NOTES
2858 [aEmix
= f (concn.) ] is apparently masked in the experi-
mental scatter. Values for BInjy, therefore, cannot be ascertained from such measurements, but the determination of the sign of the term nevertheless is possible. Thus an experimental cryoscopic constant which is greater than calculated from phase-transition heat of fusion calorimetry corresponds to a negative value for A E m i x , i e . , exothermic mixing interactions in the binary ionic melt. Similarly, fluctuations corresponding to a cryoscopic constant less than the calorimetric value correspond to a positive energy of mixing effect (endothermic) in the ionic mixtures. The signs for S m i x thus predicted for each solute in molten NaN03 are given in column 4 of Table I. Enthalpies of mixing data have been reported elsewhere by Kleppa and his eo-workers for a large series of binary nitrate The signs for the enthalpies of mixing thus determined by direct soIution calorimthat etry are in cplumn 3, Table I. Inspection shows the prediction of the ionic interaction term, AEmix, corresponds exactly with the signs of the directly observed enthalpy of mixing with one exception, i e . , Ca(NO&. The results leave little doubt that an indication of the nature of the ionic interactions and enthalpies of mixing ( i e . , next nearest neighbor repulsion, partial covalency, packing effects7-'1) can be gained directly from the fluctuations in the v-values of cryoscopy if the freezing-point data are precise, and accurate heat of fusion data for the solvent are known. Acknowledgment.-This work was made possible, in large part, through financial support from the U. S. Air Force, Office of Scientific Research, Air Research and Development Command, Washington, D. C.
sJmix
(7) 0. J. Kleppa a n d L. S.Hersch, J . Chem. Phys., 36, 213 (1962). (8) 0. J. Kleppa and L. S. Hersch, tbzd., 36, 544 (1962). (9) 0. J. Kleppa, R. R. Clarke, and L. S. Hersch, ibtd., 35, 175 (1961). (10) 0. J. Kleppa, J . P h y s . Chem., 66, 1668 (1962). (11) 0. J. Kleppa and S. V. Meschel, ebid., 67, 669 (1963).
PHOTOCHEMICAL h S D RADIATION CHEMICAL REDUCTIOX OF CERIC 10s I N AQUEOUS SULFURIC ACID SOLUTIOKS. EFFECT OF FOR,MIC ACID' BY THOMAS J SWORSKI Chemistry Division, Oak Ridge National Laboratory,2 Oak Ridge, Tennessee Received Julu S, 1968
The OH radical was postulated3 as an intermediate in the photochemical reduction of ceric ion in sulfuric acid solutions. A comparative study of the radiolysis4 and photolysis5 of ceric ion-cerous ion-thallous ion mixtures yielded kinetic evidence in support of this postulate. Although both thallous ion4,6 and formic enhance the radiolytic reduction of ceric ion in 0.4 AI sulfuric acid solutions, there is one striking difference. (1) Preeented a t the 145th National Meeting o f the American Chemical Society, New Pork, N. Y . , Sept., 1963. (2) Operated for the Atomic Energy Commission by Union Carbide Nuclear Company. (3) T. J. Sworski, J . A m . Chem. Soc., 77, 1074 (1955). (4) T. J. Sworski, Radiation Res., 4, 483 (19.56). (5) T. J. Sworski, J . A m . Chem. Soc., 79, 3655 (1957). 16) T. J. Sworski, ibid., 77, 4689 (1955). (7) H. E. Spencer and G. K. Rollefson, ibid.,77, 1938 (1955). ( 8 ) T. J. Sworski, ibid., 7 8 , 1768 (1956). (9) T. J . Sworski, Radialion Res., 6 , G45 (1967).
Vol. 67
While enhancement by thallous ion is essentially dependent only on [Tl+J/[Ce+3], the enhancement by formic acid is dependent not only on [HCOOH]/ [Cef3]but also markedly on the total concentration of cerous ion and formic acid at any particular [HCOOH]/ ~ , reaction ~ [Ce f 3 ] . This difference was a t t r i b ~ t e dto of OH radical with sulfuric acid. The effect of formic acid on the photochemical reduction of ceric ion in sulfuric acid solution was investigated to obtain further evidence for the OH radical as an intermediate and for reaction of OH radical with sulfuric acid. Experimental The experimental procedures employed were previously described in more detail.4~z~Q The source of ultraviolet light was a General Electric 4-watt germicidal lamp which emits almost entirely light of 253.7 m*.lo The flux incident upon the solutions was measured with an equal volume of actinometer solution .I1 The source of ?-radiation was a nominal 300-c. Coo0 source.12 Radiation dosimetry in 0.4 M sulfuric acid solutions was based on a yield of 15.6 ferric ions per 100 e.v. for the ferrous sulfate dosimeter.1a A molar extinction coefficient of 2210 a t 25" was used for ferric ion at 305 mp.4 Formic acid solutions were prepared by addition of Baker and Adamson reagent grade sodium formate. The concentration of undissociated formic acid was assumed to be equal to the concentration of added sodium formate. Ceric ion and ferric ion concentrations were determined spectrophotometrically with a Cary hfodel 11 recording spectrophotometer. A molar extinction coefficient of 5580 at 320 mp was used for ceric ion in 0.4 M sulfuric acid solutions13 and relative values of 5380 and 5710 were determined for 0.16 and 0.8 M sulfuric acid solutions, respectively. Formic acid has no measurable effect on these molar extinction coefficients a t the concentrations employed.
Results The notation employed is that previously used4 for radiation chemical studies: subscript notation for quantum yields and 100 e.v. yields of assumed intermediates such as @OH and GOH, respectively, and parenthetical notation for quantum yields and 100 e.v. yields of observed products such as @(Clef3) and G(Ce +9, respectively. Photochemical Studies.-Initial @(Ce+3) values, a t constant light intensity, are markedly dependent on formic acid concentration as shown in Fig. 1 for 0.4 Af sulfuric acid solutions. A similar dependence was observed for 0.16 and 0.8 $1 sulfuric acid solutions. Changes in ceric ion concentration were followed by use of intermittent irradiations and the data shown in Fig. 1 for any particular formic acid concentration were obtained from a single solution. @(Ce+3)decreases with increasing time of photolysis in all solutions due to the internal filter action of cerous ion and to reaction of cerous ion with OH radical. In the photolysis of ceric ion-thallous ion mixtures in 0.4 M sulfuric acid solutions, initial @(Ce+3)was shown4 to be equal to 2+0H. Initial @(Ce+3)values of 0.208, 0.204, and 0.195 were determined for ceric ion-thallous ion mixtures in 0.16,0.4, and 0.8 M sulfuric acid solutions, respectively. Iio significance is attached to the variation in @(Ce+3)values and the assumption is made that @OH is independent of sulfuric acid concentrations employed in this investigation. Radiation Chemical Studies.-Initial G(Ce +3) values are also markedly dependent on formic acid concentration as shown in Fig. 2 for 0.4M sulfuric acid soh(10) (11) (12) (13)
J. P. H u n t and H. Taube, J . A m . Chem. Soc., 74, 5999 (1962). W. G. Leighton and G. S. Forbes, ibid., 6% 3139 (1930). J. .4. Ghormley and C. J. Hochanadel, Rev. Sei.lnstr., 2 2 , 473 (1951). C. J. Hochanadel and J. A. Ghormley, J . Ciieni. Phgs., 21, 8S0 (19.53).
2859
Dec., 1.963
-
200
1
L
a
160 \
v,
-a 0
5
420
++-
L
.-E
0
v
+
80
+
a
V
40
0
0
2
4
6
12
40
8
I R R A D I A T I O N TIME ( m i n u t e s ) .
Fig. 1.-Effect of formic acid concentration on the photoreduction of ceric ion in air-aaturated 0.4 M sulfuric acid solutions. $1. Initial Ce4f concentration in all solutions was 3.25 X M ; (3, 2 X Initial formic acid coneentrationa were: 0, 2 X M; e, 3 X lo-' M ; 0 , 10-8 M ; C), 8 >: 10-4 M ; 8 , 5 x 2 x 10-4 M . I
I
I
-
OO
I
2
0.001/(HCOOH) hv
Ce4+
I
of $( Ce+3) on formic acid concentration.
Fig. 3.-Dependence
I
5
4
3
+ HnO -+
+
C C + ~ I
