'I
J. W. CODDING', W. 0.HAAS, Jr., and
F. K.
HEUMANN
Knolls Atomic Power Laboratory, Schenectady, N. Y.
Tributyl Phosphate-Hydrocarbon Systems O r g a n i z i n g Equilibrium Data
F O R SOLVENT EXTRACTION systems, equilibrium data of course, should be obtained and organized with minimum effort. Also, such data should describe the system sufficiently for its subsequent use, and should be in a form convenient for calculations. The organic phase of the system studied was 30 volume % of tributyl phosphate in a kerosine-type diluent; the aqueous phase was 0 to 4M nitric acid; and the extracting species were the nitrates of uranyl, plutonyl, tetravalent plutonium, and thorium. Of the extracting species, only uranyl ions were present in macro concentrations. All combinations of concentrations for the transferring components were considered, including nitric acid used as a salting agent. After the scouting work necessary with any new system, simple assumptions regarding mechanisms were valuable for interpolation, correlation, and estimation of the kind and amount of data required. Now, however, recent work on extracting metal nitrates in tributyl phosphate systems should permit work of this kind to proceed even more efficiently than was possible in this study. In obtaining the data of Tables I and I1 on which the equilibrium curves are based, care and accuracy with individual analysis was emphasized, rather than a Present address, Phillips Petroleum Co., Idaho Falls, Idaho.
large number of routine anaiyses. With the exception of tracer-thorium distributions at higher acid concentrations, the equilibrium curves are adequate for critical flowsheet calculations. Experimental Butanol-free n-tributyl phosphate obtained from the Commercial Solvents Corp. was used in this work. Both Gulf BT obtained from the Gulf Oil Corp. and Amsco 123-15 from the American Mineral Spirits Co. were used as diluents. No significant difference in distributions could be attributed to change in diluents. Chemically pure nitric acid was used throughout. Uranium-233 and thorium-230 were employed in measuring the distribution of trace amounts of uranium and thorium nitrates. Equilibrations were carried out a t 25' C. in the usual manner by placing the organic phase in contact with successive portions of aqueous phase having a previously determined composition. After this, the aqueous as well as the organic phase was analyzed to be sure that the aqueous phase retained the original concentration of acid. Then for tracer-distribution measurements, small amounts of tracer were added. The concentration of tributyl phosphate in the solvent was adjusted by
density. Before use in tracer measurements, the solvent was pretreated by shaking with an equal volume of aqueous, 0.1M in sodium dichromate and 0.1M in nitric acid, for 30 minutes, shaking twice with 1.OM sodium carbonate for 15 minutes each time and following this with two water washes. The nitric acid-dichromate treatment was eliminated in pretreating the solvent used for determining most of the distributions of macro uranyl nitrate and nitric acid. For a few of the macro distributions determinations made a t a later date, the solvent was pretreated by washing successively with 1.OM sodium hydroxide, 0.1M nitric acid, and water. For plutonium nitrate distribution measurements, the tracers were freshly prepared and oxidation state analyses were made at the beginning and end of each equilibration. When the original distribution measurements were made, the role played by small amounts of pyrophosphate was not known. It is thought now that the dichromate-acid wash may have removed any pyrophosphates present; trouble from that source is not apparent in the data. Equilibrations with trace amounts of uranyl nitrate were made following spiking of the original aqueous phase to 0.001M in natural uranium to swamp out the effect of any small amount of dibutyl phosphate remaining after the VOL. 50, NO. 2
FEBRUARY 1958
145
~~~~~~
~
Table I.
~~~~~
~
~~~
~
Equilibrium Concentrations of Uranyl Nitrate and Nitric Acid
(Distribution at 25' C. between aqueous solutions and 30 vol. % tributyl phosphate in kerosine) Corrected to 30 Vol. % TBP, Moles/L. ___ Solvent
Exptl., Moles/L. Soln. Solvent Phase "Os CO~(NO~)Z TBP 1.095 1.095 1.084 1.084 1.084 1.095 1.095 1.084 1.084 0.084 1.095
5.00 3.00 3.00 2.00 1 .oo 1.00 0.50 0.50 0.20 0.10 0.10 4.00 3.50 3.50 3.50 3.06 3.00 3.00 3.00 3.00 3.00 2.00 2.00 2.00 2.00 2.00 2.00 1.00 1.00 1.00 1.00 1.00 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.050 0.050 0.050 0.050 5.05 4.05 3.05 2.05 1.05 0.52 0.097 0.048
0.021 0.020 0.208 0.533 0.0034 0.018 0.042 0.104 0.383 0.448 0.0057 0.0187 0.048 0.049 0.107 0.362 0.0154 0.047 0.101 0.350 0.439 0.0282 0.049 0.049 0.096 0 * 098 0.294 0.344 0.010 0.049 0.099 0.352
Trace
Table II.
0.92 0.64 0.64 0.45 0.22 0.21 0.079 0.078 0.016 0.0045 0.0049
1.103 1.103 1.103 1.103 1.070 1 084 1.103 1.103 1.070 1.084 1.070 1.084 1.103 1.084 1.103 1.070 1.070 1.103 1.103 1.070 1.084 1.070 1.103 1.084 1,103 1.084 1.084 1.070 1.070 1.103 1,103 1.070 1.070 1.070 1.070 1.070 1.070 1.070 1.070 1.070
5.91 3.31 3.31 2.13 1.03 1.03 0.51 0.51 0.20 0.10 0.10 0.336 0.284 0.448 0.472 0.083 0.244 0.337 0.381 0.466 0.490 0.085 0 * 202 0.313 0.293 0.369 0.451 0.097 0.204 0.283 0.423 0.455 0.080 0.126 0.128 0.211 0.209 0.372 0.392 0.0016 0.024 0.109 0.340
0.31
0.22 0.080 0.045 0.51 0.33 0.21 0.15 0.080 0.074 0.39 0.25 0.16 0.178 0.112 0.070 0.17 0.096 0.071 0.050 0.033 0.068 0.050 0.013 0.042 0.011 0.021 0.044 0.002 0.016 0.016 0.032
I
40.4 37.3 31.1 18.9 7.50 2.56 0.24 0.081
4.58 3.93 3.99 4.11 3.38 3.31 3.32 3.34 3.42 3.44 2.13 2.14 2.14 2.14 2.15 2.20 1.03 1.04 1.04 1.06 1.07 0.51 0.51 0.51 0.51 0.51 0.52 0.52 0.050 0.050 0.050 0.051 5.99 4.63 3.37 2.19 1.09 0.53 0.097 0.048
Distribution of Tracer Plutonium and Thorium Nitrates
(Betmeen aqueous nitric acid and 30 vol. % TBP in kerosine, 25' C.)
146
Ha
E P ~ ( I(ala) V)
Ha
EpucvI)(o/a)
Ha
0.015 0.10 0.49 0.99 2.00 3.00 4.00
0.0038 0,023 0.64 2.9 6.3 16.0 21.5
0.017 0.062 0.110 0.51 1 .oo 2.00 3.99 5.00
0.00066 0.021 0.048 0.45 1.14 2.55 4.35 5.30
0.50 1.07 2.08 2.55 3.00 3.10 3.55
INDUSTRIAL AND ENGINEERING CHEMISTRY
ETH(O/a)
0.15 0.53 1.57 2.05 1.85 2.20 1.95
0.96 0.66 0.67 0.46 0.23 0.21 0.079 0.079 0.016 0.0045 0.0049 0 024 0.022 0.237 0.626 0,0038 0.020 0.046 0.116 0.436 0.513 0.0061 0.0200 0.051 0.053 0.115 0.398 0.0159 0.049 0.105 0.371 0.468 0.0287 0.050 0.050 0.098 0.100 0.306 0.359 0.10 0.049 0.100 0.362
Trace
0.32 0.23 0.084 0.047 0.54 0.35 0.22 0.16 0.085 0.078 0.41 0.25 0.17 0.185 0.115 0.074 0.18 0.97 0.072 0.053 0.034 0.071 0.051 0.013 0.043 0.011 0.022 0.047 0.002 0.016 0.016 0.033
0.348 0.292 0.464 0.490 0.088 0.258 0,347 0.394 0.500 0.519 0.091 0.211 0.321 0.307 0.380 0.479 0.103 0.206 0.288 0.451 0.480 0.084 0.127 0.132 0.213 0.216 0.389 0.417 0.0017 0.024 0.109 0.360 36.7 35.1 30.1 18.7 7.61 2.65 0.25 0.085
washing procedure. Trace uranium, thorium, and plutonium distributions were determined by alpha counting. Concentrations of macro uranium in both phases were determined by optical density measurements on suitable dilutions using a Beckman DU instrument and making appropriate corrections for the effect of nitrate and hydrogen ion concentrations. Acid concentrations were determined by direct, potentiometric and conductometric (5) titrations and by pH methods when warranted.
NUCLEAR TECHNOLOGY Effect of Macro Component Extraction
100
This system is simplified by low mutual solubility of the solvents; however, it is complicated by competition of the uranyl nitrate and nitric acid for tributyl phosphate and the contribution of both species to nitrate salting. These relations are apparent in the extraction mechanisms and equilibria proposed by Moore (4). When these components are present in macro concentrations, calculations for both must proceed simultaneously. UOz++a
+ 2NOs-, + 2TBPo
I
I
I 1 1 1 1 1 1 1
I
I I 1 1 1 1 1
I
I
I
1 1 1 1 1
t
Figure 1, A, B, C. Coefficients of tracer uranyl nitrate as a function of aqueous nitric acid concentration; 25” C.
$
UOz(NOa)z.2TBPo
ha
(MOLES “0,
/LITER HZO)
Kx[NOa-]a. [TBPIo For constructing straight operating lines on McCabe-Thiele-type plots, concentrations were expressed on a solutefree basis in terms of moles per liter of solvent. The following conversion equations were based on linear density equations shown to be adequate at higher concentrations where the effect of solutes on solution volume is significant: 1 - ---ho- U, H, 1 - 0.086 Uo - 0.046 Ho 1 0.086 uo f 0.046 h, ~o
+
~a
- ha Ha
_ _ - = -
U,
1
1
- 0.076 Ua - 0.031 Ha -
1 T j = To - (2
T, =
1
-
+ 0.076 u, + 0.031 ha
+ 0.086 To) Uo (1 + 0.046 To) H, To
1
-
- 2 uo
- ho
+ 0.086 uo + 0.046 h,
Density of the aqueous phase is :
D
=
1.001
+ 0.318 U , + 0.032 Ha
ILg
(MOLES URANYL NITRATE 1 LITER H20)
F
and of the organic phase: = p (0.394 - 0.086 p)Uo (0.063 - 0.046 p)Ho
D
+
+
Uranyl Nitrate Distribution and Aqueous Acid Concentration The apparent decreasing effectiveness of nitric acid-salting at higher concentrations results primarily from reduction in amount of uncomplexed tributyl phosphate. Starting with values from Figure 1,A, and with additional data obtained from equilibrations at constant aqueous acidities, the log-log plot of Figure 1,B, was constructed to describe the distribution of uranium at low concentrations. Straight lines of unit
a
0
0.1
0.2
0.3
0.4
VOL. 50, NO. 2
0.6 0.7 I L I T E R H20)
0.5
UcL (MOLES URANYL NITRATE
FEBRUARY 1958
147
slope are obtained in the region where the aqueous nitrate concentration remains essentially constant. This is not true for the lower acid concentrations where the amount of salting contributed by uranyl nitrate is a significant part of the total. In Figure 1,C, the data extend into the region where the amount of tributyl phosphate complexed with uranium is large, compared to that complexed with acid, and affects distribution markedly. The curves approach 0.52 mole per liter of solvent asymptotically-this represents complete complexing of 30y0 tributyl phosphate with uranyl nitrate. Because uranium is more readily extracted, nitric acid is replaced in the solvent as the uranium content increases. Even from a limited qualitative desciiption, it is apparent that acid will reflux in a system extracting macro concentrations of uranium into the organic phase. This results from the relatively high extraction coefficients of acid in those parts of the system where the uranium concentration is low and the ‘(squeezing out” of acid by the preferential extraction of uranium where the uranium concentration is high. As a result, the steady state concentration of nitric acid in most of the system can be maintained much higher than would be apparent from concentrations of the feed or effluent streams. Both the amount of acid introduced and that leaving in the raffinate can be reduced by taking advantage of this
increased salting strength within the system. T o design such a system which will also remain stable within operating and analytical limits, relations between the distribution of uranyl nitrate and nitric acid must be known with some accuracy. Also, the data should permit simultaneous stage-to-stage calculations for the two species. Figure 2 represents a method of showing these relations. In neither plot do the solid curves cross each other or the broken curves cross each other; therefore, equilibrium points on the two plots are single valued. This is not necessarily true at low acid concentrations where the extraction of acid is enhanced by adding small amounts of uranyl nitrate. This region, however, is not significant for calculations and is not shown on the diagrams. The single-valued points in Figure 2 mean that with a given concentration of uranyl nitrate and nitric acid in the aqueous phase, the equilibrium concentrations of uranium and acid in the organic phase are uniquely determined. Not only does the reverse hold true, but with any two of the four concentrations fixed, the remaining two equilibrium concentrations are again uniquely determined. Also, there is a point-to-point correspondence between the two diagrams. Any equilibrium point is represented by only one equilibrium point on the other diagram. Therefore, equilibrium for
both uranium and acid in a hypothetical stage can be represented by a single point on either diagram. For example, points A on the two plots represent the approximate coordinates: u, = 0.1 ; U, = 0.4; h, = 3.0; h, = 0.16. The rectangular coordinates of one plot (Figure 2) become two sets of curves when transferred to the other plot; these curves may be considered a distortion of the original rectangular grid. Therefore, operating lines representing material balance of one extracting component may be constructed on the distorted grid as well as on the rectangular grid. Straight operating lines on the rectangular grid become curves in transfer, but they still bear the same relation to grid. Using these properties in solving extraction problems is straightforward if somewhat complicated. Ordinarily, the position of one operating line is knowne.g., essentially complete recovery of uranium may be assumed as one boundary condition. I n such a case, the position of the uranium operating line is fixed and may be plotted as a straight line on the plot at the left. The effluent acid concentration is then estimated, and the corresponding linear operating line is constructed on the plot a t the right, This line is then transposed to the uranium diagram, using corresponding equilibrium points for orientation, where it appears as a curve. Starting from the operating point on the uranium
.5
.4
.3
UO 2
.I
Figure 2.
Equilibrium distribution of uranyl nitrate and nitric acid on a solute-free basis at
-
A.
Uranyl nitrate Constant aqueous concentration
148
INDUSTRIAL AND ENGINEERING CHEMISTRY
B.
Nitric Acid. Constant organic concentration
. .. ..
25OC.
NUCLEAR TECHNOLOGY 7
v
Figure 3. Stage-tostage calculations on a distorted acid grid
OPERATING POINTS
h LZ
EQUILIBRIUM POINTS ---*
diagram, a line is constructed which represents the product concentration of uranium. A similar line is constructed on the same diagram to represent the constant concentration of acid in the product phase leaving the first stage of the system; here, this line is a curve originating a t the intersection of the transposed acid operating line and the ha curve which represents the concentration of acid in the entering aqueous scrub stream proportionately spaced between constant h. curves of the distorted grid. The point where the straight line, representing constant organic uranium concentration, and the curve, representing constant organic acid concentration, intersect corresponds to equilibrium conditions in the first stage, if the original choice of acid effluent concentration was correct. In any case, the construction is continued by drawing a vertical line from this point to the respective uranium operating line and a curve proportionately spaced between constant h, curves to the acid operating line. This new line represents constant uranium and acid concentrations in the aqueous phase going from the first stage to the second. This construction is shown in Figure 3. The abscissa is broken to expand construction details. Stage equilibrium points for acid and uranium coincide while the operating points do not. As these operations are repeated for subsequent stages, it soon becomes
apparent whether or not the original choice of effluent acid concentration was correct. If the construction can be continued for more than three or four stages, the acid operating line chosen is close to representing an operating system at steady-state conditions. For most systems, convergence of successive uranium operating and equilibrium points at expected pinch points can be used as a test. Beyond four or five stages, construction usually is limited by the analytical accuracy of the original equilibrium data. However, this is no problem; a t this point in construction, the effluent acid concentration is usually determined to the same degree of accuracy. Tracer Component Distribution and Simplifying Approximations Calculations involving tracer components are relatively simple because these species do not contribute significantly to salting nor will their extraction change the concentration of available tributyl phosphate appreciably. However, data is necessary to indicate the effect of macro components on the distribution of those components present in very low concentrations. When nitric acid is present without uranyl nitrate, the plotting of data is similar to that for tracer uranium. Calculations involving distribution of tracer components, shown in Figure 4,
CORRESPONDING DIAGRAM FOR URANYL NITRATE
for a system not having macro uranium present then require stage-to-stage calculations for nitric acid, using the acid equilibrium curve for u, = 0 of Figure 2. Next tracer extraction coefficients in the various stages should be determined from the calculated aqueous acid concentrations at these stages and data from Figure 4. Lastly, an X-Y plot should be constructed or algebraic equations should be solved for the distribution of tracer in the system assuming an arbitrary product concentration. When both nitric acid and uranyl nitrate are present in macro concentratrations, relations derived from equilibrium equations are used to simplify stage-to-stage calculations. The generalized equilibrium equation for extracting metal nitrates by tributyl phosphate is divided by the equilibrium equation for extracting uranyl nitrate. The resulting relationship should be not only less dependent on concentrations of tributyl phosphate and nitrate ion but also less affected by changes in activity coefficients than either of the individual equations: the distribution of metal nitrates in the presence of uranyl nitrate acid is x(NOa), . nTBP,
K, [NO;].". [TBP]: VOL. 50, NO. 2
FEBRUARY 1958
149
metal nitrates will complicate this picture. As expected, the ratio for plutonyl nitrate is nearly constant (Figure 5). The relative constancy of the ratio for thorium nitrate is somewhat surprising, especially because it remains essentially constant-both where thorium is in macro concentrations and uranyl nitrate is a tracer, and where uranyl nitrate is in macro concentrations and the thorium is in trace quantities. However, that the ratio remains essentially constant with changes in concentration of tributyl phosphate and temperature, is more surprising. This behavior is partially explained by Alter, Heumann, and Zebroski (7) who found that the first two nitrates were quite tightly bound to the thorium and found no evidence of higher complexes. The equilibrium at reasonably high nitrate concentrations apparently is represented by:
I
O
O
m
3
a w
Ha
Ha
Figure 4. Distribution of plutonium and thorium nitrates as a function of aqueous nitric acid concentration; 25" C.
Healy and McKay (2) have reported that plutonyl nitrate, plutonium(1V) nitrate. and thorium nitrate complexes contain one molecule of metal nitrate and two molecules of tributyl phosphate. Katzin, Ferraro. LVendlandt and McBeth (3) investigated thorium nitrate complex formed by equilibrating tributyl phosphate with a saturated aqueous solution of the salt and found that the complex contained two to three moles of tributyl phosphate to one mole of metal
nitrate. They also found, by freezing point depression measurements, that only one metal atom was present in the complex molecule. Assuming then that the complexes contain only two molecules of tributyl phosphate under processing conditions of interest, the ratio for extraction coefficients of tracer and uranyl nitrates will be independent of the concentration of available tributyl phosphate. For plutonyl nitrate, the ratio of extraction coefficients also ivould be expected to be independent of the aqueous nitrate concentration and thus remain constant with changes in uranyl nitrate and nitric acid concentrations. Actually, the differences in degree of hydrolysis and nitrate complexing of uranyl nitrate and of the different
I
?4
i
3 0 Figure tration
1 50
I
2 3 4 MOLARITY OF AQUEOUS "03
5. Ratio of tracer extraction coefficients INDUSTRIAL AND ENGINEERING CHEMISTRY
5
vs. aqueous nitric acid concen-
Th(N03),+,
+ 2NO;, + 2TBPo Th(NOs)a.2TBP0
and
In Figure 6, a family of curves was expected ; however the data scattered without dependence on the relative amounts of nitrate contributed by thr nitric acid and uranyl nitrate. The difference in plutonium(1V) curves of Figures 5 and 6 indicates that the ratio will vary as uranyl nitrate is substituted for nitric acid and that this variation should be measurable at lower uranium concentrations. Despite the lack of absolute accuracy, the assumption of constant ratio between the extraction coefficients of plutonyl and thorium nitrate and those of uranyl nitrate has proved useful for solvent extraction calculations. 'The relation expressed in Figure 6 for plutonium(1V) nitrate has proved equally satisfactory. Calculations involving these tracer components in the presence of both uranium and acid are similar to those described previously for the case when uranyl nitrate is not present. Following stageto-stage calculations for uranium and acid, the extraction coefficients for tracer components are determined by the relations described, and the distribution of these components in h e solvent extraction system is then calculated using either graphical or algebraic methods. Small Variations in Tributyl Phosphate Concentration and Extraction Coefficients
In determining the stability of flowsheets within operating and analytical limits, the expected variation in concentration of tributyl phosphate must be considered. An equation has been derived for variation in equilibrium con-
@
NUCLEAR TECH N 0 LO 0 Y Figure 6. Ratio for tracer plutonium(lV) with macro concentrations of uranyl nitrate and nitric 3 \ acid vs. aqueous2 nitrate concentration
--
-ers j
-
.3
-
.2
-
-
-
-
=
0,
w
.4
w.I
0 0
1
0
uo
+
These equations were derived assuming that the simple equilibrium expressions presented earlier are substantially true in the region of interest and that the activity coefficients of the species in the solvent phase do not change significantly with small changes in concentration. Because u, and h, represent known data, the larger part of the error introduced by substituting concentrations for activities is eliminated. Proceeding from this, it can be shown that
i
I
terms. This requires calculations involving the use of the previously presented conversion equations, or better, use of density tables to determine the exact amount of water in the stock solutions employed in preparing the aqueous phase. Unfortunately, this was not realized when the data of Table I were obtained; final construction of the plots was more difficult than they would have been, had the acid concentrations been constant on a solute-free basis. The following methods assume that the solute-free basis is used. For the lower concentrations of uranyl nitrate, the data can be plotted and interpolated directly as in Figures 1, A and B. At higher co&entrations, another plot (Figure 7), was useful because the distribution curves for constant aqueous acid on this plot are nearly straight, having only a slight S shape. Figure 8 is a profile derived from Figure 7 and is used to determine the distribution of uranium at acid concentrations intermediate to those of equilibration. A useful relation for the extraction of
Organizing Data Equilibrium data are readily obtained and organized for single extracting species with essentially immiscible solvents. Conversion from concentration, based on volume of solution to concentration based on volume of solvent, is straightforward and interpolation is simple on either basis. A second extracting species present in trace quantities creates no difficulty and equilibrium diagrams such as those of Figures 1 and 6 are readily constructed. When two extracting species are present in macro concentrations and each affect the distribution of the other, the situation is more complicated. It is then desirable to keep the concentration of one component constant in one phase for each of a series of equilibrations. Ordinarily the concentration of salting agent in the aqueous phase is kept constant as in Figures l,B and C. For ease of interpolation and correlation, this concentration should be constant on a solute-free basis if the final data is to be presented in such
2AT, To 2 ~ 0
-
I
Methods for Obtaining and
centrations of uranyl nitrate and nitric acid in the solvent phase with small changes in the original tributyl phosphate concentration and constant aqueous concentrations : Auo -
I
-
By converting to solute-free terms and neglecting Auo and Ah, when they occur in the expression,
I
1 f 0.086 (uo f A u o ) f 0.046 (h, f Aho)
in the denominator, the following relations are derived
T Tff ' =
+
I
0.1
ua
AT, - 2AuO - Aho To - 2u, h,
-
By substituting
0.01
Au o Aho = hL 2UO
in the derived equation A u ~ ATo - ~ A u ,- Ah, Go= To - 2u0 h,
-
the desired conversion equations are obtained.
Figure 7. Interpolation of uranyl nitrate distribution data at higher concentrations
t 0.001 0.1
0.2
0.3
0.4
0.5
UO
VOL. 50, NO. 2
FEBRUARY 1958
151
solvent phase, mnlcs p c v liter solution = concentration of tributyl phosphate in pure solvcnt, moles per liter solution = density a t 25' C. of solution and pure solvent, respectively = uranyl nitrate molarity in solvent and aqueous phasc, respectively = nitric acid molarity in solvent and aqueous phasc, K spectively = uranyl nitrate Concentration in solvent and aqueous phase, respectively ; inolcs per liter = nitric acid concentration in solvent and aqueous phasr, respectively, in moles per liter = variation of molarity of tributyl phosphate in purc solvent from To = molarity of unassociatecl tributyl phosphate in solvent phase corresponding to concentration in pure solvent of To - ATo
At aqueous acid concentrations above
0.5Mand within the analytical accuracy
Figure 8. Derived profile for determining uranyl nitrate distribution at intermediate acid concentrations
nitric acid in the presence of uranyl nitrate was derived from the equilibrium expressions of Moore (4). For nitric acid only, Ho = K " Z , ( T o
- Ho)
of the data, the final equation was good over a wide range of temperatures and tributyl phosphate concentrations. I t is also approximately true when macro thorium is substituted for uranium a t aqueous acid concentrations above 1M (6). Figure 9 indicates how well the empirical results fit the equation for the case of uranium and acid. Had the analytical accuracy of nitric acid determinations in the organic phase been better, a family of curves would undoubtedly have been obtained, For the actual interpolation of acid data, the plot of Figure 10 was used. I t may not be an improvement over the expression just described, but it does have virtue of using the data directly without assumptions regarding the validity of the expression. The relation, however, is useful to test consistency of data or to estimate acid distribution when data are lacking.
Literature Cited
Acknowledgment This was performed a t the Knolls Atomic Power Laboratory, operated for the U. S. Atomic Energy Commission by the General Electric Co. under Contract No. W-31-109-Eng. 52.
K
The ratio is
Empirically, a t aqueous nitric acid concentrations above 0.5M,
equilibrium constant for extraction; subscripts denote extracting material E(o/a) = extraction coefficient. Molar concentration in solvent phase divided by that in aqueous phase; subscripts denote material [TBP], = activity of unassociated tributyl phosphate in the solvent phase T, = concentration of unassociated tributyl phosphate in the =
Figure 9. Correlation of nitric acid distribution data
1 52
INDUSTRIAL AND ENGINEERING CHEMISTRY
(2) (3)
(4)
Nomenclature
Figure 10.
H. W., Heumann, 1'. K., Zebroski, E. L., J . A m . Cficni. 506. 73, 5646 (1951). Healy, T. W., McKay, €I. A. C., Trans. Faraday Soc. 52, 633 (1956). Katzin, L. I., Ferraro, J. R., Wendlandt, W. W., McBeth, R . I,., J . A m . Chem. Sac. 78, 5139 (1956). Moore, R. L., U. S. Atomic Energy Commission, AECD-3196, June 1951. Pepkowitz, L. P., Sabol, W. W., Dutina, D., Anal. Chem, 24, 1956
(1) Alter,
(5)
(1952). ( 6 ) Siddall, T. H., 111, U. S. Atomic Energy Commission, DP-181, Octo-
ber 1956.
RECEIVED for review September 3, 1957 ACCEPTED December 18, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, 131st Meeting, ACS, Miami, Fla., April 1957.
Plot for obtaining distribution curves
