VOLUME 106, NUMBER 32, AUGUST 15, 2002
© Copyright 2002 by the American Chemical Society
ARTICLES Reactions of the Dihydroxylamine (HNO2-•) and Hydronitrite (NO22-•) Radical Ions Sergei V. Lymar,*,† Harold A. Schwarz,† and Gidon Czapski‡ Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000 and Department of Physical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: May 13, 2002
The dihydroxylamine radical anion (HNO2-•, with the H atom on the nitrogen atom) is produced by the reaction of H atoms in the pulse radiolysis of nitrite solutions and decays in water with a rate constant of 5.0 × 103 s-1. Its absorption spectrum has a maximum at 270 nm with molar absorptivity of 2.8 × 103 M-1 cm-1. The decay of HNO2-• is catalyzed by both acids and bases. The hydronitrite radical (NO22-•) produced by the reaction of eaq- with nitrite exhibits no absorption spectrum between 270 and 550 nm. It is much shorter-lived than HNO2-•, disappearing at 1.6 × 106 s-1 without generating any HNO2-•. Neither radical species has any observable pKa’s, but the pKa for HNO2-• has been estimated to be lower than 9.3 from the rate data on the hydroxide-catalyzed decomposition of this radical. Both HNO2-• and NO22-• reduce methyl viologen, and the reduction by HNO2-• has been used to measure its yield, which is found to be 0.63 radicals per 100 eV. Thus, the production of HNO2-• by H atoms is quantitative. Collectively, these data represent a major revision of the reductive radiation chemistry of nitrite.
Introduction The reactions induced by radiation in aqueous nitrite are of considerable importance for environmental chemistry and for radiation chemistry in certain types of nuclear wastes. All three primary radicals from water radiolysis rapidly react with nitrite
eaq- + NO2- f NO22-•
(1)
H• + NO2- f HNO2-•
(2)
OH• + NO2- f OH- + NO2•
(3)
The rate constant for the first reaction is k1 ) 3.5 × 109 M-1 * To whom correspondence should be addressed. Phone: (631) 3444333. Fax: (631) 344-5815. E-mail:
[email protected]. † Brookhaven National Laboratory. ‡ The Hebrew University of Jerusalem.
s-1.1 According to Yost and Russell,2 a stable salt of the NO22-• radical has been prepared and is called hydronitrite; the name is derived from a hypothetical hydronitrous acid, N(OH)2•. An earlier pulse radiolysis study by Gra¨tzel and co-workers3 found UV transient absorption with a maximum at 260 nm, which was assigned to NO22-•. From the pH dependence of this absorption, they concluded that the two protic equilibria
N(OH)2• / H+ + N(O)OH-• / H+ + NO22-•
(4)
with consecutive pKa of 5.7 and 7.7 could be attained within the lifetimes of these radicals. However, these values do not conform to the well-established general rule for oxoacids of the type HnXOm, according to which their pKa values depend mainly on the m - n difference.4 The same rule should apply to radical species5,6 and predicts pKa ) 8.5 ( 1 for N(OH)2• and, thus, about 13 for N(O)OH-• because the second pKa is usually 4-5 units above the first.
10.1021/jp026107l CCC: $22.00 © 2002 American Chemical Society Published on Web 07/24/2002
7246 J. Phys. Chem. A, Vol. 106, No. 32, 2002
Lymar et al.
Undoubtedly, the NO22-• radical eventually decays to NO•, for this is the only stable form of nitrogen(+2), i.e., the reaction
NO22-• + H2O f NO• + 2OH-
(5)
However, the literature on the lifetime of NO22-• is controversial. The half-lives of 12 µs3 and 16 µs7 were determined from the decay of transient UV absorption. Other studies that used timeresolved conductivity and photoelectrochemical techniques have found such a long lifetime inconsistent with observations and suggested values
