Maintenance of platinum crucibles, dishes and molds used in fusion

Platinum crucibles, dishes and mold are essential labware items of considerable monetary value. It is therefore very important to use and maintain them in such a way to ensure an optimal lifespan, especially considering their exposure to thermodynamic strain and corrosive molten samples during the process of fusion. Furthermore, damaged and poorly cleaned platinum labware can have a detrimental effect on the accuracy of measurements during analyses.

How to improve the life of platinum labware

In order to improve the service life of platinum labware, it is essential to use it correctly as well as to ensure meticulous cleaning.

To achieve desired fusion and analytical results, the surface of platinum crucibles, dish and molds need to be entirely smooth and scratch-free without any contaminants such as oil, dust or remnants of previous samples. Without proper care, both in cleaning and usage, the mold and crucible surfaces will gradually deteriorate which can result in:

• a negative impact on analytical results
• a decrease in fusion success rates
• incomplete transfer from crucible to the mold as particles adhere to scratches and imperfections
• contamination resulting in chemical leaching drastically reducing accuracy
• nonhomogeneous heat transfer
• nonhomogeneous sample
• cracks in labware which can leak molten fluids and cause extensive damage to fusion machines

Important guidelines for using platinum labware correctly

Following careful protocols for the correct use of platinum labware will help ensure it does not become damaged and in turn damage fusion machines, result in wasted samples or lead to invalid or inaccurate results.

Some important guidelines include:

• To prevent contamination while handling a mold, dish or crucible, always use properly cleaned platinum-tipped tongs.
• Always use crucibles and molds on clean surfaces to avoid base contamination.
• Never fill a crucible with a cold metal that will expand when heated and cause cracks.
• Always temper a crucible before use:
  • heat it to approximately 260°C for about 20 minutes
  • gradually cool to allow any moisture to be evaporated
• Labware needs to be meticulously checked for cracks or other damage before use:
  •  Any damaged items can be sent for recycling.
  • The damage can be checked using a microscope to assess the composition of the contaminant and prevent future recurrence.
• In spite of platinum being highly non-reactive, be aware of the substances that do corrode and/or combine with platinum, sometimes even at relatively low temperatures, for example:
  • Chromium (Cr), nickel (Ni), and zinc (Zn) react with platinum from 1200°C whereby molecules affect platinum’s chemical structure, causing cracks.
  • Fusion samples with lead (Pb), silver (Ag), bismuth (Bi), antinomy (Sb) and tin (Sn) content can lead to a significant reduction in the melting point of the platinum crucible due to unintended alloy formation
  • Other elements which can cause damage to platinum labware include: silicon (Si), sulfur (S), phosphorous (P), arsenic (As), zinc (Zn), copper (Cu), nickel (Ni), chromium (Cr) and iron (Fe)
• To avoid metal formation, always ensure oxidizing conditions are maintained:
  • a temporary lack of oxidizing conditions reduce silicon dioxide to its elemental form which easily reacts with the platinum
  • oxidant additives can be incorporated in fluxes to make sure that compound oxidation happens before the melting of the flux.
  • It is also possible to pre-oxidize the mixture before fusion
• Make sure that there is no unnecessary material present (not even in tiny amounts)
• Prevent the sample, disc or bead from sticking to the platinum surface:
  • By adding ammonium iodide tablets to the fusion process
  • Adding a release agent to the sample before fusion
• Direct fusions with cyanides, nitrates, caustic alkalis or nitrides should always be avoided
• It is essential to measure the weight of platinum labware daily. If it has reduced in weight by more than 10% then it needs to be recycled and replaced

Important guidelines for cleaning platinum labware correctly

Using potassium bicarbonate

1. Remove any residues from previous samples
2. Fused, solid potassium bicarbonate is added to the crucible to a point above the line of leftover materials from previous use
3. Heat the crucible until the bicarbonate melts, stirring frequently until a surface layer of red potassium salt appears (approximately 1 minute).
4. Remove from the heat and pour out the melt
5. Rinse in hot water, dry with a clean cloth and allow to cool.

Using an acid bath

The preferred method here is to use a weak acid:

Citric Acid

1. Put the platinum labware in an ultrasonic bath (using a holder to ensure there is no cross-contamination)
2. Add citric acid (20%) to the bath and vibrate at 50°C for ten minutes to remove any flux adhesions.
3. Dry the labware using a clean air stream or a drying oven.

If citric acid does not work completely, possibly a strong acid can be used as:

Diluted hydrofluoric acid

1. Soak for at least 24 hours
2. Boil for 3 hours
3. Once cooled, rinse with water and use an ultrasonic bath if necessary.
4. Repeat the rinse and boil in distilled water to remove all traces of hydrofluoric acid
5. Baked at 900°C for one hour

You are welcome to contact us with any questions you might have. We also offer our clients a full recycling service at excellent rates.

In this article we give possible options to improve the lifetime of your platinum labware. Please note that each test and test environment is different. Not al options mentioned could be applicable to your situation. Please consult your labware and machinery supplier for advice.