Guidelines for Selecting Rochoet Labware Materials

Primary uses of laboratory vessels such as melting, calcinations, etc require that the vessels

contain the following properties and are available in varying degrees in Rochoet Labware:

 

  • High Melting Point
  • High Temperature Strength
  • Oxidation resistance in both air and oxygen
  • Corrosion Resistance
  • Ductility

Platinum - Melting point 1772 Celsius. This is a ductile material with excellent resistance to

     chemical attack by fusion mixes and acids. Available in 99.95 to 99.99 fineness.

 

Platinum with 5% GoldMelting Point 1675 – 1745 Celsius. This alloy has a higher temperature strength than pure Platinum and takes on a special non-wetting property when in contact with molten glass and usually results in the easy removal of the sample after fusion and allows for many reproducible assays. It is thus accepted as the best material for crucible and casting molds for spectrographic analysis by x-ray fluorescence (XRF).

 

Platinum with 3.5% - 10% Rhodium Melting Point 1789 – 1850 Celsius. Rhodium alloys provide greater hardness and higher strength than other platinum alloys making them more suitable for more aggressive conditions. The higher the Rhodium content the longer the crucible should maintain its shape under the hottest furnace conditions. Caution / The cost of rhodium however must be considered in a cost vs. benefit analysis.

 

Platinum with 3.5% Iridium – Melting Point 1789 Celsius. Mechanical strength and thermal corrosion resistance increases as the iridium content increases. Caution / Platinum Iridium alloys suffer from increased weight loss when exposed to oxidizing atmospheres.

 

IridiumMelting Point 2447 Celsius. Iridium’s excellent corrosion resistance and high temperature strength and melting point makes it the preferred material for oxide melts in an inert atmosphere. It is ideal for growing single crystals such as aluminum trioxide, gadolinium-gallium-garnet and lithium tantalate. It is also used to draw high temperature ceramic fibers.

Various Fusion Agents can affect platinum as described below:

  • Alkali bifluorides - The loss of weight of platinum is negligible.
  • Alkali chlorides or alkali earth chlorides - Both will attack platinum above 1000EC when  air is present, which will liberate chlorine from the fused salt. These chlorides are inert in a neutral atmosphere.
  • Alkali Bisulfates – This material attacks platinum at temperatures slightly above 700EC. The effects can be reduced by adding ammonium sulfate.
  • Sodium carbonate – The loss of weight of platinum is minimal, equaling only a fraction of a gram.
  • Sodium carbonate mixed with sodium nitrate / nitrite – Under ordinary laboratory conditions, platinum loss should not exceed two milligrams.
  • Sodium Borate or sodium metaphosphate -  Sodium metaphosphate attacks platinum at only very high temperatures or under reducing conditions.

 

Guidelines for Use and Maintenance of Platinum Labware

Rochoet platinum crucibles are meticulously manufactured to precise tolerances to provide the best results when properly used. However, under certain conditions, their effectiveness and useful lifespan can be compromised. By following the recommendations discussed in these guidelines you should be able to maximize the useful life of your Rochoet Labware.

Warning

Platinum crucibles are most often damaged when the crucibles are heated in the presence of certain materials. In particular, damage can occur when platinum is heated with other metals that can alloy with platinum or in the presence of an oxide or compound of a metal that can combine with platinum.

Fundamentals of Care

  • Always heat platinum crucibles in an oxidizing atmosphere.
  • Handle them with platinum-tipped tools. If you only have access to base metal tongs,  wait for the vessel to cool down before handling.
  • Set them down only on a suitable clean surface or a platinum/iridium triangle.
  • Do not heat a platinum crucible in contact with another platinum vessel, as this may cause the two to weld together.
  • Avoid using a smoky flame: The free carbon will enbrittle the platinum.

Cautions

  • As a general rule, do not heat materials whose composition is unknown. The following elements when heated with platinum will cause the vessel to become brittle: antimony, arsenic, lead, phosphorus, selenium, tellurium and zinc.
  • In their molten state, silver, gold and nearly all base metals will dissolve platinum. At temperatures above 1000EC oxides of iron and lead will adversely affect platinum, as will silica and silicates under reducing conditions.

Evaporation of Solutions

Most alkaline and acid solutions do not attack platinum. The exceptions are alkaline solutions that contain silica, even in small amounts, and acid solutions containing hydrochloric acid with nitric acid or other oxidizing solutions. In addition, some damage will occur if a solution containing concentrated phosphoric or concentrated sulfuric acid is heated for a long period of time.

It is imperative the fusions be carried out under oxidizing conditions, and that the heated material comes in contact only with platinum or a clean refractory material. While the melt is cooling, it is advisable to tilt the crucible on its side, to enable the material to be more easily removed when it solidifies. This prevents the crucible from becoming distorted.

 

Ignitions

Always maintain oxidizing conditions when carrying out ignitions. Surface damage can occur when heating platinum around ammonia, sulphur dioxide and halogens.

Cleaning after use

  • Always clean platinum vessels after use. Immerse vessels in a chromic acid cleanser to remove organic matter and other impurities.
  • Insoluble carbonates or metal oxides may need to be removed by boiling in hydrochloric acid. Thoroughly rinse and boil in nitric acid.

         Failure to adequately rinse may allow the hydrochloric acid to attack the platinum.

  • Silica, silicates, metal and metal oxides may be removed by using the solvent action of fused potassium bisulfate. After the fusion gently pour the molten bisulfate over the inner surface and allow it to cool. Dissolve the melt in boiling water and thoroughly rinse. It may in addition be necessary to boil in hydrochloric acid.      The Detailed Process for removing silca/silacates is as follows:
    1) Add potassium bisulphate to the crucible. Fill the crucible to about 1/3 the capcity of the crucible being cleaned.
    2) Heat the crucible over an open flame and allow the potassium bisulphate to melt.
    3) Swirl the crucible with a pair of tongs over the flame until you see a red melt of potassium salt.
    4) Pour the molten salt out onto a dry stone or iron slab.
    5) The residue in the crucible is then boiled up in 20% (v/v) hydrochloric acid.
    6) Remove the crucible and rinse with plenty of de-ionized water, then dry.
    Notes: Potassium bisulphate can be replaced with sodium carbonate for light cleaning or for tough cleaning by disodium tetraborate. The melting fusion process should take about 1 minute and all reagents must be analytical grade.
  • Always follow cleaning by polishing the platinum vessel by gently rubbing it with an alumina-impregnated nylon webbing.
Photo 1

Hydra-forming Platinum labware

Like any valuable asset the proper care and maintenance of Rochoet platinum labware is a must in order to extend the life of the crucible and make the most of the laboratory's material investment.

Scott Anderson / Director of Marketing