H. BERNHARD POQGE AND F. T,JONEB
1700
The Effects of Temperature and Additives in the Radiolysis of Potassium Nitrate' by H. Bernhard Pogge and F. T. Jones Department of Chemistry and Chemical Engineering, Stesens Institute of Technology, Hoboken, New Jersey 07030 (Received December d 9 1969)
The y radiolysis of KNO3 was studied as a function of radiation dose and temperature, and with added NO2 -, C104-, and (N1803)incorporated in the KNO, lattice. 02 and NOz- are formed as products by at least two mechanisms, one of which involves 0 atoms. Both the KNOa.KNOz and KNO3.KC104 systems indicate an 0-atom yield of GO = 0.45. 02 and XOZ- are also produced by processes not involving 0 atoms at yields Goz = 0.30 and G N O = ~ 1.15. The isotopic distribution of O2 formed in KNle03.KN180a crystals can be interpreted on the basis of GO = 0.50 and Goz = 0.25. After correcting for thermal decomposition, G(O2) and G(NOz-) in KNO3 are independent of temperature between 24' and 128O, but more than double at 128' and continue to increase with temperature up to the melting point.
Introduction The mechanism by which molecules decompose under radiation is usually derived not from studies with pure compounds, but from mixtures containing additives capable of reacting with the intermediates. This technique has been used extensively in the radiation and photochemistry of gases, liquids, and rigid glasses, but has rarely been used for crystalline solids. KN03 is an example of a solid which has been studied extensively in the pure state, but the mechanism by which the ultimate products are formed is still not ~ ~ ~NO2- and understood. It has been p o ~ t u l a t e dthat O2are formed by reactions 1-111 at room temperature
+
Nos--+ NOz0 0 NOz- +NOS-
+
0
(1) (11)
+ NOa- +NOz- + 0 2
(111)
Esr4 and X-ray diffraction6 techniques have shown that small amounts of N02- can be incorporated into KN03 without significantly changing the lattice parameters of the host crystal. It should also be possible to incorporate other molecules into KNOS if the cell dimensions of both crystals are almost the same. If additional NOz- is incorporated into the KiC'O3 lattice, reaction I1 should be favored over (111) md the O2 yield should decrease. Alternatively, in the presence of Clod-, the NOz- yield should decrease because of the competition between (111) and (Iv), a d the 0 2 yield should remain unchanged
0
+ c104-
--j
c103-
+
0 2
(IV)
O2 and NOz- may also be formed by reactions not involving the oxygen atom. The present work is an attempt to determine the mechanism for O2 and formation in KNO3 by incorporating KSOz, KC104, The Journal of Physical Chemistry
arid '*O-labeled KN03 into the KN03 crystalline lattice to act as 0-atom scavengers.
Experimental Section Materials. Baker Reagent Grade KNO3, KC104, and KNOz were recrystallized three times from warm solution of KN03 and KC104 from triply distilled water, and KNO2 from anhydrous methanol, since KN02 forms a stable hydrate with water. The crystals were then dried, crushed to an average size of less than 0.7 mm, and stored over P206 in desiccators. To check for the effect of impurities in the radiolysis, a portion of the K N 0 3 was further purified by zone refining using the technique of Sue, et al.,e passing the zone over the sample twenty times at a pass rate of 0.6 cm/hr. KN03 with high lSO enrichment was obtained from the Weizm m n Institute, Israel (84% '80 enrichment) and Isomet Corp. (70% l 8 0 ) . The crystals with 84% enrichment were zone refined, but the 70% '*O enriched crystals were used without further purification, Doped crystals (KN03.KNO2, KNOS.KC104, and KN160sKN1*03) were prepared by mixing appropriate amounts of the respective salts in the melt. The mixtures were then quenched to -195' and crushed. Chemical analysis of small random samples always agreed with the stoichiometric composition of the original mixture. +
(1) Submitted in partial fulfillment of the degree Doctor of Philosophy in Chemistry a t Stevens Institute of Technology, September 1967. (2) A. 0. Allen and J. Ghormley, J . Chem. Phys., 15, 208 (1947). (3) E. R. Johnson and J. Forten, Discuss. Faraday Sac., 31, 238 (1961). (4) H . Zeldes, "Paramagnetic Resonance," Vol. 11, Academic Press, Ino,, New Yo&, N. y , , lg63, 764. (5) J. Forten and E. R. Johnson, J . Phw. Chem. Solids, 15, 218 (1960). (6) p. Sue, J. Pauly, and A. Nouaille, Compt. Rend., 244, 1212 (1957).
THEEFFECTS OF TEMPERATURE AND ADDITIVES IN
THE
Apparatus. Two cobalt-60 y sources, of nominal strengths 1000 Ci and 300 Ci, were used in this work. An electrical furnace for irradiations at elevated temperatures could be accommodated in the 300-Ci source. The furnace consisted of a solid aluminum cylinder with a small cavity for the irradiation cells and was wrapped with Nichrome heating wire. Temperature mas monitored remotely with thermocouples placed at various positions in the furnace. Most of the irradiations were carried out in 4-cm3 Pyrex break-seal vials or, when gas analyses were not required, 2-cm3 screw-cap vials. Energy absorption was determined by the Friclie dosimeter, using the mass absorption coefficients of Hochanadel and Davis.? Analyses. The crystalline samples were degassed before irradiation by pumping at 1 X lo-? Torr for 10 hr a t 110" before sealing the vials. After irradiation the seal was broken under vacuum, deaerated water was distilled into the sample container, and volatile products were removed on refluxing the solution with mild heat. The gases were analyzed by combustion with hydrogen or by mass spectrometry. The residual solution was analyzed for nitrite ion by the colorimetric method of Shinn.8 Similar results were obtained using the method of Rider and i l l e l l ~ n . ~The extinction coefficient was determined in this laboratory to be 48,800 M -l cm-l at 538 nm. C103- and other oxidizing ions formed from KC101 were determined by the oxidation of ferrous ion in 0.8 N HzS04. KOa-, C104-, and C1- do not interfere, but Not- undergoes a slow reaction with Fe2+to form Fe3+and Fe(N0)2+,making it necessary to correct the results for NOz- formed in the radiolysis. Clog- was determined by dissolving the irradiated samples of KN03.KC104in M Fe2+and following the optical density of Fe3+ as a function of time at 305 rim; tlhe contribution due to NOn- was subtracted by extrapolating to zero time. The reliability of this method of analysis was verified with solutions containing known amounts of C b - , NOz-, and NO3-.
Results Irradiation of KNOX at 24". Yields of nitrite ion and oxygen gas were measured as a function of radiation dose up to a total dose of 6 X loz1 eV/gm. N02formation is linear with dose up to ca. 1 X loz1eV/g, but a gradual decrease in the rate occurs at higher doses. Figure 1 shows that the yield-dose relationship is a smooth curve and cannot realistically be broken into linear segments, as has been done by several previous authors. The initial yield corresponds to G(NOz-) = 1.60 i 0.02, in good agreement with 1.57 obtained by Hochanadel and Davis' and 1.65 by Forten and Johnson5 a t similar doses but somewhat lower than 1.70 recently reported by Cunningham.'O Yields were independent of radiation intensity, at least over the range 0.18-
1701
RADIOLYSIS OB KNOa
20
~
0
15
30
45
Dose (eV/gm)x lo
60
EO
-*'
Figure 1. Dose dependence in radiolysis of KNOs at 24'. Dose rate = 1.8 X lo1' eV g-1 min-l.
1.8 X lo1' eV/gm min. Crystals purified by zone refining gave the same results as those purified by recrystallization from aqueous solution. Over the entire dose range the IV0z':Oz product ratio was found to be 2.2 i 0.1 instead of 2.0 expected from stoichiometry if these are the' only products. The same NOz-,:O2 ratio was obtained when OZ was removed from the irradiated sample by briefly heating to the melting point instead of the usual HzO dissolution technique. A deviation from stoichiometry of similar magnitude has also been reported by Chen and Johnson." These authors attributed the discrepancy to a loss of O2 by diffusion out of the crystal, but the results described in the next two sections suggest a reaction with surface impurities to form CO or COZ. Thermal and Radiation Decomposition of KNO3 at Elevated Temperatures. In the absence of radiation KN03 is stable below 150" but significant thermal decomposition was observed at higher temperatures. At 340", the highest temperature used in this work, NOzis produced in the thermal reaction at an initial rate pmol/(g of K N 0 3min), corresponding to of 1.9 X a first-order rate constant of k = 2 X min-'. Between 195 and 340" the data conform to an Arrhenius equation, with an activation energy of 8 kcal/mol. 02 was not observed as a product under these conditions, (7) C. Hochanadel and T. Davis, J. Chem. Phgs., 27, 333 (1957). (8) M. Shim, Ind. Eng. Chem., Anal. Ed., 13, 33 (1941). (9) B. FaRider and M. G. Mellon, ibid., 18, 96 (1946). (10) J. Cunningham, J. Phys. Chem., 6 5 , 628 (1961). (11) T.H. Chen and E. R. Johnson, ibid., 66,2249 (1962).
Volume 74, Number 8 April 16, 1070
1702
H. BERNHARD POGGE AND F. T. JONES
but mass spectrometric analysis of the gas showed C 0 2 present in molar amounts equal to that expected for 02, i.e., NO2-:COI 2. Similar results were obtained for both zone refined and recrystallized KNO,, and using either Pyrex or Vycor vessels. Neither baking nor etching the cells had any effect on the thermal reaction. In blank runs with an empty Pyrex cell any gas released by heating was below the limits of detection. Bartholomew12 found L = 5 X lo6 exp. (- 31,00O/RT) for the extensive thermal decomposition of molten KNOI between 550 and 750', which corresponds 8 X min-' when extrapolated to 340". to IC Mole % X N O . The smaller activation energy observed in the present Figure 3. Dependence of G(O2) on KNOz concentration in work may be ascribed to either the greater mobility KNOa.KN02 mixtures. (Broken line: KO2- formed in the crystalline phase than in the liquid phase, or to a by radiolysis). reaction with impurities on the surface or leached from the glass. But the fact that E, = 8 kcal/mol for the present work is less than AH for the decomposition of ' 0 , 4 KN03, and that a platinum vessel was found to accelerate the reaction, argues in favor of a surface reaction.
-
-
-
3.0
I
1 2.0
I
IRHOMBIC
-
0
TRIGONAL
20
I
1
60
BO
IO0
Mo/e % KCLO,,
I 1
40
I
I
Figure 4. Dependence of product yields on KClO, concentration in KNOa 'KC104 mixtures.
Both Not- and 0 2 are produced in the radiolysis at elevated temperatures. If no correction is made for the thermal decomposition, the results are essentially in agreement with those of Cunningham,lo showing a gradual increase in nitrite yield with temperature and reaching a value of 5.9 NO2- ions/100 eV at 325" (compared with Cunningham's value of 5.8 at 330'). But after correcting for thermal decomposition, G(NOz-) is practically independent of temperature up to 128", the trigonal transition temperature at which a rhombic occurs in KiC'Oa,18 and then suddenly increases to G(NOZ-) = 3.4 a t 134" as shown in Figure 2. The corrected yield at 325" is G(NO2-) = 4.6. Over the entire temperature range G(NOz-) ,,,/G(02) = 2.2. Irradiation of laO-Labeled KNO3 at 24'. KNOa with 84% lSO enrichment was irradiated at various doses from 7 X lozoeV/g up to 4 X 1OZ1eV/g. G(NOz-) was consistently 13% lower than the yields from unlabeled samples, at least over this dose range.
-
The Journal of Physical Chemistry
In various mixtures of KNI608 and KW803, G(NO2-) also showed an isotope effect although less than that for KW803 alone. The sum of the 02 yields, G(16r1602) G(16z1802) G(0218*18),determined mass spectrometrically showed the same isotope effect as G(NOs-). In addition to peaks at m / e 32, 34, and 36, trace amounts of m / e 28 and, for samples containing KN1803, m / e 30 mere also observed in the mass spectra. These peaks could be due to CO, or C'*O, formed in a thermal reaction during degassing, as described in the previous section. Peaks at m / e 46, 48, and 50 (corresponding to the various isotopic modifications of NOa) were conspicuously absent, even when the gas had been removed from the irradiated crystals by melting instead of the usual H2O dissolution technique. Radiolysis of K N O , Containing Additives. a. KNOa. K N 0 2 . Oxygen yields were measured at 24" in
+
+
(12) R. Bartholomew, J. Phys. Chem., 70, 3442 (1966). (13) A. G. Maddook and S. R. Mohanty, Discuss. Faraday Soc., 31, 193 (1961).
THEEFFECTS OF TEMPERATURE AND ADDITIVES IN
THE
the radiolysis of KK03 containing various amounts of added KN02. G(02) is independent of dose up to a t least 12 X 1020 eV/g. Figure 3 shows that the yield of O2 depends on the concentration of added NOZ-, decreasing nonlinearly with [KNOz]. Oxygen is not produced in the radiolysis of pure KN02. b. KN03.KCZ04. 02,NO2-, and C103- or other oxidizing species of chlorine are produced in the radiolysis of K N 0 3 containing added KC104. Prince and Johnson14 have shown that c103- is the major product in the radiolysis of KC104, with (3102- and C10- being formed in only minor amounts. Therefore no attempt was made in the present work to evaluate the individual contributions of these other species and the total oxidizing yield is expressed as equivalents of “ClOs-.” Figure 4 shows that NO2- decreases and 0 2 and “c103-” increase with increasing KC104 concentration. The values G(02) = 2.9 and G(“C103-”) = 3.5 for 100% KC104 can be compared with those of Prince and Johnson, where G(Oz) = 2.7 and C(“C103-”) = 3.2 can be calculated.
Discussion Radiolysis of K N 0 3 . KNOz Mixtures. The amount of energy deposited by ionizing radiation in a pure compound depends primarily on the electron density of the material. If the energy absorbed in a mixture of two compounds depends only on the total averaged electron density, the measured yield of a product due to energy absorbed in each component separately can be expressed by eq 1 G(P) = G(P)A(e.f.)A
+ G(P)B(e.f.)B
‘.O
-
t
o,21
Figure 5. Dependence of 0%yields on [NOz-]/[NOs-l. Test of eq 3.
t\ NO;
1.2
8 f ,s
P
8
I
1.0
-
0.8
-
o
*
*
-
~
.
[CLOG] /[NO;]
Figure 6. Dependence of NOz- and 0 2 yields on [CIOI-]/[NO~-]. Test of eq 2.
(A), (B) are the corresponding concentrations in grams of A and B, respectively, gives eq 2
Thus, G(P)Aand G(P), can be determined from the slope and intercept when the left-hand side of eq 2 is plotted as a function of (B)/(A). The O2 yields in the radiolysis of I
