NOTES
2716
Table I :
Hydrogen Peroxide and Hydroperoxide Yields as a Function of Radiation Dose Rate and Acetaldehyde Concentration in Oxygen-Saturated Solutions Irradiated at p H 1.20 and a t pH -7 Acetaldehyde
,---------doc0
concn.,
PH
1.20
-7
2
x
7
x
2 5 7
x x x
y-ray8
G(Hz0z)
M
10-3 10-2 10-9 10-1
1.60 f 0 . 2 2.30 4.50
10-3 10-3 10-2 10-2 10-1
1.90 zt 0 . 2 2.15 2.53 3.40
G(R0zH)
...
1.50 2.15 i 0 . 4 4.50 6.50 ...
, . .
, . .
Table 11: pH Dependence of the Peroxide and Formaldehyde Yields in Oxygen-Saturated M Acetaldehyde Solutions Irradiated with 2.5-Mev. Electrons G(R0zH)
G(Ha0z)
1.20 2.80 4.50 5.80 6.40 7.00 8.80 9.80 10.20
1.40rtO.l 1.45 1.40 1.28 1.25 1.24 1.25 1.07
0 0 0 0 0 0 0
0 . '39
1 12
CHsCHO OzCHzCH(0H)z
U(HCH0)
f 0 2
10 60 65 86 85
2HCHO
0 0 0 0
80
. . ,
1.50
...
0.86 f 0 . 2 ... 1.30
The hydrogen peroxide yields shown in Table I1 are too sniall t o be accounted for on the basis of reactions such as (1) and (2). It is apparent, therefore, that reactions can occur between the various peroxy radicals which do not yield Hz02. These mechanisiiis have not been elucidated.
Kinetics of Ion Exchange in a Chelating Resin
80 82
+ HZOz +
0 2
School of Chemistry, Rzitgers, T h e State University, S e w Brzinswick, ,\'eu Jersey (Received March 5 , 1964)
(5) (6)
(7)
0 2
+ 0%
1 . 2 4 i0 . 1
, . .
by Albert Varon and William Rieman, I11
+ eaq- +CHBCHO-
CH3CHO-
, . .
73
Acetaldehyde (CH3CHO) will coinpete with H 3 0+ and/or O2 for the radiation-produced electrons in this system The fact that the H 2 0 zyields obtained for 2.5Mev. electron irradiations vary only slightly with increase in acetaldehyde concentration indicates that the peroxy radicals resulting from CHsCHO
, . .
O75fOI
+ HZO 1-CH,CH(OH)2
+ HOz +
2.5-hlev. electrons G(Hz0z) G(R0zH)
1.44 f 0 . 1 1.40
, . .
PH
-
CH3CHOH
(8)
In a previous report' from this laboratory, it was concluded that the actual chemical reaction was the rate-controlling step iii ion-exchange processes involving a chelating resin (Dowex A-1) and any cation capable of forming a chelate with the iniinodiacetate groups of the resin. On the other hand, it was recently reported2 that diffusion within the resin is the slow step in the exchange of calcium, strontium, and magnesium with the hydrogen form of the same resin. The conclusion of Turse and Riemanl had been drawn from a study of the kinetics of exchange reactions by the limited-bath method, in which supposedly equivalent amounts of the resin and exchanging solution had been taken. However, recent work in this laboratory3 revealed an error
react in a niannner stoichiometrically equivalent to 0 2
The Journal of Physical Chemistry
(1) R. Turse and W.Rieman, J . P h y s . Chem., 6 5 , 1821 (1961). (2) C. Heitner-Wirguin and G. Markovits, ibid., 67, 2263 (1963). (3) A. Varon, Thesis, Rutgers, The State University, New Brunswick, N.J., 1963.
NOTES
2717
of 14y0 in the previous determinations of resin capacity, probably because of hydrolysis of the sodium-form resin on excessive washing with water. This error was largely eliminated in the recent work by washing the sodium-form resin thoroughly with 0.01 M sodium hydroxide and finally with one bed volunie of water. The error in capacity casts doubt on the previous conclusion because the calculations were based on the assumption that equivalent quantities of resin and electrolyte solution had been taken. Therefore, further experiments were performed on the kinetics of Dowex A-1. The shallow-bed4 method was used to eliminate the need for accurate determinations of capacity. The sodium form of the resin was wet-screened. The average particle sizes of the 30-35 and the 45-50 mesh fractions were determined microscopically. A fraction with much smaller particle siee was obtained by grinding the resin and sieving it between 100 and 200 mesh. A rough measure of the equivalent mean radius of these irregular particles was calculated as the mean of the two mesh openings. The radii given in Table I are corrected for the shrinking5 of the resin on conversion to the indicated form.
F is Qt/Qm; Qm is the extent of the exchange a t infinite time; Q t is the extent a t time t; n represents the integers from 1 to a ; B is Dr2/r2; D is the apparent diffusion coefficient inside the resin; and r is the radius (cm.) of the particles. I n experiments involving only isotope exchange, where B and r remain constant during any one experiment, plots of Bt vs. t are linear.4 Linear plots (Fig. 1) were also obtained in this labora-
1.4
1.2
1.0
0.8
c4" 0.6
0.4
Table I Expt. no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a
Molarity of s o h .
r X 108 B X 108
D X 108
MgRz C a + 2 0.1065 MgRz i- Cat2 0,1065 MgRz C a + Z 0.0716 2NaR C u f 2 0.0880 2XaR C U + ~ 0.0880 2HR C u f 2 0.0880 2HR C U + ~ 0.0880 2HR C U + ~ 0.0880 0.1924 MgRz 2H+ 0.1924 MgRz 2H+ 0.0978 MgRz 2H+ 0.0978 MgRz 2H+ 0.0858 MgRz 2" 0.0858 MgRz 2H+ CsRz Mg+' 0.107 CaRz Mg+Z 0.107
21 0.60 4.5" 8.7 4.5" 5.8 6.3 26 5.6" 60 20 0.69 1.1 13 4.3' 5.6 21 5.7 14 14 21 3.4 4.5" 58 14 6.1 4.5" 46 21 0.015 4.5" 0.50
2.7 1. 1.2b 43 19b 2.8 1.9
Reaction
+
+ + + + + + + + + + + + + +
1.ob
25 28 15 12b 12 9.46 0.067 0 . 10b
These were ground resins with irregularly shaped particles.
* Subject t o a large error because of the uncertainty of r.
If diffusion within the resin is the slow step in the shallow-bed method, the rate should follow the equation4 F
=
6 - 1 1- - exp(-n2Bt) ir2 n2
(1)
0.2
0
0
100
200
300
400
t , sec
Figure 1. The data of experiments 9, 10, 11, and 13 of Table I are represented, respectively, by filled circles, filled squares, open circles, and open squares.
tory for the reaction between magnesium-form resin and hydrochloric acid in spite of the fact that the reaction causes a 15% shrinkage in the volume of the resin.6 All the exchanges listed in Table I gave nearly linear plots of Bt us. t and distinctly curved plots of -log (1 - F ) us. t. 'The latter graphs should be linear if the chemical reaction is the slow step. These plots furnish evidence against the previous conclusion that the chemical reaction is rate-controlling. Experiments 6, 7, and 8 were done under identical conditions except for the particle size. If the chemical reaction were the rate-controlling step, the veloci(4) G. E. Boyd, A. W. Adamson, and L. S. Myers, J . A m . Chem. SOC.,69, 2836 (1947). (6) "Dowex Chelating Resin A-1," The Dow Chemical Co., 1959.
Volume 68, Number 0
September, 1961,
2718
ties, as indicated by the parameter B, should be the same. The marked variation in B is further evidence against the previous conclusion. Similar comparisons may be made with experiments 1 and 2, 4 and 5 , 9 and 10, 11 and 12, 13 and 14, and 15 and 16. If diffusion inside the resin is the only rate-controlling step, experiments 6, 7, and 8 should give identical values of D. This is not the case. The value of D of experiment 8 is not reliable because of the uncertainty of r . However, experiments 6 and 7 give D values varying by more than the experinieiital error. On the other hand, experiments 9 and 10 give D values in satisfactory agreement with each other. A comparison of experiments 2 and 3 indicates that a decrease in concentration causes a decrease in the parameter B and hence in the apparent diffusion coefficient. Comparisons of experiment 9 with 11 and experiment 10 with 13 show the same effect. This dependence of rate on concentration is to be expected if the chemical reaction is the rate-controlling step, or it may be due to invasion of the resin by the external electrolyte. Schlog16 found a similar dependence of the diffusion coefficient on concentration in membranes made from formaldehyde and phenolsulfonic acid. I n summary, diffusion within the resin is the major rate-controlling step although concentration effects influence the over-all rate.
TOTES
pure iridium wire. Film thicknesses were measured at the completion of an experiment by X-ray emission spectroscopy. Standard ultrahigh vacuum techniques3 were employed such that the pressure in the vacuum system was about lO-'O torr before evaporation and never rose above torr during evaporation. After the film was deposited, the reaction vessel was isolated from the vacuum system and a known amount of hydrocarbon gas admitted. The reaction was followed as a function of time with a calibrated thermistor gauge and a small bakeable mass spectrometer. Hydrocarbon gases used were Phillips Research Grade with the following purities: ethane, 99.96%, and ethylene, 99.92%.
Results and Discussion A summary of the initial conditions for these experiments is given in Table I. The composition of the gas phase as a function of time is given graphically in Fig. 1 for CzH, I r (27'), in Fig. 2 for C2H6 Ir (lOO"), and in Fig. 3 for CZH, I r (100').
+
+
+
Table I : Summary of Initial Conditions Initial amount of gas,
Expt.
(6) R. Schlogl, 2. Elektrochpm., 57, 195 (1953).
molecules
no.
Gas
1 2 3
CZH5 C2H5 C2H4 CZH4
4
x
10-18
1.54 1 40 1 63 1 61
Film temp.,
Film thickness,
OC.
A.
27 100 27 100
306 186 182 228
Interaction of Ethane and Ethylene with Clean Iridium Surfaces by Richard W. Roberts General Electric Research Laboratory, Schenectady, N e w York (Receieed M a r c h 19, 1964)
The adsorption and decomposition of several simple hydrocarbons on clean iridium surfaces has recently been investigated.' However, because of the experimental procedures used, several questions concerning the reactions were unresolved. Additional information on these reactions has been obtained and w d l be presented here.
Experimental The experimental procedure used in these experiments has been presented in detail elsewhere.'I2 In brief, clean iridium films were de'posited on the inside surface of a Pyrex glass sphere by evaporating 99.98% T h e Journal of Physical Chemistry
Ethane. It was previously observed that both Hz and CH4 appeared as reaction products for CzH6 I r (27"), but only CH, was found for CzHs I r (100"). These product analyses were made after a contact time of about 1000 and 30 min., respectively. In order to better follow the production of gaseous products we repeated these experiments and measured the composition of the gas during the course of the reaction. There mas a rapid initial adsorption of ethane on a 27" iridium surface followed by a slow decrease of ethane in the gas phase (expt. 1, Fig. 1). Hydrogen was produced during the early part of the reaction
+
+
(1) R . W. Roberts, J . P h y s . Chem., 67, 2035 (1963). (2) R. W. Roberts, T r a n s . Faraday Soc., 58, 1159 (1962). (3) R . W. Roberts and T. A. Vanderslice, "Ultrahigh Vacuum and Its Application," Prentice-Hall, Inc., Englewood Cliffs, N. J., 1963. 14) -, W. D. Davis and T. A. Vanderslice. "Transactions of the Seventh National Vacuum Symposium, 1960," Pergamon Press, New York, N. Y., 1961, p. 417. ~
