JP2008111708A - Collation electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a collation electrode capable of performing electrochemical measurement stable over a long period of time in a high temperature and high-pressure aqueous solution within a temperature range from the normal temperature to 300°C. <P>SOLUTION: A palladium-silver alloy hydride electrode stable under a high temperature and high pressure environment is manufactured by occluding hydrogen on a palladium-silver alloy by cathodic electrolytic hydrogen charge and used as the collation electrode to perform electrochemical measurement. In one embodiment, the palladium-silver alloy hydride electrode has a hydrogen occlusion amount of at least 0.1% or above, preferably 0.3% or above. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to water quality measurement in high-temperature and high-pressure water environments such as chemical plants, thermal power plants, and nuclear power plants, and more particularly to a technique for measuring potential pH and the like with a simple electrochemical sensor.

Conventionally, the method of electrochemical measurement in high-temperature and high-pressure water has been measured and evaluated with electrodes installed outside the measurement system as described in Patent Document 1. However, due to the lack of data on the physical properties of ions at high temperatures, such as the degree of dissociation of water itself, chemical potential, and entropy, there are uncertainties associated with electrochemical measurements using electrodes placed outside the measurement system. . On the other hand, electrodes of the type using an internal solution are not suitable for long-term use because of generation of an inter-liquid potential due to liquid connection, leakage of the internal liquid, and the like.
No. 4-22290

  The problem with conventional electrochemical measurement methods using “second type” reference electrodes such as external reference electrodes and internal reference electrodes using silver / silver chloride is the high resistance that separates the internal solution (reference solution) from the test solution. The use of a junction (for example, a porous plug). This high resistance has the disadvantage that the response of the measurement is reduced.

  Moreover, since a zirconia solid electrolyte electrode has high impedance from the structural property of a solid electrolyte, it similarly has low responsiveness. On the other hand, a metal-metal oxide electrode such as mercury-mercury oxide has a low impedance and can be used temporarily as a metal electrode in direct contact with a test solution, but lacks stability in a long-term measurement.

  In an aqueous solution with a constant pH, it is possible to measure the potential using a conventional classic platinum / hydrogen electrode, but it is difficult to handle the introduction of hydrogen into the solution, and the solution is saturated with hydrogen. This adversely affects the environment, so that practical use is limited. Compared with conventional platinum / hydrogen electrodes that always require hydrogen partial pressure, this palladium-silver alloy hydride electrode occludes hydrogen inside the alloy by cathodic electrolytic charge, so hydrogen is required for the measurement system. And not.

  A palladium electrode using pure palladium as an electrode serves as a reference electrode capable of measuring up to a temperature range of about 200 ° C. by cathodic hydrogen charging. The pure palladium electrode is easy to configure and use, and has the characteristics of low impedance necessary for high-speed measurement, but in the temperature range higher than 200 ° C., the configuration range of the stable two-phase region becomes small, The diffusion loss of hydrogen into an unsaturated solution of hydrogen becomes very fast and can only withstand short use. In addition, the pure palladium electrode may be mechanically broken due to repeated α-β changes accompanied by a volume change of about 10% due to the temperature cycle.

  In order to achieve the above object, the present invention employs a palladium-silver (80 wt% palladium-20 wt% silver) alloy for the electrode used in the reference electrode sensor.

  Since palladium-silver alloy has a higher hydrogen retention capacity in a high-temperature water environment than pure palladium, it can be used in a higher-temperature environment. By making an alloy of silver and palladium that reduces the α-β miscibility gap at room temperature, it is possible to avoid changes in α-β accompanied by volume changes. This alloy has characteristics such as higher hydrogen permeability than pure palladium.

[Action]
In the present invention, electrochemical measurement is performed in a high-temperature and high-pressure water environment over a temperature range from room temperature to 300 ° C. using a palladium-silver alloy as a hydride electrode.

  Thermodynamic consideration-It is considered that the potential of the palladium-silver alloy hydride electrode in aqueous solution is due to the following equation (1).

Therefore, understanding the origin of the potential requires analysis of the thermodynamic properties of palladium hydride systems. H ⁺ represents a hydrogen ion, and e represents an electron. A phase diagram of the Pd—H system given by Lewis is shown in FIG.
FA Lewis, The Palladium-Hydrogen System, Academic Press, London (1967)

Each isotherm at a temperature of 310 ° C or lower is between the equilibrium between α (solid solution of hydrogen in palladium) and β (Pd _x H and x are given near the boundary between α + β and β regions). Show the plateau about what to do. By definition, the activity of Pd _X H is constant, and the hydrogen activity of the lattice in the α + β region is given by the following formula 2.

Where μ ⁰ _PdxH , μ ⁰ _Pd is the Gibbs energy per mole of Pd _x H and Pd, and μ ⁰ _H is the standard partial molar Gibbs energy of the lattice hydrogen, R is the gas constant, and T is the absolute temperature. is there. Since these quantities are functions only of temperature (at constant pressure), the hydrogen activity of the lattice is fixed at the value given by T.

  Here, if the palladium hydride electrode is in an equilibrium state, the chemical potential of hydrogen in the lattice is considered to be equal to the chemical potential of hydrogen in the solution. That is, the equilibrium is expressed by the equation (3).

Lattice hydrogen activity lna _{H, α + β} is given by equation 4.

Here, p _βα is an appropriate hydrogen partial pressure that is isothermal with the plateau region. The potential E _Pd-H of the palladium hydride electrode is given by

If Equation 4 is substituted according to lna _{H, α + β} , the following equation is obtained.

_Equation 6 is an equation of a hydrogen electrode having a hydrogen partial pressure equal to P _βα (on the definition of electrochemistry, the first half of the equation is 0). The reference electrode performs measurement based on the principle of electrochemical equilibrium.

  According to the present invention, since the electrode structure is simple and a small electrochemical sensor can be provided, the application range of the electrochemical measurement method in a high temperature and high pressure water environment can be greatly expanded.

  The use of a reference electrode for electrochemical measurements at high temperatures was realized by using a palladium-silver alloy.

  Embodiments of the present invention will be described below with reference to FIGS.

  FIG. 2 shows a configuration of a pH measuring device in high-temperature and high-pressure water to which the present invention is applied. The type of the measurement environment solution 5 in the measurement vessel 6 that can withstand a high temperature and high pressure water environment such as stainless steel or titanium, that is, a change in the measurement atmosphere is measured as pH. A potential measuring device 3 having a high input impedance is connected between a reference electrode 1 and a sample electrode 2 placed in a solution environment to be measured by a shielded lead wire 4, and the potential difference is measured. The potential of the reference electrode 1 measured with respect to the sample electrode 2 is converted into a hydrogen electrode potential reference. The verification electrode 1, the sample electrode 2, and the lead wire 4 are insulated from the measurement container 6.

FIG. 3 shows a hydrogen storage method and circuit diagram in a palladium-silver alloy. Occlusion of hydrogen into the palladium-silver alloy 11 is performed by a cathodic electrolytic hydrogen charging method. The hydrogen occlusion in the palladium-silver alloy 11 and the measurement of the occlusion rate are carried out with respect to the platinum electrode 12 using the palladium-silver alloy 11 as a cathode in a hydrogen charging electrolyte solution 14 such as a 1N hydrochloric acid solution in a measuring vessel 13. The battery is charged by performing electrolysis for 1 to 180 minutes at a constant current value of 100 to 200 mA / cm ² from the electrolytic charging power source 16. The current value at this time is measured by the ammeter 15. Water is electrolyzed by applying a voltage of 1.23 V or more, and oxygen is generated on the anode side and hydrogen is generated on the cathode side. Cathodic electrolytic hydrogen charging is a circuit in which a palladium-silver alloy 11 is used as a cathode and a platinum electrode 12 is used as an anode, thereby generating hydrogen on the palladium-silver alloy 11 side and storing hydrogen from the surface of the palladium-silver alloy 11. . Further, the amount of hydrogen in the palladium-silver alloy 11 is based on the fact that the resistance value of the alloy changes depending on the amount of hydrogen occluded in the palladium alloy. It can obtain | require by comparing the silver alloy 18 and resistance value. The resistance value of each alloy is measured by the bridge method. A current is applied to the bridge circuit formed by the palladium-silver alloy 11 and the comparative palladium-silver alloy 18 by the constant current applying device 19, and the resistance value is calculated from the voltage value measured by the precision voltage measuring device 20.

  FIG. 4 is an example of data obtained by measuring the amount of hydrogen with a hydrogen analyzer. The amount of hydrogen occluded in the palladium-silver alloy was analyzed twice using the method shown in FIG. 3 while changing the hydrogen charging time. The vertical axis represents the hydrogen storage amount (%), and the horizontal axis represents the hydrogen charge time (minutes). The hydrogen occlusion amount of the palladium-silver alloy with a hydrogen charge time of 0 is 0.001%, which is the detection limit of the measuring apparatus, and it can be seen that there is almost no hydrogen in the palladium-silver alloy. A palladium-silver alloy that has been charged with hydrogen for 5 minutes has a hydrogen storage amount of 0.1 to 0.2%, and a hydrogen charge time of 120 minutes is 0.3 to 0.4%. It can be seen that the hydrogen is occluded.

  FIG. 5 is a production example of a palladium-silver alloy reference electrode. The electrode holder portion 24 is made of stainless steel and has a thickness calculated for strength to withstand the use of high temperature and pressure, and a connection portion to a pressure vessel or the like. The electrode and the lead wire 21 are fixed to the electrode holder portion 24 by tightening a cap 22 that fixes the electrode. Considering the use at high temperature, the electrode holder portion 24 has a cooling jacket 23 so that cooling can be performed. By flowing a cooling medium such as water and air through the cooling portion, the pressure at the time of use in a high temperature and high pressure environment It becomes possible to suppress deterioration of the pressure seal from the container and the insulating material of the lead wire 21 and the electrode. The electrode is designed to be installed at a fixed location by the electrode guide 25. Also, a porous ceramic cover to protect the electrode from the influence of the flow velocity on the electrode surface when impurities adhere to the electrode surface from the test solution environment, poisoning due to ion species, etc., and the flow of the test solution. 26. The ceramic cover 26 is made of ceramics and does not affect the measurement itself. A reference electrode is provided inside the ceramic cover 26.

FIG. 6 is an example of data actually measured using the present electrode under the following use conditions. The vertical axis represents the SHE reference potential, and the horizontal axis represents the elapsed time. The test conditions for this test were a test temperature of 300 ° C., a test pressure of 8.5 MPa (saturated vapor pressure), 0.05 mol of sodium sulfate (Na ₂ SO ₄ ) solution. Using a static autoclave made of SUS316 with an internal volume of 400 ml, the sample was heated from the outside by a heater, and the temperature was controlled by a K-sheath thermocouple installed inside to adjust the test temperature to ± 1 ° C. The test solution was degassed with oxygen, carbon dioxide, etc. by nitrogen gas ventilation for 1 hour. As a measurement object, a pressure balanced external reference electrode (silver / silver chloride, 0.1N potassium chloride electrolyte) is measured as a reference. It shows that long-term stable data can be obtained from the measured data.

FIG. 7 shows data measured using this electrode while changing the temperature. The vertical axis represents the hydrogen electrode reference potential, and the horizontal axis represents temperature. Using a static autoclave made of SUS316 with an internal volume of 400 ml, the sample was heated from the outside by a heater, and the temperature was controlled by a K-sheath thermocouple installed inside to adjust the test temperature to ± 1 ° C. The test solution was degassed with oxygen, carbon dioxide, etc. by nitrogen gas ventilation for 1 hour. The test solution used was a 0.05 mol sodium sulfate (Na ₂ SO ₄ ) solution. The measured corrosion potential of stainless steel (JIS SUS304) at various temperatures from room temperature to 300 ° C. is reproducible and shows that stable data can be obtained.

It is a correlation diagram of hydrogen and palladium, and is an explanatory diagram showing the stability of a palladium-silver alloy hydride electrode. It is explanatory drawing which showed the basic apparatus structure in the case of implementing this invention. It is explanatory drawing of the hydrogen charge by the cathode electrolysis to a palladium-silver alloy. It is a data example of the resistance change at the time of hydrogen charge to a palladium-silver alloy. It is explanatory drawing which showed the manufacture example of the electrode. It is an example of data in which stable measurement is performed by measuring for 300 hours at 300 ° C. It is an example of the measurement data in each temperature from room temperature to 300 degreeC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Palladium-silver alloy hydride electrode 2 Sample electrode 3 to be measured 3 Potential measurement device 4 Lead wire 5 Liquid in measurement environment 6 Measurement vessel 11 Palladium-silver alloy 12 Platinum electrode 13 Measurement vessel 14 Hydrogen charging electrolyte 15 Ammeter 16 Electrolytic charging power source 17 Lead wire 18 Comparative palladium-silver alloy 19 Constant current applying device 20 Precision voltage measuring device 21 Lead wire 22 Cap for fixing electrode 23 Cooling jacket 24 Electrode holder portion 25 Electrode guide 26 Electrode and ceramics cover

Claims (10)

Palladium-silver alloy reference electrode containing hydrogen. In an apparatus including an electrolytic solution, connecting a palladium-silver alloy to a cathode and using platinum as an anode, by supplying a predetermined current to the platinum, the palladium-silver alloy is made to store hydrogen. In addition, a palladium-silver alloy reference electrode containing hydrogen. The hydrogen-containing palladium according to claim 1 or 2, wherein the hydrogen-containing palladium-silver alloy reference electrode has a hydrogen storage amount of at least 0.1% or more, or preferably 0.3% or more. Palladium-silver alloy reference electrode. In an apparatus including an electrolytic solution, connecting a palladium-silver alloy to a cathode and using platinum as an anode, supplying a predetermined current to the platinum causes the palladium-silver alloy to store hydrogen, For producing a palladium-silver alloy reference electrode comprising The palladium-silver-containing palladium according to claim 4, wherein the palladium-silver alloy reference electrode containing hydrogen has a hydrogen storage amount of at least 0.1% or more, or preferably 0.3% or more. A method for producing a silver alloy reference electrode. A collation electrode device comprising a palladium-silver alloy collation electrode containing hydrogen, an electrode support part for supporting the collation electrode, and a lead wire connected to the electrode support part. In an apparatus including an electrolytic solution, connecting a palladium-silver alloy to a cathode and using platinum as an anode, by supplying a predetermined current to the platinum, the palladium-silver alloy is made to store hydrogen. The verification electrode device according to claim 6, further comprising a palladium-silver alloy verification electrode containing hydrogen. 8. The reference electrode device according to claim 6, wherein the palladium-silver alloy reference electrode containing hydrogen has a hydrogen storage amount of at least 0.1% or more, or preferably 0.3% or more. . A measurement device for pH measurement, wherein the reference electrode device according to claim 6 is connected to a potential measurement device. The measurement device for pH measurement according to claim 9, wherein the palladium-silver alloy reference electrode containing hydrogen has a hydrogen storage amount of at least 0.1% or more, or preferably 0.3% or more. .

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