Type of electrode:
Working electrode: silver-based mercury film electrode. Silver-based suspended mercury electrode. Silver-based disc. Gold-based disc. Glassy carbon electrode. Drop mercury electrode. (Glass capillary)
Reference electrode: 218 silver silver chloride electrode. 217 saturated potassium chloride double salt bridge calomel electrode auxiliary electrode: platinum auxiliary. Glassy carbon auxiliary
Mercury electrode
dropping mercury electrode; DME
A special electrode commonly used in polarography. It is mercury flowing from the lower end of a vertical glass capillary with an outer diameter of 3 to 7mm and an inner diameter of 0.04 to 0.08mm, and forms a drop of mercury electrode (each mercury drop continuously from small to large, when the diameter is about 0.5 to 1.0mm At the time, dripping due to gravity). The height of the mercury storage bottle can be adjusted or the dripping time of the mercury drop can be controlled by a mechanical method (percussion device).
Mercury dropping electrode is used as the indicating electrode of polarography method, and it is often used as the cathode. It is a polarizing electrode, which produces concentration polarization on the surface during electrolysis. The advantage is that the electrode surface is constantly updated and the reproducibility is good; many metals can form amalgam with mercury, and their ions are reversibly reduced on the mercury electrode; mercury is easy to purify; the overpotential of hydrogen on mercury is relatively high, making Polarographic determination may be carried out in a slightly acidic solution. The main disadvantages are: the use of potential range can not be greater than + 0.4V, mercury must be oxidized; the generated capacitive current limits the sensitivity of DC polarography; mercury is toxic.
A glass capillary with an internal diameter of about 50-80 μm is connected to the mercury storage bottle, and the height of the mercury storage bottle is adjusted so that the mercury in the capillary drops continuously at the end of the capillary under the pressure of the mercury column. This is a mercury drop electrode. Interfacial tension exists at any two interfaces, and the electrode / solution interface is no exception. But for the electrode system, the interfacial tension is not only related to the material composition of the interface layer, but also related to the electrode potential. This phenomenon of interfacial tension changing with electrode potential is called electrocapillary phenomenon. The relationship between interfacial tension and electrode potential is called the electric capillary curve.
Electrode treatment:
Mercury-dropping electrode: classic polarographic capillary (period 3-7 seconds), single-sweep polarography with long-period mercury-dropping electrode (more than 10-20 seconds), the mercury drop of the mercury-dropping electrode is constantly updated, and basically takes away when it falls The diffusion layer on the surface of the mercury drop causes new mercury drops to grow in the unconsumed fresh solution. Good reproducibility. Mercury has a high hydrogen overpotential, so it can measure dozens of inorganic ions and more organic compounds in a wider pH range and negative voltage range. Mercury can form amalgam with various metals, and the electrode reaction is reversible Good, good chemical stability, non-inert electrode in many media, mercury droplet size and period adjustment are convenient. However, the capacitance current is large, the lower limit of classical polarography detection is 10-6M, and single-sweep polarography can increase 1-2 quantitative poles. The dissolution potential of mercury is +0.25 volts, which makes the available positive voltage range small and polarized. The large amount of mercury used and improper treatment will cause mercury damage. The proper use of capillaries requires attention to the following points:(1). The end surface of the second habit should be flat, smooth, and must not be inclined, so as to avoid irregular mercury drop and cause disorder.
(2). The capillary should be installed vertically, and can not only rely on the wall of the electrolytic cell, adjust the height of the mercury storage bottle, the period of mercury drop in distilled water meets the requirements, (classical polarography 3-7 seconds / drop, single sweep polarography 10 -20 drops or more per second)
(3). The inner diameter of the capillary tube is very thin, and it is very easy to block. The capillary tube should be kept unblocked. The container, catheter, capillary tube, mercury and distilled water should be clean, and even small unclean things can block it.
Capillary installation process: (for example, long-period mercury drop capillary)
(1) First immerse the mercury storage bottle and the catheter with 1: 1 nitric acid for several days, then wash and dry. (2) Filter the mercury several times before use (remove the oxide layer) (3) Dip the absolute ethanol with cotton ball Wring the rod and the dust on both ends that are used to wiping seconds (if alcohol is sucked into the capillary, use a hair dryer to blow and dry the alcohol in the tube in time) (4) Connect the mercury storage bottle to the end of the catheter, And install it on the fixed frame, then clamp one end of the catheter under the fixed table (the port is facing the sky, must be clamped through the mouth of the mercury storage bottle, there must be no water in the bottle and the catheter), and then slowly poured the filtered mercury into In the mercury storage bottle, exhaust the air in the catheter, insert the capillary, fasten the end of the capillary and the catheter, and then turn the capillary head and the catheter upside down at once to let mercury slowly flow out of the capillary to form a mercury ball. Mercury drops flow on one end of the capillary. If it is temporarily unavailable, you can first put the mercury storage bottle short and fasten it. Hold the plastic tube in one hand, hold the capillary in one hand, and slap the table in the air (patting the hand holding the capillary to expel the air from the capillary, or squeeze the plastic tube to escape the air) Be careful not to squeeze the plastic, and mercury will flow out in large quantities , Causing mercury damage). (5) If the capillary is slightly blocked, there are two treatment methods: Immerse the tip of the capillary in 1: 1 nitric acid for several minutes, discharge it under aeration, inhale and distill it, and dry it several times. Hydration. Can be directly ventilated and pressurized to blow out fine dust in the capillary(6). In use, the mercury storage bottle should be sealed with water to prevent oxidation.
(7). Insert the platinum electrode into the mercury storage bottle, and then connect the wire to the polarograph
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