Abstract
The initial investigations of the radiation chemistry
of aqueous solutions of nitric oxide by Seddon and Sutton
inferred that this solute reacts rapidly and quantitatively
with all the primary species formed in the radiolysis of
water to form only NO⁻₂ and N₂0, such that
G(NO⁻₂) = Gₑ -ₐq + Gₕ + Gₒₕ and G(N₂O) = Gₑ -ₐq- + Gₕ. These
properties suggested that NO might profitably be used as
a scavenging solute to determine the radical yields in
neutral solution and their variations with solute concentration and pH. These investigations have been greatly
facilitated by the development of a quick and reliable
technique for extracting and analysing the gaseous products N₂O and N₂.
The overall reaction mechanism originally proposed
has been confirmed by studies of solutions at pH 7. The
initial yields of the products are in complete agreement
with the stoichiometry of this mechanism, and the growth
of the products calculated from the initial yields and
rate constant data is consistent with the experimental
results up to at least 70% consumption of NO. The disappearance of hydrogen peroxide in solutions at their
natural pH at high doses is shown to be due to a thermal
reaction between HNO₂ and H₂O₂ and the amount of nitrogen
formed is demonstrated to be entirely consistent with the
fraction of Gₑ -ₐq which would be expected to react with
the accumulated N₂0. By a reasonable method of extrapolation to zero solute concentration, the radical yields
obtained from 1.9 x 10⁻³M NO solutions are found to be in
agreement with the values normally quoted in the literature, viz. Greducing = 2.85 and Gₒₕ = 2.35. These values
are supported by the data obtained for the total yield of
radicals ( G(NO⁻₂) ) over the [NO] range from 10⁻⁴M to
1.2 x 10⁻²M. G(NO⁻₂) appears to be linear with respect to
[NO]¹/³ and the intercept at [NO] = 0 corresponds to the
sum of the "dilute solution" values given above. From
consideration of this data it is argued that the variations
of the radical yields with solute concentration are due
only to the scavenging of radicals from the spur regions.
It is also suggested that these dilute solution yields
demonstrate that 0 atoms are not formed in addition to
OH radicals as proposed by Allen.
Unsuccessful attempts have been made to determine an
H atom yield in neutral solutions by kinetic analysis of
the N₂O yields obtained from NO saturated solutions containing added NO⁻₂. The results of this investigation
imply that either GH = 0, or the relative rate constants
for the reactions of e-ₐq and H with NO and NO⁻₂ are very
similar, i.e. 7 ± 1.5. Evidence which suggests that H
atoms are formed in addition to e-ₐq in systems containing
only inorganic solutes has been obtained from the product
yields from NO-N₂O solutions, viz. Gₕ = 0.4 ± 0.3.
The radiation chemistry of NO has been investigated
over the pH range from 0.45 to 12.8. In acid solutions
the N₂O yield is found to increase with decreasing pH to
3.6 ± 0.1 at pH 0.45 whilst the hydrogen yield remains
constant. Due to the rapid thermal reactions of H₂O₂
with HNO₂ and NO, it is not possible to obtain either
material balance or a direct measurement of Gₒₕ in these
solutions. Attempts have been made to determine the fate
of the H₂O₂ formed and it is suggested that after a certain nitrite (HNO₂) concentration is reached, H₂O₂ reacts
exclusively with HNO₂ but that a small fraction of the
pernitrous acid formed as an intermediate in the reaction
decomposes to OH and NO₂ which subsequently react with NO
to form HNO₂.
Measurements in alkaline solution are restricted to
pHs no greater than 12.8 by the thermal decomposition of
NO. The radical yields are found to increase with increasing pH in agreement with the observations of Dainton
and Watt, i.e. G(N₂O) increases from 3.1 ± 0.1 at pH 12
to 3.4 ± 0.15 at pH 12.8, although Gₕ₂ remains constant.
H₂O₂ reacts with two molecules of NO to form two molecules
of NO⁻₂ in alkaline solution and the observed nitrite
yields therefore correspond to Greducing
+ Gₒₕ + 2GH₂O₂. The observed values are found to satisfy the conditions
of material balance. By assuming values of GH₂O₂, Gₒₕ
has been calculated over the whole pH range investigated.
As NO must react with both H and e-ₐq to form N20,
these results indicate that there are genuine pH effects
which cannot be explained by the conversion of e-ₐq to H
and H to e-ₐq on going from neutral to acid and from
neutral to alkaline solutions respectively. The scavenging mechanisms proposed by Hayon to account for
these variations are discussed and it is concluded that
these are inadequate. The excited water molecule hypothesis of Dainton and Watt is tentatively accepted as
the best explanation of these effects.
The nitrogen yields obtained from a study of NO-N₂O
solutions were found to be entirely consistent with a
simple competition for eaq by NO and N₂O contrary to the
findings of Seddon and Sutton who proposed the formation
of the intermediate N₂O⁻ to account for the unexpectedly
low NO consumption observed in these solutions. The NO
consumption experiments were repeated and the results obtained were found to agree with the simple competition
above. It should be noted that only in this experiment
do the values obtained in the present work disagree with
those obtained by Seddon.
In an exploratory investigation of the 1849 A°
photochemistry of NO saturated water, NO is found to absorb appreciably and to decompose to products which form
NO⁻₂, N₂O and N₂ such that [NO⁻₂] = 2[N₂O] + 4[N₂] . The
most likely primary acts are considered to be (a) a contact charge transfer process as proposed by Grajower and
Jortner and in addition (b) the dissociation of NO to
N and 0 atoms in their ground states. In the former process, one of the water molecules in the solvation shell oil
an NO molecule is presumed to donate an electron to the
NO molecule and the NO⁻ and OH formed are then able to
escape from each other. However several aspects of this
photolysis have still to be resolved.
