electrochemistry

The science of studying the charged interface phenomenon formed by two types of conductors

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Electrochemistry is to study the charged interface phenomenon formed by two types of conductors and its changes science 。 The interaction between electricity and chemical reaction can be completed by batteries, or by high-voltage electrostatic discharge (for example, oxygen is transformed into ozone ）The two are collectively called electrochemistry, and the latter is a branch of electrochemistry, called radioelectrochemistry. Due to the special name of discharge chemistry, electrochemistry often refers specifically to "battery science".

Electrochemistry has now formed synthetic electrochemistry, quantum electrochemistry Semiconductor electrochemistry , organic conductor electrochemistry Spectroelectrochemistry , bioelectrochemistry and other branches.

Chinese name: electrochemistry
Foreign name: Electrochemistry
Positive pole of primary battery: One pole into which electrons flow

Negative pole of primary battery: One pole of electron outflow
Electrolytic cell anode: Connected to the positive pole of the power supply, the first stage of the electronic outflow
Electrolytic cell cathode: Connected to the negative pole of power supply, the first stage of electron inflow

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seven Discipline Application
eight Electrochemical protection

electrochemistry

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Electrochemistry has been widely used in chemical industry, metallurgy, machinery, electronics, aerospace, light industry, instrumentation, medicine, materials, energy, metal corrosion and protection, environmental science and other scientific and technological fields. Current research topics of great concern in the world, such as energy, materials, environmental protection life sciences And so on are associated with electrochemistry in various ways.

Comparison between primary cell and electrolytic cell

Primary battery

The primary battery is produced by using the difference in the metallicity between the two electrodes Potential difference So as to make electrons flow and generate current. Also called non battery, it is a kind of electrochemical battery. Its electrochemical reaction cannot be reversed, that is, it can only convert chemical energy into electrical energy. In short, it can not re store electricity Battery relative^[1] 。

Primary battery Yes will chemical energy Into electric energy Device. So, by definition, ordinary dry battery 、 fuel cell Can be called Primary battery 。

Basic conditions for primary battery:

1. Two metals with different activity (i.e., one is active metal and the other is inactive metal), or one metal and graphite (Pt and graphite are inert electrodes, that is, they will not lose electrons), etc Inert electrode insert electrolyte In solution.

2. Connect with wires and insert them into electrolyte solution to form a closed circuit.

3. Spontaneous Redox reaction 。

Operating principle of primary battery

The primary battery is to generate current in the external circuit by the oxidation reaction and reduction reaction of a spontaneous redox reaction on the negative pole and positive pole of the primary battery respectively.

Judgment of electrode of primary battery:

Negative pole: the pole from which electrons flow; The pole where the oxidation reaction occurs; One pole of highly active metal.

Positive pole: the pole into which electrons flow; The pole where the reduction reaction occurs; One pole of a relatively inactive metal or other conductor.

In the primary battery, the external circuit is Electronics Conductive, in electrolyte solution Ions electric conduction.

Determination of primary battery:

(1) First, analyze whether there is an external circuit. If there is an external power supply Electrolytic cell , galvanic battery may be used if there is no external power supply; Then, according to the analysis and judgment of the formation conditions of the primary cell, it is mainly "four looks": look at the electrode - the two poles are conductors and have different activity (the electrode of the fuel cell is generally an inert electrode); Look at the solution - the two poles are inserted into the solution; Look at the loop - form a closed loop or direct contact between two poles; See the essence - whether there is oxidation-reduction reaction.

(2) When multiple cells are connected, but there is no external power supply, the cell with the greatest difference in activity between the two poles is the primary battery, and the other cells can be regarded as electrolytic cells.

Electrolytic cell

Electrolytic cell is a device that converts electric energy into chemical energy.

Electrolysis is caused by the current passing through the electrolyte solution (or molten electrolyte) at the anode and cathode Redox reaction Process.

Conditions for electrolytic reaction:

① Connect DC power

② Negative and positive electrode cathode: connected to the negative pole of the power supply as the cathode

Anode: connected to the positive pole of power supply as anode

③ The two poles are in electrolyte solution or molten electrolyte

④ Two electrodes form a closed loop

Energy conversion during electrolysis (device characteristics):

cathode : Must not participate in the reaction, not necessarily inert electrode

anode : Not necessarily participate in the reaction or inert electrode

Electrolysis results:

New substances are formed on both poles

Writing of electrode reaction equation of electrolytic cell:

anode : Active metal - electrode electron loss (except Au, Pt, Ir); Inert electrode - anion in solution loses electrons

Note: electron loss ability: active metal (except Pt Au)>S2 ->I ->Br ->Cl ->OH ->oxyacid radical (NO3 ->SO42 -)>F-

cathode : electron of cation in solution

Note: electron yield capacity: Ag+>Hg2+>Fe3+>Cu2+>H+(acid)>Pb2+>Sn2+>Fe2+>Zn2+>H2O (water)>Al3+>Mg2+>Na+>Ca2+>K+(i.e. the reverse of the sequence table of active metals)

Correspondence : The anode is connected to the positive pole of the power supply, and the cathode is connected to the negative pole of the power supply (see the high school textbook * Chemistry Elective · 4)

law : The ions before aluminum (including aluminum) do not discharge, the ions after hydrogen (acid) discharge first, and the ions after hydrogen (acid) discharge depend on the conditions.

Electrolytic law of four types of electrolytic type ① Electrolytic water type (strong base, oxy acid, oxy acid salt of active metal), pH is determined by the acidity and alkalinity of the solution. If the solution is alkaline, pH will increase, if the solution is acidic, pH will decrease, and if the solution is neutral, pH will not change. Electrolyte solution recovery - add proper amount of water.

② Electrolytic electrolyte type (anaerobic acid, anaerobic salt of inactive metal,), the pH of anaerobic acid increases, and the pH of anaerobic salt of inactive metal remains unchanged. Electrolyte solution recovery - add proper amount of electrolyte.

③ Hydrogen releasing and alkali generating type (anaerobic salt of active metal), pH becomes larger. Electrolyte solution recovery - add acid with the same anion.

④ Oxygen releasing and acid generating type (oxygen containing acid salt of inactive metal), pH value decreases. Electrolyte solution recovery - add alkali or oxide with the same cation.

development

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Electrochemical polishing of stainless steel pipe fittings

In 1663, Otto von Guericke, a German physicist, created the first alternator , static electricity is generated by friction in the machine. This generator is made by putting a huge sulfur ball into a glass ball and fixing it on a shaft. Rotate the ball by shaking the crankshaft. When a gasket rubs against the rotating ball, an electrostatic spark will be generated. The sphere can be disassembled and used as a source of electrical tests.

In the middle of the 17th century, French chemist Charles Franç Ois de Cisternay du Fay discovered two different kinds of static electricity, that is, the same kind of charges repel each other and the different kinds of charges attract each other. Du Fay released that electricity is composed of two different liquids: "vitreous" (Latin for "glass"), or positive; and "resinous", or negative. This is the double liquid theory of electricity, which was rejected by Benjamin Franklin's single liquid theory in the late 17th century.

Charles Augustine, 1781 Coulomb (Charles Augustin de Coulomb) developed the law of electrostatic attraction in the process of trying to study the law of charge repulsion proposed by British scientist Joseph Priestley.

1791 Gavani The "animal electricity" phenomenon that metal can make frog leg muscles shrink was published, which is generally believed to be the origin of electrochemistry. 1799 Volta On the basis of Gavani's work, he invented a "stack" composed of different metal sheets and wet paper, which is now called“ Voltaic pile ”。 This is Chemical power supply The prototype of. stay DC motor Before the invention, various chemical power sources were the only ones that could provide constant current. 1834 Faraday Electrolytic law The discovery of β - cyclodextrin lays a quantitative foundation for electrochemistry.

In the second half of the 19th century, Helmholtz And Gibbs' work, which is attributed to the "starting power" of the battery (now called“ emf ”）With explicit thermodynamics meaning; 1889 Nernst The relationship between the concentration of the substances involved in the electrode reaction and the electrode potential is derived by thermodynamics nernst equation ； 1923 Debye and Huckel It puts forward the strong electrolyte dilute solution static electricity Theory has greatly promoted the development of electrochemistry in theoretical discussion and experimental methods.

After the 1940s, electrochemistry Transient technology The application and development of electrochemical methods, together with optical and surface technologies, enable people to study rapid and complex electrode reactions and provide information about molecules at the electrode interface. Electrochemistry has always been an active branch of physical chemistry. Its development and the development of solid physics, catalysis, life science and other disciplines promote and penetrate each other.

Among many branches of physical chemistry, electrochemistry is the only discipline based on large industry. Its applications mainly include: Electrolytic industry , of which Chlor alkali industry Is second only to synthesis ammonia And inorganic basic industry of sulfuric acid; aluminum , sodium, etc Light metal The smelting of copper, zinc and other refining also use electrolysis; Use in machinery industry electroplate , electropolishing, electrophoretic painting, etc. to complete the surface finishing of components; Environmental protection available Electrodialysis The method of removing cyanide ion, chromium ion and other pollutants; Chemical power supply ； Corrosion prevention of metal, most Metal corrosion yes electrochemical corrosion Problems; Many life phenomena such as muscle movement and nerve information transmission are related to electrochemical mechanism. Various kinds of electrochemical principles Electrochemical analysis It has become an indispensable means of laboratory and industrial monitoring.

research contents

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The battery is composed of two electrodes and the electrolyte between them, so the research content of electrochemistry should include two aspects: one is the research of electrolyte, that is, electrolyte science, which includes the conductivity of electrolyte, the transport properties of ions, the equilibrium properties of ions participating in the reaction, etc electrolyte solution The physical and chemical research of "Li" is often called the electrolyte solution theory; On the other hand, it is the study of electrodes, that is, electrodynamics, including the equilibrium properties of electrodes and the polarization properties after electrification, that is, the electrochemical behavior at the interface between electrodes and electrolytes. Electrolytics and Electrodynamics The research of Chemical thermodynamics 、 chemical kinetics And material structure.

Material protection

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According to the principle of electrochemical corrosion, depending on the inflow of external current potential So as to reduce the corrosion rate of metal

Sp500 electrochemical marking machine

Operation. According to the trend of metal potential change, electrochemical protection is divided into cathodic protection and Anodic protection Two types. ① Cathodic protection. Cathodic protection is achieved by reducing the metal potential. According to the source of protection current, cathodic protection includes impressed current method and sacrificial anode method. The impressed current method is that the external DC power supply provides the protection current negative pole Connect the protection object, and the positive pole is connected to the auxiliary anode, through electrolyte The environment constitutes a current loop. Sacrificial anode method relies on the metal whose potential is negative to the protected object（ sacrificial anode ）The protection object is directly connected with the sacrificial anode to form a protection current circuit in the electrolyte environment. Cathodic protection is mainly used to prevent metal corrosion in neutral media such as soil and seawater. ② anode Protection. By improving passivation The potential of a metal makes it enter a passive state to achieve the purpose of protection, which is called Anodic protection 。 Anodic protection is to use anodic polarization current to keep metal in a stable passive state, and its protection system is similar to Impressed current cathodic protection System, but the polarization current is in the opposite direction. Only corrosion systems with activation passivation transition can adopt anode protection technology, such as concentrated sulfuric acid storage tank, ammonia storage tank, etc.

separation method

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Use electrochemical means to separate metal ions Organic molecule There are four types of methods:

Electrolytic separation method for controlling potential

If there are two or more metal ions in the solution Reduction potential If it is similar, it will be reduced and precipitated during electrolysis, and the purpose of separation cannot be achieved. The choice of potential depends on the experimental conditions. When applying this method, the concentration of ions to be electrolyzed later cannot exceed the concentration of ions to be electrolyzed first.

Mercury cathode electrolytic separation

When H □ is reduced on the mercury cathode, there is a large overvoltage, so some metal ions that are easy to be reduced can be separated in acid solution, so that some heavy metals (such as copper, lead, cadmium, zinc) can be deposited on the mercury cathode to form Amalgam At the same time, a small amount of ions that are not easy to be reduced are retained, such as alkali metals, alkaline earth metals, aluminum, iron, nickel, chromium, titanium, vanadium, tungsten, silicon, etc.

Internal electrolytic separation method

In acid solution, by using the difference of metal oxidation reduction potential, an internal Electrolytic cell That is, electrolysis can be carried out without external voltage. For example, to separate trace copper from a large amount of lead, Cu is reduced first than Pb in sulfuric acid solution, so the lead plate can be used as an electrode and connected with the platinum electrode to form an internal electrolytic cell, which produces a spontaneous emf , from the oxidation of Pb and the reduction of Cu. This electromotive force enables the reaction to proceed until the current approaches zero, and the internal electrolytic cell will no longer function. Internal electrolysis can separate a small amount of metal ions that are easy to be reduced, but the disadvantage is that electrolysis is slow, so it is not widely used.

Electrodialysis

The ions or charged particles in liquid can migrate under the influence of electric field. Due to the different properties of ions rate The positive and negative charges move in different directions. When a DC voltage is applied to the two poles of the battery, the mixture of some organic matters can be separated. For example, this method is commonly used in clinical experiments protein Put the sample on a carrier. After applying an electric field, the charged particles move along the carrier to the electrode with opposite charge. They are separated due to their different moving speed. Generally, the serum protein can be divided into five parts. By improving the experimental technology, the width of the concentration spot can reach about 25 microns, and then electrodialysis can be carried out to serum albumin Divide into twenty clear parts.

analytical method

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Chemical analysis method based on electrochemical properties of solution. Electrochemical analysis was first introduced into the field of analysis by German chemist C. Winkler in the 19th century. Instrumental analysis began in 1922 Czech Republic Chemist J. Helovsky established Polarography 。 The foundation of electrochemical analysis method is that electrochemical reaction 。 The electrochemical cell is composed of electrolyte solution and two electrodes immersed in it External circuit Switch on. Occurs on two electrodes Redox reaction The electrons flow from one electrode to the other through an external circuit connecting two electrodes. Depending on the electrochemical properties of the solution (e.g electrode potential , current conductance , electricity, etc.) and the chemical or physical property (such as chemical composition, concentration Oxidized state And the ratio of the reduced state, etc.).

According to the initiative of the International Union of Pure and Applied Chemistry, electrochemical analysis methods can be divided into three categories: ① Neither double electric layers nor electrode reactions are involved, including Conductivity analysis 、 High frequency titration ② Double electric layer is involved, but not electrode reaction , e.g. by measuring surface tension Or not Faraday impedance The analytical method for determining the concentration. ③ When it comes to electrode reaction, it can be divided into two categories: one is that the electrolytic current is 0, such as potentiometric titration ； The other is that the electrolytic current is not equal to 0, including timing Potentiometry 、 Chronoamperometry 、 Anodic stripping method 、 Alternating current polarography 、 Single sweep polarography 、 Square wave polarography 、 Oscillopolarography 、 Coulometric analysis Etc.

Discipline Application

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Among many branches of physical chemistry, electrochemistry is the only discipline based on large industry. Its application can be divided into the following aspects: ① electrolysis Industry, among which chlor alkali industry is the basic inorganic industry next to synthetic ammonia and sulfuric acid Nylon 66 Middle of Monomer Adiponitrile It is synthesized by electrolysis; The smelting of light metals such as aluminum and sodium, and the refining of copper and zinc also use electrolysis; ② For machinery industry electroplate 、 Electropolishing , electrophoretic painting, etc. to complete the surface finishing of parts; ③ Environmental protection available Electrodialysis The method of removing cyanide ion, chromium ion and other pollutants; ④ Chemical power supply ；⑤ Corrosion prevention of metals, most of which are electrochemical corrosion; ⑥ Many life phenomena such as Muscle movement The information transmission of nerve involves electrochemical mechanism; ⑦ Various kinds of electrochemical principles Electrochemical analysis It has become an indispensable means of laboratory and industrial monitoring.

Electrochemical protection

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oil field Oil water separator It is an important equipment for separating gas (upper part), crude oil (middle part) and water (lower part, accounting for half of the liquid level of the separator). However, the internal structure of the separator is complex, and the average water content of the oil wells in the later stage of oil production and development is about 85%, so more than half of the bottom of the separator is in the separated sewage medium, corrosion The problem is very serious.

Previously, the internal wall of separator was generally Sacrificial anode cathodic protection Method, but separator The temperature of the inner wall is high, the composition of the water on the inner wall is complex, and the water quality may be acidic or alkaline. therefore anodic sacrifice protection The service life is short, and it is generally consumed within half a year. Sacrificial anode protection still exists Dead angle 。 On the other hand, sacrificial anode Cathodic protection potential It cannot be measured, the protection current cannot be adjusted, and the length of the protection period cannot be predicted.

The impressed current cathodic protection method for the inner wall of the oil-water separator. Mixed in titanium base tube metallic oxide The anode is an auxiliary anode fixed on the support at a certain height in the water phase at the bottom of the separator, which is made of silver/ Silver chloride reference electrode Or high-purity zinc reference electrode. The method has the advantages of convenient installation, long service life (up to more than 10 years), uniform protection potential and adjustable protection current output.

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