1895
J . Phys. Chem. 1990, 94, 1895-1899
Formation and Reactivity of Hypophosphite and Phosphite Radicals and Their Peroxyl Derivatives L. V. Shastri,' R. E. Huie,2 and P.Neta*'2 Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, and Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: June 29, 1989; I n Final Form: September 12, 1989)
The formation and properties of the radicals derived from hypophosphite and phosphite ions in aqueous solutions have been studied by pulse- and y-radiolysis techniques. Hydroxyl radicals and hydrogen atoms abstract H rapidly from the P-H bond to yield P-centered radicals. The radical derived from hypophosphite, 'PHOT, exhibits an optical absorption with A,, 240 nm and tmax3400 M-I cm-l and decays by a second-order reaction, 2k = 3.3 X lo9 M-l s-I . I t reacts with 02,k = 1.6 X IO9 M-I s-', to give a peroxyl radical, '02PH02-. The radical 'PHOT and those derived from phosphite, 'P03H- and 'POj2-, transfer an electron to riboflavin, anthraquinone-2-sulfonate,methyl viologen, and several pyridinium and nitro compounds relatively rapidly. From their low reactivities with pyridinium compounds of lower electron affinity, we estimate the reduction potential for PH02/'PH02- to be between -1.1 and -1.2 V and that for P03-/'P032- between -1.3 and -1.4 V vs NHE. The peroxyl radical '02PH0T oxidizes ascorbate and hydroquinone as rapidly as does ' 0 2 S 0 3 - ,but 'O2PO,2- oxidizes them much more slowly. The radical '02PH0, abstracts H from the parent hypophosphite to propagate a chain reaction. Radiolysis in the absence of O2 also gave chain reactions involving hypophosphite or phosphite with H 2 0 2or S2082-.
Introduction Hypophosphite ions are known to reduce Ni2+ to metallic Ni in an overall process ( I ) that is used for electroless nickel ~ l a t i n g . ~ 2H2P02-
+ Ni2+ + 2 H 2 0
-
2H3P03+ Ni
+ 2H2
(I)
The process is catalyzed by Ni and was suggested to involve the following reactions. H2P02-
+ Ni2+ + H 2 0 H2P02-
H2P03- + Ni
+ H2O
H2P03-
+ 2H+
+ H2
(2) (3)
This process was found to take place with high yields in irradiated aqueous solutions of Ni2+ and hyp~phosphite.~ Radiolysis may initiate the above process by reduction of Ni2+ to metallic Ni catalyst' or by formation of reducing intermediates. For example, the hypophosphite radical or Ni+ species may be involved in the mechanism of reduction by reactions such as 'PH02-
+ Ni2+ + OH-
-,
H2PO3-
+ Ni+
(4)
or by electron transfer from these reducing radicals to the Ni catalyst, which subsequently reduces Ni2+ ions to Ni or water to H2. The hypophosphite radical, in turn, is produced in the radiolysis by the reaction of OH radicals with hypophosphite ions: H2P02-
+ *OH
-
'PH02-
+ H2O
(5)
Reaction 5 was found6 to have a rate constant of kS = 1.8 X IO9 M-I s-I and is known to proceed by H abstraction from the P-H bond to form the phosphorus-centered radical o P H O ~ - . If~ the reaction with Ni2+ does indeed take place, it indicates that the hypophosphite radical is a strong reductant. ( I ) BARC, visiting scientist at NIST. (2) NIST. (3) (a) Brenner, A.; Riddell, G.J . Res. Nutl. Bur. Stand. 1946, 37, 31; 1947, 39, 385. (b) Van Wazer, J . R. Phosphorus and Its Compounds; Interscience: New York, 1961; Vol. I, p 363; Vol. 11, p 1888, and references therein. (4) Philip, W. H.; Marsik, S. J. Report No. N69-24940 (NASA-TN-D5213), 1969. Also: Shastri, L. V. Un ublished results. G values for Ni as high as N I O 5 were found at 0.1 M N$+. (5) Marignier, J. L.; Belloni, J.; Delcourt, M. 0.; Chevalier, J. P. Nature (London) 1985, 317, 344. (6) Adams, G.E.; Boag. J. W.; Michael, B. D. Trans. Faraday Soc. 1965, 61, 1417. (7) Behar, D.; Fessenden, R. W . J . Phys. Chem. 1972, 76, 1706, 1710.
0022-3654/90/2094- 1895$02.50/0
Phosphite ions also react with OH radicals rapidly8 by hydrogen abstraction from the P-H bond7 H2P03- + 'OH 'P03H- H 2 0 (6)
-
HP032-
+ *OH
+ 'P032- + H 2 0
(7) with k6 = 1.9 X IO9 and k7 = 3.7 X IO9 M-I s-I.~The phosphite radicals formed in these reactions may act as reductants or oxidants toward different compounds.8 In the present study we examine the reactivity of the hypophosphite radical and compare it with that of the phosphite radicals. -,
Experimental Section9 Sodium hypophosphite was obtained from'Baker and Adamson and phosphorous acid from Aldrich. Potassium persulfate was from Merck, hydrogen peroxide was from Baker, and the other inorganic compounds were from Fisher or Mallinckrodt. Riboflavin was from Calbiochem, metronidazole (I-(2-hydroxyethyl)-2-methyl-5-nitroimidazole) was from Searle, the 4-substituted N-methylpyridinium ions were a gift from Dr. A. Harriman, and the other organic compounds were from Aldrich. The compounds were of the purest grade commercially available and were used as received. Water was purified by a Millipore Super-Q system. Fresh solutions were prepared before each experiment and were purged with N 2 0 . This procedure is used to remove the oxygen and to convert the hydrated electrons into hydroxyl radicals. When the presence of oxygen was required, the solutions were purged with a mixture of N 2 0 / 0 2 of known composition, generally 4: 1. Pulse-radiolysis experiments were carried out with a Febetron 705 as a source for 50-11s pulses of 2-MeV electrons and by using the optical detection and computer system described before.I0 Absorbances measured at h I250 nm were corrected for scattered light in the monochromator. (The correction varied from 4% at 250 nm to 30% at 220 nm.) Dosimetry was done with N 2 0 saturated SCN- solutions, taking molar absorptivity for (SCN)T of 7580 M-I cm-' at 472 nm." y-radiolysis experiments utilized a Gammacell 220 W o source with a dose rate of 24 Gy/min determined by Fricke dosimetry. (8) Schafer, K.; Asmus, K.-D. J . Phys. Chem. 1980, 84, 2156. (9) The identification of commercial material or equipment does not imply recognition or endorsement by the National Institute of Standards and Technology nor does it imply that the material or equipment identified are necessarily the best available for the purpose. (10) Huie, R. E.; Neta, P. J . Phys. Chem. 1984, 88,5665. (1 1) Schuler, R. H.; Patterson, L. K.; Janata, E. J . Phys. Chem. 1980,84, 2088.
0 1990 American Chemical Society
Shastri et al.
1896 The Journal of Physical Chemistry, Vol. 94, No, 5 , 1990 0.05 I
TABLE I: Rate Constants for Reactions of Radicals with HvooDhosDhite Ions radical PH k , M-' s-' 'OH 10.7 1.8 x 109' 'H I 4 x 109* so;7 1.8 X H2P04' 4 3.9 x lO8d HPO4'7 5.9 x 107d p0,'212 7.9 x 1 0 7 d Cl21.3 2.8 X 1.4 X Cl20.3 co,*11.4 5 x 105' BrO,' I < 3 x 105'
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i
Results and Discussion Formation and Decay of 'PH02-. Pulse radiolysis of N 2 0 saturated solutions of hypophosphite ions leads to formation of the 'PHOT radical (reaction 5). The optical absorption spectrum of this radical, recorded at pH 5.7 immediately after the pulse (Figure I ) , has a maximum at 240 nm with molar absorptivity of t = 3400 M-' cm-I. An identical spectrum was obtained at pH 1.2, where the production of H atoms from the edaq+ H+ reaction is important? This confirms previous results showing that H atoms abstract hydrogen from hypophosphite rapidly (although the rate constant reported, k8 = 4 X lo9 M-' s-l, appears to be too high by comparison with other values; see Table I).I4 (8 )
Hypophosphite ions are also known to react with SO4- radicals ( k = 1.8 X lo8 M-I s-I)l5 and with phosphate radicalsI5 about an order of magnitude more slowly than they react with OH, indicating that these strong oxidants also react with hypophosphite by H abstraction rather than electron-transfer oxidation. We have examined other strong oxidants, Cl,, CO), and BrO,, by following the decay of these radicals at the proper wavelength (350, 600, and 475 nm, respectively) as a function of [H2P02-]. We found that the rate constant for reaction of hypophosphite with C1y was 2.8 X 1 O6 and 1.4 X 1O6 M-I s-l at pH 1.26 and 0.3, respectively, with C 0 3 - at pH 11.4 it was 5 X lo5 M-I s-l, and with BrO, at pH 7 it was 1O0.24 Similarly, irradiation of deoxygenated solutions containing 0.01 M N a H 2 P 0 2and 7 X M K2S208at pH 7.0 or 2.6 resulted in G(-S2082-)= 2 X IO2 at 50 Gy and 9 X 10' at 250 Cy. In this case the pH gradually decreased upon irradiation (to pH 2.2) due to formation of protons in reaction 17. The finding that the chain length with peroxydisulfate is lower than that for hydrogen peroxide probably indicates that reaction 16 is slower than reaction 15. The fact that reaction 17 is 10 times slower than reaction 5 may have little effect on the chain length since these reactions are the faster and not the rate-determining step in the above chain processes. Comparable experiments with phosphite instead of hypophosphite gave somewhat lower G values, G(-H?02) = 3.5 X IO2 and G ( - S 2 0 g 2 - )= 5 X l o i under identical conditions, indicating the operation of similar mechanisms with possibly lower rate constants. When the radiolysis of the H2PO2-/H2O2solution was carried out in the presence of oxygen, no substantial decrease in [H202] was detected. This indicates that reaction 11 competes effectively with reaction 15 to prevent the reduction of hydrogen peroxide and the chain process. From the concentrations of O2and H202 and from k , , determined above, we can estimate k I 5to be
