E. HAYONAND J. J. MCGARVEY
1472
composition range is good. It must be borne in mind that the success of this treatment in fitting the data is not offered as proof that the assumed compounds exist in the liquid state. Whether they persist in the liquid state is a matter that can be settled only by
independent physical measurements which bear directly on compound properties.
Acknowledgment. We are grateful to Drs. Carl E. Crouthamel and Elton J. Cairns for helpful discussions.
Flash Photolysis in the Vacuum Ultraviolet Region of SO4%,CO,s, and OHIons in Aqueous Solutions
by E. Hayon and J. J. McGarvey Pioneering Research Division, U.S . Army Natick L d o ~ a t o r b sNatick, , Massachusetts (Received Odober 11, 1966) ~~~~~~
~~~
The flash photolysis of aerated aqueous solutions of sulfate, carbonate, and hydroxyl ions has been studied. The primary photolytic act in the three systems has been shown to lead to the detachment of an electron from the anions, M,,-% hv Maq-('-') e8, -f with the subsequent formation of the respective radical anions so4-, COa-, and OH. The hydrated electron formed then reacts with 0 2 to give 02-. The formation and decay kinetics of the transient species so4-, COa-, 0 2 - , and 0 8 - were studied. It is shown that for SO4-, Am,, is 4550 A, E * ~ ~ ( S Ois~450 - ) f 45, ,%(SO4- soh-) = 4.2 X lo* M-' sec-'; for C03-, Amax is 6000 A, k(C0102-) = 1.3 X lo*M-' sec-l; for Oa-, Amax is 4300 A, k(O3OI-) = 6 X 10' M-' sec-'; and for 0 2 - , Xmax is 2400 A, k(O202-) values range from 2.2 to 5.8 X lo7 M-' sec-l.
+
+
Introduction The observation and reaction kinetics of transient species produced in photochemical reactions using the flash photolysis technique has been studied for many years. However, relatively few investigators have carried out flash photolysis studies in the vacuum ultraviolet region. One of the main reasons for this has been the experimental difficulties encountered in working at wavelengths below 2000 A. With the recent availability of very pure silica, transparent down to about 1650 A, it has become possible to devise simple and sturdy experimental setups. A research program has been started in this laboratory to study the primary photolytic processes in gases and liquids resulting from the absorption of radiation in the vacuum ultraviolet region. A concentric arrangement, described Ths Joutnal of Phueical Chembtry
+
-+
+
+
+
below, has been used in which the cell containing the liquid or gas is surrounded by the flash tube. In this paper, experiments are described on the flash photolysis of aqueous solutions containing inorganic anions. Previous work on the flash photolysis below 2000 A of water and aqueous systems has been reported by Baxendale' and by Matheson, Mulac, and Rabani.2 The anions sod2-, cos2-, and OH- studied in the present work absorba below 2000 A, and their absorp
(1) J. H.Baxendale, Radiation Res., 17, 312 (1962). (2) M. 8.Matheson, W. A. Mulac, and J. Rabani, J . Phys. Chem., 67, 2613 (1963). (3) (a) L. E. Orgel, Quart. Rar. (London), 8, 422 (1964); (b) J. L. Weeks, G. M. A. Meaburn, and S. Gordon, Radiation Res., 19, 669
(1963).
FLASH PHOTOLYSIS OF SOcz-, Coal-,
AND
OH- IONS
1473
tion spectra have been characterized‘ as electrontransfer spectra. This charge transfer to solvent (ctts) mechanism in the photolysis of these anions can be represented as hr
AIm-“ --+
+ e-,.
d
a
(1)
Using known electron scavengers such as NzO, indirect chemical kinetic evidence was obtained in the photclysiv of S042-5.6and OH-,’.’ supporting reaction 1. hlatheson, et aL,z reported the observation of the absorption spectrum of the hydrated electron in the flash photolysis of OH- ions. No work has apparently been done on either the photochemistry or flash phctolysis of carbonate ions. Reported below are the optical absorption spectra of the transient primary species produced in the photolysis of Solz-, COaz-, and OH- ions and the reaction kinetics of these species in aqueous solutions. Experimental Section The water used was purified by distillation, radiolysis, and photolysis as described earlier.* All reagents used were the best analytical research grade available and were used without further purification. The spectra of the anions S O P , Coal-, and OH- all have their absorption maxima below 2000 A in the vacuum ultraviolet region, and their extinction coefficients have been determined.” The linear extinction coefficient of liquid water has also been obtained.”bg,’0 The experiments carried out in the presence of S O P , Coal-, and OH- ions were done using concentrations of anions such that essentially all the radiation was absorbed by the anions and none by the water. The reaction cell mas completely filled with the appropriate aerated solution through inlet h, Figure la. Flush Photolysis Setup. The concentric flash lamp reaction cell assembly is illustrated in Figure la. The outer tube was constructed from Pyrex, 27-mm i.d., wall thickness 1.5 mm, and the inner section which forms the reaction cell of Spectrosil (Thermal Syndicate Co.), 14-mm i.d., wall thickness 1.5 mm. The optically flat end windows of the cell were sealed on the Spectrosil tubing and were also of Spectrosil. The over-all length of the assembly is 73 cm with an effective optical path length of 63 cm, measured from the centers of the electrodes. The demountable electrode assembly, constructed from brass, with stainless steel electrodes, is shown in detail in Figure Ib. The flash discharge takes place in the annular space between the Pyrex tube (e) and the Spectrosil cell (d), vacuum-tight seals being made as indicated by brass couplings bearing on neoprine O-rings. The space is filled with argon a t 15 torr pressure and the flash discharge produced from
Figure 1. a: Schematic diagram of reaction cell-flash lamp assembly. b: Details of electrode assembly: (a) stainless steel electrodes, (b) brass couplings, (c) neoprene O-rings,(d) Spectrosil reaction cell, (e) Pyrex jacket, (f) optically flat Spectrosil end window, (9) connection to vacuum line for pumping annular flash space, (h) inlet joint to reaction cell, (k)electrode terminal.
a 100-pf capacitor bank normally charged to voltages in the range 3.7-7.5 kv corresponding to electrical energies between 700 and 2500 joules. For most of the work to be described here, flash energies between 1500 and 2000 joules were used. The characteristics of the flash produced from this lamp are: rise time, 5 psec; duration a t half-peak-height, 30 psec; total flash duration, 80 psec. Optical-Detection System. The continuous light source used for monitoring transient species was a high-pressure xenon arc (Osram XBO 450-w) with the required regulated power supply provided from a 6ov battery bank. Collimated light from the arc passed once through the reaction cell ont.0 the entrance slit of a monochromator (Bausch and Lomb, Catalog No. 33-864j-59;5Wmm model) with a 1200 grooves/mm plane grating, linear dispersion 16 A/mm. The emerging light beam was monitored by an EM1 9552B photomultiplier tube, and traces were recorded on Polaroid film (3000 speed, Type 47) using a Tektronix RbI 565 oscilloscope. Optical absorption spectra were obtained by the pointby-point method. No (4) E. Rabinowiteh. Rev. M o d . Phua., 14, 112 (1942): Solar Emw. 4. 20 (1960).
R. J. Marcus,
(5) J. Berrett, M. F. Fox. and A. L. Mansell. J . Phy.. C h . . 69. 2996 (1965). (6) F. S. Daintm nnd P. Fowles, Proc. Roy. Soe. (London). AZ87, 312 (1965). (7) J. Jortner. M. Ottalenghi. and G . Stein. J . Phyr. C h m . . 68. 247 (1664). ( 8 ) E. Hayon, Tiam. Foradmu Soc.. 60, 1059 (1964).
Barrett and A. Mansell. N d u r e . 187. 138 (1960). IO) W. Price. P. Harris, 0 . Heaven. and E. Johnson. ibid.. 188,46
(9) J.
1960).
E.HAYONAND J. J. MCGARVEY
1474
The assignment of the transient with Xmax 4500 A to the SO4- radical anion is strengthened by the observation18 of an identical spectrum in the flash photolysis at X >2100 A of aqueous solutions of persulfate ions. Here the primary photolytic act involves the rupture of an 0-0 bond
WAVELENGTH
-0,s-0-0-so,- h', 2504(4) Furthermore, in the course of this work, Heckel, Henglein, and Beck14 reported a spectrum with Xmax 4500 A which they assigned to the 804- ion formed in pulse radiolysis of aqueous solutions of bisulfate by the reaction OH HS04-+ SO4H2O. The decay kinetics of the 0 2 - and 804- radical anions were observed in neutral solution and the rate curves found to fit the second-order rate law
Figure 2. Optical absorption spectrum produced in flash photolysis of aerated 2 X IO-* M N ~ S O (a) I at pH 5.5 and (b) at pH 10.4. The OD was measured 40 psec after the start of the flmh. Open squares are derived from the complete SO&- spectruml* and the filled squares from the 02-spectrum;ll each spectrum was obtained in the absence of the other spectrum.
bog
difference was observed on the decay kinetics of the transients species formed with variation of flash energies between 700 and 2500 joules.
Results and Discussion Solutions Containing Sul'fate Ions. The flash photolM sulfate ysis of aerated aqueous solutions of 2 X ions a t pH 5.5 gave rise to the formation of two transient species. One species has a maximum a t about 240 n ~ pand the other at 450 mp. The optical absorption spectra of both species were recorded over the wavelength region 23Ck-545 mp and are shown in Figure 2. The two species decay by second-order kinetics. The transient absorbing in the far-ultraviolet region is identified as the 0 2 - radical. Its absorption spectra, lifetime, and decay are in agreement with the results of Czapski and Dorfman" and Adams, Boag, and Michael12 for this species, formed in their case in the pulse radiolysis of aerated water. The other transient has a maximum at 450 mp and a broad region of absorption at lower wavelengths; this is assigned to the soh- radical anion. Since under the experimental conditions used here essentially none of the flash radiation is absorbed by the water, the observation of the optical absorptions of 02- and S o h provides conclusive evidence for the primary photolytic process
sod2-
804-
+ cap-
(2)
followed by 0 2
+ eaq-
The Jour& of Phyeical Chemietry
40 2 -
+
+
i
(3)
=
(%)1+ [log (91 0--I
where I , and Io are the optical densities a t times equal t and zero, respectively, k = rate constant, c = molar extinction coefficient of transient, 1 = optical path, and t = time. In radical recombination reactions, k = 2k in all cases, and the rate constants have not been corrected for ionic strength effects. The decay kinetics correspond to the reactions 02-
+
02-
%o
+ O2+ 20H-
Hz02
(5)
and S04-
+ S04-
4products
(6)
The second-order plot for decay of SO4- in neutral solution is shown in Figure 3a. The values obtained for k6 and ICs are shown in Tables I and 11, respectively. Since the quantum yield is not known, the extinction coefficients for 0 2 - and Sodcannot be derived directly in this system. Values of e(Oz-) obtained by Czapski and Dorfman" have been used, and a em(S04-) value of 450 f 45 has been deduced on the basis that equal yields of 0 2 - to S04are formed and observed under the experimental flash photolysis conditions described above. This value compares favorably with a e4m(S04-) value of 460 25 obtainedl3by quite a different method in the flash photolysis of persulfate ion.
*
(11) G. (1964). (12) G.
Czapski and L. M. Dorfman, J . Phys. C h a . ,
68, 1169
E.Adams, J. W. Boag, and B. D. Michael, PTOC. Roy. SOC. (London), A287, 321 (1965). (13) L.Dogliotti and E. Hayon, submitted for publication. (14) E. Heckel, A. Henglein, and G. Beck, Ber. BUn8enge6. Phydk. Chem.,70, 149 (1966).
Table I: Rate Constants for the Bimolecular Decay of 01-in Aqueous Solutions
Systems
2 X 10-'MSO4'Hz0
10-1 M Cos'-
OH a
ks values from pulse radiolysis, M-1 sec-1 (ref)
ks,
PH
A, mlr
5.5 5.7
260 250
2.4 5.6
12.8 10.5
260 260
1 . 6 X l@ 6 . 5 x 104
k/
e
x x
104 104
M-1 sec-1
used"
900 1030
2 . 2 x 107 5 . 8 X 10'
900 900
1 . 4 X lo8 5 . 8 x 107
... 2.9 X 3.0 X 3.4 x 5.3 x 1.5 X
lo7 lo7 107 107 109
...
(15a) (15b) (11) (12) (12)
Values of e derived from ref 11.
Table 11: Rate Constants for the Bimolecular Decay of
soh-, Cos-,
and 0 8 - in Aerated Aqueous Solutions
PH
Species
mr
k/e
e used
k, M-1 aec-1
5.5 12.8
sodcos -
455 600
9 . 3 x 106 7 . 1 x 104
450" 1830
4 . 2 X lo8 1 . 3 X 108
os -
470
4.5
x
135OC
6.1
A,
System
2 x lo-* M SO,'10-1 M Cos2-
5
x
a
Based on $W(Oo-) 900.
lo-* M OH-
104
x
Lit. k values, M-1 8ec-1 (ref)
... 1 . 5 X loo (12) 1 . 3 x 1076(21) 1 x 108 (12) (in 10-1 M KOH)
107
* Determined in absence of oxygen. ' Bmed on c4m(Oa-) 2000.12 For comparison with k5 values in the literature, water alone was flash photolyzed at pH 5.7 in the presence of 0 2 in order to determine kg under our conditions. The photolysis of water gives rise to H and OH radicals, with the H atoms subsequently reacting with O2
H2O h',H H
t
2i I 0
4
*2
13
*i
-6
t
a7
ab
mi
40
(I
msec
Figure 3. Plots of [log (ZO/ZJ-1 against t to verify second-order decay kinetics for the transients OS-,sod-, COS-, and Os-: (a) aerated 3 X lo-* M 804'; pH 5.5, SO4- transient absorption a t 4300 A; (b) aerated water, pH 5.7, Oz-transient absorption a t 2500 A (ordinate displaced up by 5 units); (c) aerated 10-1 M NrtlCOs, pH 12.8, COatransients absorption a t 6000 A; (d) aerated 10-1 M N&C08, pH 12.8, 0 2 - transient absorption a t 2600 A; (e) 5 X 10-pM OH-, aerated, 0 8 - transient absorption a t 4700 A; (f) 5 X 10-3 M OH-,aerated, Os-transient absorption a t 2600 A. Each plot was drawn separately on a different ordinate scale but $1 are shown here on the same figure for convenience.
+
0 2
+ OH
+0 2 -
+ H+
The transient absorbing at 240 mp is mainly 02-since the pK(H0J = 4.4 f 0.4.15 The second-order plot is shown in Figure 3b, and k g = 5.8 X lo7 M-l sec-I was obtained, which is in fair agreement with the k6 values obtained in the literature (see Table I). M Na2SOc in the On flash photolysis of 2 X presence of oxygen at pH 10.4 (OH- ions absorb only a small fraction of the radiation in this system), two species are again observed (see Figure 2). One at 240 mp is 02-,and the other is a transient decaying much more slowly with an absorption maximum a t about 430 mp. Similar results have been observedIa in the flash photolysis of persulfate ions irradiated in alkaline solutions in the presence of 0 2 : the transient with a maximum of 450 mp has disappeared, and instead a relatively slow-decaying species, with a maximum at 430 mp, is (15) (a) K.Schmidt, Z . Nuturforsch., 16, 206 (1961); (b) G. Caapski and B. H. J. Bielski, J. Phye. Chem., 67,2180 (1963).
Volume 71, Number 6 April 1967
E. HAYON AND J. J. MCGARVEY
1476
found which is identical with that observed in the flash photolysis of aerated alkaline solution of sod2-. In the absence of 02,however, a t pH's above 9.0, the SO4radical anions formed in persulfate solutionsla decay rapidly with a pseudo-first order, dependent on the OHion concentration. At pH's about 10.4-10.5 all the SO4- radicals have disappeared, and no absorption due to transient species can be observed up to 6000 A. These observations have been interpreted18 as due to the conversion of SO4- radical anions to OH radicals in alkaline solutions, with the subsequent formation of ozonide ions 0 3 - in the presence of 02
sod2-+ OH + H+
sod- + H2O
+ OH-+ 0.- +
0 2
0.-
+ H20
+0 3 -
(8)
0
~
-% 2 - COS-
The Journal of Phyaicd Chemistry
+ eaq-
1 WAVELENGTH
8
Figure 4. Optical absorption spectrum produced on flash photolysis of aerated aqueous solutions of (a) 10-1 M Na&Oa at pH 12.8 and (b) 5 X 10-9 M NaOH. The OD was measured 100 psec after the start of the flash.
(9)
Values of ks = 6.2 X 108 l6 and k~ = 2.6 X log M-' sec-' l7 have been reported. This absorption spectrum with a peak at 430 mp is similar to that assigned to 03- in the pulse radiolysis of alkaline, oxygenated waterll*lzand in the flash photolysis of alkaline, oxygenated hydrogen peroxide. The decay of 0 3 in aerated Sod2-solutions does not follow a secondorder plot, but appears to follow a first-order decay. The mechanism leading to the disappearance of 0s- is not clearly evident. Support for the sequence of reactions 7-9 was obtained from the addition of lom2M methanol to 2 X M Sod2-at pH 10.4. The absorption of 0 3 at 430 mp was reduced by a factor of 6 compared to the formation of 03-in the absence of methanol. This reduction is explained on the basis that 0 . - or OH radicals are the species which give rise to the formation of 03-. The addition of methanol, a good reactant for OH or o*-,would therefore be expected to reduce the yield of formation of Os-. Methanol has also been shownla to react with SO4- with a rate constant k = 2.5 X lo7M-' sec-'. Solutions Containing Carbonate Ions. The absorption spectrum of COa2- was recently measured,ab and an extinction coefficient at 1850 A of about los obtained. To our knowledge the photochemistry of carbonate ions has not been studied. On flash photolysis of aerated lo-' M Na2COs solutions at pH 12.8, t w o transients are observed (Figure 4), one with a maximum at 240 mp due to 02-, and the other at 600 mp considered to be the C 0 3 - radical anion. The primary photolytic process is represented by ~
0.6
(7)
followed by
OH
0.q
(10)
A transient optical absorption with a maximum at 6000 A and identical with the one obtained above has been reported in the pulse radiolysis of carbonate ions. l B This radical anion formed in pulse radiolysis from the C032- -+ COSOH- has been reaction OH studied in the absenceZ0r2land presence of oxygen.12 Extinction coefficients at 6000 A of 2.9 X 10s,201860,21 and 180Ol2 have been obtained. The results obtained here have been calculated using taooo(C03-)1830 i 30. Both species 02-and co3- formed in the flash photolysis of carbonate ions decay by second-order kinetics, and the plots are shown in Figure 3c and d. The rate constants for the bimolecular decay of at 6000 A was found to be 1.3 X lo8M-' sec-' and that for 02-at 2600 A to be 1.4 X lo8 M-l sec-' (Tables I and 11). Both of these values are higher than those reported for k(Oz- 02-), and for k(CO3COa-) = 1.3 X lo7 M-l sec-' 21 and 1.7 X lo7 M-' sec-' l 3 in the absence of oxygen. Adams, et a1.,12have suggested that in the pulse radiolysis of aerated aqueous solutions of carbonate ions, COS- radical anions react with 0 2 -
+
+
cos-
+
+
COS-
+
02-
+products
(11)
(16) E. Hayon, Trans. Faraday Soc., 61, 734 (1965). (17) G. E. Adams, J. W. Boag, and B. D . Michael, Nature, 205, 898 (1965). (18) L. J. Heidt and V. R. Landi, J. Chem. Phya., 41, 176 (1964). (19) 8. Gordon, E. J. Hart, M. 8. Matheson, J. Rabani, and J. K. Thomas, J . Am. Chem. SOC., 85, 1375 (1963). (20) J. P. Keene, Y . Raef, and A. J. Swallow in "Pulse Radiolysis,"
M. Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press Inc., New York, N. Y . ,1965, p 99. (21) J. L. Weeks and J. Rabani, J . Phys. Chem., 70, 2100 (1966).
HEATSOF ADSORPTION ON BORON NITRIDE
1477
The results obtained in this work support reaction 11, since it is found that the bimolecular rate constant l e z - ~ = k s m ~ . However, Adams, et a1.,l2 report a value of kll = 1.5 X lo9M-I sec-I which is a factor of 10 larger than the rate constant of 1.3 X los M-‘ sec-l derived in this work. Solutions Containing O H - Ions. The flash photolysis of aerated aqueous solutions of 5 X lov2M NaOH (“low carbonate”) leads to the formation of two transients decaying at different rates. The transient with a maximum absorption a t 240 mp is that of the 02-radical anion, and the transient with a maximum at 430 mp is identical with the one observed in the flash photolysis of aerated alkaline sulfate ions and assigned to the ozonide ion 03-. The reactions occurring are considered to be
OH- h’, OH
+ e,-
(12)
followed by reactions 3, 8, and 9. Both species decay by second-order kinetics, as shown in Figure 3e and f.
The transient species absorbing in the far-ultraviolet region is made up of two components: a relatively rapidly decaying component due to 02-absorption and a slower-decaying component. The species giving rise to the longer lived ultraviolet absorption has not been elucidated. It is considered that, at least in part, this slow-decaying absorption in the region of 2500 A is due to the ozonide 03-ion since it was foundla to have an ultraviolet absorption in this very region, in addition to the reported maximum at 4300 A. Furthermore, since ozone is also known to absorbz2with A, -2600 A and an extinction coefficient of about 3.5 X lo*, it is possible that it may be formed as a transient product in this system. Similar observations have been found in the pulse radiolysis of aerated alkaline solutions,11,12and it is suggested that the slowdecaying transient(s) absorbing in the 2500-A region could also be due to 03- and 0 3 absorption. (22) H.Taube, Trans. Faraday SOC.,56, 656 (1957).
Heats of Adsorption on Boron Nitride
by G. Curthoys and P. A. Elkington Department of Chemistry, University of Newcastle, New South Wales, Australia
(Received October BO, 1966)
Theoretical interaction energies between an adsorbate molecule and boron nitride are calculated using the procedure proposed by Kiselev for graphite. The results obtained for simple molecules are in reasonable agreement with those obtained by other workers. Calculations for heats of adsorption of hydrocarbons are compared with those determined by gas chromatography.
The physical adsorption of gases and vapors on solids, particularly graphitized carbon black, has been the subject of intense investigations‘-a because such studies clarify the nature of the forces that cause adsorption. The surface of graphitized carbon black is sufficiently uniform to reveal both adsorbate-adsorbent and adsorbate-adsorbate interactions4 and is sufficiently large for precise measurements of adsorption isotherms and
heats of adsorption to be determined and compared with theoretical predictions.6 The graphite surface (1) A. V. Kiselev, Quart. Rev. (London), 15, 99 (1961). (2) A. V. Kiselev, Rues. J . Phys. Chem., 38, 1501 (1964). (3) M.M.Dubinin, B. P. Bering, and V. V. Serpinskii, Recent Progr. Surface Sci., 2 , 1 (1964). (4) A. V. Kiselev, J. Phys. Chem., 66, 210 (1962). (5) A. V. Kiselev and D. P. Poshkus, Trane. Faraday SOC.,59, 176 (1963).
Volume 71, Number 6 April 1967
