How to use different crucibles in the laboratory?

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A crucible is a vessel or melting pot made of an extremely refractory material such as clay, quartz, china clay, or a more refractory metal. It is mainly used for evaporation, concentration or crystallization of solutions, and burning of solid substances. So, how to use different crucibles?

What should we use the crucible for?

When there are solids to be heated with high fire, crucibles must be used. When the crucible is used, the crucible cover is usually placed obliquely on the crucible 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 mud triangle to be directly heated by fire. The crucible can be placed upright or obliquely on the iron tripod, and it can be placed by itself according to the needs of the experiment.

Do not place the crucible on a cold metal table immediately after heating to avoid it from cracking due to rapid cooling, and also do not place it on a wooden tabletop immediately to avoid scalding the tabletop or causing a fire. The correct way is to leave it on an iron tripod to cool naturally, or put it on an asbestos net to let it cool slowly. Please use crucible tongs to take out the crucible.

Main purpose:

(1) Evaporation, concentration or crystallization of the solution;

(2) burning solid matter.

Precautions for use:

(1) It can be directly heated, and cannot be quenched after heating, and it can be removed with crucible pliers;

(2) Put the crucible on the iron tripod when it is heated;

(3) Stir when evaporating; evaporate to dryness with residual heat when nearly evaporating.

Crucibles can be divided into three categories: graphite crucibles, clay crucibles and metal crucibles.

Let’s talk about the crucibles commonly used in the laboratory

Platinum crucible

Platinum, is more expensive than gold, and 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 being burned in the air, nor does it absorb moisture, and most chemical reagents have no erosive effect on it.

A. Characteristic:

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

B. The use of platinum utensils should comply with the following rules:

(1) A strict system 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 too much force when holding platinum utensils to avoid deformation. When demelting, do not use sharp objects such as glass rods to scrape from the platinum vessel to avoid damage to the inner wall; nor can you suddenly put the hot platinum vessel into cold water to avoid cracks. The deformed platinum crucible or vessel can be corrected with a water mold that matches its shape (but the brittle platinum carbide part should be corrected evenly).

(3) Platinum utensils cannot be in contact with any other metals when heating, because platinum is easy to form alloys with other metals at high temperatures, so the platinum crucible must be burned on a platinum tripod or on a support such as ceramics, clay, quartz, etc., it can also be heated on an electric heating plate or electric furnace with an asbestos board, but it cannot be in direct contact with the iron plate or electric furnace wire. The crucible pliers used should have platinum tips. Nickel or stainless steel pliers should only be used at low temperatures.

C. Cleaning method of platinum utensils:

If the platinum utensils have spots, they can be treated with hydrochloric acid or nitric acid alone. If it is invalid, potassium pyrosulfate can be melted in a platinum container at a lower temperature for 5-10 minutes, and the molten material is poured out, and then the platinum container is soaked in hydrochloric acid solution. If it is still ineffective, you can try sodium carbonate melting treatment again, or you can use wet fine sand (passing through a 100 mesh sieve, that is, a 0.14 mm sieve) to gently rub it.

Gold crucible

The price of gold is cheaper than platinum, and it is not corroded by alkali metal hydroxides and hydrofluoric acid, so it is often used to replace platinum utensils, but the melting point of gold is low (1063°C), so it cannot withstand high temperature burning, and generally must be used below 700°C. Ammonium nitrate has a significant corrosive effect on gold, and aqua regia should not be in contact with gold utensils. The principle of using gold utensils is basically the same as that of platinum utensils.

Silver crucible

A. Characteristic

Silverware is relatively inexpensive, and is not attacked by potassium hydroxide (sodium), and is only slightly eroded at the edge close to the air in the molten state. The melting point of silver is 960°C, and the use temperature is generally not higher than 750°C, and it cannot be directly heated on fire. After heating, a layer of silver oxide will be formed on the surface, which is unstable at high temperature, but stable below 200°C. The silver crucible that has just been taken out of high temperature must not be cooled with cold water immediately to prevent cracks.

Silver is easy to interact with sulfur to form silver sulfide, so it is not allowed to decompose and burn sulfur-containing substances in a silver crucible, and alkaline vulcanizing reagents are not allowed. Metal salts such as aluminum, zinc, tin, lead, mercury, etc. in the molten state can make silver crucibles brittle. Silver crucibles are not used for melting borax. When using sodium peroxide flux, it should only be sintered, not melted.

B. Extraction and washing

Do not use acid, especially concentrated acid, when leaching the melt. When cleaning silver utensils, slightly boiling dilute hydrochloric acid (1+5) can be used, but it is not advisable to heat the utensils in acid for a long time. The quality of the silver crucible will change after burning, so it is not suitable for the weighing of precipitation.

Nickel crucible

The melting point of nickel is 1450°C, and it is easily oxidized when burned in air, so the nickel crucible cannot be used for burning and weighing precipitation.

Nickel has good corrosion resistance to alkaline substances, so it is mainly used in the melting treatment of alkaline flux in the laboratory.

A. Temperature control

Alkaline fluxes such as sodium hydroxide and sodium carbonate can be melted in a nickel crucible, and the melting temperature generally does not exceed 700°C. Sodium oxide can also be melted in a nickel crucible, but the temperature should be lower than 500°C and the time should be short, otherwise the corrosion will be severe, which will increase the content of nickel salt brought into the solution and become an impurity in the measurement.

B. Pay attention

Acidic solvents such as potassium pyrosulfate and potassium bisulfate and solvents containing sulfide cannot be used in nickel crucibles. When melting sulfur-containing compounds, it should be carried out in an oxidizing environment with excess sodium peroxide. Metal salts such as aluminum, zinc, tin, and lead in the molten state can make nickel crucibles brittle. Compounds of silver, mercury, vanadium and borax cannot be burned in nickel crucibles. The new nickel crucible should be burned at 700°C for several minutes before use to remove oil and form an oxide film on the surface to prolong the service life. The treated crucible should be dark green or gray black. In the future, boil and wash with water before each use. If necessary, add a small amount of hydrochloric acid to boil for a while, then wash with distilled water and dry for use.

Iron crucible

The use of iron crucible is similar to that of nickel crucible. It is not as durable as nickel crucible, but it is cheaper. It is more suitable for sodium peroxide melting and can replace nickel crucible.

Iron crucibles or low-silicon steel crucibles should be passivated before use, first soaked in dilute hydrochloric acid, then lightly rubbed with fine sandpaper, and rinsed with hot water. Then soak it in a mixed solution of 5% sulfuric acid + 1% nitric acid for several minutes, then wash it with water, dry it, and burn it at 300~400°C for 10 minutes.

PTFE crucible

A. Characteristic

PTFE is a thermoplastic, white in color, waxy feeling, stable chemical properties, good heat resistance, good mechanical strength, the maximum working temperature can reach 250 ℃.

Generally used below 200°C, it can replace platinum vessels for treating hydrofluoric acid.

In addition to molten sodium and liquid fluorine, it can resist the corrosion of all concentrated acids, concentrated alkalis and strong oxidants, and it will not change when boiled in aqua regia. It can be called the “king” of plastics in terms of corrosion resistance.

PTFE crucibles with stainless steel covers are used in pressurized heat treatment of ore samples and digestion of biological materials. PTFE has good electrical insulation properties and can be machined.

B. Pay attention

However, it decomposes rapidly above 415°C and emits toxic perfluoroisobutene gas.

PTFE cruciblesStainless steel digestion canister

Porcelain crucible

The porcelain utensils used in the laboratory are actually glazed pottery. It has a high melting point (1410°C) and can withstand high temperature burning. For example, a porcelain crucible can be heated to 1200°C, and its mass changes little after burning, so it is often used for burning and weighing precipitation. Tall porcelain crucibles can process samples under air-isolated conditions.

Precautions:

The thermal expansion coefficient of porcelain utensils used in the laboratory is (3~4)×10-6

During the high-temperature evaporation and burning operations of thick-walled porcelain vessels, sudden changes in temperature and uneven heating should be avoided to prevent cracking. Porcelain utensils are more stable than glass utensils to chemical reagents such as acid and alkali, but they cannot be in contact with hydrofluoric acid. Porcelain crucibles are not resistant to caustic alkali and sodium carbonate corrosion, especially they cannot be melted.

Use some substances that do not interact with porcelain such as MgO and C powder as fillers, and use quantitative filter paper to wrap alkaline flux in the porcelain crucible to melt the silicate sample, which can partially replace platinum products. Porcelain ware has strong mechanical properties and is cheap, so it is widely used.

Corundum Crucible

Natural corundum is almost pure alumina. Artificial corundum is made of pure alumina sintered at high temperature. It is resistant to high temperature, has a melting point of 2045°C, has high hardness, and has considerable corrosion resistance to acid and alkali.

Precautions:

Corundum crucibles can be used for melting and sintering of certain alkaline fluxes, but the temperature should not be too high, and the time should be as short as possible. In some cases, it can replace nickel and platinum crucibles, but it cannot be used when measuring aluminum and aluminum interferes with the measurement.

Quartz crucible

Transparent quartz glass is smelted from natural colorless and transparent crystal at high temperature. Translucent quartz is made of natural pure vein quartz or quartz sand, which is translucent because it contains many air bubbles that are not exhausted during smelting. The physical and chemical properties of transparent quartz glass are superior to those of translucent quartz, and are mainly used in the manufacture of laboratory glass instruments and optical instruments.

The coefficient of thermal expansion of quartz glass is very small (5.5×10-7), which is only one-fifth of that of extra-hard glass.

Therefore, it can withstand rapid heat and rapid cooling. After burning transparent quartz glass to red heat, it will not burst when placed in cold water.

The softening temperature of quartz glass is 1650°C, and it has high temperature resistance.

Quartz crucibles are often used for melting acidic fluxes and sodium thiosulfate, and the operating temperature should not exceed 1100°C. Its acid resistance is very good, except for hydrofluoric acid and phosphoric acid, any concentration of acid has little effect on quartz glass even at high temperature.

Quartz glass is not resistant to hydrofluoric acid corrosion, and phosphoric acid can also react with it above 150 °C. Strong alkaline solutions including alkali metal carbonates can also corrode quartz, but the corrosion is slow at room temperature, and the corrosion is accelerated when the temperature rises.

Quartz glass instruments are similar to glass instruments in appearance, colorless and transparent, but they are more expensive than glass instruments, more brittle, and easily broken. Special care must be taken when using them. It is usually stored separately from glass instruments and kept in a safe place.

Use of Crucibles in Analytical Chemistry

Ceramic crucibles with a capacity of 10-15 ml are commonly used in quantitative analysis of analytical chemistry. Generally, it is used to make the analyte fully react at high temperature, and then quantitatively measure it through the difference in mass before and after.

Ceramics are water-absorbent, so in order to reduce errors, the crucible should be strictly dried before use and then weighed on an analytical balance. Sometimes the analyte is filtered with ashless filter paper, and the filter paper is put into the crucible together; This filter paper decomposes completely at high temperatures without affecting the results. After high temperature treatment, the crucible and its contents are dried and cooled in a special desiccator, and then weighed again, with clean crucible tongs throughout the process.