MECHANISM OF DECOMPOSITION OF INORGANIC NITRATES
Nov., 1962
Reactivity is highest when the halogens are not on a. terminal carbon atom. The rate of both chlorine and hydrogen removal is faster from 2,2dichlorobutane than from the 1,l-isomer, %milarly both rates are faster for 2,3-dibromobutane than for the 1,4-isomer. Halogenated Esters.-The increase of reactivity with halogenation of methyl aliphatic esters (Table IV) again is in the order iodides > bromides > chlorides. Significantly, the rates are higher than for the aliphatic halides; obviously, the carboxyl group andl the halide are augmenting one another in weakening both the C-X and C-H bonds.
2249
The activation energies ( E ) for the abstraction
of a halogen from alkyl haliden by a hydrogen atom generally are quoted as 3-6 kcal., with E decreasing: chloride -+ bromide -+iodide.? Except perhaps in the case of alkyl chlorides, these values are too high to correspond to the rate data
presented in the present paper. The rates of reaction with iodides are among the highest ever found for non-ionic reactions in the liquid phase. Further detailed comment on this will be made in a subsequent paper. (7) E. W. R. Steacie, “Atomic and Free Radical Reactions,” 2nd Edition, Reinhold Publ. Corp., New York,N. Y., 1954, p. 733.
MECHANISM OF THE DECOMPOSITION OF INORGANIC NITRATES’ BY TUNG-HO CHENAND EVERETT R. JOHNSON Chemistry Department, Stevens Institute of Technology, Hoboken, New Jersey Receieed June I 8. 1968
The radia.tion induced decomposition of several inorganic nitrates has been studied at room temperature. It has been found that tbe experimental results are compatible with a simple kinetic scheme, oz’z., MN03 ---f MNOz 0 ; 0 MNOa -+ O2 MNOa; 0 MNOz 3 MNOa, lor all the nitrates studied except silver nitrate.
+
+
+
The ino~ganicnitrates appear to decompose when exposed to1 ionizing radiation to yield nitrite ion and
viz. MK03 --+ MKOz
+ ‘/zOz
The decomposition has been shown to be complex with a dependence on lattice parameters as yet not clarified. Initial G-values (molecules decomposed per 100 e.v. absorbed) have been shown to be independent of intensity8 but dependent on the linear energy transfer (l.e.t.)9 and temperature.1° Studies of the oxygen isotope effectlo and heats of solution measurements6 have indicated the importance of lattice changes which occur during the irradiation of some of these salts. Kinetic studies have revealed that the decomposition, a t least in the case of NaN03, follows a fairly simple mechanism! It is the intention of this paper to investigate the application of this mechanism t o the decomposition of other nitrates.
Experimental The radiation source was a fixed cylinder of cobalt-60 of approximately 1000 c. The source was located a t the base of an 8-ft. shaft (“hole in the ground” type of installation) and shielded by a toroid filled with sand. Access to the source was obtained through five aluminum tubes welded together. Samples were placed in aluminum holders and I
(1) Research supported by AEC contract number AT(30-1-)-1824. (2) (a) A. 0. Allen and J. H. Ghormley, J. Chem. Phys., 16, 208 (1947); (b) 0. Hennig, R. Lees, and M. S. Matheson, ibtd., 21, 664 (1953). (3) J. Cunningham and H. G. Heal, Trans. Faraday SOC.,54, 1355 (1958). (4) C. J. Hoohanadel and T. W. Davis, J . Chem. Phys., 27, 333 (1957). (5) E. R. Johnson, J . A n . Chem. Soc., 80, 4460 (1958). (6) E. R. Johnson and J. Forten, Dzscussiuns Faraday Soc., 31, 238 (1961). (7) A. G. Maddook and S. R. Mohanty, ibid , 31, 193 (1961). (8) E. R. Johnson, J . Phys. Chem., 66, 755 (1962). (9) C. J. Ho.hanade1, Radzataon Res., 16,286 (1962). (IO) 3. Cunningham, J. Phus. Chsm., 66, 638 (1961).
+
then lowered into the radiation area. Positioning in the source was completely reproducible. The dose rate varied slightly from the base of the source to the top. Dosimetry was determined using the Fricke dosimeter. Each position was carefully calibrated and dose rate was determined with an over-all precision of better than &2%. (In previous communications6Jl a mobile co-60 source was used and the experimental results using this source were less reproducible. ) A G-value of 15.45 molecules of ferrous ion oxidized per 100 e.v. absorbed was used in all calculations. Absorbed doses were calculated using the true mass absorption coefficients reported in the l i t e r a t ~ r e . ~The value of 0.025 was used for the mass absorption coefficient of 0.8 N I32S04 solution. Nitrite ion was determined by the method of Shinn.12 The molar extinction coefficient at 546 mp was 53,200 1. mole-1 cm.-l. I n determining nitrite yield, concentrations of NOZ- were used such that when the sample was treated with the reagents and read in the spectrometer, optical density readings of about 0.2 to 0.4 were obtained. All chemicals except &NO3 were C.P. and used without further purification. The samples were ground to fine powders. CsNOa was recrystallized twice from triply distilled water. Gas analysis was determined by the method described previously.5 The results for oxygen analysis were obtained with over-all precision of better than f5%,
Results and Discussion Figures 1 through 10 show the nitrite yield vs. dose for the various nitrates. Oxygen analyses are appropriately indicated. In all cases (except AgNOa) the solid line represents a theoretical curve, which was obtained by applying the following kinetic scheme to the decomposition MNO3 --+- MXOz 0 ki@ (1)
+
0
+ NOz-
KO3 le2 (2) 0 f NOa- --f KO,0 2 k3 (3) The kinetic expression for the appearance of NO2therefore is given by eq. 4. --3
+
(11) J. Forten and E. R. Johnson, J . Phys. Chem. Solids, 15, 218 (1960). (12) M. B. Shinn, I n d . Eng. Chem., Anal. Ed., 1 3 , 3 3 (1941).
TUNG-IXo CHENAND EVERETT R. JOHNSON
2250
Vol. 66
00
36 32
26
s
22
0
p
$
20
8 4
0 0
4
8
20
6
I2
28
24
DOSE e
Y
19
32 I
36
40
44
48
52
c I'
and oxygen yields in KNOI.
Fig. 1.-Xitrite
004
0 02
I-
C/ /
0 0
I 002
l
l
004
1
l
006
l
DOSE I N e v i 9 x
Fig. 4.-Initial
,0
(n
.
6 o
l
ooa
l
l
010
l
l
012
io-z'
nitrite yield in &NO3.
A220, 0 NS;
[
5 0
Ol
4 0
3 0 LL 0
w Ln
2 0
10
0
0
004
002
006
DOSE
Fig. 2.-Initial 0
ev/g
008 K
012
010
4
8
12 16 20 24 28 32 36 40 4 4 48 52 56 60 64 68 DOSE e v / g
10."
Fig. 5.-Nitrite
nitrite yield in KNO,
A = 20, 14
0
014
x 10'~'
and oxygen yields in NaN03.
i?
.&
c
cn
0 12
3
8 a 8 E
010
E
008 006
i
2
6 0
2
4
6
8
10
12
DOSE
Fig. &-Nitrite
14 e Y
/9
I8 2 0 22
16 X
24 26 28 30
004 032
n 0
0 0 4 008 012 016 020 024 028 032 0 % 040 044 048 C52
IO-''
DOSE e v / Q x
and oxygen yields in CsNOa.
Fig. 6-Initial
nitrite yield in NaNOt.
+
d(N02-) _ _ _-- k1@(X03-) - kz(KOz-)(O) dt /iE(XOS-) (0)
(4)
Substituting for the oxygen atom concentration (using a steady-stnte approximation), one obtains eq. 5 .
Integrating under the assumption that in the decomposition range studied the NOS- concentration is essentially constant, one obtains eq. 6
Nov., 1962
2251
~WECEANIBNOF DECOEI2POBITION OF INORGANIC NITRATES
\ z"
k
04
3
0 0
a a
0 3
LL
0 v) W
g
0 2
W v
0
3
01
0 2
4
8
6
IO
12
14
16
IB
40
30
DOSE
20
70
60
50
e v / g x IO-''
and oxygen yields in AgN03.
Fig. S.-Nitrite
DOSE e . " / g x
Fig. 7.-Nitrite
20
10
0 0
yield in. Ba(NO&.
DOSE e v / q x
Fig. %-Nitrite
yield in Pb(NO&.
(6) or
ax2
+ x = bT
(7) and
where x = molecules of SO,-/g. X T = dose in e.v./g. X The constanls a,b may be evaluated from the appropria,te nitrite yield vs. dose curves. Equation 5 may be integrated without assumption of thle S O 3 - concentration remaining constant. I n this case eq. 5 is rearranged to give
2klk3@ dt
I O
0
:
,
004
008
I
~
012
016
DOSE e v / g x 1 0
Fig. lO.-Initial
l
ozo
~
1
~
OZ4
-'
nitrite yield in AgN03.
KNOi.-Figure 1 shows the nitrite yield us. dose for the decomposition of KNOi for the dose range 2 X 1O2I to 50 X loz1 e.v./g. The solid curve (theoretical) was calculated from the following expression, which was obtained using eq. 9
(8)
is the concentration of KO3- in where molecules/g. X 10-lg a t zero dose, which upon integration yields
&L KOz-) 2
-2.30310g
2.303 log
[1
-
(XOs-)o_I
=
2k@t (9)
[1
-
~
1
=
0.001812'
(10)
(N~s-)o The dosimetry for these particular results had a precision of =!=0.5%. The initial G-value for this
~
I
TUNG-HO CHENAND EVERETT R. Jownrso~
2252
decomposition is not constant, but varies as can be seen by reference to Fig. 2. Cunninghamlo used a first-order plot to summarize his results on the radiolysis of KNOB and obtained two straight lines intersecting a t a dose of about 80 X lozoe.v./g. This apparent break in the nitrite yield curve also was observed by Forten and J0hnson.l' Although no such break is apparent in Fig. 1, it is possible, because of the small variation in G-values (small curvature), t o redraw Fig. 1as two or three intersecting straight lines. This would be especially so if the over-all experimental error were of the order of about =k5% (this certainly was true of the results in ref. 11 and 6, except the results on NaN03). The fact that Cunningham obtained first-order plots for his results up to a dose of about 20 X loz1 e.v./g. and that a linear plot has been obtained by others may be explained in part as follows. For the low dose region (up to about 6 X loz1 e.v./g.), eq. 10 can be modified using the assumption that In (1 - x) for x < 1 g -x with a maximum error of 10% to give z = bT
(11) This gives a linear relationship between yield and dose. A first-order plot in the low dose region also will give a linear plot if one applies the condition In (1 G ) = G when G
