2108
R. W. AHRENS
Vol. 66
yIRRADIATION OF AQUEOUS SOLUTIONS OF Fe (11)-HYDRAZINE1 BY R. IN.AHRENS Savannah River Laboratory, E. I . d u Pont de Nemours & Co., Aiken, South Carolina Recewed February 8, 1961
?-Induced oxidation of Fez+in 0.4 M HzS04 in the presence of oxygen is inhibited by hydrazine. The decrease in Feat yield due to hydrazine is greatest a t the highest dose rates. The results are explained quantitatively by the assumption that hydrazine reacts with OH radicals to form hydrazyl radicals, which either oxidize Fez+ or combine to form ammonia. At a dose rate of 2 X lo6 rad/hr. about 90% of the hydrazyl radicals react to form ammonia.
Introduction Interference with the aqueous processing of irradiated reactor fuel is often encountered from chemical reactions originating in the radiolysis of the solutions. A possible method of minimizing the interfering reactions is the addition of a radical scavenger to the solution. For initial studies, the effect of hydrazine on the y-oxidation of ferrous ion was selected, since many studies relating to this system have been made. The oxidation of ferrous ion in sulfuric acid solutions has been the object of many investigations and the reactions involved are well understood.2 The radiolysis of hydrazine in aqueous solutions has been investigated prev i o u ~ l y and , ~ the reduction of ferric ion by hydrazine also has been studied by several investig a t o r ~ . ~ The - ~ present investigation was designed to determine the mechanism of the protection of Fez+ by hydrazine during radiolysis: the results of this work also provide supporting evidence for some of the reactions proposed in the reduction of ferric ion by hydrazine. Experimental Water was pursed according to the process developed by the L. I. Biological Laboratory? distilled water was successively distilled from alkaline permanganate and acid dichromate through columns packed with Pyrex helices. The vapor, mixed with washed oxygen, was passed through a quartz tube packed with quartz fragments that had been heated to about 900’ in a tube furnace. The vapor then was condensed and redistilled into glass-stoppered bottles for storage. Ferrous sulfate dosimeter solutions prepared with and without 10-3 M chloride showed that organic impurities in this water were below the detectable concentration. 9110 The solutions were prepared with Mallinckrodt “Reagent Grade” sulfuric acid. “Baker Analyzed” Reagent Grade ferrous sulfate, FeS04.7Hz0, was recrystallized from 0.4 M &So4 in some of the final experiments. The hydrazine was obtained from a 35% stock solution of “Hyzeen,” manufactured by Betz Laboratories, Inc., and was used without further purification. The sodium chloride used in some of the experiments was “Fisher Certified Reagent” grade. The concentration of FeS+ in irradiated solutions was de(1) The information contained in this article was developed during the course of work under contract AT(07-2)-1 with the U. S. Atomic Energy Commission. (2) (a) H. Fricke and 8. Rlorse, Phzl. M a g . , 7, 129 (1929); (b) A. 0. Allen, “The Radiation Chemistry of Water a n d Aqueous Solutions,” D. Van Nostrand Co., Inc., Princeton, N. J., 1961. (3) H. A. Dewhurst a n d M. Burton, J . Am. Chem. Sac., 77, 5781 (1955). (4) F. H. Pollard and G. Kickless, J . Chromatog., 4, 196 (1960). ( 5 ) J. W. Cahn and R. E . Powell, J . A m . Chem. Soc., 76, 2568 (1954). (6) W. C. E. Higginson a n d P. Wright, J . Chem. Sac., 15.51 (1955). (7) D. R . Rosseinsky, zbzd., 4685 (1957). ( 8 ) H. Fricke, E. J. Hart, a n d H. P. Smith, J . Chem. Phye., 6, 229 (1938). (9) H. A. Dewhurst, ibzd., 19, 1329 (1951). (10) D. M. Donaldson a n d h’. Miller, J . chzm. phys., 62, 578 (1955).
termined spectrophotometrically with a Beckman DU spectrophotometer. Measurements were made a t 304 mp. The absorbancy index” used in calculating the Fe3t concentration was corrected to the temperature of the sample (room temperature). The procedure of Dewhurst and Burton3 was modified to give an improved analysis for NHB at the concentrations encountered in this work. Hydrazine in the irradiated samples was destroyed by adding an excess of KIO4 in.the presence of about 3 M HzS04 and heating (without boiling) until the iodine color disappeared. The solution was diluted with water immediately after this treatment to prevent formation of 1 2 by decomposition of the concentrated KIO4. Mossy zinc was added, and the solution was made alkaline with NaOH (the mossy zinc provided stirring by the liberation of H2). The ammonia was distilled from the alkaline solution and was absorbed in a solution of HC!. The latter solution was diluted to 100 ml. in a volumetric flask, and the NH8 in an aliquot of this solution was determined spectrophotometrically with Nessler.’s reagent.lZ The calibration curve used in this determination was prepared by analyses of known concentrations of ammonia in solutions which had the same composition as the samples. The calibration curve was linear for the amounts of NHI investigated (0 to 5.0 X 10-smeq.). The radiation source consisted of five slugs l l J / s in. long, each containing 2200 c. of Coco, surrounding the bottom portion of a 3-a/,,-in. i.d. aluminum sample tube which was 13 ft. long. The slugs and the major portion of the sample tube were immersed in a water pit containing water to a depth of about 12 it. for shielding. The 11,000 c. of CoB0 provided a maximum dose rate of 2.2 X 106 rad/hr Samples were placed in a sample carrier that was grooved to fit four ribs extending down the inside wall of the sample tube. With this arrangement it was possible to position the samples reproducibly inside the sample tube. Lower dose rates were obtained by attaching spacers t o the bottom Ff the sample carrier to position the samples at various distances from the cobalt slugs. The duration of irradiations could be timed to within 1sec.
.
Results Hydrazine reduces the yield of Fe3+in the radiolytic oxidation of Fez+ in 0.4 M HzS04. Typical data for the radiolytic yield of Fe3+ as a function of N2H4 concentration are shown in Fig. 1. The yield of Fe3+ approaches a constant value at) N2H4 concentrations above about 0.04 M (SZH4/Fe2+ 30). The difference between the yield of FeS+at this constant value and the yield in the absence of N2H4 increases with increasing dose rate (Fig. 2 ) . The data in Fig. 2 show the fractional decrease in yield due to NzH4, (-AFe3+/ Fea3+),as a function of dose rate. The quantity (- AFe3+/Feo3+) approaches a value between 0.46 and 0.49 a t high dose rates. In some of the experiments 10-3 M C1-was present for the purpose of suppressing the effect of minute amounts of organic imp~rities.9~10The difference found be(11) C. M. Henderson a n d N.Miller, Radzation. Res., l s , 641 (1980). N. H. Furman, E d . , “Scott’s Standard Methods of Chemical Bnalysis,” Vol. 1, 5 t h ed., D. Van Nostrand Co., New York, N. Y., 1939, p. 630. (12)
Xov., 1962
Y-IRRADIATION O F
AQUEOUS SOLUTIONS O F Fe(II)-IIYDRAZINE
2109
tween the results of experiments with and without X C1- was small. However, the data in Fig. 2 show consistently higher values of (- AFe3+/Feo3+) in the presence of C1- so that separate curves could be drawn through the two sets of points. The re- 2 action between $'e3+and NZH4 was neglected since 8 most of the irradiations lasted between 30 sec. + and 20 mjn., and solutions were analyzed for Fe3+ immediately af tier irradiation. The calculated4 e decrease in Fea+ concentration due to this reaction was less than 2%/hr. On standing overnight, a decrease of about 10% was observed for a 0.1 M I I I 1.2 1 N2H4 sample. 0 0.02 0.04 0.06 0.08 0.10 Dewhurst and Burton3 observed the radiolytic Initial NlH4 ooncn., moles/l. production of NH3 in the irradiation of air-free aqueous solutioris of hydrazine. The G-values Fig. 1.-Effect of NzH4 on radiolytic production of Fe3'. for KH3 increased with increasing initial hydraI0 7 I I' I I zine concentration and reached a constant value I I II I I I II of 5.2 above 2 X 110-2i!4 hydrazine. I I I I t I I I I I l l G N H as ~ a function of initial S Z Hconcentration ~ also was measured in the present work. However, in this case the solutions were saturated with either air or oxygen. The yield, G"!, for a constant total dose increased with increasing initial hydrazine concentration up to about 4 X M; Id thereafter, the yield of ammonia was nearly constant with increasing hydrazine concentration (Fig. 3). In the solutions that were initially saturated with oxygen, the constant value attained for G N H ~was 6.2. I n the solutions that were initially saturated with air, the constant value attained was 4.6. In both of these experiments the total dose received by each sample was 1.33 X lo6 rad. All the dissolved oxygen was consumed in to5 the air-saturated solution after a dose of about 5 X lo4 sadla; the oxygen-saturated solution contained some dissolved oxygen throughout the entire irradiation. 3
w
? . .
I
I
I
I
Discussion Radiolytic oxidation of Fez+ in acidic aqueous solution (0.4 fM H2S04)containing dissolved oxygen occurs as a result of reaction of the Fez+ with OH radicals, HOZ radicals, and H202 molecules according to the scheme14 HzO M+H, OH, H2, Hz02
+ Fez+-+ Fe3+ + OH13 + 0 2 +HOz H + + HOZ -tFez+ +Fe3+ + HzOz HzOz + Fez++ Fe(OH)2+ + OH OH
(1)
(2)
(3) (4) (5)
The OH formed in (5) reacts with another Fez+ as in (2). The part played by hydrazine in reducing the radiolytic yield of Fe3+ can be exphined by the scheme OH
+ NzH4
+HzO
+ NzH3
(6)
(13) J. Weiss, A. 0. Allen, a n d H. A. Schwarz, "Use of the Fricke Ferrous Sulfate Dosimeter for Gamma-Ray Doses in the Range 4 t o 40 kr." Proc. Intern. Conf. Peaceful Uses At. Energy. Ceneva, 14, 179 (1955). (14) E.J. Hart, J . Chem. Edw., SB, 266 (1959).
Fig. 2.-Dose
H+
-A Fe3*/ Fez+. rate dependence of fractional loss in F e a t yield due to N2H4.
+ NZH3 + Fez+
--.f
Fe3+
+ NzH4
2xzH3 --+ N4H6 X4H6 --+ 23"
+
+ Nz
+
1
(7) (8)
H NZH4 HZ N2H3 (9) For simplicity, the formulas used here are designed to represent the oxidation state of the nitrogen atoms rather than the actual structure of the radicals. Reaction 9 would compete with (3) since the --+
R. FT. AHRENS
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66
VOI.
the decomposition of water by radiation with three
solutions always were initially saturated with air or 02. Reactions 6 and 9 were postulated by Dewhurst and Burton3 to occur in deaerated solutions. Since 0 2 is an efficient scavenger for the H atom it is likely that reaction 9 is negligible in the present system. Reaction 7 has been proposed by Pollard and Nickless* and by Higginson and Wright6 as part of the reaction scheme for the slow reaction between Fe3+ and hydrazine. A steady-state treatment of the rate equations for reactions 2, 6 , 7, and 8 results in the expression ks(NdL)~ R I MFe2+) h(N2H4) kS('N2H3)2
+
+ k7(Fe2+)(NzH3)
(I)
studied. 'The oxidation of Fez+ according to the mechanism of reactions 2 through 5 results in a 8E 4R. yield (GF~I+= 15.6) given by 2F If all OH radicals disappear via reaction 6 and all NzH, radicals via reaction 8, the fractional loss in Fe3+ yield is given by (GH GOH G~,o,)/l5.6. The value of (-AFe3+/Feo3+) thus can be calculated from the equivalent expression (2R 3E F)/15.6. The value obtained, 0.48, is approached a t high dose rates by both curvas shown in Fig. 2. A graph of Rs/R7, calculated for each point in Fig. 2, as a function of the square root of the dose rate allows an estimate for k ~ ' / ~ / kof. i 0.05. Chloride ion present in some of the experiments M had a small but measa t a concentration of urable effect on the kinetics. Chloride ion participates in the oxidation of Fez+ through a radical transfer mechani~m.~
+
+
+
+
+
+
The quantity, ~ R Iis, defined as the rate of formation of OH radicals from the decomposition of water and includes OH radicals formed by reOH C1H + + C1 H20 (10) action 5 ; I is the radiation intensity. At high concentrations of hydrazine (SzH4/Fe2+ > about The C1 atoms formed in reaction 10 oxidize Fez+ 30), reaction 2 is assumed t o be negligible compared quantitatively. Most organic impurities that are to reaction 6, and k2(Fe2+)
