4470
Inorg. Chem. 1985, 24, 4470-4471 Contribution from the Savannah River Laboratory, E. I. du Pont de Nemours and Company, Aiken, South Carolina 29808
Oxidation of Hydrazine by Nitric Acidt David G . Karraker Received January 7, 1985 Hydrazine is oxidized by hot nitric acid in a first-order reaction to produce N,, N20, HN,, and NH4'. The rate law for the reaction is -d In (N2H4)/dt = ~C(NO,-)(H')~where k = 5.8 X M-' min-l at 100 OC in 5.44 M HNO, and = 6. The data are consistent with a reaction mechanism that involves HN,, HNO,, and the N2H2free radical as intermediates and N,, N,O, and NH4+as products. The Arrhenius equation constants over the temperature range 70-100 OC were A = 1.2 X 10" M-' min-' and E = 26 kcal/mol. The reaction is catalyzed by Fe3', and the rate data are correlated by the semiempirical expression In [(N2H4)/(N2H4),,]= -[a'(Fe3+) + b'(Fe2+)](H+)rwhere a'= 0.1 14 M-, min-I and b'= 0.065 M-, m i d . A reaction mechanism is proposed for the ferric-catalyzed reaction whose major reactions involve the reduction of Fe3+to Fez' by hydrazine and the oxidation of Fe2' to Fe3' by nitric acid.
Introduction Hydrazine is used in nuclear fuel reprocessing as both a reducing agent and a nitrous acid scavenger.',2 As a reducing agent, hydrazine normally reacts slowly a t room temperature but has reaction rates a t higher temperatures rapid enough to be useful for the reduction of Pu(1V) and Np(V)., The rapid reaction of hydrazine with HNOz is applied to the stabilization of U(IV) nitrate, ferrous sulfamate, and hydroxylamine solutions in solvent-extraction and ion-exchange processes for the separation of uranium, actinides, and fission products. A recent application is the use of hydrazinium nitrate-HN0,-KF solutions for dissolving plutonium metal.4 Hydrazine prevents the precipitation of plutonium(1V) oxides during the dissolving of plutonium metal. Many of these applications involve temperature and acid concentrations where the stability of hydrazine is unknown and where the maintenance of a hydrazine concentration is critical to the success of the process. This study was begun to provide more detailed information on the reaction rates and products of the hydrazine-nitric acid reaction. Since Fe(I1) is used as a reagent in the chemical processing, the effect of Fe(I1) and Fe(II1) on hydrazine oxidation was also investigated.
Experimental Section Reagents. Hydrazinium nitrate solution was purchased by the Savannah River Plant (SRP) from Fairmont Chemical Co., Newark, NJ, as a 3.6 M solution. The solution for the study was obtained from the plant stock. Nitric acid, sodium azide, sodium hydroxide, and ferric nitrate were CP grade reagents. Ferrous nitrate solution was prepared by dissolving iron metal in 3 M HNO3-0.5 M N2H4.HN0, at a temperature below 50 OC. Analyses. Hydrazine was determined by the indirect iodate method. The sample was added to a measured excess of standard KI03 solution, acidified with 2 M H2S04,and mixed for 1 min or more. Unreacted KIO, was reduced to I2 with an excess of 0.1 M KI solution,and liberated I, was titrated with standard Na2S203solution. Hydrazoic acid was determined by two methods. For small samples, the solution was mixed with HN03-Fe(N03), solution and the concentration of the FeN,,* complex determined spectroph~tometrically.~ When large amounts of sample (-5 mL) were available, HN, was nitrogen-sparged from an acid solution and collected by absorbing the vapor in a measured volume of standard NaOH solution. A few drops of Fe(N03), solution were added to the sample before sparging; the disappearanceof the red-brown FeN3Ztcolor indicated completeremoval of HN,. Hydrazoic acid was then determined by titration with standard Ce4+.6 Nitric acid was determined by titration with standard base with a methyl red indicator. Ammonium ion was determined with an ammonia electrode (Orion Associates, Cambridge, MA). Gas samples were analyzed by a Hewlett-Packard Model 5750 gas chromatograph with a Carbosieve column. Results were corrected for air leakage from oxygen analyses. Ferrous ion was determined by titration with standard ceric sulfate solution in 2 M H2S04. Tests found that interference from HN, or N2H4 was negligible under these conditions.
The information contained in this article was developed during the course of work under Contract No. DE-AC09-76SR00001 with the U.S.Department of Energy. 0020-1669/85/1324-4470$01.50/0
Procedure. The reaction vessel was a two-necked 100-mL flask immersed in a thermostated oil bath. One neck of the flask was used for reagent addition and liquid sampling; the other neck was fitted with a reflux condenser that was connected at its upper end to a gas sample bulb. The gas sample bulb was in turn connected to a water-filled flask. Evolution of gas during the reaction displaced water into a graduated cylinder for measurement. The reaction mixture was magnetically stirred. Gas volumes were determined in parallel experimentsthat were not disturbed by solution sampling. The initial solution volume in all experiments was 52 mL. Eighteen milliliters of concentrated nitric acid (15.7 M) and a combined volume of 26 mL of water, I O M NaOH, and 1.3 M Fe(N03), solution were mixed and brought to the selected temperature in the oil bath; the reaction was initiated by adding 8 mL of 3.6 M N2H4.HN03solution. NaN3 and Fe(N03)*were added after the hydrazine addition in experiments where their effects on the reaction were explored. The initial ionic strength (HNO, + NaNO, + N2H4.HN0,) was 6.0 M; the final ionic strength (HNO, + NaNO, + HN, + NH4N0,) was -5.6-5.8 M. The concentration of NaN03 in this study is always the difference between 5.44 M and the acid concentration. For example, a reaction in 4.41 M HNO, in this paper is a reaction in 4.41 M HN03-1.03 M NaNO,.
Results A. Nitric Acid Oxidation of Hydrazine. Acid Dependence. Experiments in 6.0 M NO3- a t 100 "C established initially that the reaction was first order in hydrazine. Figure 1 shows a typical first-order plot for the concentration of hydrazine with time for the initial conditions 4.38 M HN03-0.55 M N,H4-HN03. The reaction rate was also found to depend on the square of the acidity (Table I). Reaction Products. The products of the hydrazine-HNO, reaction are N2, N20, HN3, and NH4+;no NO or NO2 was detected in any of the gas samples. Figure 2 shows the concentrations of NzH4,NH4', and HN, during the reaction and the rate of gas evolution during the reaction in 5.44 M HNO, at 100 "C. The decrease in HN, concentration near the end of the reaction indicates that HN3is being destroyed as well as produced in the system. A 1-h test in 6 M HNO, a t 100 "C found that there was no change in the concentration of NH4+within the limits of error of the experiment, i3%. Ammonium ion is therefore considered to be a final product. Data for the solution products of the reaction are presented in Table I. HN3 Volatility. Pure HN, is quite volatile (bp 37 "C)' and is easily removed from acid solutions by a sparging gas stream.*-1° (1) Kelmers, A. D.; Valentine, D. Y."Search for Alternate Holding Reductants to Stabilize Plutonium(II1) Solutions", Report ORNL/TM 6521; Oak Ridge National Laboratory: Oak Ridge, TN, Sept 1978. ( 2 ) Perrott, J. R.; Stedman, G. J. Inorg. Nucl. Chem. 1977, 39, 325. ( 3 ) Karraker, D. G. "Hydrazine Reduction of Np(V) and Pu(IV)", USDOE Report DP-1601; E. I. du Pont de Nemours & Co., Savannah River Laboratory: Aiken, SC, Nov 1981. (4) Karraker, D. G. "Dissolution of Plutonium Metal in HN0,-N,H4-KF", USDOE Report DP-1666; E. I. du Pont de Nemours & Co., Savannah River Laboratory: Aiken, SC, July 1983. ( 5 ) Dukes, E. K.; Wallace, R. M. Anal. Chem. 1961, 33, 42. (6) Arnold, J. M. Ind. Eng. Chem., Anal. Ed. 1945, 17, 215. (7) Yost, D. M.; Russell, H., Jr. "Systematic Inorganic Chemistry"; Prentice-Hall: New York, 1946; p 119. (8) Maya, B. M.; Stedman, G. J. Chem. SOC.,Dalton Trans. 1983, 257. (9) Templeton, J. C.; King, E. L. J. Am. Chem. SOC.1971, 93, 7160. 0 1985 American Chemical Society
Inorganic Chemistry, Vol. 24, No. 26, 1985 4471
Oxidation of Hydrazine by Nitric Acid Table I. Reaction Data for N2H4 Oxidation at 100 "C"
(HNO,), M 5.44 5.20
final concn, M HN, 0.077 0.078 0.030 0.096 0.072 0.13 0.090 0.095 0.077
N2H4
0.083 0.035 0.041 0.12 0.074 0.22 0.18 0.19 0.33
5.05
4.41 3.90 3.41 3.04 2.98 2.56
NH4+ 0.061 C
0.085
0.067 0.063 0.062 0.070 C
0.055
reacn half-time, h 1.08 1.2 1.4 1.6 2.1 3.0 3.3 3.9 5.5
half-time X (HN03)2 32.0 32.4 35.7 31.1 31.9 34.9 29.7 34.6 36.0 33.2 i 1.9 (av)
105k,bM-3 min-' 6.00 5.96 5.41
6.21 6.05 5.53 6.49 5.58 5.36 5.83 f 0.33 (av)
"Initial conditions: 0.55 M N2H4.HNOp,/I = 6.0. b k for d In (N2H4)/dr= /c(NO
