Rhodium Testing: Methods, Standards, and Applications

This article has been updated to reflect ASTM B616-23 purity grade specifications for refined rhodium; current EPA Clean Air Act vehicle emissions compliance context; and ICP-OES as a distinct multi-element technique alongside ICP-MS.

Rhodium is one of the rarest platinum group metals (PGMs) and among the most industrially consequential. Its exceptional corrosion resistance, high-temperature stability, and catalytic selectivity for nitrogen oxide (NOx) reduction make it essential to automotive emission control, jewelry plating, and high-temperature thermocouple manufacture. Verifying that rhodium meets specification requires selecting the correct analytical method and matching it to the applicable purity standard.

Key Takeaways

• ASTM B616-23 defines three purity grades for refined rhodium: 99.80%, 99.90%, and 99.95%, each requiring chemical analysis of 25 specified impurity elements.
• XRF delivers rapid, non-destructive elemental screening; ICP-MS quantifies trace impurities down to the parts-per-trillion level.
• Fire assay is the benchmark gravimetric method for PGM content determination in mining and refining operations.
• Rhodium’s role in three-way catalytic converters ties its testing directly to EPA Clean Air Act compliance requirements.
• Coating thickness testing by XRF is essential for jewelry plating and electronic contact qualification.

Why Rhodium Purity Matters: Catalysts, Jewelry, and Thermocouples

Rhodium’s value and scarcity make purity verification essential across the supply chain, from refinery output to finished component. In catalytic converter washcoats, impurities that displace or deactivate rhodium atoms reduce NOx conversion efficiency. In rhodium-plated jewelry, off-spec deposits produce uneven reflectivity and accelerated wear. In Type B, R, and S thermocouples, rhodium concentration in the platinum-rhodium alloy determines electromotive force (EMF) output; any deviation from specification introduces measurement error in high-temperature industrial processes.

Fraud risk is equally significant. Rhodium’s historically elevated price relative to platinum and palladium makes substitution with lower-cost PGMs or base metal alloys a well-documented problem in the bullion and industrial components markets. Third-party testing with validated methods independently confirms purity for purchase verification, refinery settlement, and supply chain certification.

ASTM B616-23: The Purity Specification for Refined Rhodium

ASTM B616-23, Standard Specification for Refined Rhodium, defines three commercial purity grades for rhodium sponge and powder:

• Grade 99.80 (UNS P05980): 99.80% minimum purity
• Grade 99.90 (UNS P05981): 99.90% minimum purity
• Grade 99.95 (UNS P05982): 99.95% minimum purity

The specification requires chemical analysis to confirm the concentrations of 25 impurity elements, including platinum, iridium, palladium, ruthenium, lead, tin, zinc, arsenic, bismuth, cadmium, iron, nickel, and copper. Electronics-grade sputtering targets and high-precision catalyst applications require Grade 99.95. Grade 99.80 suits mining and refining intermediates where downstream processing will increase purity.

For platinum-rhodium thermoelements, ASTM E1159 governs composition and purity requirements for Type R, Type S, and Type B thermocouples. Rhodium content varies by thermocouple type: 13% in Type R, 10% in Type S, and either 30% or 6% in the two legs of Type B, giving an overall range of 6%–30% by weight. Compliance ensures EMF output meets performance tolerances for high-temperature measurement.

What Analytical Methods Are Used for Rhodium Testing?

Method selection depends on sample form (solid, powder, dissolved solution, or coating), required detection limit, and whether the test must be non-destructive. The table below compares the principal techniques used across industries.

XRF for Rapid, Non-Destructive Screening

X-ray fluorescence (XRF) analysis determines elemental composition by directing a primary X-ray beam at the sample and measuring the characteristic secondary X-rays it emits. XRF is the dominant screening method in metallurgical analysis for precious metals because it delivers results in seconds without sample destruction. Handheld XRF instruments confirm the identity of rhodium-bearing alloys directly at the scrap yard or refinery floor. Bench-top instruments measure rhodium plating thickness on jewelry and electronic contacts with high repeatability and minimal sample preparation.

XRF has meaningful limitations for trace-level certification. At 0.01–0.1 wt%, XRF detection limits fall short of the full ASTM B616-23 impurity panel for Grade 99.95 certification. For high-purity applications, XRF functions as a first-pass screening tool rather than a standalone certification technique.

ICP-MS and ICP-OES for Trace and Multi-Element Analysis

Inductively coupled plasma mass spectrometry (ICP-MS) ionizes a dissolved rhodium sample in a high-temperature argon plasma and separates the resulting ions by mass-to-charge ratio. ICP-MS delivers detection limits in the sub-parts-per-billion to parts-per-trillion range, making it the method of choice for certifying high-purity rhodium and for quantifying trace contaminants in spent catalyst materials. Sample preparation varies by matrix: laboratories digest spent catalyst matrices with microwave-assisted aqua regia, while pure rhodium metal resists aqua regia alone and requires concentrated hydrochloric acid (HCl) combined with perchloric acid for complete dissolution. See the Contract Laboratory guide to aqua regia digestion for dissolution chemistry applicable to precious and base metal matrices.

ICP-OES (inductively coupled plasma optical emission spectrometry) measures characteristic light emitted by elements excited in the plasma, providing reliable quantification in the parts-per-million range across a broad multi-element panel. ICP-OES is well-suited for intermediate-purity rhodium lots and for chemistry and compound analysis tasks where ppt sensitivity is not required.

Fire Assay for Rhodium and PGM Content Determination

Fire assay is the traditional gravimetric method for determining PGM content in ores, concentrates, and recycled alloys. A furnace fuses the sample with flux materials that collect and separate the precious metals; analysts then recover and weigh the resulting fraction to determine PGM content. Fire assay remains the reference method in refinery settlements because its accuracy and industry acceptance support contractual and regulatory use. Refineries processing spent catalytic converters use fire assay for lot settlement, then ICP-OES for elemental breakdown of the recovered bead. See the Contract Laboratory guide to analytical testing for urban mining and scrap recycling for more on PGM certification in the recycling chain.

When Are AAS, EPMA, and TGA Used for Rhodium Testing?

Atomic absorption spectroscopy (AAS) quantifies element concentration by measuring light absorption at characteristic wavelengths in a vaporized sample. AAS provides precise single-element quantification in the ppb–ppm range, useful in facilities without ICP instrumentation or when a single specification limit needs confirming.

Energy-dispersive X-ray spectroscopy (EDS), paired with scanning electron microscopy (SEM), maps elemental composition across rhodium coating surfaces, identifying contamination and plated-layer defects. Electron probe microanalysis (EPMA) is a distinct standalone instrument using wavelength-dispersive spectrometry (WDS); it provides higher energy resolution and lower detection limits than SEM-EDS, making it preferable for quantitative spot analysis of rhodium alloy phases. Both complement XRF in materials evaluation and testing workflows requiring micro-area spatial resolution.

Thermogravimetric analysis (TGA) measures sample weight change as a function of temperature. For rhodium coatings and alloys, TGA characterizes oxidation onset temperature and mass change during thermal cycling, directly relevant to qualifying components in high-temperature furnace and combustion environments.

Rhodium Testing and EPA Clean Air Act Compliance

Rhodium is the catalytically active component responsible for reducing NOx in three-way catalytic converters (TWCs). Under the Clean Air Act, the Environmental Protection Agency (EPA) sets progressively more stringent NOx emission limits for light-duty vehicles. Catalytic converter manufacturers must demonstrate that rhodium loadings meet specification to ensure the TWC performs within required emission limits over the vehicle’s certified useful life.

Converter manufacturers qualify incoming rhodium and verify washcoat concentrations as routine quality assurance steps across the emission control supply chain. Suppliers delivering off-spec material risk product rejection and downstream compliance liability for converter manufacturers.

Choosing the Right Rhodium Testing Laboratory

No single analytical method covers all rhodium testing scenarios. Refineries processing spent catalysts use fire assay for settlement and ICP-OES for elemental breakdown. Jewelry manufacturers rely on XRF for plating thickness and ICP-OES for solution quality. Electronics and thermocouple manufacturers purchasing Grade 99.95 material require full ASTM B616-23 chemical composition reports from an ISO/IEC 17025-accredited laboratory.

For heavy metals testing laboratories handling precious metals, The standard workflow pairs XRF for initial screening with ICP-MS for final certification. Confirm that the selected laboratory participates in interlaboratory comparison programs for precious metals and can provide reference material traceability documentation.

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