caused by the solubilization into the hydrophilic polyoxyethylene portion of the micelle. In the cases of non-ionic surfactants, the partition is very favorable to incorporation into the micelle (7). When more and more amounts of the surfactant are added to a solution containing a definite amount of the dye, micelles will begin to form at certain concentration; the cmc and some of the dye molecules will then be incorporated into the micelles. At the same time, the (7) K. Shinoda, “Colloidal Surfactants,” Academic Press, New York, N.Y., 1963, p 158.
wavelength of maximum absorption of the solution begins to shift abruptly. ACKNOWLEDGMENT
The authors are thankful to W. U. Malik, Chemical Laboratories, University of Roorkee, Roorkee, for valuable suggestions and for providing research facilities. RECEIVEDfor review February 3, 1972. Accepted May 16,1972.
Chemical Enrichment and Exclusion with Ion Exchange Membranes Walter J. Blaedel and Thomas R . Kissel Department of Chemistry, Uniuersity of Wisconsin, Madison, Wis. 53706
THEHIGH QUALITY of ion exchange membranes as permselective devices is well established. However, the materials are still fairly new, and their capabilities as practical analytical separation or concentration devices have not yet been fully realized. The purpose of this note is to illustrate some of these capabilities. Modern ion exchange membranes are remarkably effective devices for the manipulation of ionic species. The membranes are easily permeable to counter ions, while co-ions of opposite charge are practically excluded and pass through only very slowly. The rates of diffusion of counter ion and co-ion species are so different [typical fluxes of about and 10-9 moles cm-2 sec-1, respectively ( I , p 348)] that an ion exchange membrane acts as a gate for counter ions and as a barrier for co-ions. Membrane thicknesses are only about 0.001 inch and counter ion transport through a membrane from one solution to another is quite fast. With efficient stirring, halftimes to steady-state are usually only a fraction of a minute. Lakshminarayanaiah (2) discusses the general properties of ion exchange membranes, and a model describing steady-state counter ion transport has been formulated (3). In the following work, the gating, exclusion, and enrichment of ionic species by ion exchange membranes are illustrated by noting the response of a membrane-wrapped ion selective electrode when it is dipped into various solutions. EXPERIMENTAL Apparatus. Four kinds of electrodes were used : a Sargent S-30050-19 flat glass electrode, a Sargent S-30072-15combination glass electrode, a Beckman 39137 monovalent cation electrode, and a Sargent 3-30080-15A saturated calomel reference electrode. A Corning Model 12 expanded scale pH meter was employed for all pH or millivolt readings, and the values were recorded on a Sargent Model SR recorder. A Sargent temperature bath and Thermonitor served as the thermostating system during measurements. (1) F. Helfferich, “Ion Exchange,” McGraw-Hill, New York,
N. Y., 1962.
(2) N. Lakshminarayanaiah. “Transport Phenomena in Mem-
branes,’’ Academic Press, New York, N. Y., 1969. (3) W. J. Blaedel, T. J. Haupert, and M. A. Evenson, ANAL.CHEM., 41,583 (1969).
m-
CATION SELECTIVE ELECTRODE
8
ION EXCHANGE MEMBRANE 0.001“ THICK
+TEFLON
TAPE
O-RING
INSIDE SOUTTION
A 0.1 M SALT
W SOUITION, A DISC, SOAKED 0.0005“ INTHCK INSIDE
Figure 1. Ion selective electrode wrapped with an ion exchange membrane
Reagents. All chemicals were of reagent grade. All water was triply distilled : once-distilled water was redistilled first from alkaline permanganate and then from dilute H2S04. Membranes were obtained from RAI Research Corporation (Hauppauge, L.I., N.Y.): PlOlO (a sulfonated fluorocarbon cation exchanger), P191-50 (a poly 4-vinyl, N-methyl pyridinium iodide) anion exchanger, and an inert polyvinyl alcohol (PVA) membrane having no exchange sites. Dry thicknesses were about 0.001 inch for the ion exchangers and about 0.0005 inch for the PVA. The ion exchange membranes were conditioned by soaking them for a few minutes each in 1 M HC1 and in 1M NaOH (3 cycles). To convert to a particular ionic form, the membrane was given 30-minute soakings in each of 4 successive portions of 1-2M counter ion salt solution. The converted membrane was then stored in a 0.1M solution of the counter ion salt until needed. PVA membranes were washed in triply distilled water and then stored in a 0.1Msolution of the proper salt until needed. Procedure. A membrane-wrapped electrode (Figure 1) consisted of the electrode, a PVA spacer film, and an ion exchange membrane. The PVA film served not only as a spacer, but also as a reservoir for inside solution. It allowed the ion exchange membrane to be tightly wrapped around the electrode while preventing complete extrusion of the inside solution.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2109
6-
A
I/
I
I
I 2 20 40 TIME, MINUTES
(C)0 (A) 0
3 60
Figure 2. Permeability of a cation (C) or anion (A) exchange membrane to hydrogen ion (Data of Experiments 1 and 2, Table I)
of membrane permselectivity, and the inside composition rapidly approached that of the outside solution. The progress of the cleaning could be followed by measuring the decrease in the sensed ion as it left the inside solution. When the major part of the sensed ion had been flushed out, the wrapped pair was immersed in successive portions of the 0.1M inside salt. The process was stopped when the concentration of the sensed ion in the inside solution fell to the blank value and no longer changed. Total cleaning time using this procedure after a counter ion enrichment run (high level of sensed ion inside) took 10-15 minutes. No attempts were made to minimize this cleaning time. Electrodes standardized before wrapping did not change during the course of a single experiment, so calibration of the wrapped pair was not necessary. After long periods of storage, a wrapped pair was calibrated by adjusting the meter reading with the standardization control so that the wrapped pair in the 0.1M salt solution gave the same pH or millivolt reading as the previously unwrapped pair in the same salt solution. To make a measurement, the wrapped electrode pair was withdrawn from the standby solution, rinsed with a stream of distilled water, blotted with a paper tissue, and immersed in the sample solution. To illustrate enrichment or the speed of ion penetration, the calibrated and rinsed wrapped electrode pair was immersed in 100 ml of a solution containing an ion to which the electrode was directly (H30+ or NH4+) or indirectly (OH-) sensitive. All solutions were stirred mechanically, and were thermostated at 25 + 0.02 "C. Electrode response was recorded as a function of time after immersion, and was taken as an indication of how fast the concentration of the sensed ion built up in the inside solution. Some systems were chosen to illustrate the slowness of co-ion penetration, while others were chosen to illustrate the rapid penetration of counter ions or their enrichment in the inside solution. RESULTS AND DISCUSSION
zY 0
-21 I I I -2 -3 -4 -5 LOG OUTSIDE COUNTER ION CONCENTRATION, M
Figure 3. Dependence of initial rate of counter ion buildup in inside solution upon outside counter ion concentration (Data of Experiments 1 and 5, Table I. Letters on curves denote electrode types)
To assemble, the electrode bulb was wetted with the 0.1M solution of the salt in which the membrane was stored. Then the ion exchange membrane, upon which was placed a small disk of 0.1M salt-wetted PVA, was stretched tightly around the bulb, and its upper folds were fastened with 1/4-in. Teflon (Du Pont) tape. In this way, a laminated construction was achieved with an inside volume estimated at 0.01 ml or less in contact with the sensitive electrode surface. The reference electrode was wrapped similarly to compensate for the membrane bi-ionic concentration potential which existed in these systems ( I , p 378). The wrapped pair was stored in a 0.1M salt solution identical to that contained in the inside solution. After preparation of the wrapped electrodes, or after a measurement, the composition of the inside solution was indefinite. To clean, the wrapped pair (ion selective electrode and reference electrode) was immersed for several minutes in two successive pxtions of a 0.5-1M solution of the original inside salt. This high ionic strength caused partial breakdown 2110
Figure 2 shows the response curves when a glass electrode wrapped with a cation or anion exchange membrane is immersed in 100 ml of a 0.001MHCl solution. Wrapped with a cation exchange membrane (Curve C), the inside solution reaches the same pH as the outside solution within 10 seconds. Within 3 minutes, the pH of the inside solution has dropped a unit below the pH of the outside solution, which corresponds to a 10-fold enrichment of hydrogen ion in the inside solution over that in the outside solution. Because of the large volume ratio of outside to inside solution (about lo4), the outside solution undergoes no significant change during the measurement. However, when wrapped with an anion exchange membrane (Curve A ) , the inside solution changes hardly at all, even after 1-hour immersion in 0.00lM HC1, indicating that the anion exchange membrane is a very effective barrier to the diffusion of hydrogen ion. Results for other systems studied are summarized in Table I. In general, all of the experiments demonstrate that ion exchange membranes show high permeability to counter ions and virtually no permeability to co-ions in dilute solutions. Short time enrichments of counter ions ranged from 5 to 100 for the systems studied. It is the permselectivity of the ion exchange membranes that causes these effects, which are embodied in the Donnan equilibrium, and which have been explained and demonstrated before, though not on so short a time scale as in the present work ( 4 , 5 ) . (4) W. J. Blaedel and T. J. Haupert, ANAL.CHEM., 38,1305 (1966). (5) R. M. Wallace, Ind. Eiig. Chem., Process Des. Decefop.,6 , 423 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
Table I. Experiments Illustrating Enrichment and Permeability of Ion Exchange Membranes to Counter Ions and Co-Ions
Wrapped electrode comExperi- bina- Sensed ment tiona ion 1 A-D H+ 2 B H+
Time (min) required for inside solution to reach N x outside concentration of sensed ion N=0.1 N = l N = 10 0.034.05 0.05-1 2.5-10
Solution compositions ~. Inside Outside O.lMKC1, pH 6 10-l to 10-5MHCl 0.1M KCl, pH 6 to 10F4MHCl b b b 0.1MKC1, pH 6 10-1M HC1 60 b b 3 A-D OHAnion-acetate0.1M KOAc, pH 8 to 10-4M KOH 0.8-2.8 1.8-5.5 >11, N = 5 4 A-D OHCation-K+ 0.1M KOAc, pH 8 & 10-4MKOH b b b O.lMKOAc, pH 8 10-2M KOH 60 b b 0.1M KOAc, pH 8 10-lM KOH 5 60 b 5 C-D NH4+ Cation-Tris+ 0.1M Tris-sulfate to 10-4M(NH&SO4 0.2-1 2.5-9 10-20 buffer, pH 8 6 C-D NH4+ Anion-SO120.1M Tris-sulfate & 10-4M(NH~)zSOI b b b buffer, pH 8 0.1M Tris-sulfate 10-'M (NHa)iSOa 15 b b buffer, pH 8 0.1M Tris-sulfate 10-'M (",),SO4 2 30 b buffer, pH 8 a A = Sargent S-30050-19 flat glass electrode. B = Sargent S-30072-15 combination glass electrode. C = Beckman #39137 monovalent cation electrode. D = Sargent S-30080-15A S.C.E. reference electrode. Denotes that concentration of ion in the inside solution had not reached this level after 60 minutes elapsed time. Membrane type and form Cation-K+ Anion-C1-
Figure 3 is a log-log plot of the rate of buildup of counter ion in the inside solution against its concentration in the outside solution. There is fair linearity of the plot over about 2 decades of concentration, and the average slope is 1.1, indicating that the rate of buildup inside is proportional to the outside concentration. Such proportionality is also shown by the transport of a trace counter ion from one solution to another through an ion exchange membrane (3). With suitable control over rate determining parameters, and with more uniform wrapping of the membrane around the electrode, the buildup rate might be of analytical use in measuring outside concentrations that are too low to be directly measurable with the sensor electrode. CONCLUSIONS The preceding experiments show that thin modern ion exchange membranes exhibit high permeability to counter ions,
and virtually prohibit diffusion of co-ions. Further, ion exchange membranes may give concentration enrichment of counter ion species by factors as high as 100, which may be of analytical use in sampling and in increasing the sensitivity of ion selective electrodes. Work is presently under way in our laboratories on coupled ion exchange-immobilized enzyme membrane systems which would increase sensitivity and reduce interferences. ACKNOWLEDGMENT The cooperation of V. F. D'Agostino, of RAI Research Corporation, in supplying membranes and technical information is highly appreciated. RECEIVED for review April 17, 1972. Accepted June 12, 1972. Financial assistance in the form of a grant from Miles Laboratories is gratefully acknowledged.
Quantitative Conversion of Cyclamate to N, N - DichIorocyclohexyIamine, and UItravioIet Spectrophotometric Assay of Cyclamate in Food Dolan Hoo and Chu-Chuan Hu Department of Biochemistry, and Biophysics, University of Hawaii, Honolulu, Hawaii THEOLDEST and most widely known method for determining cyclamate is using nitrous acid to convert cyclamate to sulfuric acid and cyclohexene, and subsequent formation of barium sulfate precipitate (I). The other product, cyclohexene, is, however, of little use in evaluating cyclamate, since the gas chromatographic method for cyclohexene determination is lengthy and laborious ( I ) . Johnson et al. (2) found that (1) M.L. Richardson, Tulanta, 14,385 (1967).
quantitative hydrolysis of cyclamate to cyclohexylamine can be achieved by heating cyclamate in aqueous 1.3N HC1 at 125 O C (15 psi) for at least 7 hours. This procedure for determining cyclamate is time-consuming. While searching for a faster and better approach to split cyclamate, we found that an equivalent amount of N,N(2) Darryl E. Johnson, Helmut B. Nunn, and Stanley Bruckenstein, ANAL.CHEM., 40,368 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2111
