High-Pressure High-Temperature Cell for Potentiometric Measurement in Aqueous Electrolyte P. Becker and B. A. BilaP

Geochemical Group, Nuclear Chemistry Division, Hahn-Meitner-Institut ffir Kernforschung Berlin GmbH, Glienicker Stral3e 100, D-1000 Berlin 39

Solutions

Zelle ffir potentiometrische Messungen in w~iflrigen Elektrolytliisungen bei hohem Druck und hoher Temperatur

Zusammenfassung. Es wird eine Zelle beschrieben, die ftir potentiometrische Messungen in wfil3rigen Elektrolytl6sun- gen bei Drficken bis 1 kbar (Gasdruck) und Temperaturen bis 250~ entwickelt wurde. Kurzschliisse und Kriechstr6me, die durch die elektrische Leitung (fiber die Dampfphase) zwi- schen den Elektroden und den Autoklavenw/inden entstehen, wurden mit Hilfe einer speziellen Konstruktion der Elektro- dendurchffihrung beseitigt.

Summary. A high-pressure high-temperature cell employing hydrogen electrodes has been developed for measuring the electrochemical potential in electrolyte systems at pressures up to I kbar (gas pressure) and temperatures up to 250 ~ C. A special construction of the electrical lead-through assembly solved the problems of short-circuits or erratic potentials and currents due to electrical contact between electrodes and autoclave walls, caused by condensed steam and conducting vapour.

Introduction

There is increasing interest for in situ measurement of the electrochemical potential of aqueous solutions under high pressure and elevated temperature as well. Such measure- ments will be a valuable tool in the investigation of the different hydrothermal systems in science and technology, for instance in the study of thermodynamic or geochemical systems and of corrosion processes in the pressure vessels and turbines of power plants.

Up to now, as far as a gas medium is used for establishing the pressure, cells are described in the literature [ 1 - 4] for measuring the emf of aqueous solutions either at elevated temperature and low pressure (saturation pressure up to 100 bar) or vice versa. Measurements under high-pressure high- temperature conditions require a special construction of the electrical lead-through assembly to overcome problems like short-circuits or erratic potentials and currents due to electri- cal contact between electrodes and autoclave walls caused by conducting vapour and condensed steam. This construction 1 is the essential development of the apparatus described below.

Offprint requests to : B. A. Bilal I Application filed with German Patent Office on Nov. 19th, 1982

Fresenius Z Anal Chem (1984) 317:118-120 �9 Springer-Verlag 1984

Fig. 1. High-pressure high-temperature cell cross-section : 1 x 5NiCrTi 2615 pressure vessel; 2 Teflon inner vessel; 3 Teflon tubes for electrical insulation; 4 Platinum electrodes; 5 Teflon stopper; 6 Teflon tube; 7 Shrinkable Teflon tube; 8 Liquid junction (rough Teflon or sapphire plug); 9 Sample solution; 10 Intermediate electolyte; 11 Electrical lead-through assembly

1. Apparatus

Figure 1 shows a cross-section of the cell. It consists of an autoclave (1) which is constructed for operation at I kbar and 300~ Because Teflon is used as material for the inner vessel

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Fig. 2. Electrical lead-through assembly: 1 Platinum electrode; 2 Stainless steel plug; 3 Cone-shaped end of the plug; 4 Insula- tion Teflon tube; 5 Shrinkable Teflon tube; 6 Overlapping point; 7, 8 Boreholes; 9 Cone-shaped transition part between 7 and 8; 10 Vespel double cone; 11 Connecting piece; 12 Vespel ring; 13 Teflon tube; 14 Screw; 15 Central borehote

(2) and for the electrical insulation tubes (3), the maximum operating temperature is restricted to about 250~ Two identical cylindrical compartments containing the sample and the reference solutions are arranged at the same distance from the autoclave axis. A liquid junction between the sample solutions (9) and the intermediate electrolyte (10) is estab- lished by two diaphragms (8) consisting of rough Teflon or sapphire plugs tightly fitted in a shrinkable Teflon tube. In this arrangement the two solutions have the same tempera- ture, which may be a little bit different from that of the intermediate electrolyte. This, however, is not of serious disadvantage. The temperature is measured by means of a Chromel-Alumel thermocouple which is situated in the inner vessel at the same distance from the autoclave axis like that of the two compartments. It is coated with Teflon of the same thickness like that of their walls. Due to the use of two

diaphragms connecting the sample solution as well as the reference solution with the intermediate electrolyte instead of one connecting the solutions directly, a higher flow rate through the junctions can be tolerated, since only the composition of the intermediate electrolyte but not that of the sample and of the reference solution will change.

A compartment consists of a Teflon tube (6) (13 mm i KS, 15 mm o ~ ) , which is closed at the top with a Teflon stopper (5). The hole in that stopper, through which the electrode (4) is inserted, is large enough in diameter to allow for pressure balancing between the compartment and the inner vessel. The Teflon tube is coated tightly with a little bit longer tube of shrinkable Teflon (7) which takes the rough Teflon or sapphire plug to form the diaphragm. The insertion of the plug into the shrinkable tube takes place under the surface of the solution to be measured, in order to obtain a conducting film of the solution between the plug and the tube. On using a Teflon plug for the diaphragm the electrical contact is sometimes interrupted on cooling down the apparatus due to the bad wetability of Teflon. Whenever the composition of the sample allows it, it is better to use a rough plug of another material like quartz or sapphire. On using I M NaC1 as an intermediate electrolyte the cell resistance of the arrangement lies between 20 and 100 kt? at room temperature. The liquid junction flow rate is some microliter per hour per cm height difference of the liquid in- and outside the compartments.

The electrical lead-through assembly (11) is the most critical point of the apparatus. To avoid the problems of short-circuit mentioned above, the electrodes had to be insulated by means of Teflon tubes without interruption through the gas-tight fitting in the autoclave head down into the sample solutions. The construction details of the lead- through assembly are shown in Fig. 2. The Pt-electrode (1) is hard soldered with a stainless steel plug (2) which is cone- shaped (3) at one end. The electrode is inserted in the insulation tube (Teflon) (4) which is overlapped at the point (6) by a shrinkable Teflon tube (5). The thus insulated electrode is then inserted in the double cone (10) (made of Vespel) and led through the boreholes (7) and (8) down into the sample solution. The coneshaped transition (9) between (7) and (8) takes tightly the double cone. The boreholes (7) and (8) can be situated directly in the autoclave head or appropriately in the connecting piece (11). After covering the plug (2) with a Vespel ring (12) and insulating the upper part of the electrode with a Teflon tube (13), it is led through the central borehole (15) of the screw (14) which squeezes the parts together to establish the gas-tight fitting.

For heating the autoclave two resistance elements are mounted on the autoclave body. The temperature is increased either continuously (0.5~ per min) or stepwise on using a precision temperature regulator.

2. Application

Employing hydrogen electrodes, the apparatus shown in Fig. 1 was used to determine the dissociation of hydrofluoric acid in the system HF/H20 at pressures up to 1 kbar and temperatures up to 250~ by measuring the emf of the concentration cell

Pt, H2 [ solution 1, [F-], [H+]I intermediate 1 r (sample) electrolyte I

[H+]2, solution 2 H2, Pt. (reference) (1)

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EMF versus Temper~ure/l o 35...72 bar ( 3 e x p e r i m e n t s l g l

/ x 230... 370 bar �9 583...1082 bar

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20 50 100 150 200 T(~ Fig. 3. Potential of cell (1) as a function of the temperature at different pressures

Before each run the Pt electrodes were cleaned by short immersion into dilute aqua regia and hot concentrated nitric acid. After cathodic cleaning in very dilute sulphuric acid for 10 rain (about 20 mA/cm 2) the Pt electrodes were platinized for 15 rain (20 mA/cm 2) in a 2 ~ solution of PtC14 in 2 N HC1.

In the above cell (1) the composition of the sample solu- tion 1 was i M NaC1, 0.0102M NaF, 0.0102M HC1, that of the intermediate electrolyte and of the reference solution 2 was 1.01 M NaC1, 0.0102 M HC1.

The cell potential was first amplified by a high impedance (Ri > 1013 O) amplifier (Knick). The output of the amplifier was displayed by a Precision Voltmeter (Keitley) and re- corded on a mV-recorder (Linseis LS 4). Simultaneously the emf of the thermocouple was recorded.

Figure 3 shows the measured cell potential as a function of the temperature at different pressures. In curve 1 the potential values of three experiments are plotted. All values ((3, A, O) fit the curve very well indicating a very good reproducibility of the measurements. The standard deviation of the potential values O, A, �9 is about _+ 0.1 mV at the temperature range 2 5 - 1 0 0 ~ and _+ 0.25 mV at the temperature range 1 0 0 - 200 ~ C.

The first establishment of the electrode potential takes a relatively long time (up to 12 h) if solutions with high ionic strenghts are used. However, the equilibrium values cor- responding to the different temperatures of a measurement series are obtained very fast.

This work is to be published in J. SoL Chem. Only a short contribution on the procedure and the measurements illus- trating their reproducibility is given here.

Very pure hydrogen (> 99.999 %) was used to obtain a H 2 partial pressure of about 35 bar in all parts of the apparatus. The oxygen content in the hydrogen did not exceed one volume part per million. Purified nitrogen was utilized for pressurizing to the desired higher pressure. The pressure was measured by means of a calibrated precision Heise gauge. The chemicals used were supra pure NaC1, HC1 and NaOH of pro analysis quality (all from Merck) and NaF as 0.1 M fluoride standard (Orion, Nr. 94-09-06).

References

], Mesmer RE, Baes CF Jr, Sweeton FH (1970) J Phys Chem 74:1937

2. Baranowski B, Szymczyk T (1980) (High pressure hydrogen electrode) Polish J Chem 54:1019

3. Hainsworth WR, Rowley HJ, Mac Innes DA (1924) J Am Chem Soc 46:1437

4. Lietzke MH et al. (1960) J. Phys Chem 64:652, 1445, 1861; (1964) 68:3043; (1965) 69:2395

Received June 21, 1983

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