Electrodes

Content
1. What is electrodes?
2. What is electrodes used for?
3. Types of electrodes
4. How to choose electrodes?
5. How do clean electrodes?
6. How to order electrodes?

Electrodes are a kind of electronic or electrical device, a part of the equipment, used as a conductive medium (solid, gas, vacuum, or electrolyte solution) in the input or export current of the two ends. One pole of the input current is called the anode or positive pole, and one pole of the discharge current is called the cathode or negative pole. There are various types of electrodes, such as cathodes, anodes, welding electrodes, furnace electrodes, etc.
In a battery, the electrode generally refers to the location where the redox reaction occurs with the electrolyte solution. There are positive and negative electrodes, generally, the positive electrode is the cathode, which gains electrons and undergoes a reduction reaction, and the negative electrode is the anode, which loses electrons and undergoes an oxidation reaction. The electrode can be a metal or a non-metal, as long as it can exchange electrons with the electrolyte solution, it becomes an electrode.

Laboratory measurements are generally done by conventional composite PH electrodes. 
The following conditions are usually required. 
The measurement range is between PH2 and PH12, temperature between 10°C and 50°C, and ion concentration between 0.5 and 4 mol/L. Furthermore, the measured material is a homogeneous aqueous solution that has been buffered. 

Samples with ionic concentrations of a few mmol or less are low ionic. Such samples will have very poor electrical conductivity. The liquid contact section in a low ionic sample will produce an increased resistance. This transfer resistance can lead to contact problems between the reference electrolyte and the measured solution, which in turn can lead to diffusion potentials. Moreover, the signal can be affected by stirring.
The above problem can be solved by using an electrode with an annular grounded liquid contact, which achieves contact between the reference electrolyte and the measured solution.

The electrode in anhydrous solutions (less than 5% water) is only possible to measure the relative pH value. Semi-anhydrous solutions are basically also of the low ionic type. If the sample contains more than 5% water, the traditional definition of PH determination is applied, i.e., the value obtained is a value and not a relative value.
The contact area between the electrolyte and the solution under test (liquid interface) is usually separated by phases, which causes signal instability. Moreover, there is a possibility of precipitation in the liquid contact area. This can easily happen when a saturated KCL solution is used as a reference solution.
To prevent this phenomenon to the maximum extent possible, it is necessary to ensure the fluidity and compatibility between the electrolyte and the sample under test.
The use of ethanol containing LiCL or acetic acid containing LiCL as reference electrolyte for anhydrous samples can solve the appeal problem.

When a high concentration of protein comes in contact with the KCL reference electrolyte, it may precipitate at the liquid junction. The use of an electrolyte can resolve this conflict. The protein-contaminated junction can generally be cleaned by soaking the electrode in a mixture of pepsin and HCL for several hours.

When an electrode is used with the Ag/AgCL reference system, the reference electrolyte always contains dissolved silver. When the solution silver in the liquid junction comes in contact with the sulfide in the measurement solution, insoluble silver sulfide is produced. The silver sulfide blocks the liquid junction and turns it black, which results in a slow and unstable measurement signal.
In the case of a reference system with a silver ion trap, an AgCL-free electrolyte can be used. Sulfide-contaminated joints can sometimes be removed by soaking the electrode in a sulfur-based urea/HCL solution.

Hydrofluoric acid damages the glass film and also prevents the formation of a gel layer at low concentrations. This leads to unstable measurement values and shortens the lifetime of the electrode. Hydrofluoric acid has a damaging effect only at pH values below 5.
When the total fluoride ion concentration is between 0.2 g/L (PH3; 20°C) and 1 g/L (PH1; 20°C). There are electrodes suitable for this type of application. When the fluorine ion concentration is higher antimony (Sb) electrode and reference electrode must be used.

Hydrofluoric acid damages the glass film and also prevents the formation of a gel layer at low concentrations. This leads to unstable measurement values and shortens the lifetime of the electrode. Hydrofluoric acid has a damaging effect only at pH values below 5.
When the total fluoride ion concentration is between 0.2 g/L (PH3; 20°C) and 1 g/L (PH1; 20°C). There are electrodes suitable for this type of application. When the fluorine ion concentration is higher antimony (Sb) electrode and reference electrode must be used.

The contact of any metal with an electrolyte generates an electric potential (potential), which is the most characteristic property of an electrode. If there is a rapid exchange of electrons in a single redox pair at the electrode interface, i.e., there is a single electrode reaction with a large exchange current (see migration over superpotential), this electrode can quickly establish electrochemical equilibrium and is called a reversible electrode. The potential of a reversible electrode remains stable for a longer period of time, is more resistant to interference, and can be measured accurately. It is a necessary part of a reversible cell, the core component of a potentiometric measurement device, and has important practical significance. There are the following types of reversible electrodes.
Metal electrodes, which are characterized by redox pairs that can migrate across the phase interface.
Redox electrodes, such as Pt|Fe, Fe electrode
Pt|Mn,MnOþo, H electrode, etc. Its redox pair can not move across the electrode phase interface, the Pt of the electrode only means that the electrode metal is inert, it only provides the site of electron exchange, any inert metal can be used in practical application. ③ Gas electrode, is a redox electrode when one of the components of the redox pair is a gas, such as hydrogen electrode (Pt|H2|H), chlorine electrode (Pt|Cl2|Cl), etc. In order to accelerate the equilibrium, the platinum metal needs to be coated with platinum black to increase the surface area and to act as an electrocatalyst.

Spark machine electrodes, also known as copper males, are also indispensable in spark machine processing.
In spark machine processing, the electrode and the workpiece are connected to the two poles of the pulse power supply. The pulse voltage applied to the electrode and the workpiece produces a spark discharge. The instantaneous temperature of the discharge can be as high as 10,000 degrees Celsius or more, and the high temperature causes localized vaporization or melting of the workpiece surface.
The next pulse voltage is followed by a spark discharge between the electrode and the workpiece, and the process is repeated.
By repeating the pulse discharge countless times, the shape corresponding to the electrode shape is finally processed. Therefore, by changing the shape of the electrode, a variety of complex shapes can be machined.
In spark machining, the role of the electrode is to deliver the processing pulse, and to etch the workpiece with the minimum loss of the electrode itself. Commonly used electrode materials are copper, graphite, copper-tungsten alloy, silver-tungsten alloy, steel, brass, cast iron, etc.

The shell of pH electrode generally adopts PC plastic (polycarbonate) shell and glass shell, PC shell is resistant to collision and impact, but the applicable temperature is <80oC. It is also easy to be corroded in high alkali solution and some media. The glass shell is suitable for temperature 0-150oC, except for hydrofluoric acid solution, it is generally not corrosive, but easy to be damaged by collision.
Conventionally, Ag/Agcl reference electrodes are used, but general Ag/Agcl reference electrodes are easy to dissolve at high temperatures and have unstable potentials, while Ag/Agcl reference electrodes with capillary structure have very stable reference potentials and are suitable for use under high temperature and long-term continuous testing conditions.

ORP electrode sensitive element selection mainly considers the nature of the measured medium, general electrode is used for oxidizing media, such as oxidation of lv compounds, oxidation of nitrite, ozone measurement, and hydroperoxide measurement. A platinum electrode is used for reducing media, such as chromate reduction, swimming pool chlorination, etc.

Metal ion adhesion: Dip the measuring end of the electrode into 0.5mol/l hydrochloric acid solution for 5 minutes, shake the electrode quickly several times (stirring) before taking out, rinse with deionized water after taking out, and soak in 3.0mol/l potassium chloride solution for 4 hours.

Organic contamination: Dip the measuring end of the electrode into anhydrous ethanol (or solvent that can dissolve the organic substance) for 15 minutes, shake the electrode rapidly several times (stirring) before taking out, rinse with deionized water after taking out and soak in 3.0mol/l potassium chloride solution for 4 hours.
Note: The electrode shell material is polycarbonate, the measured sample or cleaning solution containing organic matter may corrode the electrode shell.

Inorganic cleaning: the measuring end of electrode is immersed in 0.1mol/l EDTA solution or 0.1mol/l hydrochloric acid solution for 15 minutes, the electrode should be shaken rapidly several times (stirring) before taking out, rinsed with deionized water after taking out and immersed in 3.0mol/l potassium chloride solution for 4 hours.

Protein precipitation: The measuring end of the electrode is immersed in 1% pepsin hydrochloric acid solution (the concentration of hydrochloric acid is 0.1mol/l) for 15 minutes, and the electrode should be shaken rapidly several times (stirring) before taking out, rinsed with deionized water after taking out, and immersed in 3.0mol/l potassium chloride solution for 4 hours.

Oil and grease adhesion: Dip the measuring end of electrode in weak alkaline detergent for 15 minutes, shake the electrode quickly several times (stirring) before taking out, rinse with deionized water after taking out, soak in 3.0mol/l potassium chloride solution for 4 hours.

Regeneration of glass sensitive film: the measuring end of electrode is immersed in 4% hydrofluoric acid solution for (3-5) seconds, then rinsed with 1:1 hydrochloric acid solution for 10 seconds, washed with distilled water and stored in 3.0 mol/l potassium chloride solution for 24 hours.

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