Applications of Semipermeable Ion Exchange Membranes to Trace Analysis of Metal Ions by Electrochemical and Neutron Activation Techniques Uri Eisner, J. M a r k Rottschafer, Francis J. Berlandi, a n d H a r r y B. M a r k , Jr. Department of Chemistry, The Unicersity of Michigan, Ann Arbor, Mich. 48 104
BOTH ANODICSTRIPPINGvoltammetry (1) and neutron activation analysis in conjunction with electrodeposition (2, 3) have been shown useful in the determination of trace metal ions in aqueous solutions. It would be desirable to apply these techniques directly to natural water and biological systems where the electrode reactions are generally inhibited by the surface-active organic materials present. I t was thought that an electrode system employing a semipermeable ion exchange membrane as a barrier mounted between the solution and the working electrode might circumvent this problem. The ion exchange membrane itself may be used successfully as a preconcentration matrix for neutron activation analysis as it is composed of elements that are essentially inert to neutrofl activation. Thus, the gamma spectrum of any element held up at the ion exchange sites is easily obtained. EXPERIMENTAL
The experimental apparatus including the working electrode employed was similar in most aspects to that described previously (2-7). However, the original Teflon plunger of the working electrode was replaced by a stainless-steel plunger which was found to move more smoothly in the Teflon sleeve and gave better displacement reproducibility. Also, a Teflon cap with an opening of the same area as the electrode surface was screwed over the Teflon sleeve as shown in Figure 1. In this way the membrane was held in place between the cap and sleeve and gave a reproducible area of the membrane surface exposed to the solution. The pyrolytic graphite electrode cylinders were pretreated in the same manner as previously described (4). The ion exchange membranes were supplied by the American Machine and Foundry Co., (Springdale, Conn.). The cation exchange (and cellophane) membranes were first pretreated by washing in 1N KOH t o remove oils and surface contaminants. They were then equilibrated in 1N “ 0 3 , washed with triply distilled water, and then equilibrated with the supporting electrolyte before use. The anion exchange membranes were washed with KOH and retained in triply distilled water before use. After the usual surface polishing procedure ( 2 , 3), the graphite electrode cylinder was pressed into the Teflon sleeve (1) I. Shain, “Treatise on Analytical Chemistry,” Part 1, Volume 4, Chapter 50, I. M. Kolthoff and P. J. Elving, Eds., Interscience,
New York, 1963. (2) H. B. Mark, Jr., F. J. Berlandi, B. H. Vassos, and T. E. Neal,
“Proceedings, 1965 International Conference: Modern Trends in Activation Analysis,” Texas A. and M. Press, College Station, Texas, 1965, p. 107. (3) F. J. Berlandi and H. B. Mark, Jr., ANAL.CHEM., 36, 2062 (1964). (4) F. J. Berlandi, Ph.D. thesis, University of Michigan, 1966. ( 5 ) B. H. Vassos, Ph.I). thesis, University of Michigan, 1966. ( 6 ) B. H. Vassos, and H. B. Mark, Jr., J . Elecrroanul. Chem., 13, l(1967). (7) B. H. Vassos, F. J. Berlandi, and H. B. Mark, Jr., ANAL. CHEM., 37, 1653 (1965). 1466
ANALYTICAL CHEMISTRY
TEFLON
POLYPROPYLENE
I
I LOCK NUT
1
CAP I MEMBRANE
STAINLESS STEEL
I TEFLON SCREW
PYROLYTIC GRAPHITE
Figure 1. Membrane barrier electrode assembly
until the electrode surface was recessed about 0.1 mm. A drop of supporting electrolyte was placed in this recession. A disk of the membrane of slightly larger diameter than the graphite cylinder was placed over the drop of supporting electrolyte, and the cap was then screwed in place with care being taken to eliminate bubbles between the membrane and pyrolytic graphite electrode. A drop of supporting electrolyte was placed in the recession between the membrane and the face of the cap. With surface tension holding the drop in place, the electrode was inverted and placed in the cell. After electrolysis the working electrode was removed from the cell, the screw cap was removed, and, in some cases, the membrane was washed and encapsulated for irradiation. In the anodic stripping analysis experiments, the electrode assembly (without the cap) was then inserted in a second electrolysis cell and medium, and the stripping curve was obtained in the usual manner (8). In other experiments, the graphite electrode was cleaved yielding a disk 1 mm thick. This was treated prior to irradiation as has been previously described (2, 3). After irradiation, the samples were counted and the gamma spectra obtained were corrected in all cases for background (both counter background and that due to the graphite or membrane alone) and the total number of counts in the gamma photo peak of the species of interest was then determined.
RESULTS AND DISCUSSION
Response Characteristics of Barrier Electrode. Experiments using the cellophane membrane barrier electrode were carried out with to 10-5M Ag+, 0.1M K N 0 3 solutions. The working electrode was potentiostated at -0.4 V us. SCE for 4 minutes and then the deposited material was stripped by applying a 10 mV/second anodic sweep. Reproducibility of the areas under the stripping peaks was k.5 (relative standard deviation). The medium exchange method (@-Le, the removal of the membrane and insertion of the electrode into a second medium (8) M. Ariel, U. Eisner, and S . Gottesfeld, J. Elecrroanul. Chem.,
7, 307, (1964).
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SILVER ION CONCENTRATION IN BULK SOLUTION ( M O L A R I T Y 1
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Figure 2. Gamma spectra of neutron-activated cation exchange membranes a.
C-103 membrane before electrolysis
b. C-103 membrane after electrolysis in a dilute Ag+
solution Conditions: Irradiate 4 minutes. Pause 1-1.5 minutes. Count 2 minutes for dissolution-must be applied here for quantitative work. It appears that the membrane acts as a diffusion barrier to the cation leaving the electrode surface in the stripping step in solutions of high concentrations. This results in a nonlinearity of stripping peak area with initial solution concentration. The anodic stripping peak areas were found to be proportional to concentration in all cases using the medium exchange method (8). A quantitative study of this effect is in progress and will be reported later. Applications of Ion Exchange Membrane as a Preconcentration Matrix. The activation analysis for the metal ion can be done by two simultaneous but independent techniques using the membrane and the graphite electrode as follows: As equilibria in ion exchange membranes follow at least to a first approximation the Donnan Theory (9), the amount of any ion held in the membrane should be proportional to its concentration in the bulk solution. The membrane itself could be used as a preconcentration matrix for an ion of interest. Irradiation of the membrane containing the ion of interest would yield a gamma spectrum from which the concentration of the ion can be determined. Experiments were conducted over a ( 10-5-10-4M)range of silver ion concentrations in the bulk solution containing 0.1M KNO, as supporting electrolyte. The electrode was potentiostated for 7 minutes at -0.4 V us. SCE. After electrolysis the cation membrane was removed from the electrode, rinsed, and encapsulated for irradiation. After irradiation (4minutes) the capsule was removed (to eliminate any counts due to the capsule itself) and the membrane-ion matrix counted. Figure 2 shows a typical gamma spectrum (9)
F. Helfferich, “Ion Exchange,” McGraw-Hill, N e w York, 1962.
Figure 3. Variation of silver activity after irradiation as a function of silver concentration in bulk solution a.
Membrane
b. Pyrolytic graphite
of the cation exchange membrane (AMF C-103) before (curve a) and after (curve b) electrolysis in the silver solutions. Note that the pure treated membrane background count is low and reproducible. The membrane after electrolysis shows a distinct Ag peak a t 0.63 MeV which is employed to determine the silver concentration in solution. As shown in Figure 3, the number of counts under the silver photopeaks was linearly proportional to the bulk silver ion concentration. Equilibration of the membranes in different concentrations of copper solution showed that the copper concentration in the membrane was about 1000 times greater than in original solution. For cobalt(I1) and silver ions this factor was about 100. Neutron activation of the pyrolytic graphite electrode showed a linear dependence of electroplated silver on the bulk concentration of silver ion. Effects of Surface Active Protein on Barrier Electrode Response. The analysis of trace metal ions in biological and natural water is usually very cumbersome and/or laborious as it is generally necessary to separate or destroy the organic matter. In electrochemical techniques such organic materials (usually proteins) strongly chemisorb on the electrode surface, blocking the electrodeposition of the metals, and, thus, must be removed before electrolysis. Experiments carried out using lO-bM range Ag+ solution (0.1M KNO, supporting electrolyte) without a membrane in the electrode assembly, showed essentially no distinct anodic stripping peak after addition of 0.01% egg albumin to the solution. Using a cellophane membrane barrier before the electrode, there was also a decrease of about 80% of the stripping peak area after the addition of egg albumin, which is probably due to some blocking of the semipermeable membrane. Using the cation exchange membrane, which had been equilibrated in silver ion solutions containing 0.01 egg albumin, as barriers, it was found that large, well defined anodic stripping curves resulted which gave about 10 times greater sensitivity than the cellophane membrane barrier. VOL. 39, NO. 12, OCTOBER 1967
1467
It was shown by activation analysis that the concentration of silver in the membrane was about one third of that which was found in membranes equilibrated with silver solutions containing no albumin. This indicates some blocking of the membrane sites by the albumin, but sufficient sensitivity remains for trace determination of many metal ions by activation and anodic stripping analysis by taking advantage of the preconcentration of the Ag+ ion in the cation exchange membrane. Thus, it appears that ion exchange membrane electrodes could possibly be used in biological and natural solutions without a significant loss of sensitivity and response resulting from adsorption of organic molecules. Also, the neutron activation electrodeposition technique ( 2 ) of trace analysis is feasible for application in biological fluids, etc. Work is presently being carried out on biological fluids, and will be reported in a future detailed paper. The effects of complex formation between bioorganic molecules and metal ions of interest is also being investigated. CONCLUSIONS
The membrane barrier electrode might allow one to extend the methods of anodic stripping voltammetry and neutron activation electrodeposition to the analysis of trace metal
ions in solutions containing organic surface active agents which deactivate the electrode and, possibly, in untreated biological and natural solutions. The membrane simply acts as a barrier to large organic molecules but not to hydrated metal ions or even some coordination compounds. The same membrane-electrode assembly configuration can be employed in three separate methods for the analysis of the trace metal ions. By employing selective complexation, solvent or medium choice, and choice of ion exchange type in the membrane, selectivity could possibly be achieved for one desired ion(s) in a mixture with several others. ACKNOWLEDGMENT
The authors thank American Machine and Foundry Co., Springdale, Conn., for donating the ion exchange membrane materials used in this research, and the Michigan Memorial Phoenix Project for supplying laboratory space and reactor time. RECEIVED for review April 10, 1967. Accepted July 27, 1967. Research supported in part by Grants from the National Science Foundation (NSF GP 4620 and GP 6425) and from the U. S. Army Research Office-Durham (DA 31124 ARO-D 284).
Adsorption Isotherms of Xylene Isomers on Zinc Oxide by Gas Chromatography Herbert Malamud, Ralph Geisman, a n d Seymour Lowell Department of Chemistrjq, C. W . Post College of Long Islund Uninersity, Brookcille, L. I., N . Y STOCK(1) succeeded in testing the theory developed by Glueckauf (2-6), that elution of a volatile adsorbed substance from a packed column by an inert gas could be detected by a katharometer and that the resulting chromatogram could be used to develop the adsorption isotherm. Gregg (7) outlines this method, giving several appropriate examples and an explanation of the theory. This method of frontal analysis has been used and discussed by Cremer and Huber (8). I n the present study, the authors were concerned with the effective molecular areas of the three xylene isomers on zinc oxide and the relationships betweeen the effective molecular areas, the dipole moments, and the steric requirements of the isomers o n the zinc oxide surface. METHODS
Consistent with the experimental method of Stock, the authors have developed the apparatus shown in Figure 1. This design allows the column, packed with zinc oxide adsorbent, to be charged with xylene adsorbate to any desired R. Stock, Ph.D. Thesis, London University, 1955. E. Glueckauf, Proc. Roy. SOC.,(London) 186A, 35 (1946). E. Glueckauf, J . Cheni. SOC.,1947, 1308. 1315, 1327. Zbid., p. 1302. (5) E. Glueckauf, Nature. 156, 748 (1945). (6) Zbid., 160, 301 (1947). (7) S. J. Gregg, “The Surface Chemistry of Solids,” Reinhold, New York, 1961. (8) E. E. Cremer and H. F. Huber, “Gas Chromatography,” N. Brenner, J. E. Callen, and M. D . Weiss, Eds., Academic Press, New York, 1962, p. 169.
(1) (2) (3) (4)
1468
ANALYTICAL CHEMISTRY
I
FLOW’
M E T E RS
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Figure 1. Diagram of gas flow apparatus Valve ( W ) 5 balances pressure drop due to bubblers. Valve 6 balances the drop due to the ZnO sample and the B detector. Valves 1-4 direct flow to load or unload ZnO sample with xylene vapor. (Load, open 1 and 4, shut 2 and 3; reverse for unload.) Valves 7 and 8 bypass sample and 9 balances pressure drop due to sample
equilibrium relative vapor pressure up to unity and provides for a rapid and efficient means of switching the gas flow between pure carrier gas and carrier gas containing adsorbate vapor. The isotherms were measured at 50” C. This temperature is over 60 times greater than the critical temperature of the helium carrier gas; thus, the possibility of adsorption of this gas is precluded. Flow rates of about 40 cc per minute were used and measured with a soap film flow meter (9). Flow rates of two and one half times this value did not alter the (9) A. T. James, and A. J. P. Martin, Biochern. J., 50, 679 (1952).
