Vol. 67
1466
THE "DIRECT EFFECT" I X THE RADIOLYSIS OF AQUEOT:S SODIUJL STTR.4TE SOLUTIOKS BY H. A. ~ I A H L M A X Chemistry Division, Oak Ridge National Laboratory,' Oak Ridge, Tennessee Received December 66, 1862 Oxygen-I8 enriched water was used to determine the source of molecular oxygen formed during the Co-60 ?-radiolysis of aqueous sodium nitrate solutions. Mass spectrographic analyees ot the 0 2 indicated that the oxygen came from two sourres: (1) the joint partiripation of H2O and XOa- and ( 2 ) from the nitrate ions. The oxygen formed by decomposition of the nitrate ion is directly proportional to the dissolved solute. Since nitrite ion is the only major'nitrogen-containing radiolysis product, the over-all decomposition is consistent with NOa--the Ce+4-0.4 M H2S04-SaN03system and, like the G(Cet3) in NO20.502. ,??he G(O2) TTag measur this system, increabed rapidly as a funct the N a S 0 8 concentration a t low KaXO3 concentrations and a t tly proportional to K a y o 3 concentration, a t high SaNOI concentrations. The G(02) from Os- in H201cNaKOa solutions is correlated with the directly proportional regions of the 02) measured in the Ce+"0.4 M HzS04-KaNOa system.
+
Intrdduction The radiation chemistry of dilute aqueous solutions has been interpreted on an initial decomposition of solvent into oxidizing and ing species and their subother or with dissolved sequent reactions with solutes. The observed ical reactions are consistthese species as the OH ent with the identificat radical and the H atom and/or solvated electron. When concentrated aqdeous solutions are irradiated, however, some product yields increase and $an no longer be explained by the currently accepted radical yields observed in dilute' Aqueous solutions. These iiicreased yields have beel; descriptively ter action" and are in general directly proporti concentration of the dis8olved solute. The this paper is to'present evidence for a direc tioiial direct action effect;011 the nitrate ion K a y o 3 solutions and to correlate these the previous work of the author. The enhanced cerous yield after corrections have been made for k n o h Cef4 reduction reactions iii the Cef4-0.4 M H2SOrSaS03 ~ o l u t i o nhas ~ , ~been divided into two individual contributions that will be discussed in terms of iiitratedecomposition by direct action 2nd by scavenging Experimental Reagent grade Baker and Adamson sodium nitrate and potassium bromide, C.P. sulfuric acid, and G. Frederick Smith analytical reagent ceric acid sulfate were used in the experiments. The sodium nitrate was recrystallized three times from specially distilled water recovering oily the middle 507, on each crystallization. The potassium bromide, however, was used without recrystallization. The sulfuric acid was redistilled in the presence of ceric ion using only the middle '/a portion for the preparation of a ceric ion stock solution. used for the recrystallization of SaNOa and for the acid solutions was purified by distilling water from a Barnste from acid dichromate solution, a basic permanganate solution, ahd then in an all-silica system. Storage was in silica vessels. The acidic ceric solutions were prepared for irradiation from the ceric stock solution to kontain 0.0004 M ceric ion, 0.4 M HzS04, and the desired concentratiob of NaN03. They were deaerated by repeated freezing, evacuating to 10-6 mm. pressure, and thawing and irradiated in a 600-curie cobalt-60 -/-ray source. Quantitative oxygen determinations were made by ignition with hydrogen on a platinum filament. For the isotopic experiments, oxygen-18 enriched water, containing 1.6% 0-18, was obtained ftom the Operatione Division of the Laboratory. This water was purified by distillretion from (1) Operated by Union Carbide Corporation for the Atomic Energy Commission. (2) T. J. Sworski, J. Am. Chem. Soc., 77, 4689 (1955). (3) H.A. Mahlman, J. Phys. Chem., 64, 1598 (1960).
an acidic potassium dichromate solution and then in an all-silica system. Further purification was effected by irradiating the airsaturated water in the cobalt-60 y-ray source and then photolyzing any peroxide that was formed. After repeating the irradiationphotolysis cycle three times, the water was used to prepare the KaNO3 solutions. Solutions were prepared to contain 10-3 AI potassium bromide, to protect the molecular hydrogen formed during irradiation from OH radical attack, and the desired amount of recrystallized sodium nitrate fy the concentration range 1.0 to 6.0 molar in sodium nitrate. These solutions were deaerated aB previously described. After irradiation the irradiation cells were opened and the permanent gases, hydrogen and oxygen, were recovered and analyzed by the Mass Spectrographic Laboratory. The possible molecular oxygen masses 32, 34, and 36 were monitored. In other similarly prepared samples the quantity of hydrogen and oxygen was determined by ignition on a platinum filament with oxygen and hydrogen, respectively. The dose rate of the cobalt-60 -pray source was determined by the oxidation rate of ferrous ion in an air-saturated 0.4 M H2SOc solution. It was assumed that 15.6 ferrous ions were oxidized per 100 e.v. of absorbed energy.4 The increase in the ferric ion spectrophotometric absorption was measured on a Cary recording spectrophotomtter, using the 25' molar extinction coefficient of 2240 a t 3050 A.4 Since the solutions contained large amounts of solute, the increase in energy absorption was calculated using y mass absorption coefficients.6 The yields, G(product), defined as the number of molecules formed per 100 e.v. of absorbed energy, are reported on the basis of total absorbed energy. The experimental details and techniques utilized to determine the G(C+a) in the Ce+4-0.4 M H~S04-NaX03solutions have been reported.3
Results and Discussion Although the exchange of oxygen atoms between H201*and KO3- takes place in acid solutions it should not be a factor a t the pH used for theee experiments.' The presence of the molecular oxygen mass 34 above that predicted from the natural abundance of the oxygen isotopes in nitrate ion indicates that the water has participated in the solution decomposition. In Table I, the per cent of molecular oxygen mass 34 observed is given for the NaN03 concentrations 1.0 to 6.0 M . Using oxygen mass 34 as the criterion to determine the extent of water participation the balance of the molecular oxygen must come from the only other oxygen-containing molecule or ion in the solution-the nitrate ion. The G(On)from the nitrate ion, G(Oz)mi;-, as calculated from the mass spectrographic data is also given in Table (4) C. J. Hochanadel and J. A. Ghormley, J . Chem. Phys., 21, 880 (1983). (5) H. A. Blahlman and G. K. Sohweitzer. J . Inorg. Nucl. Chem., I , 213 (1958). (6) hl. Anbar, M. Halmann, and S. Pinchas, J. Chem. SOC.,1242 (1960). (7) Xi, Anbar, personal communication.
DIRECTEFFECTIN RADIOLYSIS OF AQUEOUS SODIUM NITRATE SOLUTIONS
July, 1963
1467
I and plotted in Fig. 1. An explanation of the positive G(OZ)NO,-intercept equal to 0.05 is not apparent; however, it is readily seen in Fig. l that the incrdase in the G(O~)NO,is directly proportional to the NaN03 concentration in the aolution. This change in the G ( 0 2 ) ~ o swill - be utilized in the interpretation and conclusions of this paper. TABLE I OXYGEXAND HYDROGEN YIELDS -4s -4 FUXCTION OF Na?;Os CONCENTRATIOS NaNOa,
M
0 0.5 1.0 2.0 3.0
4.0 5.0 6.0
HzO'LXaKOa % 0 % Fraction Total mass 34 oxygen obsd. obsd. mass 34 G(Or)
C(OZ)NO,-
..
..
..
..
.. ..
.. ..
1.84 1.39 1.30 1.37 1.28 1.29 1.12 1.02
0.57 .43 .41 .43 .40
0.20 .31
.40
.35 .31
Ce +40.4 M H2S@4NaNOa G(Hz) G(0d
0.09 .I8
0.45 .154 .098 .066
0.80 1.17 1.41 1.48
.42
.25
.055
1.54
.50 .56 .63
.30 .3B .43
.045
1.61 1.66
,041
...
I
0
I
I
1.0
2.0
I
I
I
3.0 4.0 5.0 NaN03 MOLARITY.
I
6.0
I
7.0
G ( O a ) ~ 0 3as- a function of the NaN03 concentration.
Fig. 1.-The
..
Since nitrite ion has been shown to be the only major nitrogen-containing productls stoichiometric amounts of KO2- and O2 may reasonably be expected to be formed according to the over-all equation
Koa-
soz- f
hW--L
(1)
'/202
If this reaction or its equivalent takes place it should be easily detected in a Ce+4-0.4 fl4 H2S04-TU'aN03solution, since the Ce+4ion is readily reduced in acid solution by nitrite ion. In Fig. 2, curve A represents the observed reduction of ceric ions in a Ce+4-0.4 M H2S04-NaN03 solution.3 There is a rapid initial increase in the G(Cef3) with increasing nitrate concenttation. At higher nitrate concentrations the G(Ce+3) increases less rapidly and becomes a linear function of the Xa?J03 concentration. Let us discuss the Ce+P0.4 M HzS04NaN03 system by first presenting the radiation induced ceric reduction mechanismg that takes place in the absence of K a S 0 3 .
0
1.0 2.0
3.0 4.0 5.0 6.0 7.0 8.0 9.0 NaN03 MOLARITY.
Fig. 2.--Isolation of the enhanced G(Ce+3) in the Ce+40.4 M H2SOrNaN03 system: curve A, observed G(Ce+3); curve B, equivalent G(Ce+a) due t o the suppression of G(H9); curve C, G(Ce+3) observed in the absence of NaNOa equal t o ~GH,o, GH GOH 4- 2i0.45 G(Hz)].
+ -
-
The reaction of Ce+4 with H atoms (reaction 2) can successfully compete with the recombination of €1 atoms to form molecular hydrogen.1° Thus the G(Ce+3) is given by 2 G ~ ~ oGH~ - GOH 2[0.45 G(H,)]. Since the GOH has been shown to be constant as the Sa?u'03 concentration was varied over a wide range,, it was assumed that the ceric reduction was also constant and equal to the same redyction rate as in 0.4 M HzS04. Thus, the reduction of %e+.4by the radiolysis of solvent is given by curve C in Fig. 2 as G(Ce+3) = 2.38.3
The suppression of the molecular hydrogen yield, l1 G(H2), indicates that the rate constant for the reaction YO3H is sufficiently large to successfully compete with H H t o form molecular hydrogen. In Table I the G(HJ observed in Ce+P0.4 M H2S04-NaN03 solutions are given. The decrease in the G(H2) manifests itself by forming nitrite ion12 or a precursor which reduces a stoichiometric equivalent amount of ceric ion. In Fig. 2, the difference between curve C and curve B represents the G(Ce+3)attributable to the suppression of molecular hydrogen by nitrate ions. The difference between curve A and curve B is a measurement of the corrected enhanced cerous yield and is plotted in Fig. 3. We see that as the Xa;?T03concentration increases there is an initial rapid increase in the ceric reduction yield which then becomes lipearly dependent upon the sodium nitrate concentration. If we extrapolate this linear portion, ie., the cerous yield a t KaN03 concentrations 2 molar and greater back to
(8) H. A. Mahlman, unpublished results, Chemistry Divisibn Information Meeting, Oct., 1961. (9) A. 0. Allen, Radzotion Rea., 1, 87 (1954). (101 H.A. Mahlman, J. Am. Chem. Soc., 81, 3203 (195s).
(11) (a) H. A. Mahlman and J. W. Boyle, J . Chem. Phgs., 27,1434 (1967); (b) H. A. Mahlman, ibid., S1, 993 (1959). (12) N. A. Bakh, Conference of the Academy of Scienoe of the U.S.S.R. on the Peaceful Uses of Atomic Energy, 1955.
+ H -+ Ce+3 + H + (2) Ce+4 + H202-+ Ce+3 + H f + H02 (3) Ce+4 + HO, -+ Cef3 + H + + 02 (4) Cef3 + OH + Ce+4 + OH(3 Ce+4
+
+
+ +
H. A. MAHLUN
1468 5.0 I
I
I
f.0
2.0
1
I
I
I
I
I
4.0
*-
+
3.0
0 3 -0
2
a
P ~
0
3.0 4.0 5.0 6.0 NaN03 MOLARITY.
7.0
Fig. 3.--Isolation of the direct effect in the Cef4-0.4 M HzS04B, molecular dataNaN03 system: A, ionic data-G(Ce+3).
0
1.0
2.0 3.0 4.0 5.0 NaN03 MOLARITY.
6.0
7.0
Fig. 4.-Comparison of G(Oz)No8- stoichiometric equivalent G(Ce +a) with the G( Ce +3) and the stoichiometrically equivalent G(Cef3) from G(Oz) in the Ce+4-0.4 M H2S04-NaN03 system: 0, G(Ce+3) equivalent to G(O2) mass 32 originating from 1-03-; A, G(CeCS)equivalent to G(O2) in the Ce+4-0.4 M HZSO4NaNOa system; 0 , isolated G(Cef3) direct effect in the 0.4 M H2SOa-NaN03 system.
zero XaN03 molarity, we have divided the corrected enhanced Ce+3yield into two contributions: (1) a contribution directly proportional to ?;aSOs concentration (the direct effect) and (2) a contribution that increases with the NaN03 niolarity until a maximum yield is reached and then remains constant. The directly proportional region may be correlated with the oxygen-18 enriched water-NaX03 studies. As indicated in Fig, 1, we have a directly proportional rela-
Vol. 67
tionship between the molecular oxygen arising from the sodium nitrate and the sodium nitrate concentration. If the over-all reaction given as eq. 1takes place to form molecular oxygen and nitrite, then we should be able to correlate the directly proportional increase in the ceric reduction with the formation of O2 from nitrate ion. For each molecule of oxygen found two nitrite ions are formed, each of which may reduce two ceric ions. With this stoichiometry, Fig. 4 is used to compare the direct effectfrom the oxygen-18 enriched water-NaK03 studies with the direct effect isolated from the Ce+4-0.4 ill H2S04-NaN03 studies. In Fig. 4 the open circular points (0)represent the equivalent G(Ce+3)as determined from the G(02) (mass 32) in the oxygen-18 enriched water-KaY03 system while the open square points (0) are those of the isolated direct action cerous yield, G(Ce+3)Nos-. As readily seen, the agreement between the G(Ce+3)~o,measurements and isotopic oxygen measurements is well within experimental error. The molecular oxygen evolved during the cobalt-60 7-radiolysis of Ce +4-0.4 114 H2SOh-KaS03 solutions may also be correlated with the G(Ce+3). The G(Oz), listed in Table I are presented in Fig. 3. As in the ionic Ce+3analyses there is an initial rapid increase in the G(O2) which then becomes linearly dependent upon the S a K 0 3 concentration. Extrapolation of this linear portion of the G(02) to zero NaN03 molarity separates the region of direct action from the other contributions. If reaction 1 represents the formation of O2 and NO2- by direct action, then the ionic cerous yield and the G(O2) in this Ce+40.4 M H2S04-NaSOdsystem should be equivalent. The solid triangular points (A) in Fig. 4 are the equivalent G(Ce+3) calculated as discussed previously from the G(O2). The agreement is equally good. The other mechanism, a proposed scavenging mechanism for the reduction of Ce+4,has not been deduced. We may, however, delineate some criteria for this mechanism. These are (1) the magnitude of the effect is about G = 2.92, which is essentially the OH radical yield; (2) the non-linear nature of the increased G(Ce+3) suggests a scavenging reaction; and (3) the GOHis constant12 independent of the sodium nitrate concentration. Thus, we may conclude that if the OH radical is scavenged by nitrate ion, as has been sugone of the products gested by Challenger and of the reaction reacts in the same manner as the OH radical, Le., by oxidizing Ce+3and Tl+. It is difficult, indeed, to conceive and delineate a mechanism for the initial rapid increase in the cerous yield consistent with the criteria outlined above.
Summary The use of oxygen-18 enriched water was utilized to ascertain the origin of the molecular oxygen evolved during the irradiation of deaerated aqueous sodium nitrate solutions. The directly proportional relationship of O2 formation from nitrate ions with the sodium nitrate concentration in oxygen-18 labeled water solutions was compared with the 02 formed during the radiolysis of Ce-t4-0.4 M H2S04-NaS03solutions. It was also compared with the increased Ce+4 reduction in the same system after the appropriate corrections (13) G. E. Challenger and 3.J. Masters, J . Am. Ghem, BOG.,77, 1063 (1955),
July, 1963
COKVECTIVE MASSTRAXSFER IN DIAPHRAGM DIFFUSION CELL
were made for the known contributions to the ceric reduction. Corrections in the observed G(Ce+3) included reduction of ceric ions by molecular and radical radiolysis products, ttnd the reduction of the G(H2) caused by the scavenging action of the nitrate ion for hydrogen atoms. Isolation of a G(Ce+3) directly proportional to the N a S 0 3concentration was accomplished after these corrections were made. Excellent correlation was obtained between the isolated G(Ce f3) which is directly proportional to the NaN03 concentration, the stoichiometrically equivalent G(Ce+3) as deduced from the G(O2) measured in Ce+4-0.4 M HzSOpXaS03 solutions, and the stoichiometrically equivalent G(Ce+3)
1469
calculated from the G(Oz) originating from the radiation-induced decomposition of nitrate ion in enriched oxygen-18 HzO-NaK08 solutions. This correlation supports the over-all decomposition as N 0 3 - ~ - t
xo2-
+ '/z
02.
Acknowledgment.-The author wishes to thank W. Et. Ragland and co-workers of the Mass Spectrometer Special Samples Laboratory for the mass spectrographic analyses upon which the interpretation presented depends. He also wishes to thank J. w. Boyle, C. J. Hochanadel, H. W. Kohn, and P. S. Rudolph for their critical evaluation of this work and their suggestions in the preparation of the manuscript.
CONVECTIVE MASS TRANSFER I N A DIAPHRAGM DIFFUSIOX CELL B Y JOHK T. HOLMES,~ CHARLES R. WILKE,-4ND DOSALDR. OLANDER Department of Chemical Engineering, and the Lawrence Radiation Laboratory, University of California, Berkeley, Cal. Received December 26, 1961
A fundamental assumption implicit in the analysis of a diaphragm diffusion cell is that the sole resistance to transport resides in the porous glass disk, through which the solute moves by molecular diffusion. This paper presents an experimental examination of the resistance to mass transfer in the fluid regions adjacent to either side of the porous disk. The presence of an external mass transfer resistance has been observed by others who have recommended that all diffusion measurements be conducted a t stirring speeds greater than an empirically determined value where the cell constant becomes independent of the stirring rate. However, the external mass transfer coefficients depend not only on the stirring speed, but also upon the viscosity and diffusivity of the solute-solvent system as well. There is no justification for assuming that operation above the critical speed found for the calibration fluid will eliminate the external mass transfer resistance in a system of larger viscosity and small diffusivity. Both free- and forced-convection mass transfer characteristics of a vertical diaphragm cell were investigated and the results correlated in terms of stirrer speed and the fluid properties. The data indicate that operation above a critical speed is appropriate for a number of common systems but that significant errors can arise for systems of high viscosity and lop; diffusivity and that for high reproducibility and accuracy, cells stirred by free convection alone should not be employed.
Introduction h fundamental simplification implicit in the analysis of a diaphragm diffusion cell is the hypothesis that the sole resistance to transport resides in the porous glass disk, through which the solute moves by molecular diffusion. More precisely, this model involves two independent assumptions: first, that there is no bulk convective flow through the pores of the disk, and second, that the resistance to mass transfer in the fluid regions adjacent to either side of the disk is negligible. This paper presents an experimental examination of the second assumption. Bulk convection through the pores has been observed,2but mill not be considered here. The presence of an external convection mass transfer resistance has been observed by Stokesza and Lewis3 as an increase in the cell constant with the rotational speed of stirring bars in the two compartments. Above a certain speed, the cell constant becomes independent of stirring rate and these workers recommend that all diffusion measurements be conducted a t speeds greater than the empirical determined critical value. However, the external mass transfer coefficients depend not only on the stirrer speed, but upon the viscosity and diffusivity of the solute-solvent system as well. There is no justification for assuming that operation above t,he critical speed found for the calibration fluid (generally 0.1 N KC1 in water a t 25") will eliminate the external (1) Brnonne National Laboratory, Argonne, Illinois. (2) (a) R. H. Stokes, J . A m . Chem. Soc., 72,768 (1950); (b) A. Emaneel
and D. R. Olander, J . C h e m Eno. Data, 8,31 (1963). (3) J. B. Lewio. J . A p p l . Chem. (London), I , 218 (1955).
mass transfer resistance in a system of larger viscosity and smaller diffusivity, e.g., solute dodecane in tetradecane a t 15". Therefore, the free-and forced-convection mass transfer characteristics of the vertical diaphragm cell, shown in Fig. 1, were investigated and the results correlated in terms of stirrer speed and fluid properties. Experimental Apparatus.-Ace Glass E porosity disks were employed in a vertical position. The diaphragms were 5.0 cm. in diameter, 0.5 cm. thick, and with a nominal pore size range between 3 and 8 p . The cell compartments were each approximately 125 cc. in volume and were stirred with 1-in. long Teflon-covered bar magnets. The cells were calibrated with 0.2 AT HCI using the data of Stokes28 as reference diffusion coefficients. Densities were measured with a Westphal balance, and were reproducible to 3~0.2%. Viscosities were measured with an Oswald viscosimeter calibrated with distilled water and were reproducible to 3Z0.5y0. All data were taken a t 25.0 f 0.1". The four stirring speeds obtainable with the equipment were measured with a stroboscope. For additional details of the equipment and experimental procedure, see ref. 4.
Effect of Fluid Properties and Stirring Speed.-Solute transfer from one compartment of the cell to the other occurs in the following sequence: (a) convective transfer through the boundary layer on the disk in the compartment containing the concentrated solution; (b) molecular diffusion through the sintered glass disk; (e) convective transfer through the boundarylayer on the disk in the compartment containing the dilute solutions. Assuming pseudo steady-state operation, the mass transfer rate is given by eq. 1 (4) J . T, Holmes,
USAEC Report UCRL-9145
(1960).
