Anal. Chem. 1991,63,480-486
480
Electrodialytic Eluent Production and Gradient Generation in Ion Chromatography Douglas L. Strong and Purnendu K. Dasgupta* Department of Chemistry a n d Biochemistry, Texas Tech Uniuersity, Lubbock, Texas 79409-1061
Keith Friedman and John R. Stillian Dionex Corporation, 1228 Titan Way, Sunnyvale, California 94086
A membrane-based electrochemical approach for the in situ production of ultrapure ionic substances has been developed. This is illustrated with specific reference to the production of NaOH solutions and their use in ion chromatography. Two basic types of generators are described. I n the first, a currentsfficient single-membrane device generates a stream of NaOH and H,. This stream is degassed by a membrane-degasser en route to the chromatographicpump. I n the second type, multiple membranes are used and the product NaOH channel does not contain gas. The product purity is excellent-an electrodialytic membrane suppressor (EMS) produces a typical exchanged conductance of 340 f 40 nS/cm for 0-175 mM NaOH. The system permits direct electrical control of the eluent concentration; gradient chromatography is accomplished without mechanical proportioning. The characteristics of various generator devices and a performance comparison of electrodialytic and conventional eluents are described.
The most successful eluent used for anion analysis in the pioneering work ( I ) on ion chromatograph (IC) was a COB2-/HC03-solution. During the intervening 15 years, major improvements have been made in columns, suppressors, and other necessary hardware (2, 3 ) and IC has become the preeminent technique for anion analysis and a significant commercial success. Despite journal articles and manufacturers’ notes (4-9) regarding its superiority, hydroxide is still not as widely used as eluent as C0:-/HCO3-. Consider the following: (a) under typical conditions, the suppressed product conductance (SPC) is much lower with OH-, proportionately lowering detection limits (LOD), (b) unlike CO;-/HCO;, OHis not expected to show analyte response nonlinearity ( l o ) , and (c) OH- is unquestionably better for gradient IC. However, a reproducible OH- eluent may be more difficult to prepare. Commercial standard NaOH/KOH solutions contain varying amounts of dissolved COS2-,which affect the retention behavior. Better NaOH eluents can be made from “COS2--free”50% NaOH solutions. Such an eluent can be further purified by treatment with Ba(OH)2,centrifugation, and dilution of the supernatant, all conducted under He (7). However, few are willing to go to such lengths and an alkalimetric assay is still necessary. Keeping CO, out of purified eluents over a long period is also nontrivial. Further, while OH--gradient IC is certainly far more practical than any other alternative, impurities, present even in purified eluents, initially concentrate on the column and elute later as artifact analyte peaks; an appreciable baseline rise occurs as well. An impurity-trapping column before the injector valve and/or storage and subtraction of a blank background can help, but it is obviously desirable to start with the purest possible eluent. We show here convenient in-line generation of NaOH solutions of a purity heretofore unequalled. The generated
concentration is electrically programmable, allowing gradient IC runs with isocratic pumps.
PRINCIPLES Consider a noble-metal anode flow channel bearing regular NaOH, separated by a cation-exchange membrane (CEM) from a cathode flow channel bearing pure water. With sufficient applied voltage, Na+ migrates across the CEM and forms NaOH at the cathode channel:
(Na+ + ) HzO + e- = (Na+ + ) OH- + y2H2
(1)
At the anode, O2 is evolved and OH- is electrolytically neutralized: 2 0 H - - 2e- = H20 + y2O2
(2)
The purity of the cathodic NaOH is ensured by the inhibition of anion transport across the CEM by the Donnan potential and the applied field. If transport of Na+ to and through the CEM is not a limiting factor, the amount of electrodialytically generated (EDG) NaOH is faradaically controlled. A programmable current generator can thus directly govern the concentration of the EDG NaOH. One caveat is necessary: Donnan rejection of the principal anion, hydroxide, is not perfect and some zero-current penetration (ZCP) of NaOH is unavoidable. The amount increases with increasing surface area of the membrane, decreasing membrane thickness, increases nearly exponentially with increasing concentration of the feed NaOH solution (11),and also depends on the membrane formulation; CEM compositionsthat better reject anions are of considerable interest (12). The EDG NaOH above contains significant (mL/min) amounts of H2. To make a practical system, such gas bubbles can be removed by a hydrophobic porous membrane (13,14) en route to the pump inlet. In the following, the sequential combination of a single membrane electrodialysis unit and a degassing device is designated a type 1 NaOH generator. Generators without significant gas formation in the product channel are also possible. Consider three flow channels formed by two parallel membranes (Figure 1). Water is fed to both the bottom cathode channel and the central channel. The latter is isolated from electrolytic gases, and the product NaOH output is taken from it. For meaningful product NaOH (PNaOH) concentrations, Na+ transported to the central channel through the donor CEM must be significantly retained there while current conduction through the barrier membrane (BM) must be dominantly via cathodically generated OH-. An anion-exchange membrane (AEM), which allows passage of OH- and not Na+, is an obvious choice for BM. A CEM may also be usable if the NaOH concentration on its cathode side becomes high enough to overcome the Donnan barrier, allowing significant OH- transport from the cathode (15). Accumulation of NaOH on the cathode side of the membrane may be achieved by a diffusion barrier. In such a case, the CEM basically acts as a bulk flow restrictor while permitting
0003-2700/91/0363-0480$02.50/0 0 1991 American Chemical Society
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ion transport. Other nonionomeric bulk flow barrier membranes can therefore be used as well. Since the ratio of ionic mobilities of OH- to Na+ is 4:1, up to 80% of the EDG NaOH can theoretically remain in the product channel. Actual PNaOH may be less due to bulk flow across the B M and the lack of sufficient cathodic OH- available for transport. These multiple membrane devices without gas in the product channel are hereinafter designated type 2 generators. T h e current-voltage behavior of such devices reveal important characteristics. If a higher voltage is required to establish a given level of current, the susceptibility to the undesired side reaction (breakdown of water without Na+ transport) increases. Type 1 devices, offering only one barrier to ion transport, should be superior in this respect. We also define two quantities: ieff,the current efficiency for electrodialysis
(3) where TNaoHis the total Na+ concentration dialyzed across the donor CEM normalized to 1mL/min, i is the current, and K is the faradaic conversion factor (1.6 mA/mM NaOH at 1 mL/min), and for type 2 devices, R, the retention efficiency of the central channel, R being given by =
PNaOH
/ TNaOH
(4)
EXPERIMENTAL S E C T I O N Material and Reagents. Sodium hydroxide used as the feed solution was diluted from 50% NaOH stock (J. T. Baker). Ionexchange membranes were generously donated by the manufacturers. In the following, abbreviations of membranes used in described devices represent the first entry within parentheses. Sheet CEM's included Nafion 117 (N, perfluorosulfonate, Du arenesulfonate) Pont, Wilmington, DE) and Selemion CMV (S, and Flemion (F, perfluorocarboxylate), both from Asahi Glass, Tokyo, Japan. The sheet AEM was a radiation-grafted membrane (Dionex Corp., Sunnyvale, CA). Tubular CEMs were Ndion 810 (Nl, -2.75 mm id., -200-pm wall), 815 (N2, -1000 pm id., -125-pm wall), 811 (N3, -675 pm i.d., -100-pm wall), and 014 (N4, -350 pm i.d., -40-pm wall), and a Dow perfluorosulfonate ionomer (D, same dimensions as Ndion 811), all from Perma-Pure Products, Toms River, NJ, and the AEM's were of the radiation-grafted polytetrafluoroethylene (PTFE) type (R, 500 pm i.d., -200-pm wall, RAI, Hauppage, NY) and a perfluorocarbon ionomer (Toyo Soda, Tokyo, Japan). Sheet BM's included the cellulose acetate type with molecular weight cutoffs (MWCO) of 100 and loo00 (CA100 and CAlOK, Spectrapor, Spectrum Medical Industries, Los Angeles, CA), and tubular BM's were either polyacrylonitrile (PAN, MWCO 13000, 800 pm i.d., 300-pm wall, Asahi Kasei, Tokyo, Japan) or polysulfone (PS, MWCO 5O00,900 pm i.d., 50-pm wall, A/G Technologies, Needham, MA). Microprous BM's included polypropylene (PP) of 0.2-pm mean pore size and 70% surface porosity [Accurel R5/2 (Al, 1.2 mm i.d., 300-pm wall) and Accurel S6/2 (A2, 1.8 mm i.d., 400-pm wall), both from Enka AG, Wuppertal, FRG] and a planar PP membrane Celgard 3401 (CG, 0.02-pm mean pore size, 38% surface porosity, 25-pm thickness, coated with a proprietary agent ot render the membrane hydrophilic, Hoechst-Celanese, Charlotte, NC). Hydrophobic porous membranes (HPM) used for degassing in type 1 generators included 100 pm thick PP mesh-backed porous 25-pm PP (Celgard 4400) and a porous membrane of
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i* - 10 Resistors of 24K Figure 2. Schematic for computer-controlled constantcurrent source.
proprietary composition designed especially for gas/liquid phase separations (Genie, A+ Corp., Prairieville, LA), as sheets and tubular membranes included porous PTFE (Gore-Tex TA 001, 2-pm mean pore size, 50% surface porosity, lo00 pm i.d., 400-pm wall, W. L. Gore and Associates, Elkton, MD) and porous PP (Accurel R5/2). Equipment. Figure 2 shows the constant-current source (0-500 mA). The control voltage input (0-5 V), provided by a 12-bit resolution interface board installed in an AT-class computer programmed in Pascal, was linearly converted into a constant current by using an operational amplifier and two driver transistors. The software (41 kbytes) displayed the programmed current profile, status of execution and permitted external event control, and any preset number of repeats. Calibration involved shorting the generator cell using SW2 and adjusting the current source to the desired full-scale current value using SW1 and R1. In the experimental setup, high-purity water from a recirculating water polisher fed the generator (for type 1 devices, the degassing units are not separately shown). Feed NaOH (150 mM at 1.5 mL/min, except as stated) was delivered by N2 pressure; beyond supplying an adequate amount, the flow rate was not critical and was controlled by a screw clamp. The (degassed) catholyte product output was directly connected to a Model 4000i pump, bypassing all its low-pressure gradient generation valving. The concentration of PNaOH was determined by on-line conductance measurement between the chromatographic column (AS5A) and the suppressor. The commercial CDM-I1 cell was easily modified to measure the high specific conductance: the two original electrodes were tied in common and used as one, while the grounding block on the cell was used as the other electrode. These measurements were interpreted by a 23-point calibration plot spanning 0-250 mM NaOH. For type 2 devices, the concentration of the catholyte NaOH was also determined by conductance detection after degassing for device characterization. A chemically regenerated (12.5 mM H2S04)suppressor (AMMS) was used for eluent suppression (except as stated) and was followed by a CDM-I1 conductivity detector (all components above from Dionex Corp.). A dual-channel strip-chart recorder (TY-2, Knauer, FRG) and a disk-based recording integrator (C-RSA, Shimadzu Scientific, Columbia, MD) were used for data acquisition. For cases where the use of an EMS is indicated, this was a water-regenerant electrodialytic suppressor (16) constructed with sheet membranes without carbon packing; details will be published elsewhere. For long-term unattended use, should liquid flow through the generation device stop, damage to the generator is likely if power continues to be applied. In the event of flow cessation, the pump also stops because of gas in the pumphead. We mounted photodiodes on the stop/start status indicating light-emitting diodes (LED's) of the pump module and fed the outputs to a voltage comparator and used the rectified output of the latter (the LED's are pulse-driven) to disconnect power to the generator (and suppressor) in the event of pump stoppage. Generator Devices. Over 50 generators representing over a dozen designs were investigated; the choice of a tubular vs planar
482
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991
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Figure 3. Terminal end of a tubular type device: (A) product channel in/out; (B) SS jacket tube; (C) polypropylene tee; (D) membrane tube; (E) wire crimps; (F) polypropylene tee; (G) wire crimp; (H) Pt wire; (I) space-filling PVC tube. PLATINUM
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design was often dictated by the availability of a membrane in a specific form. Only examples of the simplest tubular and planar devices are detailed here. Sealing of membranes in low-volume configurations is nontrivial; details appear in previous publications (16-21). The device designations include type (1/2), tubular or planar (T/P), initials of the membrane (proceedingfrom the anode to the cathode for type 2 devices),and for tubular devices, length (cm) and diameter (mm). For example, the designation 2-TNS-A1-Nl-13.5-4 indicates a type 2 tubular device with Nafion 811 as the anode membrane, followed by an Accurel R5/2 and a Nafion 810 membrane, the length and i.d. of the outermost stainless steel (SS) jacket serving as the cathode being 13.5 cm and 4 mm, respectively. Type 1 Cenerators. Figure 3 shows one end of device 1-TN3-18-1.1. Apertures A, 1.5 cm from each end, were made with a cutting wheel into the walls of an 18 cm length, thin-walled, 17 gauge (1.1 mm i.d.), SS needle tube (B; Small Parts, Miami, FL). The tube was then thoroughly cleaned and deburred. Two PP tees (C;0406TEEP, Ark-Plas, Flippin, AR) were drilled out and part B was push-fitted therein, the tee-arm aligned with aperture A providing catholyte access. A length of prestretched (22) Nafion 811 tube (D) was inserted through tube B. The protruding ends of tube D were swollen in hot ethanol and folded back over tube B to -5 mm. One barbed end each from two tees (F, identical with parts C) were cut and the bores widened by drilling and by a heated tapered tool until they could be pushed over the tube ends and sealed by wire crimps (G). Platinum wire H (0.254-mm diameter) centered in the device, was sealed in the F termini with PVC tube I and a wire crimp. Analyte flow was through ports F, countercurrent to catholyte flow. Planar device 1-P-N was fabricated by using components of the Dionex micromembrane suppressor (MMS) (17). The shell holds membranes ca. 15 X 1.7 cm in size. Except for M2 and the screen atop it, the design is the same as for type 2-P devices (Figure 4). Pt wire (50 cm, 0.254-mm diameter) woven on a cation-exchange (catex) screen constituted the anode; electrode connections were through 1 mm diameter SS rods push-fitted through undersized holes at the top and bottom of the shell. A second catex screen isolated the Nafion 117 membrane M1 from direct contact with the anode. A 250-pm mesh PP screen and a 75-pm mesh SS screen cathode (Small Parts) completed the device with seals provided by Parafilm gaskets hard-pressed along the peripheries of each screen (17). The MMS operates with a central eluent channel and two flanking regenerant channels;
I
WATER
Figure 6. Type 2-T generator, shown schematically: (Ml) anode membrane; (M2) cathode membrane; (SSJ) stainless steel jacket. W and L are the i.d. and lengths of SSJ.
minor modifications to the shell were made for the present design. Degassing Deoices. The tubular device was a -30 cm long HPM tube containing a space-filling PTFE filament inside a jacket through which COB-free(soda-lime guard cartridge) N2 or house air flowed at -200 mL/min. Without air flow, condensation occurred and the HPM eventually leaked. In the planar device (Figure 5), a flow channel (L; 1.9 cm X 15.2 cm) cut into Nylon block K snugly accommodated the porous polyvinylidene fluoride sheet J (Porex Technologies, Atlanta, GA) and HPM sheet I atop. The top half (H) containing a 125 pm deep flow channel aligned with channel L, was affixed to block K through screw holes N. In the vertically placed device, the gas-liquid mixture enters through the top port in H and the degassed liquid exits through the bottom port while H 2 gas leaves through bottom port 0,vented to the atmosphere via a soda-lime guard tube. Type 2 Generators. Figure 6 shows the general tubular design. Membrane M1 is the anode CEM and M2 may be an individual membrane or a combination thereof. The outer SS jacket and the inner Pt wire serve as the cathode and anode, respectively. Direct adaptations or minor modifications of previously described sealing techniques are used to isolate the fluid channels from each other and to provide access to individual flow channels. When a HPM is used as part of M2,it must first be wetted, e.g., with methanol, and then used with aqueous solutions. Planar devices, with the configuration shown in Figure 4, were built inside MMS shells. M1 was the anode CEM and M2 was one or more membranes except in device 2-P-N-N-PPTFE which contained a perforated PTFE plate (0.5mm thick, four rows of 1.0 mm diameter holes, 72/row) placed on the electrode side of the cathode Nafion 117 membrane.
RESULTS AND DISCUSSION Device Performance. Type 1 Deuices. Table I presents data for three planar generators along with EMS SPC's. Recalling that the theoretical specific conductance of pure water is 56 nS/cm, all three membranes can be used to produce high-purity EDG NaOH, albeit the product is less pure with Selemion. Electric field induced anion rejection should
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991
Table I. Type 1 Planar Generators
E, V
membrane
Nafion
i , mA
0 2.7 3.2 3.6
Flemion
4.9 50 156 260
0 2.8 3.3 3.7
Selemion
49 142 270
0 3.0 3.5 4.1
electrically suppressed PN~OH,conductance (EMS SPC), mM @S/cm
52 140 280
33.8 99.5 176.0 2.1 32.9 95.3 176.0 0.1 33.6 91.2 181.0
0.37 0.36 0.40 0.34 0.16 0.30 0.31 0.34 0.16 0.36 0.47 0.80
increase at higher applied voltages, and the apparently greater relative purity a t higher P N a O H levels can be rationalized. All three devices exhibit linear i-V behavior, and the extrapolated threshold for electrolysis is 2.50, 2.63, and 2.77 V, respectively, for Nafion, Flemion, and Selemion. The slope is also lower for Selemion, 208 mA/V vs 232-243 mA/V for the others. The ieffvalue is 0.99 for Nafion and 1.03 for the others (subtracting the ZCP NaOH). The difference from unity is essentially within the combined experimental uncertainties of flow, current, and concentration measurements. It is also desirable to reduce the ZCP NaOH to
