Short communication
Extraction of Zinc from Water into Nitrobenzene Using Strontium Dicarbollylcobaltate in the Presence of Valinomycin
Emanuel Makrlika* and Petr Varnrab
a* Corresponding author: Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic
E-mail: Petr.Vanura@vscht.cz
b Prague Institute of chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic
Received: 19-02-2007
Abstract
From extraction experiments and y-activity measurements, the extraction constant corresponding to the equilibrium Zn2+(aq) + SrL2+(nb) ZnL2+(nb) + Sr2+(aq) taking place in the two-phase water-nitrobenzene system (L = valinomycin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (Zn2+,SrL2+) = -0.1. Furthermore, the stability constant of the valinomycin-zinc complex in nitrobenzene saturated water was calculated for a temperature of 25 °C: log P„b(ZnL2+) = 6.2.
Keywords: Zinc, valinomycin, water-nitrobenzene system, extraction and stability constants.
1. Introduction
The antibiotic valinomycin was discovered in cultures Streptomyces fulvissimus1. It is a macrocyclic dep-sipeptide exhibiting the threefold symmetry.2 It forms complexes with alkali metal ions, the stability of which shows a pronounced dependence on the ion radius.2-5 Because of the lipophilic outer envelope of the complex, valinomycin enables univalent cations to be transported across membranes of cells and cell organelles3, and consequently, it is a powerful uncoupler of oxidative phosphorylation in mitochondria6. The very large difference in stability of the potassium and the sodium complexes is the cause of the high potassium selectivity of the valino-mycin-based ion-selective electrode.7
Up to now, a complex species of zinc with valino-mycin has not been obviously proved. In the present communication, stability of the valinomycin-zinc complex in the organic phase of the water-nitrobenzene extraction system was determined.
2. Results and Discussion
Regarding the results of previous papers8-11, the two-phase water-Zn(NO3)2-nitrobenzene-Sr(DCC)2ex-traction system can be described by the following equilibrium
Zrr*(aq) + Sr2+ (rib) o Zrr+ (nb) + Sr2+ (aq);
(1)
with the corresponding exchange extraction constant Kex(Zn2+,Sr2+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex(Zn2+,Sr2+) it can be written8
log K„(Zrr*,Sr+) - log K' ,. - log K'
(2)
where KZi n2+ and KiSr2+ are the individual extraction constans for Zn2+ and Sr2+, respectively, in the water-nitrobenzene system8. Knowing logKZn2+ = -11.610 and logKSr2+= -10.7 (inferred from References 8 and 11), the exchange extraction constant Kex(Zn2+,Sr2+) was simply calculated from Eq.(2):
log Kex(Zn^,Sr2+)=-0.9
(3)
In terms of previous results9,12, the two-phase water-Zn(NO3)2-nitrobenzene-L (L = valinomycin) -Sr(DCC)2 extraction system (see Experimental), chosen for determination of stability of the complex ZnL2+ in water saturated nitrobenzene, can be characterized by the main chemical equilibrium
with the equilibrium extraction constant Kex(Zn2+,SrL2+):
[ZnL2+USr2+L
KiX<Ztr+,SrL3+) =
[Zn3,]s q[SrLi+3„
(5)
It is necessary to emphasize that the valinomycin ligand forms-with Zn2+ and Sr2+ - the relatively stable complexes ZnL2+ and SrL2+ in the nitrobenzene phase.
Following the conditions of electroneutrality in the organic and aqueous phases of the system under study
(6)
(7)
the mass balances of zinc and strontium at equal volumes of the nitrobenzene and aqueous phases
(8) (9)
and the measured equilibrium distribution ratio of strontium
DSr=[SrL2+]nb/[Sr2+]a
(10)
then combination of relationships (5)-(10) gives the final expression for the extraction constant Kex(Zn2+,SrL2+):
Kex (Zn"+,SrL"+) = 1
p in, nb
Sr(DCC)2
(11)
D n + D	-C
Sf U T Sr /^ZnfNOj ).
lil, nh SrtDCCt.
In this study, from the extraction experiments and y-activity measurements (see Experimental) by using Eq. (11), the following value of the constant log K(Zn2+,SrL2+) was evaluated:
log K K (Zn, SrLi+ ) = - 0.1
(12)
Moreover, with respect to previous results1213, for the extraction constants Kex(Zn2+,Sr2+) and Kex(Zn2+, SrL2+) defined above, as well as for the stability constants of the complexes ZnL2+ and SrL2+ in nitrobenzene saturated with water, denoted by Pnb(ZnL2+) and Pnb(SrL2+), respectively, one gets
+ log K„{Zn;+,Sr!.;+) - log K„(Zn;+,Sr;+)
Using the constants log Kex(Zn2+,Sr2+) and log Kex(Zn2+,SrL2+) given above, the value log Pnb(SrL2+) = 5.4 (L = valinomycin)14 and applying Eq. (13), we obtain the stability constant of the valinomycin-zinc complex in water saturated nitrobenzene at 25 oC:
log ßnb(ZnLi+) = 6.2
(14)
In conclusion, it should be noted that the stability constant of the complex species BaL2+ (L = valinomycin) in nitrobenzene saturated with water is log Pnb(BaL2+) = 6.5.15 It means that the stability constants of the complexes ZnL2+ and BaL2+, where L is valinomycin, in the mentioned medium are comparable.
3. Experimental
Valinomycin was supplied by Merck, Darmstadt, Germany. Cesium dicarbollylcobaltate (abbrev. CsDCC) was synthesized in the Institute of Inorganic Chemistry, Rez, Czech Republic, using the method published by Hawthorne et al.16 The nitrobenzene solution of hydrogen dicarbollylcobaltate, HDCC, was prepared from CsDCC by the procedure described in Reference 17. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The equilibration of the nitrobenzene solution of HDCC with stoichiometric Sr(OH)2, which was dissolved in the aqueous solution of Sr(NO3)2 (0.2 mol/l), yields the corresponding Sr(DCC)2 solution in nitrobenzene. The radionuclide 85Sr (DuPont, Belgium) was of standard radiochemical purity.
The extraction experiments were carried out in 10 ml glass test-tubes covered with polyethylene stoppers: 2 ml of the aqueous solution of Zn(NO3)2 of the concentration in the range from 1 x 10-4 to 1 x 10-3 mol/l and mi-croamounts of 85Sr were added to 2 ml of the nitrobenzene solution of valinomycin and Sr(DCC)2, whose initial concentrations varied also from 1 x 10-4 to 1 x 10-3 mol/l (in all experiments, the initial concentration of valinomycin in nitrobenzene, CLin,nb, was always equal to the initial concentration of Sr(DCC)2 in this medium, Cg^Dcc ). The test-tubes filled with the solutions were shaken for2 2 h at 25 ± 1 °C, using a laboratory shaker. Under these conditions, the equilibrium in the system under study was established after approximately 15 min of shaking. Then the phases were separated by centrifugation (2 min, 2 500 rpm). Afterwards, 1 ml samples were taken from each phase and their y-activities were measured using a welltype NaI(Tl) scintillation detector connected to a y-ana-lyzer NK/350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratio of strontium, DSr, was determined as the ratio of the measured radioactivities of 85Sr in the nitrobenzene and aqueous samples.
4. Acknowledgements
The present work was supported by the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6046137307.
5. References
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Povzetek
Z ekstrakcijo in meritvami y-aktivnosti smo določili konstanto ekstrakcije v sistemu dveh faz (voda/nitrobenzen), ki jo ponazorimo z ravnotežjem Zn2+(aq) + SrL2+(nb) ZnL2+(nb) + Sr2+(aq) (L = valinomicin; aq = vodna faza, nb = nitro-benzenova faza) in znaša log Kex (Zn2+,SrL2+) = -0.1. Ocenili smo tudi konstanto stabilnosti kompleksa med valinomi-cinom in zinkom v nasičeni raztopini nitrobenzena v vodi, ki pri 25 °C znaša log Pnb(ZnL2+) = 6.2.