The auxiliary electrode is also called the counter electrode. It is only used to pass current to achieve the polarization of the research electrode. When studying the cathode process, the auxiliary electrode is used as the anode, and when studying the anode process, the auxiliary electrode is used as the cathode. The area of the auxiliary electrode is generally larger than that of the research electrode, so that the current density on the auxiliary electrode is reduced, so that it is basically not polarized during the measurement process. Therefore, the platinum black electrode is often used as the auxiliary electrode, and it can also be used in the research medium. Metal materials that remain inert, such as Ag, Ni, W, Pb, etc.; in certain situations, certain electrodes are sometimes used. Sometimes, for convenience of measurement, the auxiliary electrode can also be made of the same metal as the research electrode.
Principle of auxiliary electrode
The role of auxiliary electrode is relatively simple. The auxiliary electrode is also called the counter electrode. It forms a series circuit with the research electrode set at a certain potential, so that the research electrode is smoothly connected to electricity, and is only used to pass current to realize the polarization of the research electrode. The reverse current of the research electrode should be able to pass through the auxiliary electrode smoothly, so it is generally required that the auxiliary electrode itself has low resistance and is not prone to polarization. The area of the auxiliary electrode is generally larger than that of the research electrode. In the general electrolysis reaction experiment of the potentiostat, it is hoped that the area of the auxiliary electrode is more than 100 times larger than the area of the research electrode. This reduces the current density on the auxiliary electrode and makes it In the measurement process, it is basically not polarized, so that the external polarization mainly acts on the research electrode, so the platinum black electrode is often used as the auxiliary electrode.
When studying the cathode process, the auxiliary electrode is used as the anode, and when studying the anode process, the auxiliary electrode is used as the cathode. The polarization current passes through the research electrode and also through the auxiliary electrode. When the cathode reduction current is passed through the research electrode, the electrode reaction of anodic oxidation is carried out on the auxiliary electrode. Conversely, when the anodic oxidation current passes through the research electrode, the electrode reaction of cathodic reduction will occur on the auxiliary electrode.
In order to avoid the contamination of the electrolyte solution near the research electrode by the electrode reaction occurring on the auxiliary electrode. Generally, there are two measures: ①Select inert electrode material as auxiliary electrode; ②Separate the research electrode from the auxiliary electrode. Pt is widely used as auxiliary electrode in laboratories, and it can be applied in acidic or alkaline solutions. In acidic solutions, especially in H2SO4 solutions, PbO2 electrodes can also be used as auxiliary electrodes. Ni electrode can be used as auxiliary electrode in alkaline solution. Although the above-mentioned electrode materials are used as auxiliary electrodes, hydrogen or oxygen evolution electrode reactions are still unavoidable. The precipitation of these gases will affect the convection state of the research system and the pH value of the solution, and they may also perform electrode reactions at the research electrode. Therefore, two chambers can be designed to place the research electrode and the auxiliary electrode in the electrolytic cell container, separated by sintered glass, diaphragm, gel salt bridge or piston, and pay attention to the connection of the ion conduction channel. The polarization current intensity through the auxiliary electrode and the research electrode are equal, but the current density is not necessarily equal. Therefore, when the auxiliary electrode area is relatively large, it not only ensures the uniformity of the research electrode power line distribution, but also reduces the cell pressure of the electrolytic cell.
The role of the auxiliary electrode
An important sign that determines the quality of the plating layer is the uniformity and integrity of the plating layer distribution on the part, which largely determines the protection of the plating layer Performance, dispersion ability and covering ability are commonly used in electroplating production to evaluate the uniformity and completeness of the coating distribution on the parts.
Practice has proved that even on a flat cathode with the same distance from the anode, the distribution of current density and coating thickness is not uniform. The thickness of the coating on the sharp corners and edges is obviously greater than the average thickness. The center of the flat cathode The thickness on the surface is significantly less than the average thickness. The deviation between the actual thickness and the average thickness is about 20% to 30%, and the deviation on the surface of parts with complex shapes can be as high as several times. Sometimes the edge or tip of the part is "scorched", but the deep concave part is not plated. The reason for this phenomenon is: ①During electroplating, due to the edge effect of the current, the power line is easy to be on the part. The sharp corners, edges and other places with large curvature are concentrated, and the current density is relatively large, so the thickness of the coating is greater than the average thickness; while in the middle of the surface, the curvature is relatively small, and the distribution of power lines is less, and the current density is also small, so that the thickness of the coating is Less than the average thickness.
②For complex parts with uneven or deep holes, in addition to the edge effect of the current, the distance between the two parts of the unevenness and the anode is different, so that the resistance between them is different, so the current is The distribution of the bumps is also different. If the bump is close to the cathode, the resistance is small, and the current density is high, the coating is thick; on the contrary, the coating at the recess is thin. Generally speaking, there is no requirement for the film thickness of the surface of the inner hole exceeding the depth of the hole diameter, and the surface of the inner hole exceeding the depth of twice the hole diameter is allowed to be partially unplated.
In order to improve the plating performance and deep plating performance, the measures that can be taken are:
⑴Improve the cathodic polarization of the plating solution.
⑵When the cathodic polarization of the plating solution is large, the conductivity of the plating solution is improved.
⑶The auxiliary electrode is used to improve the throwing performance and deep plating performance.
⑷After plating, mechanical processing (such as grinding) is used to ensure the uniformity of the plating thickness.
From the above analysis, once the solution formula is determined, the auxiliary electrode is indispensable in order to obtain a uniform and complete coating for the various specific parts.
Selection of auxiliary electrode
When analyzing alloy samples, the base metal of the analyzed alloy is often used as auxiliary electrode; aluminum alloy is pure aluminum; bronze and brass are copper; steel The analysis uses pure iron. The requirement for the auxiliary electrode is that it should not contain the element to be analyzed, but this requirement is not easy to meet, because it is impossible to obtain an absolutely pure auxiliary electrode. When the auxiliary electrode contains the element to be measured, the spectral line of this element will produce an additional intensity in addition to the expected intensity. This effect is when the analysis is low and the amount of the analysis element contained in the auxiliary electrode is high. , More significant, we must pay attention. The result of this effect is that the slope of the working curve is reduced, and the accuracy of the analysis is also reduced. In some cases, the low-content part of the working curve is curved. In Figure 5.3, 1 is the working curve of manganese in steel obtained by using carbon steel containing 0.5% manganese as the auxiliary electrode. The manganese content in the auxiliary electrode is in the middle of the analytical content range of 0.3 to 1.0%, which strongly affects the slope of the working curve. . 2 is the working curve using copper as the auxiliary electrode. Because steel does not contain manganese, the additional fixing strength of the manganese wire is not produced, and the slope of the working curve is improved. In order to select an auxiliary electrode that does not contain the measured element in the sample for steel analysis, in addition to pure iron, pure copper and graphite are commonly used as auxiliary electrodes.
The composition of the auxiliary electrode sometimes changes drastically The evaporation conditions in the light source. Figure 5.4 is the working curve of zinc in tin bronze samples made with different auxiliary electrodes. Ⅰ-Copper is used as auxiliary electrode; Ⅱ-Both upper and lower electrodes are samples; Ⅲ-Graphite is used as auxiliary electrode; Ⅳ-Aluminum is used as auxiliary electrode. Auxiliary electrode. It can be seen from Figure 5.4 that the strength of the zinc wire is significantly different when different auxiliary electrodes are used. Therefore, for different analysis tasks, suitable auxiliary electrodes should be selected, which can be selected through experiments. Many jobs use high-purity carbon or graphite as auxiliary electrodes. Graphite is a form of carbon, so graphite electrode is also a kind of carbon electrode. Since it is easier to make carbon electrodes to make high-purity graphite electrodes, graphite electrodes are generally used in spectral analysis. Spectroscopy workers sometimes call graphite electrodes as carbon electrodes, and the distinction is not very strict. Under the action of the light source, the carbon or graphite electrode can promote the formation of a reducing atmosphere near the surface of the sample to be analyzed, which significantly reduces the possibility of oxide layer formation on the surface of the sample, and enlarges the arcing spots on the surface of the sample. This allows the substance to evaporate more uniformly, thereby improving the accuracy of the analysis.
When doing spectroscopic analysis of steel, spectroscopy workers in my country are accustomed to using pure copper or pure graphite as auxiliary electrodes, as mentioned above. However, when using a vacuum direct reading spectrometer to analyze carbon, phosphorus, sulfur and other alloying elements in steel, the sample is excited in an argon atmosphere, and a pure silver rod with a diameter of 3 mm is used as an auxiliary electrode, and the end is ground to 120. Cone. Since this auxiliary electrode is excited in argon gas and a unidirectional discharge light source is used, the silver electrode is rarely consumed and does not require frequent grinding (about 100 to 150 samples are analyzed before grinding is required), only in analysis After several samples are wiped with a fine brush. Non-ferrous metals and their alloys use pure metals, carbon or graphite as auxiliary electrodes. According to different specific requirements, one of them can be used as auxiliary electrodes. The tip of the auxiliary electrode is generally ground into a hemispherical shape, or a conical shape with a cross-section (see Figure 5.2).
Application of auxiliary electrode
The auxiliary electrode and the working electrode form a current loop. If the auxiliary electrode is polarized by the external current, the cell pressure will fluctuate, and it will be difficult to control the current and potential of the working electrode. Therefore, the auxiliary electrode should be made of non-polarized or difficult-to-polarize materials such as platinum, graphite and other inert materials. For some systems that do not require high measurement accuracy, nickel electrodes and lead electrodes can also be used, because the passivation film they form in some media is relatively stable. The general requirements for auxiliary electrodes are as follows.
①The area should be large, so that the current density can be reduced, thereby reducing the polarization of the auxiliary electrode, and the power lines can be more evenly distributed.
②In order to prevent the mutual contamination of the reaction products on the auxiliary electrode and the working electrode, it is sometimes necessary to use a diaphragm such as sintered glass, ion exchange membrane, ground glass piston and other measures to separate the auxiliary electrode chamber from the working electrode chamber , The most common is to use a three-electrode chamber or a double-electrode chamber, see Figure 7-88.