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Anal. Chem. 1992, 64, 2406-2412
Voltammetric Sensor for Determination of Water in Liquids Huiliang Huang and Purnendu K. Dasgupta' Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Thin-film perfluorosulfonate lonomer (PFSI ) sensors overcoated with cellulose triacetate, polyvinyl alcohol (PVA)-HSPO,, or PVA-PFSI-HsP04 compositefllmsoperated In a puked voltammetric mode are described. With the proper choice of the protective film, attractive performance In terms of low limits of detection and good quantitative reproduclbllity are possible In solvents ranging In polarity from saturated hydrocarbons such as hexane, aromatic hydrocarbons such as toluene, and halogenatedsolvents such as dlchkromethane to modestly polar solvents such as dlethyl ether and higher polarlty solvents such as acetone or acetonitrile. They are not satisfactory for use In very high polarlty solvents, e.g., methanol, dlmethylformamlde (DMF), dimethyl sulfoxide (DMSO), etc. Water activity of the sample equilibrates with the sensing flhn between the analyzing pulses, and the accumulated water Is then electrolyzed by the pulse. The response current shows more than a firstorder dependence on water concentration, especially In nonpolar solvents over a large range of concentration.
tially limited by the diffusion of water vapor into the film. Liquid-phase diffusion is slow and only limited sensitivity is attainable by this mode. Additionally, too large an applied voltage causes an unacceptably large background current in many solventa, even when essentially no water is present. Presumably, electrochemical breakdown of the solvent occurs and this results in poor detection limits for water. The above problems can be addressed by (a) utilizing a second electrochemically stable polymer along with the PFSI, either as a protective external film or in a homogeneous mixture with the PFSI. This may serve the dual roles of a confining, swell-inhibitingelement that maintains structural integrity and of a selectivemolecular barrier that allowswater transport while inhibiting the transport of other, larger molecules. They can also be addressed by (b) operating in the pulsed voltammetric mode such that the water content of the sensing f icomes to equilibrium with the water activity of the solvent in between the pulses, thus improving sensitivity. Experimental investigation of these strategies has led to a class of new sensors that can be utilized in a variety of solvents and are described in this paper.
In two previous papers, we have described PFSI' and hybrid PFSI-Pz0b2 thin film amperometric sensors for the determination of water in the gas phase. The determination of water in liquid samples is in general a more challenging task. A survey of the literature indicates that the present practice is dominated by the Karl Fischer method3 while capacitive sensors based on controlled pore aluminum oxide and silicon dioxide are being increasingly used in on-line application^.^ More recently Chen and FritzS have developed a spectrophotometric method capable of determining trace quantities of water based on a shift in the acid-catalyzed equilibrium involving cinnamaldehyde, methanol, water, and cinnamaldehyde dimethyl acetal. This detection method is ideally coupled to chromatography, and a variety of applications have been demonstrated.6 There is nevertheless a need for simple inexpensive sensors that perform reliably, especially for process applications. If the PFSI thin-film sensors described previously for gas-phase applications are directly deployed for measurement of water in liquids, limited success is achieved a t best. First, physical integrity of the sensing film and electode-film contact is adversely affected by many solvents that swell the film. Second, experience indicates that adventitious impurities are present in many real liquid samples. Some of these impurities partition into the film and eventually poison the electrodes or the sensing film through electropolymerization or redox conversion. Third, in the continuous amperometric mode used with the gas-phase sensors, the operating voltage is maintained sufficiently high such that the current is essen-
EXPERIMENTAL SECTION Reagents. Nafion PFSI was obtained as a 5 wt % solution (equivalentweight(EW)1100,inamediumof90% loweralcohols/ 10%water) from Aldrich Chemical Co. or aa tubing from PermaPure Products (Raritan,NJ). The Dow PFSI, 1,1,2,2-tetrafluoro2-((trifluoroethenyl)oxy)ethanesulfonic acid (EW go), was obtained as a 2.5 wt % solution in alcohol from the Dow Chemical Co., Freeport, TX. All concentrations given in percent or ppm are compositions by weight. Platinum and rhodium wires (in diameters ranging from 25 to 200 wm) were obtained from Aesar Inc. Stainless steel needle tubing (Type 304) and rod were obtained from Small Parts Inc. (Miami, FL). Cellulose triacetate (CTA) waa obtained as a powder from Eastman Organic Chemicals. A 1% CTA solution was made by dissolving it in glacial acetic acid or a mixture of 3070 (v/v) ethanokchloroform. In acetic acid solution, CTA was stable for several months, solutions in other solvents must be freshly prepared. Eastman AQ 55D was obtained as a 28% aqueous dispersion (Kodak) and was diluted with deionized water to form a 1% solution. A 1% solution of silicone rubber was made by dissolving household silicone rubber adhesive (General Electric) in either ethylene dichloride or toluene. A 1% solution of Nylon was made by dissolving monofilament fishing line (STREN, duPont) in boiling formic acid. Poly(acry1ic acid) (PAA) and PVA (100%hydrolyzed powder from poly(viny1acetate)) were obtained from ScientificPolymer Products (Ontario, NY). A 2% PVA solution was made by dissolving it in boiling water and was then mixed 1:l with a 2% K2S208 solution. A mixture of 2% PVA and 4 % H a 0 4 waa made by dissolving PVA in a boiling aqueous solution of HaO,. A blend of 1 % PVA, 1% Dow PFSI and 4% HaPo, was made by dissolving 1g of PVA in 60 mL of boiling water containing 4.7 g of 85% HsPOl and adding 40 g of the 2.5% Dow PFSI solution; after cooling to room temperature, enough deionized water was added to make the total weight 100 g. Instrumentation. A simple inexpensive dedicated voltammetric analyzer was constructed for the present sensor application. The schematic diagram of the analyzer is shownin Figure 1. Operational amplifier AI, capacitor CI, and the resistor Rz constitute an integrator, with a scanning rate given by -dV/dt
(1)Huang, H.;Dasgupta, P. K. Anal. Chem. 1990,62,1935-1942. (2)Huang, H.;Dasgupta, P. K. A w l . Chem. 1991,63,1570-1573. (3)Scholz, E. Karl Fischer Titration; Springer Verlag: Berlin, 1984. (4)Carr-Brion, K.Moisture Sensors in Process Control;Elsevier: New York, 1986. (5)Chen, J.;Fritz, J. S. in Advances in Ion Chromatography; Jandik, P., Cassidy, R. M., Eds.; Century International: Medford, MA, 1990;Vol. 2,pp 73-91. (6) Fritz, J. S.; Chen, J. Am. Lab. 1991,23 (ll),245-24Q. 0003-2700/92/0364-2406$03.00/0
0 1992 American Chemlcal Soclew
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
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= VlIR2C1, variable from 0.2 to 6.0 V/s by means of varying input voltage VI or resistance Rz.The output of the integrator applies the desired voltageto the sensor. The sensor output is monitored by operational amplifier Az functioning as a current voltage converter and is registered on a strip chart or an X-Y recorder. The output of A2 can be varied by the switch-selectable gain resistor Rls (0.1-200 kQ). Operational amplifiers & and & were respectivelyused as triangular and square-wave generators.When & output reaches around +6 V (limited by the zener diode Z), the red light emitting diode (LED) Re is lit. During this time, A1 output is -0.46 V (limited by transistor T), because diode D1 is reverse biased and does not conduct. The period over which -0.46 V is applied to the sensor depends on the integrating capacitor C2 and the resistor R,. At the end of this period, comparator & switches and this output swings to -6 V (limited by the zener diode Z)and a green LED, Gr, is illuminated. Diode D1 is now forward biased and T conducts, causing integrator A1 to begin scanning. The maximum output of A1 is limited by the base voltage of T, this can be adjusted by potentiometer Rd. Thus a scanning voltage is applied to the sensor. The scanning time is dependent on resistor and the integrating capacitor CZ. Together, V2, R8, and Fb provide comparator & with a reference voltage. SwitchS1providesfor manual operation.When activated, a scanning voltage is applied to the sensor. When the switch is open, the operation is fully automatic and cyclic. Typically, the followingpreset values were w e d scanning time 2 s (including the time at the applied voltage limit), scan limit -4.WV,acanrate3.WV/s, initialvoltage-O.&V, and608 between pulses. The outputs of the individual amplifiers are depicted in Figure 2. It is important to note that the voltages cited above are the outputs of the amplifiers VB the common power supply ground and not absolute potentials in the electrochemicalsense. Someexperimentswere conducted with sensors containinga third electrode, a chloridized silver wire, to function as a reference alongwith appropriately modifiedelectronics. The signalto noise ratios were consistently poorer than those from the two electrode systems and this avenue was not further pursued. Sensor Fabrication. A number of different sensor configurations were explored;two are described below. The first design contains a cylindrical ceramic element (6.3-mm diameter) on which two rhodium wires (200-pm diameter, 200 mm long) are wound in parallel on precut threads without contacting each other
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(center to center spacing 320 pm). The base material for this device was obtained from EG&G Inc., Chandler Engineering Division (Tulsa, OK). A variation of the above design utilized 7-mm-diameterthreaded (64or 46 threadslin., doublelead, center to center spacing 400 or 550 pm, respectively) Nylon rods on which electrode wires (typically Pt, 25-200-pm diameter, 250 mm long) were wound in parallel without contacting each other. The film was formed on these sensors by dip coating the polymer. After the solvent was evaporated at room temperature, the film was thermally cured at 80-120 OC for 4 h. Without the thermal curing step, the sensor behavior changed with time. Sensors with increasing f i i thickness were fabricated by repeating the steps of dip coating, room temperature evaporation, and heat curing. In the second design, referred to as the coil sensor, a O.&mmi.d. and 1.0-mm-0.d. Nafion tube segment (20mm in length) was swelled in methanol and then slipped over a 0.9-mm-diameter stainless steel rod segment (7 cm in length). After thermal treatment at 120OC, 20 cm of 200-pm-diameter Pt wirewas wound on the membrane with a turn spacing of -300 pm. The excess lengths of the platinum wire and the stainless steel rod were respectively insulated with an encapsulating Teflon tube and Teflon tape. Teat Arrangements. To determine sensor performance, the sensor was affixed to a cap which in turn was sealed to the vial containing the liquid sample. The entire assemblywas enclosed in a glovebox (Dri-Lab HE 113, Vacuum Atmospheres, Los Angela, CA). The lead wires of the sensor gassed through sealed conduits to the outside of the drybox and connected to the analyzer electronics. The moisture level in the drybox was maintained well below 100 ppm water by means of nitrogen gas circulated through a drying train by a blower integral to the drybox. The dry train was periodically regenerated by the passage of a gas mixture containing 10%hydrogen and 90% nitrogen while the train was heated. Samples. Solvents tested as sample matrices were generally purchased in the driest grade available (typically in septumsealed bottles packed under nitrogen and opened only inside the drybox) and in other cases were dried in-house by distilling over molecular sieves, metallic sodium or magnesium,and anhydrous KzC03, etc., following procedures outlined in reference compilations.' Relatively high concentrations of water standards were prepared gravimetrically by adding known amounts of water to the dry solvents. In solvents in which water solubility is limited, saturated solutions of water in the solvent were prepared; saturated water concentrations in such solvents have been reported in the literature.8 Microaliquots of these high-concentration standards were added with a microliter syringe to the known amount of the sample contained within the test vial. All manipulations were conducted within the drybox. Incremental additions were made, and the data were interpreted following (7) Perrin, D. D.; Armarbgo, W. L.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1990. (8) Soremen, J. M.; Arit, W. Liquid-Liquid Equilibrium Data Collection. Binary S y s t e m ; Chemistry Data Serires; Deutache Gesellachaft fur Chemisches Apparatewesen: Dechema, Frankfurt, Germany, 1979; VOl. v, Part I.
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standard addition procedures. It is important to note that for many of the test solvents, significant amounts of water remain in the solvent despite best efforts to dry it. Standard addition is therefore the only means to judge the true response of the sensor to water in the particular sample matrix.
RESULTS AND DISCUSSION Estimation of Sensor Film Thickness. In the cylindrical sensors, the PFSI film is formed not only on the span of the wire electrodes (0.5 and 1 cm, respectively for the ceramic and Nylon substrate sensors), the active region of the sensor, but also above and below this region. Consequently, the mean film thickness in the active region is difficult to estimate. Based on microscopic examination, we estimate that from 70% to 80% of the applied PFSI occupies the active region. On the basis of gravimetric measurements and a polymer densityg of 2 g/cm3 and under the assumption of a uniform film, a sensor with three dip coatings has a -4-pm film when coated thrice with the 2.5% solution of the Dow PFSI and a -8-pm film when coated thrice with a 5% Ndion solution. If sensors of similar film thickness but composed of the two different PFSI materials are compared, the Dow PFSI sensors bearing the lower EW polmer show higher response. However, the latter polymer is not commercially available and Ndion can be substituted for the Dow PFSI in all the recipes given here with only a marginal loss of sensitivity a t identical film thickness. Film thicknesses of other composite films or polymer film overcoats were similarly estimated from gravimetric measurements and polymer density. Typically, a single coat of CTA, PVA-H3P04, and the PVA-H3P04-PFSI composite (vide infra), respectively, produced approximately 2-, 3-, and 1.5-pm-thick films. Protective and Composite Membranes. As previously mentioned, bare PFSI sensors are generally impractical for long-term use. Even when swelling-induced loss of integrity is not a problem, intrusion of undesirable species can foul the measurement system. For example, in the early stages of these studies it was discovered that drying of acetone by molecular sieves leached some material from the desiccant that results in a slow discoloration of the PFSI film. This darkening is accompanied by a decrease in sensor sensitivity. Furthermore, a bare PFSI sensor may have a large enough response to many of the more polar test matrices to make any meaningful measurement of water impractical a t low levels. If the sensing film can be protected by a barrier that has a greater selectivity for water transport, the detection limit for water can be effectively improved. The absolute current levels in the PFSI sensors operated in the pulsed voltammetric mode are relatively large; consequently, the absolute sensitivity can be substantially sacrificed if the discrimination against the matrix can be increased. However, an acceptable compromise between overall transport efficiency and transport selectivity is not obtained with any arbitrarily chosen barrier layer. When an 8-pm PFSI sensor was coated by a single dip in 1% AQ 55D, the decrease in response was acceptable (-40 % of original)but it provided little protection against transport of the matrix solvent. At the other extreme, if coated with 28% AQ 55D solutions, the matrix transport was inhibited but the sensor essentially lost its response to the analyte as well. Barrier layers of silicone rubber and Nylon were largely unusable for similar reasons. PVA or PVA-PAA layers cured by initiating cross-linking with UV irradiation of the persulfate-containing solutions produced the only overcoats among the above to resist swelling by high polarity solvents like methanol, DMF, or DMSO. (9) E. I. dePont de Nemours and Co. Product Information Literature. Ndion Perfluorosulfonic Acid Product. Wilmington, DE,1976.
Unfortunately, it also reduced the response of the underlying PFSI film to water by a factor of 1OOO. Thinner films of such material may be more useful. Nonetheless, an appropriate overcoat on the PFSI or a composite sensing film does produce attractive sensors; these are described below in more detail. Cellulose Acetate Protective Film. It is well recognized that CTA films can be subjected to controlled hydrolysis to govern the effective size cutoff of molecules that permeates the film."JJ1 Wang and Tuzhi12 have shown that determinations of small analyte molecules in complex matrice with Ndion-coated electrodes are simplified by an overcoat of a hydrolyzed CTA film. Our experience indicates that unhydrolyzed CTA films are the best for discriminating against the relatively small solvent molecules of major concern to us. Barrier layers of CTA over PFSI have been found to be particularly well suited for use in polar solvents such as acetone, acetonitrile, and diethyl ether and nonpolar solvents such as toluene. We have also found that CTA films formed from ethylene dichloride or chloroform solutions swell easily and lose integrity in a variety of organic solvents. When formed from the solutions prescribed in the experimental section and cured for 2-4 h at 80-90 "C, the CTA films formed are vitreous and resist solvent-induced swelling. If cured at 120 OC for 2-4 h, the film becomes dark brown and is accompanied by a further increase in swelling resistance and discrimination against solvent transport, albeit at the expense of sensor sensitivity. For either cure protocol, two dip-cure cycles are recommended for use in polar solvents such as acetonitrile or acetone that exhibit large background current levels with a bare PFSI sensor. The sensors cured a t higher temperature are profitably used in acetone and acetonitrile while those cured at lower temperature are well suited for lower polarity solvents, e.g., diethyl ether, toluene, etc. CTAovercoated sensors cannot be used in halogenated organic solvents, DMF and DMSO; these solvents destroy the film. PVA-H3po4 Film. An electricallyconductivewater-soluble complex of PVA and H3P04 was reported by Polak et al.13 Oxygenated anions with multiple -OH functionalities such as borate or silicate rapidly cross-link PVA; pedagogic demonstrations based on these reactions are widely used.14-17 We have found that if PVA is dissolved in boiling dilute H3PO4and such a solution is used to form a barrier layer by dip coating, followed by curing at 80-90 "C, such a film is not only insoluble in DMF or DMSO, it is unaffected by boiling methanol or water. Equilibration of such a film with the water activity of the surrounding medium is slow, however; films thicker than 5 pm require 210 min to reflect the sample water content. Films of -3-pm thickness produce response times acceptable for most applications-approach to equilibrium with different barrier layendsensing filmsis illustrated in Figure 3. Response time also appears to depend on the tested solvent and is generally somewhat larger in less polar solvents. The PVA-H$04 overcoating is the only protective membrane that fully resists dissolution and excessive swelling by highly polar solvents such as methanol, DMF, DMSO, etc. However, even for this film, the response signal ratio to water relative to DMF, DMSO or methanol is
