Extraction of some univalent and divalent cations into nitrobenzene in the presence of calcium ionophore I

: The stability constants of the Calcium ionophore I (i.e., N , N ´ – di[(11 ′ - ethoxycarbonyl) undecyl] – N , N ′ ,4,5 -tetramethyl – 3,6 – dioxaoctane diamide) complexes with M n + (M n + =Li + , H 3 O + , Na + , NH 4 + , Ag + , K + ,Rb + , Tl + ,Cs + ,Ca 2 + , Sr 2 + , Ba 2 + . Pb 2 + , Cu 2 + , Zn 2 + ) in nitrobenzene saturated with water were determined by the extraction method. These constants were found to increase in the series of Cs + < Rb + < K + < NH 4 + < Na + < Tl + < H 3 O + < Ag + < Li + and Ba 2 + < Zn 2 + < Sr 2 + < Pb 2 + < Cu 2 + < Ca 2 + .


Introduction
The dicarbollylcobaltate anion and some of its halogen derivatives are very useful reagents for the extraction of alkali metal cations (especially Cs + ).These reagents can be used, in the presence of polyoxyethylene compounds, also for the extraction of univalent, divalent, and trivalent cations from aqueous solution into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes [1,2], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive wastes [3,4].
Several naturally occurring as well as synthetic substances have been found which increase the ion permeability of biological and artificial membranes [5][6][7].These mediators of ion translocation fall into one of two groups: they are either ion carriers or channel formers.Thus, the series of acyclic compounds derived from 3,6-dioxaoctane-dicarboxylic acid was prepared.Among these, N,N´di[(11′-ethoxycarbonyl) undecyl] -N,N′,4,5 -tetramethyl -3,6dioxaoctane diamide, also called Calcium ionophore I or ETH 1001 (1, see Scheme 1), exhibits a high selectivity for Ca 2+ when used as the neutral carrier in n-octyloxy-o-nitrobenzene as the membrane component in a poly(vinylchloride) matrix of the ion-selective electrode [8][9][10].This ligand is able to mediate the transport of Ca 2+ ions across the synthetic phospholipid bilayers and can therefore act as a Ca 2+ ion carrier.The evidence available indicates that 1:2 complexes (cation to ligand) of Ca 2+ are formed in the bulk of the membrane [7].
In our previous work, the extraction of , and AmL 3 3+ are formed in the organic phase [11].The distribution ratios D and the separation factor α(Am 3+ /Eu 3+ ) are very high.However, up to now, the stabilities of the Calcium ionophore I complexes with univalent and divalent cations have not been investigated.Some of these cations, especially Cs + , Ag + , Rb + , Sr 2+ and Ba 2+ (and, of course, nitric acid, i.e., cation H + ) represent a significant component of the raffinate from the reprocessing of spent nuclear fuel by the PUREX process.
Nitrobenzene was the first highly polar diluent investigated for the extraction of fission products and transplutonium elements from the PUREX process raffinate.Most equilibrium data and stability constants are available for this diluent.More suitable polar solvents were later found, but the extraction behavior of these solvents is like that of nitrobenzene.The individual extraction constants (see below) are available only for waternitrobenzene and water-phenyltrifluoromethyl sulfone systems.
Therefore, in the current work, the stability constants of the 1.M n+ complex species, where M n+ = Li + , H 3 O + , Na + , NH 4 method described by Hawthorne et al. [12].Acros Organics purchased the nitrobenzene (99 %, extra pure).The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity.A nitrobenzene solution of 0.2 mol/L hydrogen dicarbollylcobaltate in nitrobenzene (HDCC, see Scheme 2) was prepared from CsDCC by the procedure published elsewhere [13].The concentration of HDCC was determined by the potentiometric two-phase titration by the carbonatefree sodium hydroxide in a nitrogen atmosphere.A Radiometer RTS 822 automatic titrator with a G 2040 glass electrode and K 4040 calomel electrode was used for this titration.The equilibration of the nitrobenzene solution of HDCC with stoichiometric NaOH, which was dissolved in an aqueous solution of NaCl (0.2 mol/L), yielded the corresponding sodium dicarbollylcobaltate (NaDCC) solution in nitrobenzene while the equilibration of HDCC solution with aqueous 1 mol/L strontium acetate yielded the strontium dicarbollylcobaltate (Sr(DCC) 2 ) solution in nitrobenzene.The concentrations of the NaDCC and Sr(DCC) 2 solutions correspond to the concentration of the original HDCC solution.It was simply checked by the distribution of 22 Na, respectively 85 Sr, in the systems aqueous solution of 0.2 mol/L NaCl in water -0.2 mol/L solution of NaDCC in nitrobenzene respectively 0.1 mol/L SrCl 2 in water -0.1 mol/ L Sr(DCC) 2 in nitrobenzene (distribution ratio D = 1).
The radionuclides 22 Na and 85 Sr (DuPont, Belgium) and 133 Ba (Polatom, Poland) were of standard radiochemical purity.The acidic solutions of these radionuclides were evaporated to dryness and afterwards dissolved in distilled water.
The extraction experiments were performed in 10 mL glass test tubes with polyethylene stoppers.2 mL of an aqueous and organic phase were shaken for 2 h at 25 ± 1 °C, using a laboratory shaker.Under these conditions, the equilibria in the systems under study were established after approximately 20 min of shaking (the distribution ratios, measured after 20 min and 40 min of shaking are the same as that measured after 2 h of shaking).Then the phases were separated by centrifugation (5 min, 3000 rpm). 1 mL samples were taken from each phase.At first 1 ml of upper aqueous phase were taken using 1 ml Plastomed macro pipette with a polyethylene tip.Afterwards, 1 ml sample was taken from lower, nitrobenzene phase.The pipetting of the organic phase was done so that the aqueous phase did not stain the inside of the polyethylene tip.After pipetting, the outside of the tip was carefully wiped with a cotton swab.Afterwards, the γ-activities of both phases were measured using a welltype NaI(Tl) scintillation detector connected to an NK 350 γ-analyzer (Gamma, Budapest, Hungary).It must be pointed out that the distribution ratios were always higher than D > 0.01.
The stability constants of Na + and Ba 2+ with 1 were determined from the extraction of sodium (NaPic) and barium ((Ba(Pic NaDCC respectively C in, nb Sr( DCC) 2 , and also to the concentration of metal or ammonium nitrate in the aqueous phase.
The equilibrium distribution ratios, D, of 22 Na, 85 Sr, or 133 Ba were determined as the ratios of the measured radioactivities in the nitrobenzene and aqueous samples.

Results and discussion
The stability constants of the cation complexes of calcium ionophore I ligand in nitrobenzene, i.e., the equilibrium constants of the reactions are defined as These constants were determined for all investigated cations by the extraction methods described below.Aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively.Regarding the results of our previous papers [14,15], the two-phase water-MA n (M = Na + , Ba 2+ , A − = picrate) -nitrobenzene extraction system can be described by the equilibrium (3) with the corresponding extraction constant K ex (M n+ , nA − ); For the constant K ex (M n+ , nA − ) one can write [16].
where K i M n+ and K i A − are the individual extraction constants for M n+ and A − in the water-nitrobenzene system.Knowing the values log K i Na + = −6.0[16], log K i Ba 2+ = −10.5 [17], and log [16], the extraction constants were simply calculated from Eq. ( 5) as log K ex (Na + , A − ) = −5.2 and log K ex (Ba 2+ , 2A − ) = −8.9.
The two-phase water-MA n (A − = picrate)nitrobenzene -1 (Calcium Ionophore I) extraction system, chosen for the determination of the stability constant of the complexes 1.Na + and 1.Ba 2+ (see Experimental), can be described by the main chemical equilibrium with the equilibrium extraction constant K ex (1.M n+ , n A − ) in the form At this point, it is necessary to emphasize that 1 is a considerably lipophilic ligand, practically present in the nitrobenzene phase only, where this ligand formswith the univalent and divalent cations very stable complexes, as given below.
Employing the conditions of electroneutrality in the organic and the aqueous phases of the systems under study, the mass balances of 1 and NaA, respective BaA 2 at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratios D, combined with Eq. ( 7), we gain the final expression for the extraction constants K ex (1.Na + , A − ) and K ex (1.Ba 2+ , 2A − ) in the form where D is the distribution ratio of 22 Na respectively 133 Ba in the systems under study.Eq. is valid, because the distribution ratios of 22 Na and 133 Ba in the absence of a ligand are negligible, so [1.Na + ] nb >> [Na + ] nb and [1.Ba 2+ ] nb >> [Ba 2+ ] nb .Furthermore, the stability constants of the complexes 1.Na + and 1.Ba 2+ in water-saturated nitrobenzene at 25 °C can be simply calculated from the extraction data by Eq.
Applying the conditions of electroneutrality in the organic and aqueous phases of the system under study, the mass balances of 1, M n+ and N n+ at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratio D combined with Eq. ( 11), we gain the final expression for the extraction constant K ex (M n+ , 1.N n+ ) in the form Eq. ( 12) In the absence of the ligand, the extraction proceeds by Eq. ( 13) as For the constant K ex (M n+ , N n+ ) one can write Eq. ( 14), where K i M n+ and K i N n+ are the individual extraction constants for M n+ and N n+ in the water/nitrobenzene system Knowing K ex (M n+ , 1•N n+ ) and K ex (M n+ , N n+ ) the stability constants of Calcium ionophore Imetal complex can be simply calculated as where M n+ = Li + , H 3 O + , NH 4 + , Ag + , K + , Rb + , Tl + , Cs + , Ca 2+ , Pb 2+ , Cu 2+ , Zn 2+ and N n+ = Na + and Sr 2+ .
The stability constants of the complex 1.Sr 2+ , used for the calculations of the stability constants of divalent cations by Eq. ( 15), has been calculated using the analogical equation log K nb 1.Sr 2 ( )= log K nb 1.Ba 2+ ( ) All resulting stability constants are summarized in Tables 1 and 2. It must be pointed out that the stability constants of the complexes 1.K + , 1.Rb + and 1.Cs + are lower than 10 5 , so Eq. ( 12) is not fully valid.Therefore, these stability constants were refined using the computer program Haltafall [21].Table 1 contains these refined values.
It is clear from Table 1 that the stability constants of the univalent cations in nitrobenzene saturated with water, K nb (1.M + ), increase in the order Cs + < Rb + < K + < NH 4 + < Na + that determined in our previous work from the extraction of Eu 3+ and Am 3+ (log K nb (1.H 3 O + ) = 6.2 [11]).It is possible to prove that the affinity of the extracted cation to the nitrobenzene phase is proportional to the Table : Equilibrium constants for the M + and •M + cations in the two-phase water/nitrobenzene extraction system at  °C (M + = Li + , Na + , H  O + , NH  + , Ag + , K + , Rb + , Tl + , Cs + ;  = Calcium ionophore I; for the meaning of the constants see text).The standard deviations of the constants log K are lower than . (three measurements).product of its individual extraction constant and the stability constant of its complex with Calcium Ionophore I in nitrobenzene.These products for the univalent cations are also summarized in Table 1.It is clear from Table 1 that the maximum affinity to nitrobenzene phase have the "soft" cations Ag + and Tl + .The same products in Table 2 indicates that the highest affinity to nitrobenzene has the cation Ca 2+ .
The stability constants of divalent cations in the system under study increase in the sequence Ba 2+ < Zn 2+ < Sr 2+ < Pb 2+ < Cu 2+ < Ca 2+ .The stability constants of Ca 2+ with 1 is very high log K nb (1.Ca 2+ ) = 20.We cannot find the reason for the extraction behavior of Cu 2+ and Zn 2+ .On the other hand, the extraction of alkali earth metal cations Ca 2+ , Sr 2+ and Ba 2+ and also of the cation Pb 2+ decrease with the crystallographic radius of the respective cation M 2+ as depicted in Figure 2. Calcium cation has the highest affinity to the nitrobenzene phase (Table 2).

Conclusions
In the present work, the solvent extraction of univalent cations Li + , H 3 O + , Na + , Ag + , K + , NH 4 + , Tl + , Rb + , Cs + divalent cations Ca 2+ , Sr 2+ , Ba 2+ .Pb 2+ , Cu 2+ , Zn 2+ in a waternitrobenzene extraction system has been studied.Furthermore, the stability constants of the proven cationic complexes with Calcium ionophore I in nitrobenzene saturated with water were evaluated.The stability constants of univalent cations decrease with the ionic radíus, but the stability constants od ammonium and esppecially of the "soft" cations Ag + and Tl + are higher than that of the alkali metal cation with similar ionic radius.The stability constant of Ca 2+ with Calcium ionophore I is very high, log K nb (1.Ca 2+ ) = 20.
Because Calcium ionophore I is a very efficient extraction reagent also for Eu 3+ and Am 3+ [11] the present work is a valuable contribution also for the handling of reprocessing raffinate.

Scheme 1 :
Scheme 1: The structural formula of Calcium inophore I.

<
Tl + < H 3 O + < Ag + < Li + .The log K nb (1.M + ) values of the alkali metal cation and H 3 O + decrease almost linearly with the crystallographic radius of the respective cation M + , as depicted in Figure 1.On the other hand, the stability constants of NH 4 + and soft univalent cations Ag + and Tl + are higher than the stability constants of the alkali metal cation with similar ionic radius.This is probably caused by the presence of nitrogen atoms in the molecule of Calcium ionophore I.It must be pointed out that the protonation constant of Calcium ionophore I in nitrobenzene, determined in the present work, log K nb (1.H 3 O + ) = 6.4,agrees with  −. −. −. −. −. log K nb (•M + ) . . . . . .
152,154Eu 3+ and 241 Am 3+ by the nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of Calcium ionophore I was investigated.It was found that the particles EuL 2

Table  :
Equilibrium constants for the M + and •M + cations in the two-phase water/nitrobenzene extraction system at  °C (M + = Ca + , Sr + , Ba + .Pb + , Cu + , Zn + ;  = Calcium ionophore I).For the meaning of the constants, see text.The standard deviations of the constants log K are lower than . (three measurements).Ca +Sr + Ba + Pb + Cu + Zn +