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Choosing the Right Crucible for Accurate Analysis
Selecting the appropriate crucible for your analytical process is crucial for ensuring accurate results and preventing costly damage to equipment and labware. There are several factors to consider, both general and specific to your application, including:
· Crucible volume and shape
· Size and type of furnace
· Sample composition and potential reactions with the crucible
· Maximum melting and holding temperatures (Note: the maximum operating temperature of a metal or alloy should be between 85-90% of its melting point)
· Expected lifespan
· Temperature change rate (ramp rate) of the process
· Refining processes
· How slag or dross is removed
· Technique for emptying the crucible
Given that each user and application is unique, with different combinations of furnaces, processes, treatments, and end products, selecting the right crucible is highly individualized. At Birmingham Metal, our team of experts is available to assist you in choosing the best crucible for your specific needs.
Crucible Alloy Compositions
A crucible must withstand high operating temperatures and remain chemically compatible with the sample material, meaning it should be inert and non-reactive. We offer a wide range of platinum and platinum alloy crucibles. Platinum is the industry standard for crucibles due to its exceptional properties:
· Extremely high melting point
· Low reactivity, even at elevated temperatures
· Mechanical stability during fusion processes
By adding alloying elements to pure platinum, we can enhance specific characteristics of the crucible to suit different analyses. For instance, rhodium increases platinum’s melting point, offering enhanced performance for high-temperature applications.
We supply the following platinum crucibles:
Pure Platinum (Pt)
Pure platinum crucibles boast excellent resistance to chemical degradation and have a melting point above 1770°C, making them ideal for high-temperature applications.
Platinum-Gold Crucibles (PtAu)
These 95% platinum / 5% gold crucibles are an industry benchmark for fusion sample preparation, particularly in XRF analysis. Their structure ensures excellent mixing performance and precise analytical results. The addition of gold offers several key benefits:
· Melting point between 1780 – 1850°C
· Increased crucible strength
· Reduced wrinkling and recrystallization tendencies
· Enhanced sample removal ease
· Improved thermodynamic stability
· Non-wetting to molten glass, making them easier to clean
· Ideal for both the oil & petrochemical industries.
Platinum-Rhodium Crucibles (PtRh)
These alloys contain either 10% or 3% rhodium, with melting points ranging from 1780 to 1850°C. Platinum-rhodium crucibles are known for their enhanced hardness and strength, particularly in harsh conditions. The more rhodium present, the better the crucible retains its shape under extreme heat. However, it’s important to be aware that rhodium is sensitive to certain elements like sulfur (S), lead (Pb), and tin (Sn), which makes knowing your sample composition critical in selecting the right crucible.
Which Crucible Alloy is Right for Your Application?
We provide general recommendations, but it is essential to take your specific process, limitations, and sample composition into account when selecting a crucible.
XRF and ICP Analysis
For fusion processes in x-ray fluorescence (XRF) and inductively-coupled plasma (ICP) analysis, temperatures can reach up to 1250°C. The crucible material must endure these temperatures and ramp rates, while ensuring no cross-reactivity with the samples. Platinum and platinum alloys are the standard for high-purity crucibles used in these applications. In most cases, platinum-gold (PtAu5%) crucibles are ideal.
For higher temperature fusions, a stabilised version like PtAu5%XRS may be selected, or in some cases, platinum-rhodium (PtRh10%) crucibles can be considered, such as for applications in the glass industry or petrochemical industry.
Ash Testing
During ashing, where organic substances are incinerated, a gold-platinum alloy is preferred due to its resistance to organic materials at lower temperatures. Depending on the sample, platinum or platinum-rhodium crucibles may also be suitable.
Evaporation
For evaporation and concentration processes, platinum or platinum-rhodium crucibles (or low-profile dishes) are ideal. These dishes have a large surface area, allowing uniform heating and evaporation of samples, such as when evaporating hydrogen fluoride (HF) or carbon compounds at around 800°C.
Acids
Platinum alloys are generally resistant to acids. However, above certain temperatures, some alloys may begin to lose weight and stability, making them suitable for chemical processes that occur at lower temperatures.
Electrogravimetry
In electroanalysis, platinum or platinum-rhodium crucibles are typically the most suited due to their stability and chemical resistance.
Thermal Analysis
For thermal analysis, platinum or platinum-rhodium (PtRh10% and PtRh20%) crucibles offer the best performance under extreme conditions.