July, 19413
POLAROGRAPHY O F URANIUM IN STRONGLY
117.5
ACIDSOLUTION
G r i f f i t h ~appear ~~ to be the best; they give 216 and 218 cal. per gram atom, respectively, The agreement between these directly determined values and that computed from the equilibrium nicasurements is surprisingly good; the discrepancy (shown by Table VII) a t the lower temperatures is felt to be due in large part to the great uncertainty in the heat capacity of the two forms of iron in this region. The heat of solution of graphite in 7-iron may also be determined with the aid of equation 19 and the relation AH, =
- 4 575 d
log at/d ( 1 / T )
where a2 is activity of carbon in the austenite 'in equilibrium with graphite and is relative to a standard state such t h a t a2 = nz/nl a t infinite dilution. A plot of log a2 vs. 1/T for the experimentally determined values of the solubility of graphite is shown in Fig. 10. The slope of this line corresponds to a heat of solution of 9800 cal. in satisfactory agreement with the value (10,100 cal.) obtained previously by a different method.
Summary By determination of the carbon content of an iron phase which had been definitely equilibrated with one of a series of gas mixtures, each of constant composition, the following equilibria were measured: CH4--H2 mixtures with ferrite a t 750 and 800', with austenite a t 800 and 1000°, and with graphite a t 800 and 1000°; CO-CO2 mixtures with austenite a t 800, 1000 and 1200", and with graphite a t 800 and 1O0Oo. The limiting solubility of carbon in ferrite a t 750 and 800' was found to be 0.020 and 0.012:3 weight per cent., respectively, both of which are lower than the data commonly accepted hitherto. Austenite a t its lower limit of stability at 800" contains 0.319 * 0.004% carbon, in good accord with the results of dilatometric measurements. (38) J FI Awherv ani1 E Griffiths, Proc Roy Soc (London), 174, 1 (1940)
7
8
9
10
I04/T. Fig. 10.-Plot of logarithm of activity of carbon in austenite at equilibrium with graphite against 104/T, based on the following solubility measurements: 0 Smith, average value of carbon content, determined from plot cf austenite equilibria, at which rI = 11' and r2 = Y Z ' ; 0 Wells; Gurry, equilibrium with graphite; 0 Gurry, equilibrium with carbon from toluene.
The experimental results permitted direct calculation of the activity of carbon in austenite relative to graphite as unity and of the heat of solution of graphite in ferrite and in austenite. There is a slight apparent difference in activity according as i t is calculated from the results with one or the other pair of gases; its source is still uncertain. I n the appendix an analytic expression, based on statistical considerations, is developed which reproduces satisfactorily the measurements of activity of carbon in solid solution in y-iron, austenite, as a function of its concentration. KEARNY,N. J.
RECEIVED JANUARY 8,1946
[CONTRIBUTION FROM THE S C l l O O L OF CHEMISTRY OF THE UNIVERSITY O F MINNESOTA]
The Polarography of Uranium. 11. Polarography in Strongly Acid Solution' BY I. M. KOLTHOPF AND W. E. HARRIS In a previous paper2 the polarography of hexavalent uranium in 0.01 to 0.2 N hydrochloric acid was discussed. Two reduction waves were obtained, the first one corresponding quantitatively td the reduction of hexavalent uranium to the pentavalent state and the second to the reduction to the trivalent state. ( 1 ) From a thesis submitted by W E. Harris to the graduate school of fhe University of Minnesota in partial fulfillment of the requirements for the P h . D . degree, June, 1944. (2) W. E. H a r m and I. M . .Rolthoff, T H I SJ O U X N A L , 67, 1484 (1945).
Herasymenko3 briefly states that a high concentration of strong acid in the uranyl solution changes the relative magnitudes of the reduction waves obtained. He attributes this to a decomposition of pentavalent uranium into hexavalent and tetravalent uranium, the hexavalent uranium formed being reduced a t the dropping electrode. We have made a systematic study of the effect of acidity on the polarographic waves of uranyl uranium. The first diffusion current (3) P. Herasvmenko. ?ram. Faraday SOC..14. 272 (1928).
I. M. KOLTHOFF AND W. E. HARRIS
1170
Vol. G8
was found to increase with increasing acidity. hloreover, the sum of the two reduction waves (Uv' -+ Uv and Uv ---f U"') was unaffected by the acidity of the solution. The effect of the acid is a function not only of the hydrogen ion concentration but also of the anion of the acid. I n agreemelit with Herasynienko the effect of acids on the first diffusion current is attributed to a decomposition of the Uv into Uvl and UIV. Based on the experimental evidence we postulate the following equations as representing the Uv decomposition equilibrium. 2C02+ H+= UO*++ UO+OH (1)
(a) It is known that Uv is unstable at all acidities and that under equilibrium conditions the disproportionation of Uv into Uvl and UIv is virtually complete. (b) The increase of the diffusion current corresponding t o the first wave with increasing negative potential a t relatively high acidities (see exl)eriiiieritul part) is qualitatively accounted for by the tlisproportionation but it cannot be interpreted on the basis of the alternative assumption. Similarly the effect of tetravalent uranium on the first diffusion current cannot be interpreted on the basis of the alternative assumption. Experimental or dependent upon the acidity The apparatus, materials and technique used :!UOr+ -I- 2 H + = UOs++ U O + + H20 ( 2 ) in making the polarographic measurements have been described previously. ,411 diffusion curThe equilibrium constant Ke, of reaction (1) is rents have been corrected for the residual current (3) of the medium. Unless stated otherwise, the ternperature was maintained at 23 * 0.1'. All values From an approximation of the order of magni- of the potential refer to the saturated calomel tude of K e q , (Y. i.) it is quite apparent that U 0 2 + electrode (S. C. E.). is very unstable at all acidities. If decomposition equilibrium were established readily a t the Effect of Various Factors on Pentavalent Uradropping mercury electrode one would find a t nium Decomposition as Observed a t the Dropping Mercury Electrode various acidities two reduction waves, one corresponding to Uvl + UIv and a second UIv -+ (1) Uranyl Concentration.-The diffusion UII1. Dependent upon the experimental condi- currents obtained with the dropping mercury tions a "steady state" is attained in the region of electrode a t potentials in the region of the first potentials corresponding to the first diffusion cur- diffusion current have been used as an indication rent in which the net result is reduction of U+6 of the extent of pentavalent uranium decomposipartly to U+s and partly to U+4. tion. It is convenient t o express the extent of Equation (1) cannot be used for the quantita- decomposition in the "steady state" found a t the tive calculation of the steady state ion concentra- dropping electrode in terms of the ratio, R , of tions a t the surface of the mercury drop in the reduction of uranium to the tetravalent and pentapotential range of the first diffusion current but valent states respectively. experimental evidence is given that it allows a R = Cvd+/Cu6+ (4) qualitative prediction of the effect of various facPreviously2 it has been found, with the capillary tors affecting the steady state conditions. As a n alternative hypothesis it may be argued of characteristics used in this work, the diffusion that the UOz+ formed at the dropping mercury current in moderately acid medium is 4.08 microelectrode is partly present as such and partly as amperes per millimole per liter of uranyl chloride. U02(3H and that only one of the two forms is If reduction stopped a t the pentavalent state the reducible at the dropping mercury elsctrode, diffusion current would be equal to 4.08 C, in and further that equilibrium between the two which C is the millimolar concentration of uranyl forms is established slowly. The equilibrium in the bulk of the solution. As the other extreme, between UO2+ and UOZOH should be shifted in if reduction proceeded completely to U(1V) the favor of U 0 2 + with increasing hydrogen ion con- diffusion current would be twice as great or 8.10 C. centration. Since the first diffusihn current in- Suppose that the fraction of the U(V1) reduced to R ) , then the creases with increasing acidity the U 0 2 + should U(1V) isf, which is equal to R/(1 be reducible a t the dropping electrode and the observed diffusion current will be U02(3H should not be reducible. i,, = 4.08C 4.0SfC (5) For the following reasons the above interpreta- or tion is not probable and we attribute the increase i,, = (4.n8c + s.imc)l(i R ) (6) of the first diffusion current t o a disproportionafrom which tion of the UOZ+.
+
+
+
+
+
+
+
( 4 ) I n a private communication H . G. Heal indicates t h a t he has carried o u t a s t u d y , on t h e polarography of uranium. similar in m a n y respects t o t h a t carried out by t h e authors. Dr. Heal states he has " m a d e many experiments which p u t t h e existence of t h e ion L.02 ' , for shtxt periads, heyond d o u b t a n d (has) d a t a concerning t h e v e ~ lucity constant5 f u r 11s disproportionation under various cundition\ " H e intends t u publish his results shortly.
I
id (3) Acid Concentration.-The concentration upon diffusion current was studied with the non-complex forming perchloric acid. For a given concentration of uranium the rate FUNCTION OF of pentavalent uranium decomposition increases THYMOL with increasing acid strength (Table 11).
indicating more extensive disproportionation. According to equation (1) the fraction of pentavalent uranium decomposing should become greater with increasing uranium concentration. The first diffusion current is not proportional to the concentration of uranium in solutions of high acidity (Table I). TABLEI DIFFUSIONCURRENTOF URANIUMAS CONCENTRATION IN 2 M HCl; 2 X Uranyl chloride, moles per liter
3.6 X 1.04 x 2.11 x 3.48 x 5.63 x 9.12 x 2.13 x 4.34 x
10-4 10-4 10-4 10-4 10-4
10-3 10-3
A
microamperes
idC microamperes per millimole per liter
R
0.21 0.70 1.50 2.51 4.09 6.66 16 ..OO 35.0
6.1 6.7 7.11 7.22 7.27 7.31 7.52 8.06
0.7 2.0 2.9 3.3 3.6 3.7 5.3 40
i d a t -0.5 v.,
(2) Potential.-At the surface of the mercury drop the uranyl ion concentration decreases continually as the (negative) potential increases. According to equation (1) a decreasing uranyl ion concentration a t constant hydrogen ion concentration should cause the equilibrium t o shift toward the right. This shift would manifest itself in a continuously increasing current a t the dropping mercury electrode with increasing potential. This is found experimentally when the hydrochloric acid concentration is greater than about 0.5 N . I n 1.5 M hydrochloric acid cotitaining M uranyl chloride the diffusion current a t -0.3 v. was 6.08 microamp.; a t
TABLEI1 DIFFUSIONCURRENT m MILLIMOLAR URANYL CHLORIDE I N VARIOUS PERCHLORIC ACIDSOLUTIONS, w4% THYMOL HClO!,
moles/liter
i,i,at -0.4 v., microamperes
R
0.90 1.89 3.78
4.38 4.75 5.56
0.08 0.19 0.57
(4) Temperature.-The rate of pentavalent uranium decomposition increases as the temperature increases. This effect is shown qualitatively in Fig. 2 . Results of a quantitative in-
-0.4 -0.6 -0.8 -1.0 -1.2 Volts vs. S. C. E. Fig. 2.-Effect of temperature upon polarogram of lo-$ M uranyl chloride in 1 M hydrochloric acid: A, polarogram a t 60"; B, polarogram a t 0"; C, residual current a t 60"; D, residual current a t 0". -0.2
J"
vestigation of the effect of temperature are summarized in Table 111. TABLE 111 EFFECT OF TEMPERATURE ON PENTAVALENT URANI~W DISPROPORTIONATION -R-Solution examined
00
4
- 0.4 - 0.8 Volts vs. S. C. E. 144 uranyl chloride in 1.5 A1 Fig. 1.-Polarogram of hydrochloric acid, lo-' % thymol.
+ + +
0.1 M HCl M UOzClz 0.25 M HCl 4 X Ad UO2Clz 1 M HCI 10-3 M u o 2 c i 2
0'
25'
60'
0.01 0.09. 0 . 0 0.02 0.13 0.21 0.60 4.61
Table 111 shows that even in 0.1 M hydrochloric acid a small amount of decomposition is found a t 60". I n 1 M hydrochloric acid a t 60' such extensive decomposition takes place there is almost complete reduction of the uranyl ion to the tetravalent state a t potentials more positive than those corresponding to reduction of U f 4 to U+3. Even a t 0" the decomposition of U+5is appreci-
I. M. KOLTHOFF
117s
AND
able a t the dropping electrode in 1 AI hydrochloric acid ( 5 ) Tetravalent Uranium.-According to equation (1) the decomposition of pentavalent uranium should be retarded if an excess of tetravalent uranium were present in the solution. This retardation of the decomposition would be evidenced polarographically by a decreased diffusion current at a given acid strength. This effect has 3een found (Table IV). TABLE IV EFFECTOF TETRAVALEWT URANIUMUPOU FIRSTDIFFU$io\ CURRFZTox- l o - ? .ll U02C1, I \ 1 ,V HISOl, 10-*cG THYMOI Uranous sulfate, nioles per liter
0
Diffusion current in microamperes :it potential -0.3 v. -U.1 v. -U.d v. -0.6 v .
7.06 6.69
10-2
7.13 G.74
7.15 6.59
7.18 6 51
However, an anomaly appears in polarograms of strongly acid solutions containing both hexavalent and tetravalent uranium i i t h a t the diffusion current actually decreases with increasing negative potential instead of increasing as one would normally expect (see Table IV).. Figure :I{ oxalic acid, l W 4 70 thymol.
reduced to pentavalent uranium. As the state of complete reduction to tetravalent uranium is approached the current voltage curves begin to show a region of constant diffusion current instead of a gradually increasing current with increasing potential (compare Figs. 4 and 5 ) . In Fig. 4 reduction to tetravalent uranium is quite incomplete; hence the current in the region between -0.3 and -0.7 volt is increasing continuously. I n Fig. 5 where reduction to the tetravalent state is nearly complete the current is practically constant in this same region. A number of anions form complexes of varying degree of stability with uranyl. If the value of R with the various anions may be used as a qualitative indication of the stability of the complex
with the uranyl ion, these complexes become more stable in the order perchlorate, chloride, sulfate, oxalate (Table VII).
0
TABLE VI1 OF l? I'ALUES IN VARIOUS MEDIA SUMMARY Concentration of uranyl chloride
10-3 M 10-3 M
10-3 Af 1 18 X l o v 3 M
Medium
1 89 MHClO4 1 5 MHCI 0 . 5 ibfHrSO4 1 89 MHCIO,, 0 027 MOxalic Acid
R at
0.5 v.
0.19 1 5 3 0
3 9
0
Summary In strongly acid solution (greater than about 1 M hydrochloric acid) pentavalent uranium decomposes a t the dropping mercury electrode probably according t o the equation 2U02+
+ H +_r UOz++ + UO+OH
The polarographic steady state does not correspond t o the equilibrium state of the pentavalent uranium. The effect of the concentration of strong acid and of other factors, is accounted for qualitatively by the above equation. The rate of disproportionation of the pentavalent uranium a t the dropping mercury electrode increases with increasing concentration of pentavalent uranium, decreasing uranous ion concentration, increasing acid concentration, increasing temperature and addition of complex forming ions. MINNEAPOLIS, MI".
RECEIVED MARCH 2, 19-10
