M. ANBARAND I. PECHT
1460
The Sonolytic Decomposition of Organic Solutes in Dilute Aqueous Solutions. I.
Hydrogen Abstraction from Sodium Formate
by M. Anbar’ and I. Pecht The Weizmann Institute of Science, Rehouoth, Israel
(Xeceiued December SO, 1983)
Deuterated formate ions were sonolyzed in dilute aqueous solutions and the H D produced was determined. The yield of H D was found to be independent of solute concentration from 0.005 to 0.1 M . The yield of H D was diminished in both strongly acid and alkaline solutions. The results indicate the formation of hydrogen atoms under sonolytic conditions. The yield of ‘lmolecular”’hydrogen formed under the same conditions is 50 times greater than that of hydrogen atoms.
The analogy between the behavior of radiolyzed and sonolyzed aqueous solutions has been pointed out in several ~ t u d i e s . ~ Molecular >~& hydrogen was detected,3b and it was suggested that hydrogen atoms are also formed during ~onolysis.~It was of interest to find out whether hydrogen atoms are formed under sonolytic conditions and to determine their yield. The hydrogen abstraction from organic solutes was applied in radiation chemistry as an indicator for the presence of hydrogen atoms.5,6 Quantitative information on the yield of hydrogen atoms in aqueous solutions was recently obtained by determining the yield of H D produced from deuterated organic solute^.^-^ In the present study the sonolytic formation of hydrogen atoms was investigated by following the dehydrogenation of deuterated formate ions.
Experimental Materials. Deuterated sodium formate (>98% D) was supplied by Volk Radiochemicals Co; MgSO, and NaOH were Fluke Puriss grade; KaF was Baker Analyzed Reagent. Distilled water was redistilled from alkaline permanganate and then from phosphoric acid. Argon of highest purity (Matheson Co.) was used. The pH of the solutions was determined using a Metrohm Kompensator Type E 148 C with an accuracy of 0.05 pH unit. Ultrasonic Irradiation. A Model T 200 (Lehfeldt and Co. GMBH) ultrasonic generatar with a watercooled quartz transducer operating at 800 kc./sec. and an energy output of 1.6 ergs/cm.2 sec. was adapted to The Journal of Physical Chemistry
focus the ultrasonic energy to a limited volume. The irradiation vessel was thermostated by circulating water at 20 f 1’. A small Pyrex bulb (10.5 ml.) connected to an inlet tube was used as the irradiation vessel. The bulb, which was of minimal thickness in order to reduce sonic absorption, was fitted a t a constant position in the focus of the ultrasonic concentrator. I n all experiments 4.0 ml. of water or aqueous solution were sonolyzed. The yield of HzOz under these conditions was 4.2’ X 10-8 mole/l./min. Analysis. After sonolysis, the gas was introduced into a mass spectrometer (CEC Model 21-401) and the masses 2, 3, and 4 were determined, as well as masses 28 and 32 to check on air contamination._ _The masses _ _ _ 2,3, _ _and _ 4 were measured in the sequence 2 , 3 , 4 , 3, 2, 3, 4, 3, 2. From the mean values of the three 2 parts of the measured sequence, the ratio R.= (3 X Z)/(5 - 5) was calculated. The values obtained showed a standard deviation of f4%.
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(1) Chemistry Division, ilrgonne National Laboratory, Argonne, Ill. (2) A. Weissler, J . Am. Chem. Soc., 81, 1077 (1959); J . Acoust. SOC. Am., 32, 283 (1960). (3) (a) M. Haissinsky and R. 0. Prudhomme, J . chim. p h m . , 47, 925 (1950); (b) P. Grabar and R. 0. Prudhomme, ibid., 44, 145 (1947). (4) P.Rivayrand and M . Haissinsky, ibid., 59, 623 (1962). (5) J. T. Allan and G. Scholes, Nature, 187, 218 (1960). (6) G. Scholes and M .Simic, ibid., 199, 2 i 6 (1963). (7) G. Lifshita and G. Stein, J . Ckem. SOC.,3706 (1962). (8) iVI. Anbar and D. Lleyerstein, Israel AEC Report IA-901 (1963); J . Phgs. Chem., in press. (9) M. Anbar and I. Pecht, J . Chem. Phys., 40,608 (1964).
1461
SONOLYTIC HYDROGE s ABSTRACTION FROM SODIUM FORMATE
The absolute yield of hydrogen was determined from the value of the 3 pea,k compared with known mixtures of Hz and argon (5-15y0 H2),measured under the same conditions. Results and Discussion The rate of formation of “molecular” hydrogen (H2) from the water and of H D from the deuterated formate under different conditions, is given in Table I. It can be seen that the yield of LLmolecularll hydrogen ( Y H Jin neutral solutions of sodium formate is independent of the concentration of the solute. As formate ions are most efficient scavengers of H atoms in solution,’” it may be implied that H2 is not formed by recombination of H atoms in solution but is produced in the cavities, in complete analogy to the formation of H2Oz.l’ Y H , was found to be slightly diminished in 1 M solutions of NaF, probably owing to changes in water structure. l 2 Oxygen, even a t low concentrations (2.5%), was found to diminish Y H ,by about 60%. ~~~
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Table I : T h e Formation of HD from Sono1,yzedFormate-d in Aqueous Solutions under Argon (1 a t m . P
Sodium formate
PH
Additive
[Additive), M
R X 10%
...
6.0
...
0.005 0.01 0,025 0.05 0.1 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
6.0
1,758 1.95 2.2 2.7 2.6 1.75 2.62 2.35 0.43 1.91 2.3 2.00 2.68 2.68 0.86
a
6.0
6.0 6.0 6.0 0.6 3.0 12.1 14. Ob
7.0 7.0 6.0 6.0 6.0 6.5
HzSOa HzSOc XaOH NaOH NaF MgSOa HzOz NaNOa NaNO? 0 2
0 25 0 0025 0 01 1.0 IO 1.0 0 05 0.01 0.05 2.6%
Time of irradiation waei 60-70 min.
HZ produced, pmolesl 1. min.
HD produced, @mole/ 1. min.
25.0 25.0 22.0 24.5 23.0 25.0 16.2 22.6 25.0 28.5
18.4 24.0 23.4 25.8 22.6 9.4
... 0.44 0.43
0.54 0.53 0.65 0.27 0.59 0.59 0.12 0.35 0.53 0.47 0.69 0.61 0.08
By definition.
The effect of Oz may be interpreted by the scavenging of H atoms and the formation of H02. The H 0 2 radicals may form Hz02 02. The yield of H20zoriginating from 02,compared with the scavenging effect of 0 2 on Y H zis, however, small.11 It has to be assumed therefore, that O2 interacts with a precursor of the H atoms in the cavity or that, a t the given high transient temperatures, H 0 2 decomposes according to 2H02 --c HzO 0 2 0.
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The yield of H D given in Table I was found to be almost independent of the concentration of formate ions, which implies its formation by indirect action. The per cent H D in the evolved hydrogen was found to be independent of the time of sonolysis; this excludes an isotopic exchange between hydrogen and water under our experimental conditions. In analogy to radiation chemistry17the limiting value of HD may be taken as a measure for the yield of hydrogen atoms Y H . Y Hwas found to be diminished in acid solution. This effect is due to acidity and is not a salt effect, as is evident from the experiment with MgS04. The effect of acidity must be due to the penetration of formic acid into the cavities causing a decrease in YH. A paralle1,decrease ~ in Y H *is also observed, and a reduction in Y H ~ino the presence of formic acid has been previously reported. If electrons would be formed under sonolysis, one would expect an increase in YH with increasing acidity.13 The absence of electrons as precursors of H atoms in our system is corroborated by the finding that HzOz or NOs-, both most effective electron scavengers,14,15 do not affect Y H to any appreciable extent. The slight decrease in Y Hin the presence of H202is probably due to the oxygen formed from its decomposition. Oxygen acts as an effective scavenger of H atoms and diminishes YH far better than J“,. In strongly alkaline solutions, YH is diminished to a much greater extent than expected for a salt effect. This is due to the interaction of H atoms with OH-resulting in the formation of hydrated electrons. It is of interest to compare the over-all yield of H202 under our conditions, YH2O2= 4.9/pmoles/l. rnin.” with Y E , = 25 pmoles/l. min. The stoichiometry of the decomposition of water would require the formation of oxygen with a yield Yo, = 10 pmoles/l. min. The proportions between H20z:Hz: O2: : 5 :25 : 10 are different from those obtained by Haissinsky, et UZ.,” a t much lower ultrasonic intensities, namely 7 :9: 1.4. The relatively low yield of Hz02under high ultrasonic intensities may imply that most of the H20zproduced H2 reaction is decomposed a t by the 2H20-+ H2Oz the high transient ternperatures involved. l8 An alter-
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(10) J. Rabani, J . Phys. Chem.. 66, 361 (1962). (11)
M.Anbar and I. Perht, ihid., 68, 352
(1964).
(12) M. Haissinsky and A. Mangeot, Nuovo Cimento, 4, 1086 (1954). (13) E. Hayon and A. 0. Allen, J . Phys. Chem., 65, 2181 (1961). (14) G. Czapski and A. 0. Allen, ibid., 6 6 , 762 (1962). (15) S. Nehari and J. Rttbani, ihid., 67, 1609 (1963). (16) J. Jortner and J. Rabani, J . Am. Chem. SOC.,83, 4868 (1961). (17) M. Haissinsky, R. Klein, and P. Rivayrand, J . chim. phys., 5 9 , 611 (1962). (18) M. E. Fitagerald. V. Griffing, and J. Sullivan, J . Chem. Phys., 2 5 , 926 (1956).
Volume 68, Number 6
J u n e , 1964
M. ANBARAND I.PECHT
1462
native explanation could be the decomposition of water under high ultrasonic intensities to H and 0 atoms, forming Hz and O2without HzOzas intermediate. The absence of any deuterium isotope effect in the sonolytic formation of H, under high intensitiesg makes the
second interpretation rather likely. On the other hand, it should be noted that the yield of hydrogen atoms in solution, Y H ,is only about 1/60 of YH,.This means that less than 2y0 of the hydrogen atoms produced in the cavity diffuse into solutions.
The Sonolytic Decomposition of Organic Solutes in Dilute Aqueous Solutions. 11. The Sonolysis of Isopropyl Alcohol
by M. Anbar and I. Pecht The Weizmann Institute of Science, Rehoaoth, Israel
(Receiued December SO, 1968)
The sonolytic decomposition of isopropyl alcohol in aqueous solutions under argon has been investigated following the evolution of hydrogen and the formation of acetone. It has been found that the yield of hydrogen produced significantly exceeds the yields of molecular and atomic hydrogen. The sonolytic decomposition of isopropyl alcohol is not scavenged by nonvolatile hydrogen atom scavengers. It was concluded that the sonolytic decomposition of isopropyl alcohol takes place in the cavities, where the high transient temperatures induce the monomolecular dehydrogenation of the alcohol. Hydrogen abstraction by a bimolecular process has been observed a t high concentrations of isopropyl alcohol (>0.1 M ) .
Organic solutes in sonolyzed aqueous solutions have been shown to undergo extensive decomposition. 1-4 Certain organic solutes in aqueous solutions were found to affect the sonolytic formation of hydrogen peroxide5-’ and the sonolytic oxidation of iodide ion^.^,^ It has been pointed out that the chemical effect of volatile organic solutes on the formation of Hz02differs significantly from that of nonvolatile free-radical scavenger^.^ It was of interest to compare the sonolytic decomposition of volatile and nonvolatile organic solutes under similar conditions. The hydrogen abstraction from formate ions was investigated and was found to be induced by hydrogen atoms.9 The sonolytic dehydrogenation of isopropyl alcohol in dilute solutions was chosen as a model reaction for the behavior of volatile solutes. The Journal of Physical Chemistry
Experimental Solutions of isopropyl-2-d alcohol (>99Y0 deuterium, Merck Sharp and Dohme Ltd.) in triple-distilled water were sonolyzed under argon and the isotopic (1) 111. E. Fitzgerald, V. Griffing, and J. Sullivan, J . Chem. Phys., 2 5 , 926 (1956).
(2) D. L. Currell and L. Zechmeister, J . X m . Chem. Soc., 80, 205 (1958). (3) S. C. Srivastava, Nature, 182, 47 (1958). (4) B. H. Jennings and S. N. Townsend, J . Phys. Chem., 65, 1574 (1961). (5) A. Henglein and R. Schulz, 2 . Naturforsch, 8b, 277 (1953). (6) A. Weissler, J . Am. Chem. Soc., 81, 1077 (1959). (7) M .Anbar and I . Pecht, J . Phys. Chem., 6 8 , 352 (1964). (8) N . Ellfolk and A. Virtanen, Acta Chem. Seand., 11, 230 (1957). (9) 11.Anbar and 1. Pecht, J . Phys. Chem., 68, 1460 (1964).
