Crucible type

  1. Platinum crucible Platinum is often used because of its many excellent properties. The melting point of platinum is as high as 1774°C and its chemical properties are stable. It does not undergo chemical changes after burning in the air and does not absorb moisture. Most chemical reagents are resistant to it. No corrosive effect.

(1) Characteristics: The ability to resist the corrosion of hydrofluoric acid and molten alkali metal carbonates is an important property of platinum that is different from glass, porcelain, etc., so it is often used for precipitation burning weighing, hydrofluoric acid melting, and Melting treatment of carbonate. Platinum is slightly volatile at high temperatures and needs to be corrected after a long burning time. Platinum with an area of 100cm2 loses about 1 mg when burned at 1200°C for 1 hour. Platinum is basically non-volatile below 900°C.

(2) The use of platinum vessels should comply with the following rules:

① Strict systems must be formulated for the collection, use, consumption and recycling of platinum.

② Platinum is soft in texture, and even alloys containing a small amount of rhodium and iridium are relatively soft, so do not use too much force when picking up platinum containers to avoid deformation. When removing the frit, do not use sharp objects such as glass rods to scrape them from platinum containers. To avoid damaging the inner wall; do not put hot platinum vessels into cold water suddenly to avoid cracks. Deformed platinum crucibles or vessels can be corrected with a water mold that matches their shape (but the brittle platinum carbide part must be applied evenly with force correction).

③ When heating, the platinum vessel cannot come into contact with any other metal, because platinum easily forms alloys with other metals at high temperatures. Therefore, the platinum crucible must be placed on a platinum tripod or on a support made of ceramic, clay, quartz and other materials. For burning, it can also be heated on an electric hot plate or electric furnace with an asbestos plate, but it cannot be in direct contact with the iron plate or electric furnace wire. The crucible pliers used should be covered with platinum tips, and nickel or stainless steel pliers can only be used at low temperatures. be usable.

(3) Cleaning method of platinum vessels: If the platinum vessels have spots, they can be treated with hydrochloric acid or nitric acid alone. If it is ineffective, potassium pyrosulfate can be melted in the platinum vessels at a lower temperature for 5 to 10 minutes, and the melted material can be poured out. After removing it, soak the platinum vessel in hydrochloric acid solution. If it still doesn’t work, you can try melting it with sodium carbonate or gently rubbing it with moist fine sand (passing a 100 mesh sieve (0.14 mm mesh)).

  1. Gold crucible Gold is not corroded by alkali metal hydroxides and hydrofluoric acid, so it is often used to replace platinum vessels. However, the melting point of gold is low (1063°C), so it cannot withstand high-temperature burning. It must generally be lower than 700°C. Usage. Ammonium nitrate has a significant corrosive effect on gold, and aqua regia cannot come into contact with gold vessels. The principles for using gold vessels are basically the same as those for platinum vessels.
  2. Polytetrafluoroethylene crucible 1. Characteristics Polytetrafluoroethylene is a thermoplastic with white color and waxy feel. It has stable chemical properties, good heat resistance and good mechanical strength. The maximum working temperature can reach 250℃. Generally at 200 When used below ℃, it can replace platinum vessels for handling hydrofluoric acid. In addition to molten sodium and liquid fluorine, it can withstand the corrosion of all concentrated acids, concentrated alkali and strong oxidants. It will not change when boiled in aqua regia. It has excellent corrosion resistance. It can be called the “King” of plastics. PTFE crucibles with stainless steel covers are used in pressurized and heated mineral samples and digestion of biological materials. PTFE has good electrical insulation properties and can be cut. 2. Special Note that it decomposes rapidly above 415°C and releases toxic perfluoroisobutylene gas.
  3. Main components of porcelain crucible: alumina (45-55%), silica. It is a porcelain vessel used in laboratories. It is actually glazed pottery. It can be used with a heat resistance of about 1200 degrees and is suitable for melting samples of acidic substances such as K2S2O7. Generally, it cannot be used for melting with alkaline substances such as NaOH, Na2O2, Na2CO3, etc. to avoid corrosion of the porcelain crucible. Porcelain crucibles cannot come into contact with hydrofluoric acid. Porcelain crucibles can generally be washed by boiling with dilute hydrochloric acid.
  4. Graphite crucible Although there are more and more users of graphite crucible, you need to pay attention to the following points for its maintenance. 1. The specification number of the crucible is the capacity of copper (#/kg) 2. The graphite crucible should be strictly protected from moisture and needs to be placed In a dry place or on a wooden frame. 3. Handle it with care when transporting, and do not drop or shake it. 4. Before use, it needs to be heated in a drying equipment or furnace, and the temperature gradually rises to 500°C. 5. The crucible should be placed in the furnace Below the level of the mouth, prevent the furnace cover from wearing the upper mouth of the crucible. 6. When adding materials, the amount of dissolved material in the crucible should be adjusted. Do not add too much material, and prevent the crucible from shrinking if it is too tight. 7. The furnace lid and the crucible clamp should conform to the shape of the crucible, and the clamp should prevent the middle part from being damaged. The crucible may be damaged by force. 8. When removing the slag and coke on the inner and outer walls of the crucible, tap lightly to prevent damage to the crucible. 9. A suitable distance should be maintained between the crucible and the furnace wall, and the crucible should be placed in the center of the furnace. 10. Use an appropriate amount of combustion aid Agents and additives will shorten the service life of the crucible. 11. During use, turn the crucible once a week to extend the service life of the crucible. 12. Prevent strong corrosion flames from directly spraying on the sides and bottom of the crucible. 06. Corundum crucible A crucible with an alumina content of more than 95% is called a corundum crucible. Corundum crucible is composed of porous molten alumina, which is strong and resistant to melting. Corundum crucible is suitable for melting samples using some weakly alkaline substances such as anhydrous Na2C03 as flux, but is not suitable for melting samples using strong alkaline substances such as Na 202, NaOH and acidic substances (such as K2S 207, etc.) as flux.
  5. Stone jade crucible Transparent quartz glass is made of natural colorless and transparent crystal smelted at high temperature. Translucent quartz is made of natural pure vein quartz or quartz sand, because it contains many bubbles that are not drained during smelting. It is translucent. The physical and chemical properties of transparent quartz glass are better than that of translucent quartz, and it is mainly used to manufacture laboratory glass instruments and optical instruments. The thermal expansion coefficient of quartz glass is very small (5.5×10-7) and is only used for extremely hard glass. One-fifth of glass. Therefore, it can withstand rapid heat and rapid cooling. After the transparent quartz glass is burned to red heat, it will not burst when placed in cold water. The softening temperature of quartz glass is 1650°C and has high

Crucible type

7.temperature resistance. Quartz crucible It is often used for the melting of acidic fluxes and sodium thiosulfate. The operating temperature should not exceed 1100°C. Its acid resistance is very good. Except for hydrofluoric acid and phosphoric acid, acids of any concentration rarely interact with quartz glass even at high temperatures. Quartz glass is not resistant to hydrofluoric acid corrosion, and phosphoric acid can also act on it above 150°C. Strong alkali solutions including alkali metal carbonates can also corrode quartz, but the corrosion is slower at room temperature and accelerates when the temperature rises. Special care must be taken when using quartz glass instruments. They are usually stored separately from glass instruments and properly kept.

8.Silver crucible Silver is cheaper than gold, and it is not corroded by potassium hydroxide or sodium. In the molten state, it only works slightly at the edge close to the air. However, the melting point of silver is 960°C and cannot be heated directly on the fire. After the silver is heated, a layer of silver oxide is formed on the surface. Silver oxide is unstable at high temperatures and is stable below 200°C. Silver easily reacts with sulfur to form silver sulfide. Do not decompose or burn sulfur-containing substances in the silver crucible, and do not use alkaline sulfide flux. Metal salts such as aluminum, zinc, tin, lead, and mercury can make silver crucibles brittle in the molten state. Silver crucibles cannot be used to melt borax, and acids cannot be used when leaching the melt, especially concentrated acids. The quality of the silver crucible will change after burning, so it is not suitable for weighing precipitation.

9.Silicon carbide crucible The silicon carbide crucible is a ceramic deep-bottomed bowl-shaped container. When a solid is to be heated over a high fire, a crucible must be used. Because it can withstand high temperatures better than glassware. When the crucible is used, the molten material is usually not filled too full to prevent the heated object from jumping out and to allow air to freely enter and exit for possible oxidation reactions. Because the bottom of the crucible is very small, it generally needs to be placed on a clay triangle to be directly heated by fire. The crucible can be placed upright or tilted on the iron tripod. It can be placed by yourself according to the needs of the experiment. Do not place the crucible on a cold metal table immediately after heating to avoid cracking due to rapid cooling. Do not place it directly on a wooden tabletop to avoid burning the tabletop or causing a fire. The correct method is to leave it on an iron tripod to cool naturally, or place it on an asbestos net and let it cool slowly.

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