Silver chloride electrode

Silver chloride electrode is composed of porous metal silver covered with silver chloride and immersed in solution containing Cl -, which can be expressed as Ag / AgCl / Cl -, and the electrode reaction is AgCl + e = Ag + Cl -.

Silver chloride electrode is a common reference electrode with stable potential and good reproducibility. Its standard electrode potential is + 0.2224v (25 ℃). The advantage is that it is more stable than calomel electrode in the case of heating. There are three types of KCl: 0.1mol/l KCl, 1mol / L KCl and saturated KCl. When the electrode is used in the solution containing chloride ion, it will be interfered by oxygen content in acidic solution, so it can be protected by nitrogen in precise operation. When HNO3 or Br -, I -, NH4 -, CN - ions exist in the solution, it can not be used. In addition, it can also be used as internal reference electrode of some electrodes, such as glass electrode and ion selective electrode.

1、 Concept

Because Ag / AgCl electrode has very small solubility, high stability and reversibility in high temperature and high pressure aqueous solution system, and the electrode surface can be well protected even in the presence of hydrogen, these characteristics are incomparable to other electrodes. Therefore, the study of Ag / AgCl electrode is more in-depth and detailed.

2、 Electrochemical performance

Research background

Exploration and development of marine resources is of great significance to the development of national economy. However, some conventional geophysical methods have lost good detection effect after extending the detection object from land to ocean. With the development of marine geophysical research, it is found that the marine magnetotelluric method based on electrical exploration and reflecting the distribution law of medium in seawater with electrical parameters can provide valuable information, and the measurement of these information needs high-performance electric field sensors. Because the characteristics of submarine electric field signal are as follows:

(1) Due to the attenuation of the sea water layer, the electric field signal on the sea floor is much weaker than that on the land, and the amplitude can reach mv level;

(2) The frequency components of submarine electric field signal are mainly medium and low frequency, so the signal acquisition time is relatively long. Generally, the instrument needs to be placed on the seabed for a week or even longer. Therefore, the detection of underwater electric field signal requires that the electric field sensor has the characteristics of low self noise, high measurement sensitivity, voltage resistance and long-term stability. Therefore, it is very important to find and develop the electric field sensor with high stability in seawater.

The Ag / AgCl solid-state electrode was developed based on the analysis of commonly used electrodes and the specific requirements of deep-sea high-pressure environment. It can be directly inserted into the measured sea water, thus avoiding the generation of liquid contact potential. The preparation conditions, the factors affecting the stability of potential and the self noise performance of the electrode were studied. The stability and self noise performance of the electrode were analyzed.

Preparation of Ag AgCl electrode

According to a certain proportion, AgCl and Ag powder with the particle size of 200 mesh and purity of 99.97% are evenly mixed and granulated. The electrode body is pressed into a cylinder mold under uniaxial pressure. The initial electrode is sintered at 400-600 ℃. After grinding and hydrochloric acid activation, it is threaded connected and packaged with silver rod to make marine electric field measuring electrode.

Stability test of electrode

After considering the micro morphology of the electrodes, it can be seen that the range potential is mainly caused by the difference of the reaction active areas of the two electrodes, and the large difference of the reaction active areas leads to the range that the range potential can not be controlled within the specified range. According to the working principle of the electrode, there are two phase interfaces of Ag / AgCl electrode in seawater, i.e. Ag AgCl Cl -, and Ag and AgCl on the surface of electrode participate in the reaction process as anode and cathode respectively and tend to balance, that is, AgCl + e = Ag + CL. Because AgCl is a kind of insoluble salt, there is another equilibrium relationship in the reaction process: AgCl → Ag + + Cl -. When the electric field signal is transmitted, a small amount of current passes through the electrode interface, and the above-mentioned equilibrium appears to be deviated. The double-layer controlled by chloride ion concentration is destroyed, and the double-layer equilibrium is reestablished, resulting in the instability of the range potential.

Therefore, the range potential and its stability are determined by the electric double layer on the electrode surface, and the composition and morphology of the surface of the two electrodes determine the composition of the electric double layer. When the particle size of silver powder is between 2 and 5 μ m, the number of AgCl particles is much more than that of silver powder. The particles of AgCl are evenly distributed around the silver powder, and the small particles of AgCl are filled between the large particles of silver powder, thus effectively avoiding the generation of large pores and greatly improving the density and uniformity of the electrode.

There are few defects on the surface of the electrode, and the interface between the particles can be seen when AgCl is slightly melted. For metals, the resistivity will increase with the increase of the number of grain boundaries due to the existence of potential barrier at the grain boundary, which is attributed to the scattering effect of the grain boundary on the conduction electrons; as the sample electrode contains more silver, the electrons will be transmitted through the scattering and silver interaction of the AgCl interface to make the signal effective The conduction of.

3、 Research status

The stability of reference electrode is very important for all electrochemistry and related research in high temperature and high pressure. Before the 1990s, a wide range of reference electrodes were studied in high temperature and high pressure environment, such as hydrogen electrode, palladium hydride electrode, silver / silver halide electrode (Ag / AgCl, Ag / AgBr, Ag / AgI, etc.), metal / metal oxide electrode (Hg / HGO, Ag / Ag2O, Pt / PTO, IR / IrO, Rh / rh2o3, Zr / ZrO2, etc.), metal / metal sulfate electrode (Ag / ag2so4, Hg / hg2so4, Pt / ptso4) We have tried and studied calomel electrode and inert metal redox electrode (PT, Ni) and so on, and have been widely used in related fields.

However, the Ag / AgCl electrode has very small solubility, high stability and reversibility in the high temperature and high pressure aqueous solution system, and the electrode surface can be well protected even in the presence of hydrogen, these characteristics are incomparable to other electrodes. Therefore, the study of Ag / AgCl electrode is more in-depth and detailed. Ag / AgCl electrode has been widely used in thermodynamics and kinetics research. This paper reviews and summarizes the research of Ag / AgCl reference electrode for high temperature and high pressure aqueous solution system in recent decades, analyzes its development trend and puts forward its own suggestions.

Reference electrode for high temperature and high pressure aqueous solution system

(1) Requirements of high temperature and high pressure reference electrode

For any reference electrode used in high temperature and high pressure system, the following conditions should be met:

1. The electrode potential has good reproducibility, and the potential is stable in the process of experiment;

2. It can not react with the test system solution;

3. If the reference electrode is used for thermodynamic research, it must have correct thermodynamic behavior characteristics and satisfy Nernst equation;

4. The solubility of electrode should not be too large;

5. The electrode material will not corrode, decompose and distort the performance of electrode under high temperature and high pressure;

6. The material of liquid connection part should keep good conductivity and thermal stability under high temperature and high pressure.

Of course, when designing and using the reference electrode, a series of problems, such as liquid contact potential, interaction between solutions and pollution, should be considered in detail according to the specific requirements of the experiment. In this regard, Ives and Janz have given some suggestions on the design of high temperature and high pressure reference electrode.

(2) Form of reference electrode

The reference electrode for high temperature and high pressure can be divided into internal type and external type. The former is in the same temperature, pressure and solution environment as the working electrode; the latter is to place the reference electrode outside the test system and connect with the solution in the high-pressure container through the salt bridge. The characteristics of the two electrodes are different due to the different environment. In general, the built-in reference electrode is mostly used to study the thermodynamic behavior of electrode, while the external reference electrode is mostly used for the study of electrode dynamics.

Structure of Ag AgCl internal reference electrode for high temperature and high pressure

The biggest problem of the built-in reference electrode for high temperature and high pressure water is the aging of the electrode in a short time. This is related to the poor sealing, damage and solution circuit interruption caused by the difference of thermal expansion coefficient between the electrode body components and between the component and the internal solution. In addition, the hydrolysis of Ag / AgCl at high temperature and the blockage of micropores in the liquid junction by AgCl hydrolysate are also important reasons for electrode aging. There are many workers to solve this problem from various aspects, such as adding circulating water cooling, increasing the surface area of Ag exposed in electrode solution, etc. Iwamoto et al. Developed a new composite absorbent. The absorbent is composed of alkali metal ions and tetraphenylborate ions (or nitrogen-containing organic substitutes such as triamine salts). It is filled in porous framework structure such as porous activated carbon or silica gel. At high temperature, it can prevent the hydrolysis of Ag / AgCl, and the diameter of the complex is larger than the original ion The possibility of the hydrolysate blocking the micropore was discussed.

Structure of Ag AgCl external reference electrode for high temperature and high pressure

According to the different liquid connection methods, the external reference electrode can be divided into the following two types: the first is to take a small amount of test liquid from the high temperature and high pressure system as the connecting liquid, in order to achieve this goal, it must be constantly outflow, which is a disadvantage, but the electrode structure is relatively simple; the second is to use the pressure balanced external reference electrode, which can have a high potential It can be easily converted into she reference potential, so it is often used as reference electrode.

In the case of liquid outflow type external reference electrode, in order to ensure the conductivity of liquid connection, the pressure seal (or pressure connector) becomes a crucial problem. On the one hand, the pressure sealing part should limit the flow rate of liquid from the high pressure vessel, on the other hand, it should maintain the conductivity of the hot solution. Generally speaking, the pressure balance electrode is mainly used to evaluate the potential of the hydrothermal connection, measure and calibrate the temperature coefficient of the built-in reference electrode. However, this type of electrode is easier to handle than the liquid outflow type electrode, more stable and long service life than the built-in reference electrode, and has the characteristics of correcting the liquid potential After the corrosion research will be more widely used.

4、 Electrode maintenance

1. Preparation of copper sulfate solution of reference electrode: pour the chemical pure copper sulfate crystal into a clean glass beaker, then pour appropriate amount of distilled water (the temperature of the prepared water is 25 ℃), stir and dissolve it with clean glass rod (metal rod is not allowed), and partially deposit, so as to prepare the copper sulfate solution.

2. Open the upper cover of the reference electrode, pour out the liquid in the reference electrode, and remove the components.

3. Check whether the contact connection is good, and re connect the poor contact. Remove the surface attachment of each part, especially the copper bar, which should be polished with sandpaper. Check whether the semi permeable membrane is in good condition and replace it in time if any damage is found. In case of blockage, soak and clean it with hot water.

4. Pour in the prepared saturated copper sulfate solution to submerge more than two thirds of the copper bar, tighten the upper cover, and check the bottom semi permeable membrane for solution leakage, but no solution leakage, otherwise the semi permeable membrane should be replaced.

Shandong Linyi Bosi conductive materials Co., Ltd. is a high-tech enterprise specializing in the production of MMO flexible anode, conductive polymer flexible anode and titanium coated anode, mainly providing cathodic protection technology, engineering design, installation and technical services.