D. KATAKISAND J. KONSTANTATOS
2054
ance of ordered phases in the mixed oxides. However, as has been mentioned earlier, there does not seem to be much reason to believe that there is any valency stabilization; hence the disappearance of ordered phases on moderate addition of one oxide to the other is not surprising. The exception here is the special case of the R01.714 phase. Examination of Figures 1 and 7 will show that for the same pressure and temperature over a reasonable range, ~Pr01.714and ~Tb01.714 will be obtained. What this means for the mixed oxides is that both the praseodymium and terbium may satisfy independently at ROl .7,4 the average cation valency requirements of this phase, but the distribution of praseodymium and terbium will still be random, since cation mobility is small in fluorite and fluorite-related structures (not, however, the oxygen mobility which is considered very facile). Then, the greater the concen-
tration of one oxide in the other (maximum at Pro.5Tbo.sO,), the less ordered the R01.714 phase must be, for even though there may be the right number of the two kinds of cations (4R3+:3R4+;R = Pr, Tb), these are not distributed in the required manner. This alone may be enough to rule out the existence of a R01.714 phase in Pro.5Tbo.50,. However, for the Pro.~5Tb0.280, mixed oxide where there is also no sign of a R01.714 phase-though such a phase is found in the conjugate mixed oxide Pro.25Tbo.7eOz-thefurther assumption has been made that the system is effectively, if not in reality, transferred to higher pressures (and temperatures). Furthermore, the relatively greater stability of Tb01.714 than of PrOl.714 could govern the obvious dissymmetry.
Acknowledgment. The authors are grateful to the Atomic Energy Commission for its support of this work.
Decomposition of Aqueous Perchlorates by Radiation
by D. Katakis and J. Konstantatos Nuclear Research Center "Democritos," Aghia Paraskeui, Attilcis, Athens, Greece Accepted and Transmitted by The Faraday Society
(Received October 29, 1967)
The investigation of the radiation chemistry of aqueous concentrated perchlorate ion solutions is extended into the alkaline region, where the G values of chlorate, chlorite, and some of the HzO primary products, at various NaC104 concentrations, are determined. The G values of the primary products are calculated from data obtained with solutions containing ferricyanide, ferricyanide CH30H, and ferrocyanide NzO. It is concluded that, for a given C104- concentration, G(-C104-) remains the same over a wide range of hydrogen ion concentrations. A brief comparison is also made among H+, Na+, Zn2+, and Laa+ in relation to their role in the radiolysis of the perchlorate-water mixtures.
+
+
Introduction
Experimental Section
On the basis of the evidence obtained so farl1I2 the only mode of decomposition of the perchlorate ion in the radiolysis of its concentrated aqueous solutions is by the so-called direct excitation. Attack by the water-free radicals may be important only in the spurs, under special energetic conditions. I n the present paper the investigation of the radiolysis of concentrated aqueous solutions of C104- is extended into the alkaline region. Thus the information now covers a range of hydrogen ion concentrations of approximately 14 orders of magnitude. It was also found useful to make a brief attempt to compare H+ to other cations. Some information about Na+ is already available. I n the present work we report results obtained with Zn2+ and La*+ in large concentrations.
The irradiations and measurements were usually made in Spectrosil cells, without transferring the solutions. Co60 y rays were used. The yields refer to the total energy absorbed by the solution, calculated on the assumption that it is proportional to the electron density; more sophisticated assumptions7 are per-
The Journal of Physical Chemiatry
D. Katakis and A. 0. Allen, J. Phys. Chem., 68, 3107 (1964). J. Konstantatos and D. Katakis, ibid., 71, 979 (1967). B. Milling, G. Stein, and J. Weiss, Nature, 170, 710 (1962), M. Cottin, J . Chim. Phys., 53, 903 (1966). L. T. Bugaenko and Yu. M. Maksimov, Russ. J . Phys. Chem., 40, 976 (1966). (6) L. T. Bugaenko, W.-H. Wang, and V. N. Belevskii, ibid., 40, 1129 (1966). (7) H. A. Mahlman and G. K. Schweitser, J . Inorg. Nucl. Chem., 5, 213 (1968). (1) (2) (3) (4) (6)
DECOMPOSITION OF AQUEOUS PERCHLORATES BY RADIATION haps equally arbitrary and do not seem to eliminate the uncertainty as to exactly how much energy is really absorbed. Because of this, a quantitative comparison between the concentrated solutions of the various cations must be avoided. The c103-, CIOz-, and HzOzanalyses of the alkaline solutions of NaC104 were made after neutralization, as described previously.2 Ferricyanide was determined spectrophotometrically at 420 mp, using E 1000. Sodium perchlorate does not affect this extinction coefficient. Light exposure of the ferro- and ferricyanide solutions was avoided as much as possible. Freshly prepared solutions were used. Carbonate was removed with Ba(OH)z and subsequent filtration of the precipitate. All reagents were of analytical grade and used without further purification. The triply distilled water had a specific resistance of lo6 ohm cm. La(C104)S solutions were prepared by dissolving La203in concentrated HClO4. La203 had previously been baked at 1000” in an oxygen atmosphere in order to destroy organic impurities. La(ClO4)a SOlutions preoared from commercial La203contained an impurity which gave an absorption of light around 260 mp and which was identified as being Ce3+. Its percentage over La3+was of the order of 0.1. Various blank experiments indicated that the results reported in the present paper are not affected by the presence of Ce3+. The concentration of La3+ was determined potentiometrically with 1 M NaOH and was also determined by compleximetry with EDTA using bromopyrogallol as the indicator. Zn(C104)z solutions were prepared by dissolving zinc carbonate in perchloric acid. The concentration of Zn2+ was also determined by compleximetry with EDTA, using Eriochrome Black T as indicator. Fe3+ was measured at 240 mp with E 4185; we found that the presence of high concentrations of salts did not change (within -2%) the extinction coefficient of Fe3+ at 240 mp. Chlorate in concentrated La(C10J3 and Zn(ClO& solutions, at low HC104, was determined kinetically by following its reaction with Fez+ and extrapolating the difference of the instantaneous value of [Fe3+] from its “infinite” value to zero time. With excess Fez+ the reaction is pseudo-first order8 and the extrapolation is straightforward.
Results G(C103-) in a 6 M T\’aC104,0.1M NaOH, carbonatefree, air-saturated solution was found to be 1.50 and G(C102-) was 0.23. Chloride is formed only in very small amounts. Bromide added to the initial solutions (10 mM) has no effect. This is consistent with the conclusion of Cheek and Linnenbomg that a t pH 13 the reaction of Br- with the hydroxyl radical is very
2055
inefficient. Hydrogen peroxide is formed in appreciably larger amounts than in neutral solutions. This enhancement, nevertheless, seems to be due to carbonate: with Ba(OH)z added, the peroxide yield is suppressed to almost zero. Bromide has also no effect on the peroxide yield. Carbonate influences not only the peroxide yield but also the yields of the decomposition products of perchlorate and seems as if it induces some complicated thermal reactions which we did not attempt to investigate. I n order to obtain information on the yields of the so-called primary radiolysis products, we irradiated solutions containing ferricyanide, ferricyanide CHIOH, and ferrocyanide NzO. The ferric ion reduction G values obtained in 1 mM ferricyanide, air-saturated solutions, containing 0.1 N NaOH and various NaC104 concentrations are summarized in Table I, column A. An increase in
+
+
Table I: G Values of Ferricyanide Reduction or Formation in 0.1 N NaOH Sodium Perchlorate Solutions Conon of
NaC104,
________ G valuesa---------
Y
A
B
C
6
2.8 2 7 2.6 2.5
7.2 7 5 7.7 7.9
1.2 2 0 3.0
4
2 0
3.8
a Column A, G( -Fe(CN)63-) in 1 mM ferricyanide solutions; column B, G( -Fe(CN)2-) in 1 mM ferricyanide solutions with 0.1 M CHsOH; column C, G(Fe(CV).$-) in 1 mM ferrocyanide solutions saturated with NzO.
[NaC104] causes a slight increase in G( -Fe(CN)63-). Above a dose of 1-2 X loz1eV/l., the concentration vs. dose curves were linear up to high conversion ratios. The yields quoted in Table I are those obtained above this dose. Carbonate, bromide, and deaeration did not affect the results. The initial slope was as much as twice the final constant value. The increased initial yields are more noticeable at high xaC1o4 concentrations but they are generally irreproducible and they may be attributed to impurities in SaC104. It is relevant to point out that when controlled amounts of such impurities (e.g., CH30H) are added to the solutions, the regular yields obtained are approximately of the same magnitude as the above initial yields. Blank experiments in both acid and alkaline solutions showed that C103- does not react with ferrocyanide. It was also confirmed that in alkaline solutions HZOZreduces ferricyanide, whereas it leaves ferrocyanide unaffected. (8) L. T. Bugaenko, Russ. J . Phys. Chem., 5 , 919 (1960). (9) C. H. Cheek and V. J. Linnenbom, J . Phys. Chem., 67, 1866
(1963). Volume 72, hiumber 6 June 1968
D. KATAKISAND J. KONSTANTATOS
2056
G(-Fe(CN)P) in solutions containing 0.1 N NaOH and 0.1 M CH30H are given in Table I, column B. G(-Fe(CN)63-) is lower at higher Sac104 concentrations. The concentration us. dose lines were straight, without intercepts, up to at least 80% conversion. Carbonate, air, and bromide did not affect the results. I n certain preparations of ferricyanide CHIOH solutions, anomalously low and irreproducible G values were obtained a t room temperature; at lower temperatures (5-8"), however, even these preparations gave the normal values reported in Table I, obtained with solutions which did not show this effect. The phenomenon is presumably due to catalytic decomposition of hydrogen peroxide induced by an unknown impurity. A small decrease in absorbance, namely, less than 5y0 in 24 hr, may be attributed to hydrolysis of the cyanide salts. G(Fe(CX)&-) in solutions saturated with N20 are summarized in Table I, column C. At high [NaC104] there is a considerable decrease in the G value of formation of ferricyanide. We paid particular attention not to have any gas space above the solutions in the cell, in order to avoid possible chain decomposition of N20in the gaseous phase, induced by the high-energy radiation. lo Figure 1 gives the G values of oxidation of Fez+ (10 mM), in 02-saturated and deaerated solutions, as a function of the c104-concentration. The figure includes data at constant HC104 (1.2 M ) and varying La(C10& concentrations, as well as data at constant La(ClOJa (1.34 M ) and varying HC104 concentrations. Postirradiation reactions were not detected in these solutions and thermal oxidation of Fez+ by perchloric acid under the conditions of the experiments is negligible. I n deaerated solutions, G(Fea+) is calculated from the initial slopes of the concentration us. dose curves which were essentially linear up to the higher dose used, namely, -1021 eV/1. Some additional data on the radiolysis of aqueous concentrated perchlorates are summarized in Table 11.
I-
I
+
Table I1 : G(Fe8+) and G(Cl0a-) a t Various C101- Concentrations"
Zn(ClO& Zn(ClO4)z Zn(C104)Z Mg(C104)a La(ClO4)a
Perchlorate
Concn of
concn,
HCLO4,
M
M
2.1 2.1 2.7 1.75 2.0
1.2 2.4 0.06-0.1 0.8 0.02-0.07
-G(Fe"9--0 2
De
saturated aeratedb G(ClOa-)
25.5 26 23 21.5 28.5
18.5 19.5 16.5 15 20.5
,..
... 1.3 ... 1.6
For irradiations up to a dose of Initial [Fez+] = 10 mM. eV/l., only with deaerated solutions of Mg(C104)~was there an indication of deviation from linearity of the concentration os. dose curves. a
-1021
The Journal of Physical Chemistry
I 2
OO
4
5
8
10
CI O,-(M)
Figure 1. G(Fe3+) as a function of the perchlorate ion concentration: full symbols, Oa-saturated solutions; open symbols, deaerated solutions; circles, 1.2 M HC104 and varying concentrations of La(Cl0a)a; squares, 1.34 M La(ClO4)a and varying concentrations of HClOa.
Kinetic determination of chlorate was made only at hydrogen ion concentrations low enough for the rate of its reaction with Fe2+ to be measurable. I n Table I1 we quote values for G(C1O3-) a t 2.7 IM Zn(C104)2 in the [H+] range from 0.06 to 0.1 M and at 2.0 M La(C104)1 in the [H+] range from 0.02 to 0.07 M . I n these hydrogen ion concentration ranges, G(C103-) is, within experimental error, independent of the hydrogen ion concentration and of the presence or absence of oxygen in the solutions. I n Zn(C104)2 solutions at [H+] lower than -0.06 M , somewhat higher values of G(ClO3-) were obtained in the presence of oxygen (not reported in Table 11), perhaps because of interference by basic zinc salts.
Discussion The radiation chemistry of alkaline ferricyanide and ferrocyanide solutions has been studied by a number of investigator^.^^-'^ I n the presence of high concentrations of NaC104, the ferricyanide reduction yield is G(-Fe3+) =
Ge,,-
+ GH + G H O+ ~ 2G~901-
- GOH
Gd
(1)
We conveniently use the acid forms to represent the various species, since acid-base equilibria are not relevant to our discussion. I n eq 1, as well as in the equations given below, any yield of oxygen atoms is included in G&, and Gd represents the oxidation equivalents of the chlorine compounds formed in the de(10) G. Czapski, private communication. (11) F. 8. Dainton and W. S. Watt, Nature, 195, 1295 (1962); Proc. Roy. Soc., A275, 447 (1963). (12) G. Hughes and C. Willis, Discussions Faraday SOC.,36, 223 (1963). (13) E. Hayon, Trans. Faradau Soc., 61, 723, 734 (1965). (14) G. E. Adams, J. W. Boag, and B. D. Michael, ibid., 61, 492 (1966).
DECOMPOSITION OF AQUEOUS PERCHLORATES BY RADIATION
2057
composition of clod- per 100 eV of absorbed energy. Chlorate is not included in G d , since it does not react. I n the presence of methanol, OH is transformed into CH20H, which reduces ferricyanide either directly or, if oxygen is present, through formation of HOz Gm(-Fe3) = Ge,,-
+ GOH+ GH + G H O+ ~ ~ G H -~ O ~ (2) Gd
I n ferrocyanide solutions saturated with NZO, hydrated electrons are transformed into oxidizing radicals via their reaction with N20. Therefore
+
GNZO(Fe3+) = 1,2Geaq- 1 . 2 G o ~-
+
GH - GHO*- ~ G H ~ OGd ?
(3)
The coefficient 1.2 in eq 3 is estimated from the data of Hayon13 and is introduced in order to account for the scavenging of the hydrated electrons by NzO. It is assumed that high n’aC1O4 concentrations do not have appreciable influence on this coefficient. The scavenging by the other compounds is neglected. Bromide and oxygen in particular are not expected to interfere with the measured yields. Methanol was used at a fairly high concentration, but, from the radiation chemistry point of view, the solutions can still be considered “dilute” since k , [ S ] < 107.13 The reducing agent which is represented as HO2 in eq 1-3 originates in the direct action of radiation ~ expected to have an on c104-,and hence G H O is appreciable value only at relatively high concentrations of NaC104. At very small or zero perchlorate concentrations, no HOz is formed since the yield of direct action on Clod- is negligible or zero and HO2 is not formed in measurable amounts in the decomposition of water by y radiation. Equations 1-3 are then simplified slightly, and from the data in Table I a t zero c104-, we obtain Ge,,-
=
2.6, GoH = 2.7, GH
+ ~GH,O,= 2.6
The values for the radical yields are in good agreement with the values reported by Hayon,la but our peroxide yield seems to be considerably higher. Our values for Ge,,GH ~ G H , Qagree with the corresponding value given by Hughes and Willis;lZ the agreement with these authors for GOHis also quite good. I n the presence of NaC104 we obtain the values of Ge,,-, GOH,and G’ = GH GHO~ - Gd given in Figure 2. It is seen that G,,,- decreases with increasing [NaC104]. The oxygen, on the other hand, from the “direct” effect on clod-, appears as emerging from the spur in the form of peroxide or HOz.
+
+
+
+
’b
’
2’
4
6 Na C106(M)
Figure 2. Calculated G values in the radiolysis of concentrated alkaline aqueous solutions of NaC104.
+
The values of G(C103-) G(C1OZ-) in the absence of scavengers, which can be thought of as equal to G(-C104-), do not vary from neutral to alkaline solutions; the separate values of G(C103-) and of G(C102-) do not agree as well as their sums, yet they could perhaps still be considered within experimental error the same. Comparison with acid solutions can be attempted only at HC104 concentrations below 3 M , since at higher acid we may have complications from the thermal reactions of c103-. Below 3 M HC104 G(-C1O4-), which is assumed to be equal to G(C1O3-) in the presence of C1- or Br-, has values essentially the same as the corresponding values in neutral and alkaline solutions. This lack of sensitivity of G( - c104-) on [H+]is by no means trivial, especially in view of the large effect on G(-H20) which is measured in dilute solutions. With cations heavier than H f or Na+, the data in Figure 1 and Table I1 indicate that there is an increase in the decomposition yields, which are larger for the heavier La3+. This means either that the simple assumptions used to estimate the absorbed energy are no longer valid or that there are more specific cation effects. With Laa+, in particular, it is noteworthy that there is a considerable increase in viscosity in going to higher concentrations16 which may affect the yields through a corresponding decrease in the values of the diffusion coefficients of the various species. Acknowledgment. This work was performed under the auspices of the Greek Atomic Energy Commission. (15) A. 0. Allen, Radiation Res. Suppl., 4, 54 (1964). (16) F. H. Spedding and M. J. Pikal, J . Phys. Chem., 70, 2430 (1966).
Volume 73, Number 6 June 1068
