354
J. M. SCHREYER AND C. F. RAES, JR.
1101. 'in
electrons, then the argument becomes more straightforward due to the greater extension of the 6d orbitals. A satisfying explanation for the observed results is difficult to propose. If a direct electron transfer mechanism is postulated, it would appear that the probability for the penetration of the barrier is not primarily determined by the symmetry of the species (Franck-Condon principle). Furthermore, the small difference in the observed free energy of activation for the neptunium and iron cases is difficult to reconcile with the relatively large differ+ ence ~ in free energy of rearrangement of the hydration spheres expected in the two cases. An explanation of the observed results can be made in terms of electron transfer if i t is assumed that the determining factor is the low probability for the transfer of an electron through the intervening water molecules. -Ilternatively, an atom transfer mechanism could be p r o p o ~ e d . ~Decision ,~ as to the probable mechanism will have to await further investigation.
TABLE I1 ENERGIESA N D ESTROPIES OF ACTIVATIONFOR THE ExCHANGE REACTIONSOF N p 0 2 + AND XpOz++, A N D OF FERROUS I O V WITH VARIOUS FERRIC SPECIES cart as kcal /mole
NpOr +-iYpOt Fe + +-Fe + + + Fe++-Fe(0H) L T Fe - +-FeCl+ +
-
c.11 !95% in each sample.11 Chemical analysis gave for the first sample 61.6% U02++, 21.6% Po4-3, and by difference 16.6% HzO, while the second sample gave 61.9% UOZ+*, 21.6% Po4-3, and by difference 16.3% H20. This corresponds to the formula UO2HPO4.4Hz0, for which the theoretical percentages are 61.63% UOz++, 21.68% PO,-3 and 16.45% HzO. If the acetone dried sample prepared by the above method is heated in an oven at 110", the UOzHP04.4HzO is converted to UO2HPO4.2Hz0. A typical analysis of a sample dried in this manner gave 67.7% UO2++, 23.1% and by difference 9.0% HzO, while the theoretical percentages for U02HP04.2H20 are 67.16% UOZ++, 23.6% and 8.96% HnO. 2. Preparation of (UO&( P04)2.4Hz0.-A 100-g. sample of ( U02)3(P04)~.4H20 was prepared according to the directions of Ryon and Kuhnl* by digesting UOzNH4P04.3H20 in 0.8 M Hn'Os a t 100'. The solid was then washed with hot water and acetone and dried in air. Conversion of UOzKH4P04.3H20 to ! U O Z ) ~ ( P O ~ ) Z . ~was H ~ Oeasily observed by the change in physlcal appearance of the solid phase. Under the microscope needle-like crystals were observed characteristic of ( U O ~ ) ~ ( P O ~ ) Y ~ HX-Ray Z O . diffraction H Zbe O present to analysis indicated ( U O Z ) ~ ( P O ~ ) Z . ~to >90% in the sample. Chemical analysis of the solid gave 75.33% UOz++, 17.97% and by difference 6.70% HtO. This corresponds to a formula of ( U O Z ) ~ ( P O ~ ) Z . ~ H Z O , for which the theoretical percentages are 75.5% UOZ++, 17.72% and 6.72% H20. 3. Preparation of U02(H2P04)~.3H20.-A sample (approximately 10 9.) of U O Z ( H Z P O ~ ) Z . ~was H Z Oprepared by the addition of approximately 30 g. of UOzHP04.4HzO to 100 ml. of 85% H3PO4. The sample was shaken for 13 days a t 25" and then filtered. Preliminary experimentation showed that the wet solid would dissolve immediately upon washing with a small amount of water. For this reason, the sample was washed only with acetone followed by carbon tetrachloride. The dry sample was placed in a vacuum desiccator over CaCl2 for 12 hours. X-Ray diffraction analysis could not be made since a standard for this compound was not available, although a pattern was made for future reference. Chemical analysis of the solid gave 52.63% UOZ++,35.7% POI-^ and by difference 10.9% HzO, while the formula U O Z ( H Z P O ~ ) ~ . ~gives H Z Ofor the theoretical percentages 52.12% UOz++, 36.66.% and 10.4% HzO. If U O Z ( H ~ P O ~ ) ~ . ~isHheated Z O for 7 hours a t l l O o , it is converted to UOZ(H~PO~)ZHZO. Werther6 reported that part of the water in this compound is lost upon heating and a t red heat all of the water is removed without melting or losing H3P04. It is probable that he converted the compound to a pyrophosphate. Bernard S. Borie, Jr.,l3 X-ray Laboratory, Oak Ridge National Laboratory, by private communication has re(11) This is the limit of accuracy of the X-ray diffraction analysis. (12) A. D. Ryon and D. W. Kuhn, Report Y-315, January 10,1949. (13) Letter of October 14, 1952, from B. S. Borie, Jr., to J. M. Schreyer, Oak Ridge National Laboratory, Oak Ridge, Tenn.
3.X
J. M. SCHREYER AND C. F. RAES, JR.
VOI. 76
ported a preliminary investigation of the crystal structure ages greater than 100% were obtained for the total analysis. of U O Z ( H Z P O ~ ) Z . ~ H Z O . More nearly constant values for water were obtained by One of the yellow crystal platelets of a sample of UOZ- calculations of differences assuming the values for uranium ( H Z P O ~ ) ~ . ~ was H Z Omounted on a glass fiber with a little and phosphate to be correct. grease and then mounted on a precession camera. A series TABLE I of photographs was taken with copper KCYradiation, which showed the crystal: t o be monoclinic; a = 10.83 b., b = CHEMICALANALYSESOF URANIUM(VI)PHOSPHATES FROM 13.92 A., c = 7.48 A., and 0 = 105'45'. PHOSPHORIC ACIDSOLUTIONS If it is assumed that the unit cell contains four stoichioTheoretical compositions: (UOr)3(P04)2~6H20-73.1~% metric units, the calculated density of U O ~ ( H Z P O ~ ) ~ . ~COS+', H ~ O 17.10% P04-3, 9.75% H20; U O Z H P O ~ , ~ H Z @ is 3.17, in agreement with the measured value of 3.16. If ti1.6% UOa++,21.68% POa-3, 16.45% HzO: UOdH.>PO,)-. the formula U O Z ( H Z P O ~ ) Z . ~isHassumed, ~O the calculated 3~~01--52.18%vo4+G, 3 6 k % po4-'3; i(i.&% H ~ O . density is 3.28. The densities were calculated by using the Analyses of scilid phases formula weights and the unit cell value determined by the Total ASHs0 H20 phosphate sumed by thy dimensions given above. The agreement between the calin mother no. o f differanalculated and experimentally determined density values subSample liqmior, U?,++, POI-3. hydro- Theor. ence, ysis, ,< Ti pens % H:. F' stantiates the formula UOz(H2POr)2.3H20. no. moles,']. From the photographs, it was observed that reflections A-30 0.00100 73.17 17.4 0 0 1L4.7 ... of the type Ok0 are absent except when k is even, and that ,00625 70.79 18.5 0 0 10.71 9 . 4 8 reflections of the type h01 are absent except when 1 is even. A-26 No other extinction occurs. Therefore, the space group is A-29 .00816 71.24 18.3 0 0 10.46 9 . 4 1 C;, - P2 1I C . Since uranium is by far the heaviest element A-28 0101 71.81 1 8 . 6 0 n 9.59 8.88 present in the compound, it should be possible t o find parA-8.7" oi4n 6-5.90 19.8 . . ... .. . . ameters for the four uranium atoms in the cell from the inA-27 ,0179 61.82 22.0 1 0 . 2 8 15.95 1 6 . 5 tensities of the reflections recorded on the photographs. Approximate agreement between observed and calcuIated A-IO ,103 61.70 2 1 . 5 1 .23 16.57 . . . intensities results if the uranium atoms are in fourfold gen- A-25 10% 62.00 2 1 . 7 1 .23 16.07 1 6 . 5 eral positions, xyz; 272; f, y, ' 1 2 - z ; x , - y , A-21 2 74 61.30 21.6 1 ... .23 16.87 ' I z z , where x E 0.25, 31 2 0.4, and z g 0.25. The de2 97 61.70 21.6 1 .23 16.47 . . . termination of the light atom positions would require quanti- A-3 tative intensity measurements. A-7 6 09 1 61.90 2 1 . 6 .23 16.27 ... A-20 6 10 61.80 2 1 . 0 1 .23 16.99 . . . C. Apparatus A40 6.14 .78 10.32 14.6 54.30 3 4 . 6 4 1. SolubiIity Apparatus.-The soIubiIity flasks were as- A-44 6 67 .78 10.32 1 0 . 1 B5.10 33.8 4 sembled on a mechanical shaker in a pvater-bath thermo.78 9 . 5 2 1 3 . 2 53.80 3 5 . 9 4 statically controlled at 24.9 0.1". Agitation of the solu- A-38-3 6 78 7 21) .78 1 0 . 5 4 7 . 6 52.18 3 6 . 5 4 tion was accomplished by a pendulum-like motion with an .4-45 arc of 80" and 36 strokes per minute. After equilibration, A-86 7 31 .78 10.02 1 5 . 1 ;i3.10 3 6 . 1 4 filtration was carried out by means of the pressure filter A-41 8 88 .78 1 0 . 4 2 1 6 . 7 61.80 8 7 . 0 4 apparatus which permitted the maintenance of a constant A 4 2 10 40 .78 9.12 1 4 . 6 5A.6 3 4 . 5 4 temperature by immersion of the entire assembly under water . Solid phase obtained a t transition point between UOe2. Po1arograph.-The polarograph used in these studies HP044H20 and (UO2)?(PO4)~.6H~0. was an automatic recording instrument constructed a t the Y-12 Instrument Shop according to the specifications of The data for the composition of the solid phase for the John Horton of the Oak Ridge Sational Laboratory and normal phosphate can more nearly be explained by assumsimilar t o the instrument described by Kelley and Miller.'4 ing the formula ( U 0 2 ) 3 ( P 0 4 ) z . 6 H ~ 0There . would be no The instrument sensitivity ranged from 0.03 to 20 micro- reason to assume the tetrahydrate under these conditions amp. per full scale deflection. since the recommended preparation of (UOZ)3(P04h..4Hz0 is carried out a t 100". These chemical analyses are the only D. Solubility Studies in Phosphoric Acid evidence obtained for the formula (UOz)3(P04)2.6H20 since The solubility of uranium(T'1) phosphate solids in phos- application of the Schreinemaker's wet residue method was phoric acid alone was measured in the range of 0.001 t o not practical because of the low solubilities in this range. 14.6 d l total phosphate a t 2 5 " . Using UOzHP04.4H20 as I t was observed by comparison of the X-ray patterns of the ' 4, L e . , (UOz)a(POr)z6Hz0 solids from 0.001 to 0.014 M H&O the original solid added, it was found that a solid phase that change occurred in both low and high phosphate concentra- compared with the standard for (~02)3(P04)3~4HzO, these two hydrates have virtually the same X-ray powder tions. An extensive survey of the solid phases in equilibrium with the mother liquors was made, therefore, over the pattern. It is also of interest t o note that the crystals of ( U02)3(PO4)2.6H20 are visually identical with those of entire range of phosphate molarities. The solubility measurements were carried out by adding ( V ~ Z ) ~ ( P S ) ~ ) Z . ~ H ? ~ . Transition points between each solid phase were estabthe desired uranium(V1) phosphate to a solution of phosphoric acid, placing the solution in the solubility apparatus lished through selected solubility runs and in the case of the and (UO2),( P04)r.C,H20, and shaking the flask assembly in a mater-bath a t 25" for a transition between UOzHPO4.4H~0 :L mixture of well formed crystals was obtained. period necessary to attain equilibrium conditions. The periods of shaking of the samples in order t o attain Microscopic examination of solids provecl very useful in equilibration were established through experiments of varied detecting the tetragonal crystals of uranyl monoacid phos~ 0 used as the phate and the needle-like crystals of the normal uranyl shaking times. IVhen ( U 0 2 ) 3 ( P 0 4 ) ~ . 4 Hwas phosphate. The crystal habit of the solid phaTe UOz- added solid phase, the samples in the region of stability of (H2PO4)2.3H?Oobtained in high phosphate concentrations (G02)3(P04)2:6H20 were shaken from 7 to 11 days. I n this has not been characterized previously. The crystals varied region, the time required to convert UOzHP04.4HzO to the stable (U02)3(P04)2.6H20 was much longer, making the use from large monoclinic rods obtained a t the transition point of UOgHPO4.4HZ0as the added solid phase impractical. t o indistinguishably small crystals in concentrated phosI n the region of stability of UOzHPO4.4H20, the samples phate solutions. as the added solid phase were shaken In some of the solubility tests, chemical (see Table I ) and using UO2HPO4.4HZ0 2 to 12 days. Three determinations using (UOZ),for X-ray diffraction analyses were made on the solid phases (P04)?.4H20 as the added solid phase were run a t 0.02, 2.495 removed from the mother liquors after attaining equilibrium. and 5.527 initial [H3P04] and complete conversions to The water analyses (see Table I ) are not considered reliable COzHPO4.4H20 occurred, The samples in the region of since it is not known what happens t o the solid phases when using 2 0 LOZ(HzPO4)~~3H20 as heated at 300" for 1 hour. It is probable that some decom- stability of U O z ( H z P O 4 ) ~ ~ 3 H the added solid phase were shaken for 9 to 23 days. These position of the UOz(HzP04)2.3Hz0occurred since percentvalues were used t o definitely establish the solubility curve in this region. When UOZHP0,.4H20was used as the added (14) M. T.Kelley and H. H. Miller, Anal. Chcm.. 24. 1895 (1852). (
+
+
+
1
Jan. 20, 1954
SOLUBILITY O F U(v1) ORTHOPHOSPHATE IN PHOSPHORIC
solid phase in the region of stability of U O Z ( H ~ P O ~3Hz0, )Z the samples were shaken for 15 to 20 days with some question as t o whether equilibrium had been attained since chemical analysis of these solid phases indicated contamination with UO2HPO4.4H20 (see Table I). If such contamination is involved, it has not seriously affected the resulting solubility values for these runs since they generally conform to a smooth curve which includes runs for which the initial solid phase is the stable UOZ(H Z P O ~ ) Z . ~ H ~ O . The low uranium concentrations in mother liquors below 0.017 M H3PO4 were determined polarographically as described in the section on analysis. I n solubility runs more dilute than 0.5 M H&0'4, wherein U02HP04.4H20 was the stable solid phase, the final ZPo4-3 concentration was calculated to be simply the sum of the initial [H3P04] and the final ZUOZ++. For those runs in which (U02)3(PO4)2.4H20was the stable solid phase, ZPo4-3 was taken as the sum of [HgPOa] and 2/3ZU02+f. I n the range 0.5 to 15 M HsPOd, the final calculated ZPo4-3 was determined by taking into consideration the effect of volume change accompanying dissolution of the salt. A log-log plot of the solubility data is presented in Fig. 1. Calculated ZP04-s values were plotted where the original phosphate concentrations were known accurately, since these were probably more accurate than those obtained by analysis of the mother liquors.
ACID SOLUTIONS
3.57
Fig. 2.-A portion of the isothermal phase diagram for the system U03-HsPOa-HnO a t 25": 0, saturated solutions; 0 , slurries. in the phase diagram. In this method of plotting the data (known as Schreinemakers' wet residue method), the composition of the mother liquor, that of the slurry and that of the solid phase should lie on the same straight line. Within the limit of experimental error, the data are consistent with this requirement, indicating that these solid phases are the correct ones.
111. Conclusions As a result of solubility studies in 0.001-15 M phosphoric acid, three compounds were identified as stable solid phases in the system. These compounds with their respective stability ranges are (UOZ)~(PO~)~.~HZ
