The Journal of Physical Chemistry, Vol. 83, No. 7, 1979 709
Pulse Radiolysis of Aqueous Hydrazine Solutions
Pulse Radiolysis of Aqueous Hydrazine Solutions. The Triazene Speciest James W. Sutherland Department of Energy and Environment, Brookhaven National Laboratory, Upton, New York 11973 (Received June 15, 1978; Revised Manuscript Received October 18, 1978) Publication costs assisted by the Division of Chemical Sciences,
US. Deparfment of Energy
Pulse radiolysis of aqueous hydrazine solutions has been studied in the pH range 2-13. At times greater than about 1ms after the pulse, a single transient species is observed (Arna 230 nm). This species decayed by first-order kinetics under all conditions studied. A mechanism which accounts satisfactorily for the observed kinetic behavior is proposed. From the available evidence, this species, identified as triazene (N3H,), is amphiprotic and the following acid-base equilibria are rapidly established: N3H4+== N3H3 + H+ (pK1 = 4.95, AH,"= -4.9 kcal mol-', AS1" = -39.2 eu) and N3H3== N3H2- + H+ or N3H3+ OH- + N3H40- (pK2 = 11.37, AH2"= -4.2 kcal mol-', AS2" = -67 eu). pK values derived from the dependence of the first-order rate constant on pH, from the dependence of initial absorbance on pH, and from the dependence of the first-order rate constant on temperature at selected values of pH are in satisfactory agreement. The rate constants for decomposition of the acidic and basic forms of triazene are hN3H4+ = 1.97 X 10" exp(-12600/RT) s-l and ~ N ~ = H 2.14 ~ ; X exp(-19200/RT) s-l. For N3H3,which is stable relative to the faster reaction rates of its conjugate acid and base forms, h is estimated to be 10.001s-l at 24 "C. The dependence of the observed first-order rate constant on pH at constant temperature is expressed by the following equation: hobsd = [h3/{l+ (Kl/[H+])]]+ [ k 4 / ( l ([H+]/K2)]](h3= hNsHr+= 133 s-l, h4 = hN3H2-= 2 s-l, pK1 = 4.g5 and pK2 = 11.37). Phosphate is a catalyst for the decomposition of triazene. From studies on the salt effect, it is shown that the conjugate acid of neutral triazene has unit positive charge and its conjugate base has unit negative charge.
+
Introduction From studies on the oxidation of hydrazine (pK, = 8.1) in aqueous solutions by one-electron oxidizing agents, it wadd suggested that the mechanism of the overall reaction following the formation of hydrazyl radicals was 2NzH3 N4H6 (tetrazene) NH3 + HN=N-NHz (triazene) NH2-NH-NH-NH[:! + NHz-N=NH I"=N-NHZ N3H3 NH, + N2
--
The y radiolysisll-l4 of dilute aqueous hydrazine solutions has been explained by supposing that OH, H, and e,, attack hydrazine to yield hydrazyl radicals which then react according to the above scheme to give nitrogen and ammonia as stable end-products. For example, the following reactions occur in acidic solutions of hydrazine a t sufficiently high concentration and with low dose rates: H+
+
-+
H
It was also pointed out that these reactions may be influenced by the acid-base behavior of these intermediates. Recently, more direct evidence for the validity of the mechanism has been obtained by the pulse radiolysis technique. Pagsberg'j reported that in basic solutions attack of OH radicals on hydrazine produced a transient (A, 230 nm), identified as the hydrazyl radical, which decayed completely by a fast second-order reaction (tip 50 hs). At the same time, Sutherland and B i e l ~ k i , ~ looking a t the reaction on a much slower time scale, described the characteristics of a transient (A, 230 nm) which decayed by a first-order acid-base catalyzed reaction (tl12 I5.3 ms) and which was formed by a first-order reaction in basic solution. On the basis of the above mechanism, they suggested that the transient was triazene. These observations were good evidence for the complexities of the oxidation reactions described above, since tetrazene, which can be anticipated to have an absorption spectrum similar t o ammonia, would not be observed a t the wavelengths studied. More detailed studies by pulse radiolysiss-10 have provided more substantial evidence for the above reaction scheme and have described some of the added complexities of the scheme due to the acid-base character of the intermediates. 'This work was supported by the Division of Chemical Sciences, U.S. Department of Energy, Washington, D.C., under Contract No. EY-76-C-02-0016. 0022-365417912083-0789$0 1.OO/O
-
HzO v.w+ H, OH, HZO2, H2 H HzOz OH + HzO
-
OH 2NzH4+
+ N2H5+
+
H2
+ NzH4+
+ NzHS' H2O + NZH4' Nz + 2NH4+ (overall reaction) +
Because kH+N2H5+ is sufficiently slow compared to kH+HzOz, only a small steady-state concentration of H20zis built up (entirely negligible as far as the overall stoichiometry is concerned). Thus the radiolytic decomposition can be represented by the equation 2NzH4 Nz + Hz + 2NH3
-
and G(N2) = G(H2) = 1/2G(NH3)=
GHz
+ GH - GHzOz = (GH + G o H ) / ~
However, a t high dose rates, usual scavenger concentrations (0.01-0.001 M) of hydrazine are insufficient to scavenge all the H atoms and the detailed interpretation of the results is complicated by radical-radical reactians.14 In acid solution, the following reactions must be added to the above scheme: H+H-HH,
H
+ NzH4"
-
N2H5"
In neutral and alkaline solutions, the reactivity of eaq-with itself, with N2H5+or N2H4, and possibly with H atoms must also be taken into account, together with the different reactivities of the acidic and basic forms of the radical
0 1979 American Chemical Society
790
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
James W . Sutherland
TABLE I : Influence of Hydrazine Concentration on the Decav Kinetics of Triazene pH 1 . 9 pH 7.87 pH 1 0 . 5 3
0.1 M 0.05 M
129 130 0.01M 1 3 3 6mM 130 2 mM 130
0.1 M 0.01 M 2mM 0.6 mM 0.2 mM
0.108 0.108 0.095 0.110 0.092
0.01 M 2 mM 1mM
0.53 mM
TABLE 11: Effect of Dose Rate and Wavelength on t h e Decay Kinetics of Triazene (Solution 0 . 0 1 M Hydrazine, p H 2.40)
0.183 0.170 0.190 0.167
species, leading to a more complex set of reactions. The addition of these radical-radical reactions to the reaction scheme lowers the G(product) values, while the stoichiometry of the overall reaction and the reactions following the dimerization of hydrazyl radicals remain unchanged. In nitrous oxide saturated solutions at pH values where all the solvated electrons are converted to OH radicals, the situation is much simplified, although the fate of the molecular HzOz and that of the hydrogen atom remains uncertain. The present paper is concerned with the properties and reactivity of the "slow species", identified as triazene, obtained on pulse radiolysis of aqueous hydrazine solutions which have been made oxygen free by saturation with nitrogen, argon, or nitrous oxide.
Experimental Section Chemicals. Hydrazine solutions were made up from triply distilled water and hydrazine sulfate (Baker reagent grade). The desired p H was obtained by addition of freshly prepared carbonate-free concentrated sodium hydroxide solutions. The hydrazine solutions were deaerated by bubbling with argon or nitrogen and saturated with nitrous oxide by a similar technique. Where alkaline solutions were used, the hydrazine and sodium hydroxide solutions were deaerated separately, mixed, deaerated further, and then used. Apparatus. A pulsed 1.95-MeV Van de Graaff accelerator served as an electron source. A pulse length of 50 y s was used for most of the work, but pulses up to some 200 ys were used when high doses were given to the sample. All irradiations were carried out in a Suprasil quartz cell (2 X 2 X 0.8 cm) with one 2 X 2-cm window thinned to 0.4 mm to allow penetration of the electrons.16 Yields were determined by ferrous sulfate dosimetry (G(Fe3+)= 15.5). Analyzing light from a deuterium lamp passed through the cell three times with a total optical path length of 6.1 cm. The emerging light passed through two Bausch and Lomb f / 3 . 5 monochromators coupled in tandem onto a photomultiplier. The signal generated by the photomultiplier was fed into an oscilloscope where it was recorded photographically as a function of time. The scattered light a t 200 nm was less than 0.3% of the total light signal measured. Temperature studies were made by cooling the cell in a special holder with a stream of cold nitrogen. The temperature in the cell was monitored by a thermocouple dipping into the solution. Results When deaerated solutions of hydrazine were irradiated with short pulses of high energy electrons, a transient absorption (A, 230 nm, half-life >1 ms) was observed. This transient has been identified as tria~ene.~-lO Observations o n the Kinetics of Decay of Triazene. The decay of the transient followed first-order kinetics under all experimental conditions. The observed specific rate constant hobsdwas (a) a complex function of pH; (b) in-
a
*
0
kobsd, S-
dose/pulse ,a krd
hob?,
A. nm
210 215 220 225 230 2 35 240 250 2 60 270 280
128 139 133 135 133 128 130 133 139 128 133
4.5 4.9 7 .O 7 .I 8.5 18.21 25.56 34.71
127 127 127 137 127 139 125 139
S-
The pulse length was 56 p s ,
o O
'
PH
Figure 1. Decay kinetics of the transient, identified as triazene, as a function of pH (temperature 22 OC). Solid line is calculated from eq 5 using the following values: k 3 = 133 s-I,k, = 2.1 s-', pK, = 4.95, PKP = 11.37.
dependent of hydrazine concentration (0.5 mM to 0.1 M) at constant pH (Table I); (c) independent of total dose and dose rate at constant pH (Table 11);(d) identical in argon saturated and nitrous oxide saturated solutions a t constant pH; (e) independent of wavelength for a solution a t fixed pH under a variety of pulsing conditions, e.g., dose rate, dose, and concentration of hydrazine (Table 11); (f) increased in the presence of phosphate buffer in the p H range 6-9. Influence of p H (Temperature 22 "C).The kinetics of the transient decay were measured as a function of pH in argon saturated and nitrous oxide saturated solutions using a 0.01 M hydrazine solution. pH adjustments were made by additions of concentrated sodium hydroxide solution. The observed first-order rate constant was the same for argon and nitrous oxide saturated solutions and the results for both solutions are shown in Figure 1. Because of the weakness of the signal a t pH values above 12 and the difficulty of obtaining sufficient analyzing light onto the photomultiplier, due to the absorbance of hydrazine and hydroxyl ions, k values were determined a t much higher
Pulse Radiolysis of
Aqueous Hydrazine Solutions
The Journal of Physical Chemistry, Vol. 83, No. 7, 1978 791
doses per pulse and are less precise. Also it was impossible t o determine a complete spectrum of the transient under these conditions and it is uncertain whether the experimental observations are those of a more basic form of the same transient or of another species peculiar to highly alkaline solutions. For these reasons the discussion that follows is restricted t o the results obtained a t pH values 512. In the pH range 0.8-4.0, kobsd = 133 s-l is independent of pH, but on further increase in pH, k decreases, passes through a sharp minimum at pH ~ 9 . 0then , increases until pH 12.0 where it again is independent of pH. Finally, a t still higher pH, kobsddecreases rapidly. On the acidic side of the minimum (pH below 9.0), k is a linear function of [H+], or log k vs. pH has a slope of -1.0, while on the basic side (pH >9.0), k is a linear function of [H+]-l or of [OH-] and log k vs. pH has a slope of +1.0. Such a behavior is that expected of a species which can exist in three forms, acidic, neutral, and basic and which decomposes by acid-base catalyzed reactions in which the rate of decomposition of the neutral form is negligible a t any pH when compared to the rate of reaction of its conjugate acidic or basic form. Representing the acidic species as N3H4+,the neutral. species as N3H3,and the basic species as N3Hz-, the following mechanism is proposed:
22 N3H3+ H+
N3H4+
K1 = kl/k-l
(1)
k-1
N3H4+ N3H2-
+ HzO
k3
+ "4' Nz + NH3 + OH-
N2
k4
(3)
(4) The precise form of the basic species remains in doubt. The following equilibrium and decomposition, though considered less likely, may occur and result in identical kinetic expressions: OH- N3H3 N3H40(24 4
+
-
-+
N3H40Nz + NH3 + OH(44 Experimentally we observe that d(A,)/dt = -hobsd[At], where A , is the absorbance of the solution a t time t and kobsdis the c?xperirnental first-order rate constant. From the above reaction scheme, we find that
and on the acid side of the minimum, where the second term in (5) can be neglected
PH
L
.. I
1.0
I
T
i
-Aid -1
-
0.0
4
5
6
7
8 PH
9
1
0
Figure 2. Evaluation of pK1 and pK,: ( 0 )log [(I33 s-'/kobsd)- I ] = pH - pK,; (0)log [(2.1 s-'/kobsd) - I ] = pKp - pH.
At high acid concentrations, how = h3 = 133 s-l and a t high base concentrations, hobd = k4 = 2 d,independent of [H+] from pH 11.5 to 12.5, (Figure 1). Plots of eq 6a and 6b are shown in Figure 2 and from the x intercept, where log [(k3/kobsd)- 11 and log [(k4/kobsd)- 11 are Zero, pK1 4.95 and pK2 = 11.37. From these values and eq 5, the solid line in Figure 1 was calculated and is in good agreement with the experimental data. Because the rate of decomposition of the neutral species is negligible compared to the rate of reaction of the more basic and acidic species, pK values can be read directly from Figure 1;pK1 = 5.0 is the value of the pH where hobsd = k3/2 and pK, = 11.3, the value where hobsd = k4/2. According to the proposed mechanism, the simplified form of eq 5 , namely (7) describes the kinetic results a t or near the observed minimum. On differentiating eq 7 with respect to [H'] and equating to 0, then [H+]mi: = (k4/k3)KlK2
(8)
and PKI or log
(-* -
1) = -pK1
+ pH
kobsd
(64
In basic solutions where the acid term can be neglected
and log
(k4 kobsd
- 1) =
pK2 - pH
(6b)
+ P K ~= 2(pHmin) - log (k4/k3)
(9)
The pH a t the minimum is 9.0-9.2, k3 = 133 s-l, and k4 = 2 s-l; hence pK1 + pK2 = 16.2 which is to be compared with the value of 16.3 derived from the sum of the pK values obtained from eq 6a and 6b. Influence of Temperature on the Observed First-Order Rate Constant. The first-order rate constants were measured as a function of temperature a t pH 2.3 where kobsd = k3, a t pH 6.6 where kobsd = k3[H+]/K1,a t pH 10.6 where kobsd= k4K2/[H+],and a t pH 11.93 where hobsd = k4. Within experimental error, the data were described by the simple Arrhenius equation (Figure 3) from which activation energies and preexponential factors were calculated (Table 111).
792
The Journal of Physical Chemistry, Vol. 83, No. 7, 7979
James W . Sutherland
/
0.7
Oe3V
0.2
2.0
1.0 [H2P0i]X 10' M
LTX 1 o 4 Flgure 3. Effect of temperature on first-order rate constant at selected pH values: (1) pH 2.30, scaling factor A = 100; (2) pH 6.60, scaling factor A = 1.0; (3) pH 10.6, scaling factor A = 0.1; (4) pH 11.9, scaling factor A = 1.0.
TABLE 111: Effect of Temperature on the Decay Kinetics of Triazeneu
pH
Ehb kcal mol-'
log A kobsdC 11.33 i 0.57 kobsd = k , 13.30 i 0.48 kobsd = k,[H+]/K, 10.18 f 0.58 k o b d = k,K,/[H+] 14.22 ?r 0.59 kobsd = k, Solution 0.01 hl in hydrazine. b Observed activation energy, Definitions of observed rate constants.
2.30 6.60 10.60 11.93
12.63 i 17.59 i 15.00 f 18.98 i
0.75 0.63 0.76 0.77
Since the simple form of the Arrhenius equation adequately fits the data, then in the equilibrium reaction described by eq 1,AH1', the standard heat of the reaction, can be identified as equal to the difference in activation energies between the forward and reverse processes and AS1', the standard entropy of the reaction, can be idenare tified equal to R In (Al/A-l), where Al and preexponential factors. Thus, from the results obtained a t pH 2.3 and 6.60 AHl' = E3 - Eobsd(pH6.6) = -4.9 kcal mol-'
AS,' = R In (Al/A-l) = -39 eu and
AH'' AS'' pK1 = -- -- 4.9 (T = 298.17 K) 2.303RT 2.303R By a similar analysis, using the results obtained a t pH 10.60 and 11.93, pK2 = 11.7 ( T = 298.17 K) and, for the reaction described by eq 2, AHz' = -4.2 kcal mol-' and AS2' = -67 eu. The values for pK1 and pK2 are in reasonable agreement with those calculated from the dependence of k on pH a t 295.17 K. Influence of Phosphate Buffer Concentration on the Kinetics of Decomposition of Triazene. In initial studies on the effect of pH on k, where there was concern that a t
Figure 4. Effect of phosphate buffer concentration on first-order rate constant in 0.01 M hydrazine solution adjusted to pH 7.81. ( 0 )Solution is 0.2 M in sodium sulfate.
near neutral pH, transient pH changes might influence the kinetics of decay, hydrazine solutions, buffered by phosphate, were irradiated. In the pH range 6-8, where acid catalysis occurred, higher values for the first-order rate constant were obtained and the slope of log k vs. pH was observed to be about -0.79 instead of -1 as demanded by the mechanism. This behavior suggested that phosphate ion was a specific catalyst for the decomposition of triazene. T o test this hypothesis, the effect of phosphate buffer concentration on the transient yield and on the rate constant was investigated for a 0.01 M hydrazine sulfate solution a t pH 7.8. Under identical pulsing conditions no change in the initial yield of the transient was observed in the presence of sodium phosphate, but the first-order rate constant increased linearly with buffer concentration (Figure 4). Such behavior can be explained by the addition of the overall catalytic reactions (eq 10a and lob) to the mechanism previously described by eq 1-4. Although it
-
+ N3H3+ HP042N3H3 H2P04-
+ NH3 + HZPO4N2 + NH, + HPOd2Nz
(loa)
(lob)
is not possible to specify the mechanism of these reactions from the limited data of this study, it should be noted that, in the absence of phosphate, equilibrium is apparently m a i n t a i n e d a t t h i s pH and t h u s t h e c a t a l y t i c mechanism by phosphate does not involve simple proton transfer. At pH 7.81, the concentration of N3H4+is small and since acid-base equilibrium is assumed k3
hobsd
= -[H+l
K1
+ h10[HZP04-l
where klo = klOa+ klOb(KHzPO,/[H+]) = 27.1 M-'S-' (Figure 4). From the observation that kobsd is a linear function of concentration of phosphate, it can be deduced that klo is independent of the ionic strength of the solution. T h i s conclusion was verified by measuring the rate constant in
Pulse Radiolysis of Aqueous Hydrahe Solutions
a solution which had been made 0.2 M in Na2S04. No change is observed in the rate constant (Figure 4). Thus the transient, identified as triazene, is amphiprotic; its acidic form has unit positive charge and its basic form has unit negative charge. The same conclusion can be drawn from the observation that the rate constant is independent of hydrazine sulfate concentration. Spectra and Extinction Coefficients of Triazene. The initial absorbance of the transient was measured in nitrous oxide and nitrogen or argon saturated solutions under experimental conditions where hydrazine concentration, dose per pulse, dose rate per pulse, and pH were varied. In nitrous oxide saturated solutions, the initial absorbance yield was independent of absorbed dose a t constant pulse length in neutral and alkaline solutions but in acid solutions (pH range
