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Usage and Maintenance of Different Crucibles

Crucibles are indispensable tools in laboratories and industrial applications, serving a vital role in high-temperature heating, melting, and chemical reactions. Choosing the right crucible depends on various factors, including temperature resistance, chemical compatibility, and durability. This article covers various crucible types, their characteristics, and best practices for handling and upkeep.

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Metal Crucibles

1. Platinum Crucible

Platinum is a valuable metal with a melting point of approximately °C. It is soft and requires careful handling to avoid deformation. The following points are essential when using platinum crucibles:

• Avoid direct contact with hands to prevent deformation.
• Do not use a glass rod to scrape the inner walls to prevent damage.
• Avoid rapid cooling of a red-hot crucible by immersing it in cold water.
• Heating should be performed on an electric furnace or ceramic plate lined with asbestos, or in an oxidizing flame of a gas burner.
• Do not expose the crucible to iron, reducing flames, or certain reactive compounds such as Pb, Bi, Sb, Sn, Ag, Hg compounds, sulfides, phosphates, and arsenic compounds, as they form alloys with platinum.
• Avoid exposure to halogens, aqua regia, hydrochloric acid, and some oxidizing mixtures due to their corrosive nature.
• Alkaline oxides, hydroxides, nitrates, nitrites, cyanides, and barium oxide can erode the crucible at high temperatures, while sodium and potassium carbonate are safe.
• Keep the inner and outer surfaces clean and bright by boiling with 1:1 HCl solution. If necessary, use K2S2O7 fusion, Na2CO3, or borax fusion for cleaning.
• Always use platinum tongs to handle hot crucibles.
• If deformed, the crucible can be reshaped by rolling on a wooden surface while pressing with a horn spoon.

Further reading: Precious Metal Crucible: Types and Applications

2. Nickel Crucible

Nickel has a melting point of °C and is resistant to alkaline and corrosive environments. It is suitable for melting iron alloys, slags, clays, and refractory materials.

• The temperature should not exceed 700°C to prevent oxidation.
• Not suitable for precipitate ignition.
• Works well with NaOH, Na2O2, Na2CO3, NaHCO3, and basic solvents containing KNO3, but not with KHSO4, NaHSO4, K2S2O7, Na2S2O7, or sulfur-containing fluxes.
• Molten Al, Zn, Pb, Sn, and Hg salts can embrittle the crucible. Borax should also be avoided.
• New crucibles should be preheated in a muffle furnace until they turn blue-violet, then boiled in 1:20 HCl before rinsing with water.

3. Iron Crucible

Iron has a melting point of °C and is commonly used for cost-effective applications.

• Must be passivated before use by cleaning with dilute HCl, sanding, washing with hot water, then soaking in a 5% H2SO4 and 1% HNO3 mixture.
• Usage rules are similar to those of nickel crucibles.
• Can be cleaned with cold dilute HCl.

4. Silver Crucible

Silver melts at 960°C, with a recommended heating limit of 700°C.

• New crucibles should be preheated at 300-400°C, then washed with hot dilute HCl.
• Suitable for NaOH-based fluxes but not for Na2CO3.
• Should not be used for precipitating sulfur or burning sulfur compounds.
• Avoid rapid cooling to prevent cracks.

Ceramic Crucibles

5. Porcelain Crucible

Ceramic crucibles can withstand temperatures up to °C.

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• Suitable for melting acidic substances like K2S2O7.
• Avoid alkaline fluxes like NaOH, Na2O2, and Na2CO3 to prevent corrosion.
• Should not be exposed to hydrofluoric acid.
• Clean with boiling dilute HCl.

Further reading: An Overview of Various Ceramic Crucibles

6. Quartz Crucible

Quartz crucibles are stable up to °C but become opaque above °C. The recommended fusion temperature is below 800°C.

• Should not come into contact with HF or high-temperature alkaline substances.
• Suitable for use with K2S2O7, KHSO4, and Na2S2O7 fluxes.
• Quartz is brittle, requiring careful handling.
• Can be cleaned with standard dilute inorganic acids except for HF.

7. Corundum Crucible

Made of porous fused alumina, corundum crucibles are robust and heat-resistant.

• Suitable for weakly alkaline fluxes like anhydrous Na2CO3.
• Not recommended for use with strong alkaline or acidic fluxes like Na2O2, NaOH, and K2S2O7.

Other Crucibles

8. PTFE (Polytetrafluoroethylene) Crucible

PTFE crucibles have a heat resistance of up to 400°C but are typically used below 200°C, with a maximum safe limit of 280°C.

• Resistant to acids and bases, including HF.
• Ideal for HF-based dissolution processes, such as HF-HClO4 mixtures.
• Should not be used with smoking HF-H2SO4 solutions.
• Its primary advantage is preventing metal contamination in samples.
• Features a smooth, wear-resistant surface and high mechanical strength.

Conclusion

Choosing the right crucible depends on the chemical nature of the materials, required heating conditions, and potential reactions. Proper usage and maintenance ensure the longevity and reliability of the crucibles. By following the specific guidelines for each crucible type, users can maximize their efficiency and safety in laboratory and industrial settings.

Summary Table: Upkeep and Handling of Different Crucibles

This table summarizes key properties, compatible materials, and handling guidelines for different crucibles.

Stanford Advanced Materials—The Reliable Supplier

Stanford Advanced Materials (SAM) offers high-quality laboratory crucibles, including ceramic, pyrolytic graphite, precious metal, and metal crucibles, along with molds, at competitive prices.