The Logarithmic Diagram for the Uranyl-CarbonateHydroxide Complex Equilibria Esma Tutem and Re@ Apak Istanbul University, Faculty of Engineering, Department of Chemistry, Avcilar Campus, Istanbul, Turkey Mehmet Hulusi Turgut Fekmece Nuclear Research and Training Centre, Department of Nuclear Engineering, P.O. Box 1, Havaalani. Istanbul. Turkey Vildan Apak Fekmece Nuclear Research and Training Centre, Department of Chemistry. P.O. Box 1, Havaalani, Istanbul, Turkey When ametal cation, M, forms aseries of soluble mononuclear complexes with a ligand, L, i.e., ML, ML2,. . , ML,; the distribution fraction of any complex, MLk, is given by a h , where
(omitting charges for simplicity) and its concentration is
.
[ML,] = a, M,
where & is the cumulative formation constant of the complex MLk, [L] is the free ligand concentration, and Mt is the total metal concentration.' Such complex equilibria may be easily represented by the aid of a logarithmic distribution diagram (where the logarithm of the concentration of relevant species, log Ck, is plotted vs. log [L]) as long as the total metal concentration, Mt, the free ligand concentration, [L], and the stability constants for. the corresponding metal complexes are known. On the other hand, if a ligand gives rise t o another ligand by hydrolysis that binds the same metal in complexes of comparable stability and if some of these complexes are polynuclear, then the task of treatment of complex equilibria and depiction in a diagram is not so easy. Therefore, we thought it would be instructive t o treat such simultaneous equilibria from a standpoint of teaching metal-complex chemistry. Carbonate (COi-) forms stable complexes with the uranyl (UOY) ion, i.e., UO2CO3,U02(C03):-, and U02(C03)j-. At
the same time, carbonate gives rise t o hydroxide (OH-) ions in aqueous solution by hydrolysis, e.g., by the reaction the latter also complexing with uranyl in the form of polynuc l e a r com lexes, p r e d o m i n a n t l y a s U 0 2 ( 0 H ) + , ? 2 (U02)2(0H), ,and (U02)3(0H):.ZThus a conventional diagram with a single metal and ligand cannot be drawn for the uranyl-carbonate system due to the presence of bydroxyl ions. The analysis of the system would also be interesting for knowing uranyl speciation in natural and environmental waters and for recovering uranium by anion exchangers from carbonate leachates (obviously the cationic hydroxo- complexes would not be retained by the resin). Experimentally a series of uranyl-carbonate mixtures were prepared by mixing M uranyl nitrate and M Na2C03solutions in appropriate proportions and dilution so as to yield clear solutions of pH 4-11, The mixtures were equilibrated at room temperature, and the pH measurements were made with a Metrohm E-512 pH-meter glass electrode. Equations 1 and 2 follow from metal and ligand balances, respectively, [uO~],,, = [UOF]
+ [UO,CO,] + [uo,(co,);-]
+ [uo,(co,):-]
+ [UO,OH'] + 2[(UOJ,(OH)?l + 3[(UO&(OH):I
(1)
' Freiser H.: Fernando. 9. Ionic Equilibria in Analytical Chemistry; Wiley: 1963. New York: pp 144-167. 2Mikarni, N.; Sasaki, M.; Hachiya, K. J. Phys. Chem. 1983, 87, 5478.
Volume 68 Number 7 July 1991
589
M (experimental ~igureI. Total metal concentralon = M, = 1.017 X curves). Oistrlbutlon of species with regard to the free llgand concenbatlon and pH. log C = logarithmof the molar concentrationof relevant species, p[L] = -log (free llgand concentration), and pH = -log a ~ +Symbols . used: X: [uo?], 0 : [UO,CO,], A: [UOaCO,)P], 0:[ u o & o ~ ~ ] ,0 : [UOAOH)+l. A: [(UO,)~(OH):+], m: [(uo,),(oH)~], and +: pH = -log a"+.
[CO;-],,
= [Cot-]
+ [HCO,] + [H,CO,] + [UO,CO,] + zIuo,(co,~:-l + a[uo,(co,);-I
(2)
By using the symbols M and L for the free metal (UO;') and ligand (C0:-), respectively, and by expressing the concentrations of complex species in terms of [MI and [L] and the corresponding stability constants, we obtain eqs 1' and 2'.
Figure 2. T m l metal concentratlon = M, = 1.017 X I O F M (computational curves). (See Figure 1 for explanation of symbols.)
by the solubility product, K,,, of uranyl hydroxide at a given pH unless solid UOz(0H)z precipitates, and free carbonate should be less than the value when no uranyl were present (as some carbonate should have been bound to uranyl). A challengingalternative is to solve the system completely without measuring the pH. In this case, the three unknowns, i.e., [MI, [L], and pH, in the pair of eqs 1' and 2' should be reduced by one via introducing the eledroneutrality principle in UOz(N03)rNazCO3mixture solutions. [Nat]
+ [Htl + 2[UO:+I + [uO,(OH)+l+ 2l(UOz),(OH)~1
+ [(UO,),(OH):]
= [NO,]
+ [OH-] + [HCOJ + 2[CO$l
Mi = [MI + PtIMIlLl + PzlMIlL12+ P~IM11L13+ +ZK,
M 21012pH-28) + 3~ [&q31016~H-70) (1,) I 3.5
2 21
L, = [L]
+ ~oV'~K;~[L]+ 10@~q~K,;'[Ll + PIMIILI + 2P21M1[L12+ 3P31WM3
(2')
In the above equations, lopH-'4 has been substituted for [OH-], and the conjugate acids of carbonate are expressed in terms of [L] and pH. Since the total metal and ligand concentrations, Mt and LC, are known for each synthetic mixture and the pH values have been measured, the pair of eqs 1' and 2' can be solved simultaneously by the aid of a computer program (see Appendix, Algorithm 1)to yield [MI and [L] in each case, and the concentrations of the corresponding complexes may then be computed by using the values of [Ml and [L]. Figure mol/L. 1may be drawn for Mt = 1.017 X In the iterative search for solving eqs 1' and 2', the upper and [L] 5 LtKalKs~l(lO-zPH + limits of [MI 5 K,, 10(28-2pH' Kal 10-pH KalKaz)have been used for [MI and [L] because free uranyl may not exceed the concentration limit imposed
+
570
Journal
of C h e m i c a l ~ducailon
The corresponding symbols may be substituted for the terms of eq 3 to yield 2L, + 10-pH
+ 2[M] + C,,, + 2C2,2+ C3,5= 2M, + lopH-"
where C,,, = K,,,[M]lOiPH-"I
and Thus, the concentrations of all species may be mathematically computed (see Appendix, Algorithm 2) to yield Figure
2. The insignificant disagreements hetween the experimental a n d theoretical values m a y arise from a n u m b e r of facts s u c h a s the slow reaction kineticsof uranyl solutions t o r e a c h
LI = L,- Lb (Lf = total free ligand concentration = [L]
+ [HL]
+[WI)
equilibrium, t h e varying confidence levels of complex stahili t y constants collected from literature, t h e possible presence of other species i n aqueous solution i n addition to those used i n computations of the model, a n d differences i n ionic strength.
Appendlx :Thedratribution fraction of the complex ML,, i.e.. IhlL.llM,. ,%:The rurnuiatiwstability constant of rhemunonuclenr carhmate complex MLa, i.e., [ML~]/([MI[LIk). [MI and [L]: The concentrations of free m'etal and ligand, respecGvely. M, and L,: The total (analytical) concentrations of metal and ligand, respectively. Ck: The molar concentration of an arbitrary species in the logarithmic diaersm. K,, and K,.: The successive aridity consrants of carbonic acid. K,,:'i'he rduhiiity product of trranyl hydrorrde, VOtOHl . K, ,,: The cumulatrw stahilrty constnnts
