Chapter 4
A Continuous Flow Electrochemical Cell for Analysis of Chemical Species and Ions at High Pressure: Laboratory, Shipboard, and Hydrothermal Vent Results George W. Luther, III, Andrew B. Bono, Martial Taillefert, and S. Craig Cary College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes,DE19958
An inexpensive and rugged voltammetric flow cell has been developed to test pressure dependence and flow rate on current vs. concentration standard curves and for the simultaneous measurement of electroactive species in field samples. Standard curves show that current is independent of pressure but varies with the square root of the flow rate. The electrochemical system consists of a pump to deliver water into a voltammetric flow cell with a standard three-electrode arrangement that is controlled by a voltammetric analyzer. The flow cell can be used onboard ship or in situ at any depth including from the deep-sea submersible, Alvin. Field samples were analyzed for a variety of redox species under controlled flow conditions with a solid state working electrode at a depth of 2500 m. In this study, a 100 μm Au/Hg amalgam solid-state working electrode is used to detect dissolvedO ,S(-II),Fe(II) and Mn(II). We describe the flow cell and its capabilities. 2
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© 2002 American Chemical Society
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Introduction Voltammetric microelectrode techniques are becoming increasingly popular as a means to quantify electroactive species in natural environments. Since amperometric microelectrodes were first employed in a natural setting to study the biogeochemistry of oxygen in sediments (1) significant improvements have been made in electrode and cell design, as well as data acquisition and processing. Solid state microelectrodes have several important advantages over larger, more traditional electrodes. Smaller electrode surfaces can operate at low currents, which permits use in solutions with weak ionic strength. Furthermore, because of the decreased IR drop, the working and reference electrodes do not necessarily have to be kept immediately adjacent to one another (2). The smaller surface area results in a decrease in the charging current relative to the faradaic current and thus allows faster scan rates and increased instrumental sensitivities to reversible and semi-reversible redox reactions (2). Recently, the development of voltammetric techniques, which utilize a solid-state gold amalgam (Au/Hg) working electrode, has permitted the simultaneous quantification of several dissolved species both in laboratory and natural settings. Dissolved chemical species (0 , H 0 , S 0 ", S ', HS", I", Fe(II), Mn(II), organically complexed Fe(III), and FeS clusters) have been simultaneously measured in sediment pore waters to spatial resolutions unattainable by traditional subsampling techniques (3-8). Most have been performed in cores brought aboard ship but recent work shows that direct in situ sediment and water column work is possible (5, 9-12). The incorporation of microelectrodes into continuous flow detectors is well established, and electrochemical flow-cells designed for bench-top analyses that work under atmospheric pressure conditions have been commercially available for some time. Continuous flow systems improve over traditional electrochemical cells by permitting minute quantities of a large number of samples to be rapidly evaluated under identical conditions. Furthermore, they reduce errors due to contamination and handling, and provide opportunities for computerized autosampling, pre-analysis conditioning, and data collection (Π Ι 4). Additionally, in the case of electrochemical determinations in natural settings, flow cells offer advantages over traditional sampling and analysis techniques since samples are processed immediately and thereby minimize the effects of degassing, oxidation, and microbial metabolism associated with sample storage. The majority of electrochemicalflow-throughsystems have been developed for use in the pharmaceutical industry to quantify organic compounds (15-18). Several researchers have, however, applied voltammetric flow cell apparatus to natural systems (19-23). The predominant focus in geochemical work has been the determination of trace metals (Mn, Cu, Pb, Cd, Zn, Co, Ni, U). 9
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56 Consequently, systems have not been designed to evaluate redox-sensitive species such as 0 , Mn(II), Fe(II), S 0 ", Γ, and S(-II); although, De Vitre et al. (19) and Tercier et al. (23) examined the distribution of some redox-sensitive species (Mn(II), Fe(II), S(-II)) in Lake Bret (Switzerland) using a hanging mercury drop electrode in a flow cell design. Electrochemical flow-cell systems for geochemical analyses tend to be specialized. Most researchers have selected hanging drop mercury electrodes (HDME) to avoid potential memory problems associated with mercury film electrodes (19, 20, 22). The bulky and elaborate HDME working electrodes, however, cannot work at high pressure and preclude in situ deployment of the apparatus. Large solid state electrodes were first used in a flow cell design by Lieberman and Zirino (20), who used mercury film on a graphite tube, and more recently by Tercier et al. (23) with a mercury film on a 3 mm glassy carbon electrode. Since then, an array of mercury coated Ir electrodes (Ir/Hg) has been used for trace metal analyses (10). In addition to permitting in situ deployment, the solid state electrode simplifies the flow cell design, endures higher flow rates, and permits the miniaturization of the working electrode with the benefits that accompany the use of microelectrodes. Most flow cell designs have also involved significant pre-analysis handling of the sample, including filtration, purging or degassing, and various pre-concentration and chelation steps to determine low levels of trace metals (19-22). Completely closed flow cells free from sample pre-treatment, like the design employed by Tercier et al. (23), prevent alteration of the sample prior to analysis and are necessary for operating in situ under high pressure conditions. In this paper, we describe a portable, closed-system voltammetric cell made from the durable polymer, polyethyletherketone (PEEK™). Using the Au/Hg solid-state working electrode permits simultaneous in situ analysis of a wide range of aqueous dissolved species in both field and laboratory settings at pressures at least up to 250 atmospheres. The overall goal of this work was to determine the effect of flow and pressure to calibrate the working electrode for hydrothermal vent conditions. First, laboratory testing of the design established the effects of flow and pressure on the response of solid state Au/Hg microelectrodes. Shipboard field tests of the flow cell system were performed on waters from the Chesapeake Bay. In situ deploymentfromthe submersible DSV Alvin enabled comparison to established sampling and analysis techniques and evaluation of the utility of the apparatus as an in situ tool for geochemical study. This flow cell shows significant promise in a wide range of possible future applications including determination of respiration rates of single larvae (Glazer and Luther, unpublished) because the total cell volume is about 0.7 mL. 2
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Experimental Methods
Chemicals and solutions All chemicals used in laboratory experiments were analytical grade, and all water was Milli-Q quality (Millipore). A l l laboratory measurements were carried out in a 0.55 M NaCl solution (Fisher). Mn(II), and S(-II) standards were prepared from MnCl · 4 H 0 (Fisher), and Na S-9H 0 (Fisher). The mercury plating solution was prepared as 0.1 Ν Hg(N0 ) in 0.05N HN0 . The glassware and flow cells were cleaned and stored in a 10% (v/v) HC1 solution. 2
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Flow cell System The flow-cell is constructed entirely of PEEK™ (polyethyletherketone) and employs standard HPLC fittings (0.125 inch), which permit the rapid removal or exchange of electrodes and sample tubing, and the use of the cell at elevated temperatures and pressures. The flow cell is approximately 10 cm in length and 1 inch in diameter; a hole of 0.0625 inch (1.59 mm) diameter is drilled along its length with ports at each end for sample input / output (Figure 1). Three holes are drilled perpendicular to the lengthwise hole for a three solid-state electrode system; the total cost per cell is less than $500 including parts and labor. The flow cell is a completely closed system and permits analysis of aqueous samples without manipulation or exposure to the atmosphere. The reference is a 500 μπι Ag/AgCl or Pt electrode, the counter is a 500 μιη Pt electrode and the working electrode is a 100 μιη Au/Hg electrode. Gold amalgam PEEK™ electrodes were made as described by Luther et al (9) by fixing 100 μηι-diameter Au wire soldered to the conductor wire of BNC cable within a body of 0.125"-diameter PEEK™ tubing, which is commercially available as standard HPLC high-pressure tubing. The metal is fixed within the tubing with the West System 105 epoxy resin and 206 hardener. A portion of the black outer coat and braid of the BNC wire are removed to expose the teflon shield and Cu conductor wire so that the Au wire soldered onto the Cu conductor can be inserted into the PEEK™. The epoxy is injected into the PEEK™ tubing which contains the gold wire that was previously soldered to the conductor wire of the BNC cable. Then the teflon is inserted into the PEEK™ tubing until the black coating of the BNC wire fits against the PEEK™ tubing, and the assembly is held so that epoxy, which has a moderate setting time (-1 hr), does not drain out the lower open side. On setting, the epoxy seals the tip and the top end can be refilled with epoxy if necessary. Then the top end is coated with Scotchkote
58 (3M) electrical coating and Scotchfil (3M) electrical insulation putty. PEEK'" and high-purity epoxy fill permits the determination of metal concentrations without risk of contamination, and at temperatures as high as 150 °C. Pt counter and solid Ag/AgCl reference electrodes were made similarly but 500 μιη diameter wire was used for each. These PEEK™ electrodes are mated with standard HPLC fittings from Upchurch, Inc for insertion into the flow cell. Once constructed the working electrode (Au) surface was sanded, polished and plated with Hg by reducing Hg(II) from a 0.1 Ν Hg / 0.05 Ν H N 0 solution, for 4 minutes at a potential of -0.1 V, while purging with N . The mercury/gold amalgam interface was conditioned using a 90-second -9 V polarization procedure in a 1 Ν NaOH solution (3). The electrode was then run in linear sweep mode from -0.05 to -1.8 V versus a Saturated Calomel Electrode (SCE) or Ag/AgCl electrode several times in oxygenated seawater to obtain a reproducible 0 signal. 3
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Figure 1. A schematic of theflowcell and electrode designs. The input and output tubing also utilize the high-pressure HPLC threadedfittings andferrules. Theflowcell is 100 mm in length and 25.4 mm in diameter. For DSV Alvin work, up to three working electrodes could be placed into the flow cell. The reference electrode was Ag/AgCl and the counter electrode was Pt wire, both of which were mounted on the basket of DSV Alvin so that they would not enter sulfidic waters (5, 11). For hydrothermal vent work, the Ag/AgCl reference was silver wire, which was oxidized in seawater at +9 V for 10 sec to form a AgCl coating. This electrode was used as a solid state electrode in the seawater medium (1=0.7) so that no pressure effects on filling solutions would hinder electrode performance. Comparison of peak potentials for the analytes measured in situ and aboard ship was the same and similar to those for a saturated calomel electrode (SCE). The tubing leading into the flow cell's inlet was placed in a sensor package (11), which was held over the vent orifice and areas along the length of vent chimneys. These latter areas are termed diffuse
59 flow because water temperatures can range from 8 to 125 °C and do not emanate from the vent orifice; the cell sits in a basket at 2 °C. A submersible electrochemical analyzer (DLK-sub I) from Analytical Instrument Systems, Inc. was used for data collection (11). All laboratory and shipboard analyses were carried out using an Analytical Instrument Systems (AIS) DLK-100 potentiostat controlled by a microcomputer using software provided by the manufacturer. Chesapeake Bay field sampling was performed using a Rabbit-Plus peristaltic pump (Rainin Instrument Co., Inc.) with a flow rate of 12 mL min* through Teflon® tubing. For DSV Alvin work, the submersible analyzer was used for data collection. A General Oceanics T5 submersible pump was used to fill the cell; the cell could be used under flow conditions but was typically used under diffusion control conditions when the pump was turned off since hydrothermal vent waters may undergo rapid change (11). 1
Laboratory Experiments The effect of solution flow past the electrode surface was determined by connecting the flow cell in line with a Scientific Systems Inc. Model 200 HPLC pump, attached to a Model 210 Guardian and LP-21 Lo Pulse dampener. Experiments with this system permitted the measurement of current versus flow only. Addition of a 15 cm C-18 (3 μιη) HPLC column to the exit port of the flow cell permitted measurement of electrode response with increasing internal pressure within the cell by varying the flow rate, up to a maximum internal pressure of 200 atmospheres. In these experiments, a 150 μΜ Mn(II) solution was pumped from the solvent reservoir through the pump and flow cell (only flow rate varies), or through the pump, flow cell, and column (both flow rate and pressure vary).
Field Experiments Field work was performed in the Chesapeake Bay during the summer anoxic season on 22 August 1996. Values for the concentration of oxygen were determined by three methods: (i) an in situ 0 sensor contained within the conductivity-temperature-depth (CTD) package, (ii) analysis of waters sampled by bottle cast using Winkler titrations, and (iii) direct on-deck sampling of waters using the voltammetric flow cell. Agreement was favorable as previously published (9). Waters pumped into the flow cell from the anoxic layers of the Chesapeake were compared against samples from the bottle casts. The bottles were sampled using the syringe method described in Luther et al (24) and were 2
60 analyzed via hanging drop mercury electrode (HDME) stripping voltammetry for S(-II), Fe(II), and Mn(II). For hydrothermal vent work, the flow cell was mounted onto the basket in front of the deep-sea submersible Alvin, during a cruise to Guaymas Basin, Gulf of California in January 2000. Separate discrete samples were taken with a gas tight syringe sampler from the same waters for comparison with the flow cell measurements. The discrete samples were measured aboard ship by voltammetry (11). We typically made three to five replicate measurements per depth with the flow cell system. Electrodes were calibrated at different temperature (3) as well as for flow in this paper.
Voltammetry The three-electrode configuration was used to determine the concentration of the species present in natural waters. Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and square wave voltammetry (SWV) were typically used for analyses. CV and SWV were used to test for reversibility. SWV is the method of choice for low level detection for a reversible signal; LSV and C V were used for fast scans and for higher concentrations of analytes. The following conditions were generally applied during the LSV scans: scan rate = 200 to 1000 mV/s, scan range = -0.1 to -1.7 V, equilibration time = 5 s. Square wave voltammograms were conducted under the same conditions with a pulse height of 24 mV, 1 mV scan increment and 200 mV/s scan rate. To prevent memory effects, caused by the accumulation of sulfide and metal species on the mercury surface, conditioning steps were applied to the working electrode as per Brendel and Luther (J). To re-oxidize metals (Mn, Fe) that are reduced at the amalgam, a potential of -0.1 V was applied over 30 seconds before each scan. When sulfide was present, conditioning at -0.8 V for 10 s was employed since the metals and sulfide are not electroactive at that potential (5,5).
Results & Discussion
Effect of flow and pressure The effect of flow on an electroactive species at the electrode surface is described by the Levich equation (eq. 1): 2/3
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61 where I represents current under mass-transport (not diffusion or zero flow) controlled conditions, k is a constant coefficient, η is the number of electrons transferred, F is the Faraday constant, C is the concentration of the electroactive species, D is the diffusion constant, r is the radius of a disk-shaped but stationary electrode, U is the rate of flow through the cell, and ν is the kinematic viscosity. In Figures 2A,B the effect of flow on the current response (SWV) of the electrode at constant atmospheric pressure to a 150 μΜ Mn(II) solution corresponds closely to eq. 1, in close agreement with previous work (13, 14,20, 25, 26). The tendency of current to plateau at increased flow rates suggests that the conditions at the 100 μιη electrode surface are no longer controlled by the rate of mass transport of the analyte to the electrode surface. The relationship in Figure 2B confirms a linear relationship (r = 0.972) of I with U before current levels off at flow rates above 2 ml/min, which corresponds to 1.68 cm/s based on the diameter of the flow cell's hole. Similar results were found using CV scans (data not shown) and for 0 and H S/HS" solutions. We observed the change in current with flow-induced pressure using a 150 μΜ Mn(II) solution, using the same apparatus as the flow experiment but with the addition of an HPLC column after the cell to provide internal back-pressure. The HPLC fittings were tested to pressures as high as 200 atmospheres, corresponding to mid-ocean ridge depth (2000 m). The behavior of the electrodes under induced pressure indicates that the increase in current is comparable to that of flow without induced pressure (Figure 2C). When the current response of the electrode is plotted against both flow and flow-induced pressure, the increase in current fits the Levich relationship for flow (I
