 25” in 0.8 N H2S04. The extinction coefficient depends on pH and total sulfate concentration and care was taken of this when reactions were carried out at other pH values.1° All AD305 values reported in this paper refer to solutions in which [HzS04]was brought to 0.8 N . Ferrous ion was determined as the 1,lO-phenanthroline complex a t 510 mp using acetate buffer to maintain pH 3.6, and adding NaF, at least 150 times more than the ferric ion concentration, to eliminate the ferric phenanthroline complex.
f”,
Results Dependence of A[Fea+] on AH.-The dependence of the quantity of ferric ions produced on the quantity of hydrogen passing through the solution was investigated using a discharge in Hz a t 27 mm. (8) F. E. Littman, E. M. Carr and A. P. Brady, Radiation Res.,I , 107 (1957). (9) K.R.Jennings and J. W. Linnett, Nature, 182,597 (1958). (10) Y. Gilat and G. Stein, Proc. 2nd Int. Conf. Peaceful Uses At. Energy, Geneva, 1958.
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0.06 0.08 0.10 0.15 CO FeBO4. Fig. 1.-Dependence of the oxidation yield on initial Fez+ concentration. [H2S04] = 0.8 N . 0 and 0 represent results obtained in two separate series of experiments. The curve is the theoretical one, calculated using the data of Fig. 2.
0
0.02
0.04
and 0.01 M Fez+in 0.8 N H2SOa. The pumping rate was measured by the time elapsed and also by the pressure decrease in the reservoir. The results shown in Table I represent a separate run for every experimental point. TABLE I DEPENDENCE OF A[Fes+] O N AZT
- A p , cm.
t , aec.
130 330 585 815
ADm
6 15
0.237 .370 .625 .960
26.5 38
Thus the yield is a linear function of time and thus of quantity of gas passing the solution a t the same pressure. It indicates the degree of constancy of conditions in a series of consecutive runs. It also shows that in investigating the initial yield a t this pH the back reaction is not of importance. The Oxidation Yield as a Function of Initial Fez+Concentration.-In 0.8 N HzS04solution the amount of Fe2+ oxidized was measured as a function of the initial Fez+ concentration, keeping Hz pressure, Aow rate and discharge conditions constant. The yield increases with increasing Fez+ concentration, reaching a maximum a t higher Fez+ concentrations as shown in Fig. 1. Hydrogen atoms are generated in the discharge and I mole 1.-l sec.-l enter the solution. Without deciding what the actual mechanism or the species participating is, H atoms can be considered to disappear by reaction with Fez+, recombination or reaction with FeS+
+ + H + -+ Fe3+ + Hz kl +HP kz + -+ Fez+ + H + kl
Fez+ H 2H Fe3+ H
(1)
(2)
(3)
I n our system a t the relatively low H atom concentrations and under continuous flow, steady state conditions regarding H will prevail, compared with changes in Fez+, so that, assuming homogeneous kinetics d[Hl/dt = 0 = I
- kl[Hl
[Fez+]- kz[H12 - ka[H1[Fea+l (1)
where IC1 is the velocity constant of reaction l.,a t a constant H+. Thus [HI =
+ k3[Fe3+l - (kl[Fe+al2kz
+
+ k3[FeS+])a+ 4k2Z
d(kl[Fe2+]
2kz
(11)
GIDEONCZAPSICI AND GABEIEL STEIN
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value we calculated the theoretical curve according to equation V. As seen in Fig. 1, the experimental results fit the curve well. This relationship between k1 and kz will be true whatever the actual mechanism. However the p H dependence of kl and kz and their exact meaning will depend on the species participating in the reaction steps. Thg Dependence of the Oxidation Yield on pH.The dependence of the oxidation yield on the pH was investigated in 0.01 M Fez+ solutions, containing varying amounts of added H2S04. Thus ionic strength and sulfate ion concentration also varied a t the same time. The yield was determined a t constant discharge and flow conditions, for a fixed time of reactions and thus for a constant amount of H atoms passed. The total amount oxidized in all cases was considerably less than the final equilibrium value of [Fea+]a t that pH. Table I1 shows the results.
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3 4 5678910 20 30 40 50 Log 10yco - 1.13X). Fig. 2.-Log Co as ti function of Log (Co - PX).
TABLE I1 DEPENDENCE OF A[Fe3+]O N NH~BO,
The rate of formation of Fe3+is given by d[Fea+l/dt = -d[Fea+]/dt = kl[Fe2+l[HI
0.8
- k3[Fe3+][H] (111)
+
Denoting k3/kl by y, and defining fl = 1 y (where y and 0 are p H dependent, as is the ratio of the rate constants) we get, putting [Fe2+]o = Coand [Fe3+] = X dX/dt = kl[H](Cp - X )
- ksX[H] = ki[H]. (Co
- PX)
(IV)
so that
(V)
At high initial [Fe2+], where 4kzI/k12
