Crucible is a container or melting pot made of refractory materials (such as clay, quartz, porcelain clay or Refractory metals). Mainly used for evaporation, concentration, or crystallization of solutions, burning solid substances.
Crucible and its use
When a solid is heated by high fire, a crucible is required. When using a crucible, the lid is usually tilted on top of the crucible to prevent heated objects from jumping out and allow air to enter and exit freely for possible oxidation reactions. Due to the small bottom of the crucible, it is usually placed on a mud triangle for direct heating with fire. The crucible can be placed directly or obliquely on the Audio-Technica frame, and can be placed by itself according to the experimental requirements. After heating, the crucible should not be placed immediately on a cooled metal tabletop to avoid cracking due to rapid cooling. Do not immediately place it on a wooden tabletop to avoid scalding the tabletop or causing a fire. The correct method is to keep it on the Audio-Technica frame for natural cooling, or put it on the asbestos net for slow cooling. Please use Crucible tongs to take out the crucible.
1. Main purpose:
(1) The evaporation, concentration, or crystallization of a solution.
(2) Burning solid substances.
2. Precautions for use:
(1) It can be heated directly, and it can not be cooled suddenly after heating. Use Crucible tongs to remove it.
(2) Put the crucible on the Audio-Technica when heating.
(3) When evaporating, stir and use residual heat to evaporate.
3. Crucibles can be divided into three categories: graphite crucibles, clay crucibles, and metal crucibles.
Describe in detail the commonly used crucibles in the laboratory
Platinum crucible
Platinum, also known as platinum, is more expensive than gold and is often used due to its excellent performance. The melting point of platinum is as high as 1774 ℃, and its chemical property is stable. After burning in air, platinum will not undergo chemical changes, nor will it absorb water. Most chemical reagents have no Erosion on it.
1. Characteristics:
The resistance to corrosion by hydrofluoric acid and molten alkali carbonate is an important characteristic of platinum that differs from glass and porcelain. Therefore, it is commonly used for precipitation, combustion and weighing, hydrofluoric acid melting, and carbonate melting. Platinum has slight volatility at high temperatures and should be corrected after prolonged combustion. Platinum with a cm2 area loses about 1mg when burned at 1200 ℃ for 1 hour, and platinum is basically non volatile at 900 ℃.
2. The use of platinum vessels should comply with the following regulations:
(1) Strict regulations must be established for the collection, use, consumption, and recycling of platinum.
(2) Platinum is soft, even alloys containing a small amount of rhodium and iridium are soft, so do not use excessive force when removing platinum containers to avoid deformation. When removing the frit, do not scrape it from the platinum container with sharp objects such as Glass rod to avoid damaging the inner wall; Hot platinum containers should not be suddenly placed in cold water to avoid cracking. Deformed platinum crucibles or containers can be corrected using water molds with consistent shapes (but fragile platinum carbide parts should be uniformly and forcefully corrected).
(3) When heating, platinum containers should not come into contact with any other metal, as platinum is prone to forming alloys with other metals at high temperatures. Therefore, the platinum crucible needs to be placed on a platinum tripod or supported by materials such as ceramics, clay, quartz, etc. It can also be placed on an electric heating plate or electric furnace with asbestos board for heating, but cannot directly contact the iron plate or electric furnace wire. The Crucible tongs used shall be wrapped with platinum head, and nickel or stainless steel tongs can only be used at low temperature.
Golden crucible
Gold is cheaper than platinum and will not be eroded by alkaline metal hydroxide and hydrofluoric acid, so it is often used to replace platinum utensils. However, gold has a low melting point (1063 ℃), so it cannot withstand high temperature combustion and needs to be used below 700 ℃. Ammonium nitrate has obvious Erosion on gold, and aqua regia cannot contact gold utensils. The principle of using gold vessels is basically the same as that of platinum vessels.
Porcelain crucible
The porcelain used in the laboratory is actually glazed pottery, with a high melting point (1410 ℃) and resistance to high-temperature combustion. For example, a porcelain crucible can be heated to 1200 ℃, and its quality changes very little after firing. Therefore, it is commonly used for firing and weighing precipitates. High profile porcelain crucibles can handle samples while isolating air.
Precautions:
The thermal expansion coefficient of laboratory porcelain is (3-4) × 10-6. During high-temperature evaporation and combustion operations, thick walled porcelain should avoid sudden temperature changes and uneven heating to prevent cracking.
Porcelain has better stability to chemical reagents such as acid and alkali than glassware, but cannot come into contact with hydrofluoric acid. Porcelain crucibles are not resistant to the corrosion of caustic soda and sodium carbonate, especially they cannot melt.
Using substances not related to porcelain, such as MgO and C powder as fillers, and wrapping alkaline melt with quantitative filter paper in a porcelain crucible to treat silicate samples can partially replace platinum products. Porcelain utensils have strong mechanical properties, are cheap, and are widely used.
