4013
NOTES
A Dechlorination Reaction i n the Radiolysis of Aqueous Nonochloroacetic Acid Solutions i n the Presence of Nitrous Oxide by Akira Yokohata, Takuichi Ohmura, and Satoru Tsuda Department of Chemistry Faculty of Engineering, Hiroshima University. Senda-machi, Hiroshima, Japan (Received February 80, 1969)
It is well known1 that the formation of inorganic chloride on irradiation of a deaerated solution of monochloroacetic acid is a result of the reaction of the reducing species with the solute. I n general, the reaction of OH radicals is believed to result in dehydrogenation rather than dechlorination. However, during the course of our recent investigation on the effect of electron scavengers such as NzO, the authors found that the reaction of dechlorination caused by OH radicals should be considered and the degree of its contribution can be expressed as ' / b G(0H). The present work deals with this interesting reaction.
Experimental Section Irradiation was carried out using a 26043 6oCosource. The dose rate (1.59 X 10'' eV g-I min-') was determined by the Fricke dosimeter, taking G(FeS+) .= 15.5. The water used was triply distilled. The pH of the sample solution was adjusted with NaOH. All chemicals used were of reagent grade. NzO (99.0%) was purified by several distillations on a vacuum line, prior to introduction into the irradiation vessel containing the degassed solution. The amount of NzO in the solutions was determined by measurement of the
10-3
IO-'
10-2
[CICH2C00-],
M.
Figure 1. Relation of G(N2) and G(C1-) t o [CICHzCOO-]. [NaOIo = 8 X 10-*M, pH 6.5; A, G(Cl-); 0, G(N2).
pressure drop after the solution stands for about 24 hr a t room temperature. Irradiations were performed in sealed ampoules. Microquantities of chloride were determined by the turbidimetric methoda2 The turbidity was measured spectrophotometrically at 350 mp. The gaseous products Hz and Nz were measured with the mass spectrometer.
Results and Discussion Figure 1 shows the relations of G(Nz)and G(Cl-) to [C1CH2C00-1, where [CICHzCOO-] was changed in M to 5 X lo-' M under the the range from 1 X conditionof NZO = 8.0 X 1 0 - 3 M a n d p H = 6.5. The reactions to be considered are
+ CICHzCOO- +C1- + .CHzCOO- (1) (2) ea,- + NzO +NZ+ 0 H + CICHzCOO- +Hz + ClfiHCOO(3) C1- + H + + -CHzCOO- (4) HzO* + ClCHzC00C1- + *CHzCOO (5) eaq-
----t
The occurrence of reaction 5 is suggested by Sworski3to be important only at concentrations of ClCH2COOhigher than 0.1 M . A value of G(Nz) = 2.8 obtained at [CICHzCOO-] = loRaM shows clearly that most eaq- i s scavenged by NzO. On the other hand, the rate of H abstraction by H atoms is known to be faster by a factor of 11than the rate of de~hlorination.~Therefore, G(Cl-) should be expected to be > keas- + C H ~ O H .
(6) T. Balkas, F. S. Dainton, J. K. Dishman, and D. Smithies, Trans. Faraday SOC.,62,81 (1966). 3.2 3.0 2.8
3d Transition Metal Complexes in Molten 1.2
3
Potassium Thiocyanate Solution
1.0
> Q
0.8
by H. C. Egghart
0.6 0.4
Energy Conversion Research Division, U.S . Army MobWu Equipmen Research and Development Center, Fort Belvoir, Virginia 86060 (Received November 18,196’88)
0.2 10-4
10-2
10-~
[CH,OH].
Figure 2. Effect of additive CHaOH on G(Nz), G(Ha), and G(Cl-) in t h e system of 1 X M ClCH&OO-2.3 X M NSO. 0, G(Cl-); A, G(Hz); 0, G(N2).
I
Figure 3.
OH OH
0.5
1
[CH,OHJ
/[CiCH,COO-].
1.5
2.0
A plot of l/G(Cl-) against [CHaOH]/[ClCHaCOO-].
+ ClCHzC00- +H2O + ClCHCOO-
+ ClCHzCOO+C1OH
+R
(R: radical) (7)
+ CHBOH+H2O + ’CH20H
1 G(C1-) G(0H)
(’ -I-
k13
-I-
(6)
(8)
k7[C1CHzC00-]
Figure 3 shows a plot of l/G(Cl-) against [CH30H]/ [C1CH2C00-]. A straight line supports the validity of the simple competition mentioned above. Since 0produced via reaction 2 is considered to show the same behavior as OH radicals, G(0H) of eq 9 should be taken to be 5.0 (GeaQ-= 2.8, GOH = 2.29. Then, Figure 3 leads to k6lk.l N 4.0 and lc8/k7 N 12. I n other words, it may be concluded that the degree of the contribution The Journal of Physical Chemdstry
Since 1956 adsorption spectroscopy has been used as a technique to study transition metal complexes in molten salts such as molten alkali halides and to a lesser extent low-melting alkali nitrate Only one investigation on transition metal species in molten potassium thiocyanate solution was carried out.6 I n that study results of preliminary character on Cr(III), Co(II), and Ni(I1) were obtained. Very low molar absorbances were reported which do not agree well in every case with the tentatively proposed structures of the complexes. The low molar absorbance ( E 15) reported for Cr(III), its spectrum indicating octahedral coordination, was not too surprising since d-d transitions in a system having a center of symmetry are Laporte-forbidden and gain some intensity only by distortion from a regular octahedral symmetry or by a vibronic mechanism. A tetrahedral field has no center of symmetry and d, p, and ligand orbitals can become mixed together to some extent. I n this way forbidden transitions become allowed to a certain degree.’ Therefore, tetrahedral complexes show much higher molar absorbances than octahedral complexes. I n view of this, particularly the molar absorbance of 86 reported for the maximum of the spectrum of Co(I1) solutions in molten potassium thiocyanate seemed too small for a tetrahedral species. The present investigation was carried out (i) to resolve these questions concerning the molar absorbances, (1) D. M. Gruen, Nature, 178,1181 (1956). (2) D. M. Gruen, J . Inorg. Nucl. Chem., 4,74 (1957). (3) D.M. Gruen, ”Fused Salts,” B. R. Sundheim, Ed., McGraw-Hill Book Co., Inc., New York, N. Y., 1964,Chapter 5,p 301. (4) G. P.Smith, ‘%MoltenSalt Chemistry,” M. Blander, Ed., Interscience Publishers, New York, N. Y., 1964,p 427. (5) D. M. Gruen, Quart. Rev., 19,349 (1965). (6) G. Harrington and B. R. Sundheim, Ann. N . Y . Acad. Sci., 79, 950 (1960). (7) C. J. Ballhausen and A. D. Liehr, J . Mol. Spectroac., 2, 342 (1958).
