~~~
~~
~
~~~
~~~
~
~
Table 11. Atmospheric Concentration of Interfering Substances and Their Equivalent Concentration in Absorbing Solution Concentration Usual of pollutant Interfering concentration in absorbing substance in air, ppm solution, pg 1.-1 Dimethyl butene 0.2 0.68 Acrolein 0.1 0.23 Nitrogen dioxide 0.3 0.56 Sulfur dioxide 1 .o 2.62 Hydrogen sulfide 0.5 0.69 Formaldehyde 0.1 0.12
0.02-0.05 ppm. The method can be extended to high atmospheric concentrations of ozone by increasing the volume of the absorbing solution. Thus the method described is some 2 or 3 times more sensitive than the colorimetric method of Hauser and Bradley (3). Twelve different ozone concentrations were analyzed for ozone simultaneously by potassium iodide and 2-DIH, and 4 samples were taken at each level. From an analysis of variance, the overall precision of a single measurement was found
to be ~k0.08pg 03 per ml absorbing reagent and the standard error was k0.065 pg per ml. INTERFERENCES
The effect of interfering substances on fluorescence was determined. An ozone-enriched air stream was split in half; one half was sampled by the prescribed procedure, in the other, the absorbing solution contained microgram quantities of the interfering substance. After passing both samples through the analytical procedure, any increase or decrease in fluorescence was noted. The substances tested and the concentrations used are shown in Table 11. The concentration of pollutant in absorbing solution is such as to be equivalent to the amount of pollutant normally found in 10 liters of air. Interferences due to aliphatic aldehydes and ketones are eliminated since the chromophore produced by them fluoresces at 520 nm with excitation at 425 nm. At the concentrations of interfering substances found in air and within the limits of experimental error, none of the substances caused a significant change in fluorescence and thus the method is specific for ozone in the atmosphere.
RECEIVED for review November 10, 1969. Accepted April 3, 1970.
Liquid Anion Membrane Electrodes Sensitive to Metal Cation Concentration Giancarlo Scibona, Leda Mantella, and Pier Roberto Danesi Industrial Chemistry Laboratory, Comitato Nazionale per L'Energia Nucleare, Centro di Studi Nucleari Casaccia, Rome, Italy Liquid membranes formed by organic solutions of long chain alkylammonium salts behave as liquid electrodes sensitive to the anion concentration. Liquid anion membranes can be used as electrodes sensitive and selective to the aqueous concentration of metal cations through their anionic complexes. By means of this new use of the liquid anion membrane, it is possible to develop electrodes that are highly selective with respect to other metals that either do not form complexes in the aqueous solution or form weaker complexes than the metal ion, which anionic complex is the counterion of the alkylammonium radical. Metal concentrations of zinc or palladium are determined by means of liquid membranes formed by benzene solutions of tetrachlorozinc or tetrachloropalladium(ll) salts of alkylammonium.
LONG CHAIN alkylammonium salts dissolved in low dielectric constant solvents are known to behave as liquid anion exchangers. The electrical potential of liquid membrane electrodes (organic solutions of long chain alkylammonium salts interposed between two aqueous electrolyte solutions of suitable composition) have been theoretically described and experimentaly tested for both the cases of monoionic and biionic potentials with ions of the same charge (1-5). (1) J. Sandblom, G. Eisenman, and J. L. Walker, Jr., J. Phys. Chem., 71, 3862, 3871 (1967). ( 2 ) G. Eisenman, ANAL.CHEM., 40,310 (1968). (3) 0. D. Bonner and D. C . Lunney, J. Phys. Chem., 70, 1140 (1966). (4) C. J. Coetzee and H. Freiser, ANAL.CHEM., 40, 2071 (1968). (5) P. R. Danesi, B. Scuppa, and G. Scibona, J. Phys. Chem., in
press. 844
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
The diffusion-migration processes in the membrane phase and the exchange properties of the liquid exchangers seem to contribute to the electrical potential of the system. When an anion is strongly preferred by the membrane phase, the organic solution of the corresponding alkylammonium salt can be used as a liquid membrane electrode sensitive to it. Since some metal anion complexes are strongly preferred by the liquid anion exchangers and the concentration of the complexes depends on the metal and on the ligand concentrations, a membrane can be prepared that behaves as a liquid electrode highly selective to metal cations and sensitive to their concentrations (through their anionic complexes). In this paper, the electrochemical properties of a liquid anionic exchanger interposed between solutions containing one or more cations and one complexing anion have been studied. The simple complexing anion and the metal complex anions present in the solution will, of course, contribute (at different extent) to the membrane potential. Liquid membrane electrodes formed by benzene solutions of trilaurylammonium and tetraheptylammonium salts of tetrachlorozinc and tetrachloropalladium(I1) interposed between aqueous solutions containing zinc and palladium(I1) (as cations) and chloride (as anions) will be discussed. EXPERIMENTAL
Reagents and Solutions. Trilaurylamine (TLA) supplied by Rhone and Poulenc and tetraheptylammonium iodide (THAI) supplied by Eastman Kodak have been used as materials for the preparation of the other alkylammonium
S.C.E
mV -2
4
Figure 1. Experimental cell used with liquid membranes. I and I1 contain aqueous solutions. I11 contains the membrane phase salts. All the other chemicals used were Carlo Erba analytical grade products and have been used without further purification. The preparation of trilaurylammonium chloride (TLAHC1) and tetraheptylammonium chloride (THACl) has been described (6, 7). Preparation of Liquid Membrane. The tetraheptylamrnonium chromate [(THA)2Cr041has been prepared by shaking a THACl organic solution with a K2Cr04 aqueous solution. The organic diluent consisted of benzene and chloroform (8%) to increase the solubility of the chromate salt. The trilaurylammonium zinc tetrachloride, (TLAH)2ZnC14,tetraheptylammonium zinc tetrachloride, (THA)2ZnC14,and trilaurylammonium palladium tetrachloride, (TLAH)tPdC14, have been prepared by shaking a benzene solution of TLAHCl or THACl with a large excess of aqueous solution containing zinc or palladium(I1). This operation has been repeated several times by renewing the aqueous phase. After separation of the phases, the organic layer was filtered on a sintered glass filter, centrifuged, and slightly concentrated by evaporation. The analysis of the nitrogen, chlorine, and metal content of the organic phases agreed with the reported stoichiometric formula of the salts. The zinc aqueous solutions have been prepared by dilution of stock solutions of HCl, LiCl, and ZnC1. Aliquots of the three stocks were mixed in such a way as to give a hydrogen ion concentration equal to 1M (to avoid both hydrolysis of the aqueous zinc and of the organic salt), and a free chloride concentration equal to 3 M . The chloride concentration has been chosen to have quite a large amount of complexed zinc and a rather high distribution coefficient. The zinc-chloride complexing constants, necessary to evaluate the free chloride content and the concentration of each zinc complex, have been taken from reference (8). The palladium aqueous solutions have been prepared sirnilarly. In this case the hydrogen ion concentration was equal to 0.5M and the free chloride concentration to 0.5M. The palladium(I1) extraction coefficient, when tertiary amines are used, strongly decreases with the chloride concentration (9) which therefore has to be kept quite low. On the other hand, palladium(I1) is strongly complexed by chloride ions even at low concentration. The palladiumchloride complexing constants necessary to evaluate the free chloride content and the concentration of the palladium complexes have been taken from reference (10). Solutions containing both palladium(I1) and zinc also have been prepared. The zinc concentration ranged between O-O.lM, and the concentration of the other ions was Pd(I1) = (6) G. Scibona, S. Basol, P. R. Danesi, and F. Orlandini, J. Znorg. N L ~Chem., . 28, 1441 (1966). (7) P. R. Danesi, M. Magini, and G. Scibona, “Proc. XI Int. Conf. Coordination Chemistry,” Israel, Sept. 1968, paper J-19. (8) G. Scibona, F. Orlandini, and P. R. Danesi, J. Znorg. Nucl. Chem., 28, 1313 (1966). (9) A. A. Mazurova and L. M. Gindin, Russ. J . Inorg. Chem., 10, 263 (1965). (10) H. A. Droll, B. P. Block, and W. C. Fernelius, J. Phys. Chem., 61, 1000 (1957).
l.
i
I
O.OlM, [H+] = OSM, [Cl-] = 0.5M. All the aqueous solutions were preequilibrated with the membrane phase in the phase ratio 01W = 0.1 before performing the measurements. The aqueous solutions were then analyzed again for the metal in order to take into account possible concentration variation due to organic phase solubilization. Membrane Potential Measurements. The emf measurements have been performed by means of the cell: Hg, Hg2C12,satd KC1 I Solution Membrane phase ISolution 1 I I11 I1 satd KCI, Hg2C12,Hg (1)
I
Figure 1 shows the experimental cell used. The composition of solution I (reference solution) as well as that of the membrane phase(II1) was kept constant during each run of measurements. Two cm3 of membrane phase and 20 ,ma of each aqueous phase were sufficient to perform the measurements. The membrane potential has been measured by Keithley model 610 B and by Cary No. 31 vibrating reed electrometers at 25 f 0.1 “C. The membrane potential cell and the calomel reference electrodes were enclosed in a copper Faraday cage to reduce disturbances due to the high impedance of the system. The equilibrium EMF reading was reached in a few seconds and was stable over a period of hours. When solutions that were not preequilibrated were used, a transient potential of several minutes was observed, because of the solubilization of the alkylammoniurn salt in the aqueous phase. Liquid Junction Potential Measurements. The liquid junction potential difference existing between the two liquidliquid interfaces (saturated KC1-Solution I and saturated KCI-Solution 11) has been evaluated through EMF measurements on cells of the type:
Hg,HgC12, satd KC1 /metal variable lAgC1, Ag
(2)
where the hydrogen and the chloride ion concentrations were kept constant to the same value used in the membrane potential measurements and the metal concentration was ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
845
A
-2
-3
-1
log[Pdl
Figure 3. Lower part: membrane potential (mV) us. logarithm of palladium concentration in solution I1 for cells of the type: Hg,HgpCIz, Mtd KCII Pd 0.1M A or 0.02M1 A, H' OSM, C1- 0SMI (TLAH)zPdar0.2kf in benzene I Pd at M variable, Hf0.5M, CI-0.5MI satd KCI, Hg2CIz,Hg 0 refers to values corrected for the liquid junction potential. Straight lines with a slope of 30 mV have been drawn through the points. Higher part: membrane potential (mV) us. logarithm of palladium concentrations ratio in the two half cells
-L
-3
-1
-2 log a
Figure 4. Membrane potential (mV) us. logarithm of chromate ion activity (log a) in solution (11) for cells of the type : Hg, HgzCIZ, satd KCl IKZCrOd at Mconstant I (TEA)zCr04O.1Min benzene I KzCrO, at M variablelsatd KCl, HgeCIz, Hg changed. In the case of chromate, the total K2CrOl concentration was varied. The EMF of this cell is given by E =
- EOA,,A,,OI
+ 59-15 log [Cl-] + EJ
(3) By plotting the experimental data in the form E - 59.15 log [Cl-1 us. metal cation concentration, we found that the liquid junction potential is given by AEJ = const x (metal cation or chromate anion concentration) with const = 0.25 mV/mM for palladium(I1) and const = 0 for zinc and chromate. The contribution to the liquid membrane potential due to the liquid junction potential in the aqueous phase has been then calculated as the difference between the two liquid junction potentials. In Figure 2, a liquid junction potential determination is reported. 846
EOoai
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
RESULTS AND DISCUSSION
In Figure 3 are reported the experimental EMF data obtained with the cell: Hg, Hg2C12,satd KC1 IPd(O.lM or 0.02M),
Hf0.5M, C1- 0.5M I(TLAH)zPdCla 0.2M in benzene IPd at M variable,
H+ OSM, C1- 0.5M lsatd KC1, Hg2C12,Hg. (4) The black triangles represent the points obtained with a Pd reference solution 0.1M. By correcting for the aqueous liquid junction potential (see Experimental), a straight line
mv
rnv
Figure 5. Membrane potential (mV) us. logarithm of zinc concentration in solution I1 for cells of the type: Hg, HgX12, satd KCI /ZnO.lM, H+lM, CI-3MI (TLAH)2ZnC140.2Mxor (TEA)2ZnC14,0.2M 0 in benzene 1 Zn at Mvariable, H+lM, C1-3M I satd KCI, Hg2C12,Hg Straight lines with a slope of 30mV have been drawn through the first point. The solid curved lines have been calculated through Equation 1
with slope 30 mV is obtained. By using as reference the Pd 0.02M solution, there is no need for the liquid junction potential correction (white triangles). The higher curve shows all the experimental data in the form of ratio of the two half cells palladium concentrations. It is noteworthy that because of the numerical values of the palladium(I1) chloride formation constants (IO), nearly all the palladium in solution is in the form of PdC142-. In Figure 4 the experimental results obtained with cells of the type
I (THA)2Cr040.1M in benzene I
Hg, HglC12, satd KC1 IK2Cr04at M constant
Pz.1 = Kz/Ki
K2Cr04 at 114 variable lsatd KC1, Hg2C12Hg ( 5 ) are shown. The experimental points fall on a straight line of slope 30 mV. No correction for the junction potential is needed in this case. In Figure 5 are reported the E M F data obtained with the cell Hg, HglCI?, satd KC1 (Zn O.lM, C1- 3M, H+ 1M 1 (TLAH)2ZnC14or (TEA)2ZnC140.2M in benzene
satd KCl, Hg2C12,Hg. (6) Also in this case, no correction for the liquid junction potential has to be applied. All the points lie on a curve which limiting slope is 30 mV. For both the Zn and Pd cases, the experimental data are fitted by the equation: (7)
where (') and (") indicate the two aqueous solutions, al and a2 are the aqueous activities of the two anions with al
or
and a2 = acl -.
(8)
Pd Case. In the case of Pd, where P2.1 = 0, (in our experimental condition) Equation 7 can be expressed in terms of total Pd concentration. In fact since the experimental work is carried out at constant ionic strength, it is possible to consider the ratio of the activity coefficients as being constant. Further by means of the metal complex formation constants, pi,for the reaction MfY iX MA'i(Y-') it is possible to express the fraction of the metal complex, C P d C l , Z - / C P d , through constant terms (when a constant chloride concentration is used):
+
I
Zn at M variable, C1- 3M, H + l M I
aPdCI12-
When total metal concentrations are used at the places of at, values of P2.1= 0 and Pz.l = 1.3 x 1 0 - ~are obtained for the Pd and Zn case, respectively. An equation similar to Equation 7 has been already reported in the literature for a mixture of univalent and divalent ions (11). An equation formally equivalent to Equation 7 can be easily obtained by considering the continuity of the electrochemical potentials of the two counter ions 1 and 2. In this case the potential V is a distribution potential and the constant parameter P2.1is given only by the ratio of the distribution coefficients, Ki of the two anions
CPdCIrZ-/CPd
- P ~ ( C C ~ )f~ / [ I
~fli(cCl)']
(9)
with CFd total Pd concentration. From Equation 9, it is possible to obtain CPdClrZ- and by substituting in Equation 7 ( P z .= ~ o), we obtain V = 30 log ' c p d / " c p d . Therefore as a consequence of P2.1 = 0 , the system behaves as a simple monoionic potential for a bivalent ion. The data for the chromate electrode (Figure 4) show in fact the same behavior. Zn Case. In this case it is P2.1 # 0 and a more detailed discussion on is needed. By considering the ion pair
=
(11) G. A. Rechnitz, C&EN, 45, 146 (1967). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
847
*lo
““1
,0
0
0
0
0
-101 1
I
I
-3
-2 tog
-1
1
[zn I
Figure 6. Membrane potential (mV) us. logarithm of zinc concentration in solution I1 for cells of the type: Hg, Hg2C12,satd KC11 ZnO.lM, PdO.OlM, Hf0.5M, C1-0.5MI (TLAH)2PdCla 0.2M in benzene 1 Zn at M variable, Pd 0.01MHf0.5M, C1-0.5MI satd KCI, Hg2CI2, Hg
+
formation equilibrium, S with Kij equilibrium constant, and the ion exchange equilibrium 2S 1 with K2.1equilibrium constant (the bar indicates the organic phase), we can write
+
w+
(10)
Kz.1 = (K~J/KIJ)X (KzIKJ
By using Equation 8, it is Pz.1 = K2.1T1.2 with T1.2 = KIJ/KZJ. Values of K2.1can be obtained on the basis of simplified assumptions. us consider the simple exchange reaction - Let2Cl- + RzZn C14$ 2RC1 ZnCla2- with E l alkylammonium chloride and Kzn,C1exchange equilibrium constant. The distribution coefficient of Zn between aqueous and organic phase, D = Zn(org)/Zn(aq) can be written in first approximation as
+
+
D z ~ = pE2Rc1C2c1/Kzn,cd1 ZpiCcl*)
(11)
where the pi are the equilibrium constants for the metal complex formation in aqueous phase. Therefore by means of Equation 11, it is Pz.1 = [ p 4 c 2 ~ ~ ~ C 2 ~ ~ / Z/3iCicl)]T1,~. D~n(l By neglecting the .activity coefficients, (since we work at constant ionic strength) Equation 7, for the Zn case, gives
+
V = 3010g
+ P*2.iwhere it is P*z,I = ,’CZnCla CznCI4+ P*Z.I
+
cient of the metal. The experimental results qualitatively agree with this prediction. In fact it is D P = ~ lo3 at Ccl = 0.5M and TOAHCl0.05M in toluene (TOA = trisoctylamine) (9) and DZn= l o 2 at CCI= 3M and TLAHCl 0.05M in oxylene (12). Other Considerations. The contribution of the diffusionmigration process to the membrane potential and the possible existence of other charged species in the membrane phase beside ZnC142- and PdCla2-have been neglected. However, it has been shown that despite the apparent isotropy of chemical composition of the membrane phase, the diffusion processes, in strongly associated membranes, can play an important role as potential originating mechanism. Further from distribution and vapor pressure lowering measurements (13), it has been suggested that the following species: [(RCl) (ZnCl~)l,[(RCh ( Z n W l , [(RC1)3 (ZnCldl, [(RCl), (ZnCUd can be present in the membrane phase. The contribution to the potential of these species has to be evaluated. They can in fact exist in solution in the form of more or less complex paired charged species, like (R+ZnCl:-), (R2Z+ZnC142-), (Rj3+ZnC153-),(R+ZnC13-)+ A more detailed study on the electrochemical behavior of the system investigated in this paper is then needed to draw any conclusion on the theoretical basis of Equation 7. As far as the analytical implications are concerned, it is evident that the liquid membrane electrode formed by the alkylammonium salt of tetrachlorozinc or palladium is sensitive to the metal ion concentration (Zn2+or Pd2f) and highly selective with respect to other metals which either do not form complexes in the aqueous solution or form weaker complexes than the metal ion which anion complex is the counterion of the alkylammonium radical. In Figure 6 are reported some EMF data for the cell Hg, HgzClz, satd KCl/Zn at Mvariable, Pd O.OlM, H+0.5M, CI- 0.5M/(TLAH)2PdC14 0.2M in benzene/Zn at M variable, Pd O.OlM, H+ OSM, C1- 0.5M satd KCl, HgZClzHg. The zinc concentration in the two half cells has been changed in the range 0.1-0.001M according to several possible combinations. In all cases the EMF of the cell was very close to zero. It appears then that by the appropriate choice of liquid membranes containing selective ion exchanger extractants (the distribution coefficient can be used as a first approximation selectivity criterion), it is possible to construct equally selective liquid membrane electrodes for which use in both analytical and physical chemistry appears very promising.
[PaC2~c1C4c~/Dzn(lZPrCtci)lT1.z The total Zn concentration CZncan be used at the place of CZnclrl-. In this case P*2.1has to be replaced with P * * z , ~ = (cRclz/Dzn) X Tl.2. Equation 7 then turns to V = 30 log [(’C,, P**2,1)/(”Czn P**2.1)]with P**z.la function of the chloride concentration. Despite the simplicity of the it is clear that the selecmodel, from the definition of P**2.1 tivity of the electrode increases with the distribution coeffi-
+
848
+
ANALYTICAL CHEMISTRY, VOL. 42, NO, 8, JULY 1970
RECEIVED for review January 12, 1970. Accepted March 6, 1970. (12) D. Dyrssen and M. de Jesus Tavares, Acta Chem. Scand., 20,
2050 (1966).
(13) R. Chiarizia, P. R. Danesi, and G . Scibona “Proc. XI Int.
Conf. Coordination Chemistry,” Israel, Sept. 1968, paper 1-26.
