In-situ measurement of dissolved H2 in aqueous fluid at elevated temperatures and pressures

Geochimicaet CosmochimicaActa,Vol. 59, No. 22, pp. 4769-4773, 1995 Copyright© 1995 ElsevierScienceLtd Printed in the USA.All rightsreserved 0016-7037/95 $9.50 + .00

Pergamon

0016-7037(95)00319-3

SCIENTIFIC COMMENT

In-situ measurement of dissolved H2 in aqueous fluid at elevated temperatures and pressures K. DING and W. E. SEYFRIEDJR. Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA

(ReceivedMay 9, 1995; acceptedin revisedform August 10, 1995) Abstraet--In-situ measurement of H2 in aqueous fluid under hydrothermal conditions has long been a major concern in experimental geochemistry owing to the important role of H2 in a wide range of homogenous and heterogeneous equilibria. Although numerous efforts have been made, an effective and reliable 1-12sensor for geochemical applications is lacking, especially one suitable for measurement at supercritical conditions of water. In this paper, we discuss a sensor which was developed for in-situ measurement of dissolved 1-12in aqueous fluid at temperatures and pressures greater than previously possible. In general, the I-I2 sensor consists of two electrodes and a Hg/HgO internal reference element: a platinum electrode and a yttrium-stabilized zirconia (YSZ) membrane electrode. The electrochemical cell can be depicted as follows: Ptl H2, H +, H20 [ZrO2 (Y203) In g O Ing. A titanium flow reactor was used to test and calibrate sensor response and sensitivity. Dissolved H2 concentrations and the rate of fluid flowing through the reactor were controlled by a modified HPLC pump and computer system, respectively. Dissolved H2 concentrations of fluid leaving the reactor were determined by GC analysis. AEv. of the fluid in the reactor was, at the same time, monitored and recorded by an electrometer with high input impedance. The experiments were performed at 400 bars, and temperatures up to 400°C. Results revealed good Nernstaln response, even for unusually low dissolved H2 concentrations. Thus, at 400°C and 400 bars, the following calibration line was obtained: AE¥, (V) = 0.972 + 0.054 log mH2 ( M / k g )

(r = 0.9974, N = 9),

where mH2 ranged from 0.096 to 5.75 mM/kg. The experiments also demonstrated excellent stability of the sensor during the month-long operation. The high temperature H2 sensor may have important applications for field monitoring dissolved H2 in mid-ocean ridge vent fluids. It is also ideally suited for direct determination of redox state in hydrothermal experiments at a wide range of conditions. 1. INTRODUCTION

through a metallic Pt or Ag-Pd membrane. In their design, H2 fugacity is indirectly obtained by measuring HCI concentrations of the quench solution in the sensing capsule where the fugacity of C12 is fixed by the Ag-bearing assemblage. In contrast, Gunter et al. (1979) coupled a small-volume pressure transducer to a metallic membrane permeable to H2 for measurement of H2 in hydrothermal experiments. Although this technique is a more direct method for measuring H2, it has seen limited use owing to design problems, which affect its sensitivity and resolution (Gunter et al., 1987). Another insitu method for H2 measurement in high temperature aqueous systems, however, was developed by Macdonald et al. ( 1981 ), and makes use of the H2-dependent resistivity of palladium, which serves as the sensor. The response of this sensor to H2 was demonstrated for dissolved H2 at concentrations up to 0.72 mM in 0.1 M boric acid, and at temperatures as high as 275°C (Macdonald et al., 1981 ). Application of the palladium electrode to higher temperature aqueous system, however, has not been demonstrated. Here we report preliminary data for an in-situ H2 sensor, which uses platinum as a H2 sensing electrode in combination with a yttrium-stabilized zirconia (YSZ) membrane and Hg/ HgO internal reference element. The YSZ membrane was

Dissolved H2 is a key parameter reflecting thermodynamic equilibria and kinetic processes during water-rock interaction in hydrothermal systems. Moreover, dissolved H2 is often the only component that can be directly determined and reliably used to indicate redox state in nature and experimental system. Over the years, numerous efforts have been made to measure dissolved H2 concentrations in hydrothermal fluids. Generally, two approaches are used most often. One involves sampiing fluid at elevated temperature and pressure conditions with subsequent analysis of H2 by GC or an equivalent technique (Kishima and Sakai, 1984; Von Damm et al., 1985; Seyfried et al., 1987). Although this method has proven to be reliable and a relatively effective means of measuring H2 in fluids from laboratory experiments, serious difficulties are often encountered with field applications owing to inherent limitations of fluid samplers and potential for contamination (Welhan and Craig, 1983; Merlivat et al., 1987). Thus, the second approach, which involves in-situ measurement of Ha, is a particularly attractive alternative. Chou and Eugster (1976), for example, successfully developed the Ag-AgCI-HaO H2 sensor, which is based on Ha permeability 4769

4770

K. Ding and W. E. Seyfried Jr.

step motor power supply To IEEE board

To I/0 board

:

!YSZ I~ electrode I

~

Electrometer N~

I

~

i

o Step motor

77sampling valve

L~ reactor

Testingsolution HPLCpump

hydrogen injector

controller

FIG. 1. Schematic illustration of flow-through reactor, which was developed in this study to permit calibration of the

H2 sensor. The temperature in the reactor is controlled by a time proportioning solid state controller, while the pressure of the fluid phase is controlled by an HPLC pump where all wetted parts are constructed of titanium. The dark line indicates fluid flow line, while light line represents electronic connection. originally described b y Niedrach (1980a,b) as an oxygen i o n - c o n d u c t i n g ceramic electrode, which could b e used as a p H sensor at elevated temperatures. Indeed, the Y S Z electrode has been used extensively as a means to measure pH of aqueous solutions at subcritical conditions of water (Tsurata and Macdonald, 1982; Bourcier et al., 1983; Hettiarachchi and Macdonald, 1984; Myers et al., 1984; Hettiarachchi et al., 1985; Light and Fletcher, 1985; Macdonald et al., 1988). Recently, the viability of Y S Z electrode as a pH sensor was demonstrated at 255 bar, and at temperatures up to 390°(2 (Hettiarachchi et al., 1992). In their study, a unique method was developed by combining the Y S Z electrode with the hydrogen electrode ( S H E ) to evaluate the p H o f high-temperature aqueous fluids from the Y S Z sensor response. Their results indicate that the measured potential of the cell ( S H E Y S Z ) is a function of dissolved HE at given temperature, and insensitive to the pH of aqueous fluid. These studies suggest the possibility of using the Y S Z electrode as a reference electrode for measuring dissolved H2 at temperatures and pressures applicable to mid-ocean ridge vent fluids. To test this, we developed a computer-controlled fluid flow system and titanium reactor to determine the response of the electrode to H2 in aqueous fluid at relatively high temperatures and pressures. Experimental results exceeded expectations. Indeed, data indicate that the sensor responds rapidly and precisely to slight changes in dissolved H2. This has important implications for use of the sensor as a means to determine and monitor dissolved H2 in aqueous fluids at temperatures and pressures higher than previously thought possible. 2. EXPERIMENTAL METHOD 2.1. Sensor Construction

As a wide band-gap material, ZrO2 (Y203) is characterized by negligible electronic conductivity at relatively low temperatures (Bour-

cier et al., 1987; Macdonald et al., 1992), which means that the YSZ electrode can serve as a reference electrode for measuring H2 provided it is coupled with a H2 sensing electrode. In this study, therefore, we use platinum for the H2 sensing electrode which together with a Hg/HgO internal reference element gives an overall electrochemical cell as follows PtlH2, H + , H2OIZrO2 (Y203)IHgOIHg.

(1)

The YSZ membrane electrode used here was modified after Macdonald et al. (1992). In particular, the electrode is a impermeable YSZ tube (9% Y203, from Coors Ceramics Co.), and is 0.625 cm in diameter, 0.076 cm in wall thickness, and 7.5 cm in length. Native Hg and synthetic HgO (99.998% pure from AESAR) were sealed in the bottom part of the tube with high temperature ceramic epoxy (SiO2 base from Cotronics Corp.). A P t wire connected to the Hg/ HgO paste served as a conductor and facilitated electrode response. The electrode was then installed into a high pressure titanium reaction vessel using a Teflon sealant with a cone seal method. To prevent overheating of the Teflon sealant, water was circulated through cooling jackets attached to the upper part of the electrode body. An 8 by 3 by 0.5 mm platinum foil was used for the hydrogen electrode. The foil was welded to a Pt wire, which was coated with heat shrinkable polytetrafluoroethylen (PTFE), and placed inside a zirconia tube of 0.8 mm wall thickness. The Pt electrode was sealed into the Ti reaction vessel with the same technique mentioned above for the YSZ electrode. 2.2. Titanium Flow Reactor

The titanium flow reactor was designed to permit the electrodes to react with H2-bearing fluid at pressures and temperatures as high as 500 bars and 425°C, respectively (Fig. 1 ). Fluid flow rates could be adjusted from 0.1 to 8.0 mL/min by a titanium HPLC pump (from Shimadzu), while pressure was controlled by a computer-controlled titanium metering valve. The total cell volume was 50 mL. Fluid samples were collected at experimental conditions by actuating a titanium valve, and allowing fluid to enter a sample loop with fixed volume. At the same time, recharge fluid was delivered to the reaction vessel by the HPLC pump. A set of individually controlled band heaters were used to maintain temperature at a constant value

Measurement of

H2

(_+0.5°C). The band heaters enabled the system to achieve operational temperature within 30 rain. A band heater was also used to preheat fluid entering the reactor. That each of the band heaters could be independently monitored and controlled assured maximum thermal stability. Since conventional HPLC pumps are not suitable for use with gas-charged fluid, especially H2, we developed a high pressure and chemically inert separator, that allowed us to inject fluid containing different amounts of dissolved H2 into the reaction vessel at experimental condition.

2.3. Experimental and Analytical Procedures Two different experimental strategies were used to determine the sensor's sensitivity to dissolved H2. First, deionized water (DW) and H2-bearing DW were alternately injected into the reactor at 400 bars, and a constant flow rate of 0.5 mL/min. For the second method, the sensor was calibrated at different dissolved H2 concentrations by simply diluting a H/-bearing source fluid, which filled the reactor at the start of an experiment. Following dilution of the H2-bearing fluid, approximately 2 h were allowed for homogenization of dissolved H2 in the reaction vessel prior to sampling. Experiments using each of the strategies described above were conducted over a period of time to test the efficiency of the experimental system, and durability of the sensor. These tests showed that the sensor responded effectively, and could be operated reliably, even at temperatures and pressures as high as 400°C, 400 bars, and for time periods as long as two months.

(a)

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0.849_+0.002 0.844_+0.002 0.836_+0.001 0.824_+0.002 0.811_+0.002 0.796_+0.002 0.789_+0.002 0.7545.-0.002 0.759-Z-0.002

5.75_+0.12 4.61_+0.19 2.93_+0.19 1.67_+0.02 0.858_+0.046 0.472_+0.022 0.435i-0.025 0.096_+0.006 0.128_-t-0.011

Error: IG

The potential of the H2 electrode vs. the YSZ electrode was measured using a Keithley Model 2001 electrometer with an input impedance of 1 × 10 ~ f~. The meter was linked to a data acquisition system through a scanner (Keithley Model 2001-SCAN), and thus, the results could be simultaneously collected and recorded in a Windows-PC environment. Dissolved H2 concentrations of fluid flowing through the titanium reactor were independently determined by gas chromatograph (Perkin-Elmer Autosystem). The error from GC analysis was _+3%. The errors associated with fluid sampling, however, were considerably greater than this, and resulted in overall error up to --_8%.These errors are most likely introduced by the relatively large dead space in sampiing valve, which may collect H2 gas during sampling procedure. To improve the quality of these data, however, we increased fluid sample size, and number of samples at each concentration condition. Contamination from the reaction vessel and ceramic materials were checked by measuring full fluid chemistry by ICP-MS. Results indicate relatively low levels of material transfer from the electrodes to the fluid. At 400°C, for example, less than 8 ppm SiO2, 52 ppm Na were observed in the fluid phase. 3. RESULTS AND DISCUSSION

In the flow-through mode of operation, the sensor revealed outstanding response to dissolved H2 concentrations at temperatures o f 375 and 350°(] (Fig. 2a,b). The results from experiments at both temperatures showed sharp and virtually instantaneous response to changes in dissolved H2. At 375°C and 400 bars, for example, a fluid containing 0.48 r n M / k g H2, yielded a potential of 0.795 volts, while switching to H2free D W caused the measured potential to decrease rapidly to background values (Fig. 2a).

3.2. Sensor Calibration

400 300

0

n

Table 1. Measured potential and total dissolved H2 concentrations at 400°C and 400 bars Sample Potential (V) H2 (mM/k~)

3.1. Sensor Response During Fluid Flow

(b) 900

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5OO

400 e2 o 300 13_ 200 100 0 -100

i

in hydrothermal fluids

200 lOO o

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50 100 150 200 250 300 350 400 450 500 550 Time (min.)

FIG. 2. Measured potentials of hydrogen sensor recorded during flow-through experiments at 375°C (a) and 350°C (b), respectively. The pressure and flow rates are 400 bars and 0.5 mL/min, respectively. The dissolved 1-12concentrations obtained from GC analyses are also indicated, which demonstrate an excellent consistency with measured potentials.

In Table 1 we list measured potentials and corresponding dissolved H2 concentrations from experiments conducted at 400°C and 400 bars. The values and errors reported for measured potentials were collected and determined from 20 readings over 10 min, while H2 concentrations represent the average o f three separate analyses o f fluid samples removed from the reactor. It can be seen here that errors associated with electrode performance are very small, and thus, indicate a stable response o f the sensor to dissolved He concentrations. W h e n the measured potential is plotted against logarithm of the dissolved hydrogen concentrations, a linear correlation ( r = 0.9974, N = 9) is obtained over the entire range of dissolved H2 concentrations tested (0.096 - 5.75 r a M ) (Fig. 3). Thus,

4772

K. Ding and W. E. Seyfried Jr. 0.900

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2H ÷ + 2 e - ¢# H2

!

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0.880

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0.840 0.820

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2.303RT ~ log (f.2)

0.800

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0.780

(4)

=

KH''YH2"

mH2,

(5)

where KH (mol kg -~ atm -I ) and ~/H~are the thermodynamic Henry's law constant, and activity coefficient for dissolved hydrogen, respectively. For the YSZ electrode, the potential can be written as (Macdonald et al., 1988)

0.760 0.740 0.720 0.700

2"303RTpH, F

where f.2 is the fugacity of hydrogen, while R, F, and T stand for the gas constant, Faraday constant, and temperature ( K ) , respectively. In Eqn. 4, f.2 can be represented as

v

E o "5

(3)

with the potential (in volts) expressed in terms of the Nernst equation, as follows:

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=

2.303RT log (all:o) 2F

0

EHg/HgO

where a,:o is the activity of water and Eag/HgO 0 is the standard potential for the Hg/HgO reference element, which is defined as

FIG. 3. Experimentally calibrated relationship between dissolved H2 concentrations and measured potentials at 400°C and 400 bars. Dashed lines indicate dissolved H2 concentrations from hematitemagnetite, and pyrrhotite-pyrite-magnetite assemblages, which represent the most common dissolved H2 range in hydrothermal systems, especially in mid-ocean ridge. The values of these dashed lines axe determined from Kishima and Sakai (1984), and Kishima (1989).

HgO+2H ÷ +2e

¢*Hg+H20.

(7)

Thus, by combining Eqns. 4, 5, and 6, we can depict the potential of this hydrogen sensor as follows: AEv~

=

EOg/HgOq

~-~

log \

+ AEyH (V) = 0.972 + 0.054 log mH2 ( M / k g ) .

2.303RT pH, (6) F

a,2o /

2.303RT ~ log (mH2).

(8)

(2)

The availability of a 1-I2sensor, which can be used to reliably determine dissolved H2 at low to moderate concentration in high temperature fluid, is of great significance for many geological applications. For example, the dissolved H2 concentrations in hot spring vent fluids at mid-ocean ridges range between 0.1 and 2 mM (Von Damm et al., 1985; Craig et al., 1980; Evans et al., 1988; Bowers et al., 1988; Kim et al., 1984; Campbell et al., 1988; Lilley et al., 1993). Dissolved H2 most likely results from the interaction of seawater with basaltic ocean crust at 375-400°C and 400 bars (Ding and Seyfried, 1992; Seyfried and Ding, 1995). Experimental data shown here indicate that the H2 sensor performs well at precisely the same set of conditions responsible for the chemical evolution of mid-ocean ridge hot springs. Indeed, the sensor may be well suited for deployment on the seafloor as a means of monitoring time dependent changes in the dissolved H2 of hot spring fluids. These data could constrain fluid-mineral equilibria and phase separation processes in subseafloor reaction zones. Moreover, the availability of the HE sensor is highly relevant to a wide range of laboratory/experimental studies involving redox equilibria and kinetics in hydrothermal fluids.

3.3. Theoretical Consideration The line plotted in Fig. 3 can be evaluated theoretically using the Nernst equation. For the H2 electrode, the potentialdetermining equilibrium is conventionally written as

At a given temperature, pressure, and ionic strength, the terms in parentheses are constant, and it can be seen that Eqn. 8 gives a straight line when it is plotted against log (mH2). This is consistent with the trend shown in Fig. 3. At 400°C, the slope of Eqn. 8 can be calculated to be 0.065, which is similar to our regression value (Eqn. 2). The m m n source of errors with our calibration line (Eqn. 2) is most likely related to the difficulty of obtaining sufficiently accurate dissolved 1-12measurements by GC analysis to test electrode response. Thus, the relationship between measured potential response and dissolved H2 concentration (Fig. 3) is in agreement with the theoretical consideration. This result also confirms an ideal response of YSZ electrode to pH at 400°C and 400 bars, which has been previously documented only at the temperature and pressure conditions lower than this (see above). This result establishes the thermodynamic viability for dissolved H2 measurements by this H2 sensor. Comparing our data with those from Hettiarachchi et al. (1992), which resulted from similar cell construction at relatively lower temperature and pressure conditions, provides further evidence of the suitability of the H2 sensor for measurement of dissolved 1-12over a broad range of conditions. For instance, at 375°C and 255 bars, Hettiarachchi et al. (1992) obtained potentials of 0.750-0.800 V in fH2 range from 0.017 to 0.5. 4. CONCLUDING REMARKS Taking advantage of recent developments in material science and electrochemistry, we successfully constructed a hy-

Measurement of He in hydrothermal fluids drogen sensor for hydrothermal applications. The hydrogen sensor consists of a platinum electrode and yttrium-stabilized zirconia ( Y S Z ) m e m b r a n e electrode. W e tested the response and performance of the sensor in a newly developed titanium flow reactor at 400 bars, and temperatures from 350 to 400°C. Experimental data indicate that the sensor can be used for insitu m e a s u r e m e n t and monitoring dissolved H2 concentrations at subcritical and supercritical conditions of water. During month-long tests, the sensor revealed excellent stability and performance. Indeed, measurements performed at 400°C and 400 bars indicate that the sensor is an ideal tool for determination of dissolved H2, even at H2 concentrations as low as 0.1 raM. The temperature and pressure conditions and He concentration range tested are o f significant geochemical interest, since numerous field and laboratory applications require monitoring of redox sensitive species at precisely this range of conditions. A more detailed study of this hydrogen sensor is in progress to extend the P - T conditions, and He concentration ranges beyond present limits, and will involve a wide range of fluid chemistries, in particular, variable dissolved C1- and pH conditions, to test sensor response and performance in geochemically important chemical and physical environments. Acknowledgments--We appreciate the communications with Dr. S. Hettiarachchi, Dr. D. D. Macdonald, and Dr. P. Jing in constructing the YSZ sensor, which greatly benefited this study. N. Dong is acknowledged for developing the computer program which enables automatic control on the experimental system. We also wish to thank H. L. Barnes and two anonymous reviewers for their comments and suggestions which greatly improved the content of the paper. This research was supported by NSF grant OCE-9300119. Editorial handling: G. Faure REFERENCES Bourcier W. L., McKibben M. A., and Barnes H. L. (1983) Hydrothermal pH and H2 measurements using the zirconia electrode and Teflon membrane. Proceedings 4th International Symposium on Water-Rock Interaction, Misasa, Japan, pp. 59-61. Bourcier W. L., Ulmer G. C., and Barnes H. L. (1987) Hydrothermal pH sensors of ZrO~, Pd hydrides, and Ir oxides. In Hydrothermal Experimental Techniques (ed. G. C. Ulmer and H. L. Barnes), Chap. 7, pp. 157-188. Wiley-Interscience. Bowers T. S., Campbell A. C., Measures C. I., Spivack A. J., and Edmond J. M. (1988) Chemical controls on the composition of vent fluids at 13° - 1 I°N and 21°N, East Pacific Rise. J. Geophys. Res. 93, 4522-4536. Campbell A. C. et al. (1988) The chemistry of hot springs on the Mid-Atlantic Ridge. Nature 335, 514- 519. Chou I-M. and Eugster H. P. (1976) A sensor for hydrogen fugacities at elevated P and T and applications. Eos Trans. Amer. Geophys. Union 57, 340 (abstr.). Craig H., Welhan J. A., Kim K., Poreda R., and Lupton J. E. (1980) Geochemical studies of the 21°N, EPR hydrothermal fluids. Eos Trans. Amer. Geophys. Union 61, 992. Ding K. and Seyfried W. E., Jr. (1992) Determination of Fe-C1 complexing in the low pressure supercritical region (NaCI fluid): Iron solubility constraints on pH of subseafloor hydrothermal fluids. Geochim. Cosmochim. Acta 56, 3681-3692. Evans W. C., White L. D., and Rapp J. B. (1988) Geochemistry of some gases in hydrothermal fluids from the Southern Juan de Fuca Ridge. J. Geophys. Res. 93, 15,305-15,313. Gunter W. D., Myers J., and Wood J. R. (1979) The shaw bomb, an ideal hydrogen sensor. Contrib. Mineral. Petrol. 70, 23-27.

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