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Samples Containing Rhodium

General Information

Rhodium is the third of the six platinum group metals (PGMs –Ru, Os, Rh, Ir, Pd and Pt) to be addressed in this series. Rh is very rare and is found only in platinum ores.  Most of the world’s Rh comes from South Africa where ~75% of the world’s ~3 metric tons/year production occurs. Mining strikes can and do influence the availability and price of Rh as did the 2014 platinum miners’ strike. Rhodium is used industrially in catalysts, electrodes and alloys. In the analytical laboratory Rh is used for crucibles and as an internal standard for many ICP-MS applications. As an ICP-MS internal standard, rhodium is particularly useful for several reasons; its rarity makes it a low contamination risk, it can be converted to a chloride free nitrate solution that is compatible with common laboratory acids and reagents, it is stable at the ppb level in dilute nitric acid in common container materials such as polyethylene (e.g., LDPE), it exhibits excellent inter-element chemical compatibility, and exists as a monoisotope in a region of the periodic table where chemically compatible monoisotopes otherwise do not exist. Because of rhodium’s excellent compatibility chemistry it has become a popular choice as an ICP-MS internal standard element.

Some Rh Chemistry – A Brief Summary

  • Name – rhodium comes from the Greek work rhodon (rose).
  • Oxidation States – 0, +1, +2, +3, +4, +5, +6
  • The most stable oxidation state is +3
  • Inorganic Ventures Rh CRMs are all in the +3 oxidation state.
  • CRMs manufactured by Inorganic Ventures that are in HCl contain Rh as the Rh(Cl)6-3 complex ion.
  • CRMs manufactured by Inorganic Ventures that are in HNO3 contain Rh as the Rh(H2O)6+3 complex ion.
  • Reactions – hydroxides ppt. Rh(III) solutions as the hydroxide which is yellow. This ppt dissolves in excess NH4OH. With Rh(III) the sulfide, iodide, phosphate and borate as borax form precipitates. Cyanide forms the soluble Rh(CN)6-3.  Metallic Zn or Mg will form a fine black precipitate of Rhº.
  • Metals - Rh° as the pure metal is not soluble in aqua regia. Fortunately, other than National Metrological Institutes such as NIST the need to dissolve the pure metal, which is nearly impossible, is rare. Alloyed with Bi, Pb, Cu, or Pt it is soluble in HNO3 or aqua regia. When Rh is alloyed with Pt the sample is rolled to a thin ribbon and dissolved in aqua regia, although an alloy containing 30% Rh is practically insoluble and the aqua regia has to be replaced periodically as the alloy dissolves slowly. Using high pressure systems will speed the process. 
  • Oxides – Two oxides of Rh, which have been well identified, are Rh2O3 and RhO2. Rh2O3 is formed when Rh metal is heated in air near but not above 1150 ºC. RhO2 (black) is formed when Rhº is fused with KOH/KNO3. RhO2 is not soluble in water or the alkalis but will dissolve in HCl with the evolution of Cl2.  Rh(OH)3 (yellow) is formed by treating RhCl3 with aqueous NaOH. It should be noted that RhCl3 formed from the dry chlorination (see below for explanation of dry chlorination) of the metal is not water soluble but the hydrated salt is.

The Handling and use of Rh CRMs – A Summary

Location:  Group 9, Period 5
Atomic Weight:  102.9055
Coordination Number:  6
Chemical Form in Solution:  RhCl6-3

Storage & Handling:  Keep tightly sealed when not in use. Store and use at 20 ± 4°C. Do not pipet from container. Do not return portions removed for pipetting to container.

Chemical Compatibility:  Soluble in HCl, HNO3, H2SO4 and HF aqueous matrices. May cause AgCl precipitation when mixed with Ag+. Stable with all other metals.

Stability:  2-100 ppb levels stable for months in 1% HNO3 / LDPE container. 1-10,000 ppm solutions chemically stable for years in 10% HCl/LDPE container.

Rh Containing Samples (Preparation & Solution):  Metal (elevated temp. with aqua regia or HCl / Cl(gas) ); Ores (HF / H2SO4digestion followed by aqua regia digestion); Platinum scrap (aqua regia digestion).

Atomic Spectroscopic Information:  (red text indicates severe at ~ concs.)

Technique / LineEstimated D.L.*OrderTypeInterferences

ICP-OES 233.477 nm

0.04/0.004 µg/mL

1

ion

Ni, Sn, Mo, Nb, Ta

ICP-OES 249.077 nm

0.06/0.006 µg/mL

1

ion

Ta, Co, Fe, W, Cr, Os

ICP-OES 343.489 nm

0.06/0.006 µg/mL

1

atom

Mo, Th, Ce

ICP-MS 103 amu, monoisotopic

1 ppt

n/a

M+

40Ar63Cu, 87Rb16O, 87Sr16O, 206Pb+2

*ICP-OES D.L.'s are given as radial / axial view

Sample Preparation Methods for the Platinum Group Metals and Gold Compatible with AAS, ICP-OES and ICP-MS Measurement

An excellent review paper (Balcerzak, 2002) that discusses sample preparation methods for the platinum group metals and gold (Au) is readily available1, and this review compares the following commonly used methods for these metals:

  1. Fire assay
  2. Wet chemical acid digestion
  3. Oxidizing fusion
  4. Chlorination

The reader is encouraged to consult this paper for detailed information on these sample preparation methods.

For general information on sampling and sub-sampling see:

https://www.inorganicventures.com/sampling-and-subsampling

1 Balcerzak, M. , ANALYTICAL SCIENCES, Vol. 18, July 2002, pp 737 -750 (PDF)