Samples Containing Iridium

General Information

Iridium (Ir) is the fourth of the six platinum group metals (PGMs — Ru, Os, Rh, Ir, Pd, and Pt) to be addressed in this series. Iridium's chemistry is similar to that of rhodium. Iridium's presence in the earth's crust is very low, at ~10^-6 g/kg. Ir in the earth's crust, along with Rh and Pt, is of geochemical interest as an indicator of meteorite impacts. Ir deposits are associated with Pt, and it is also found alloyed with Os in osmiridium (syserkite) and iridiosmium (nevyanskite). Most of the world's Ir comes from South Africa, where ~75% of the world's ~3 metric tons/year production occurs. Ir alloyed with Pt is used industrially to make electrodes, crucibles, electrical contacts, etc., as these alloys can withstand long periods of high current and temperature. For example, crucibles made from Pt/Ir alloys can withstand prolonged temperatures of 2000 °C, which is required for growing single-crystal oxides. In the spectroscopy analytical laboratory, Ir is alloyed with Pt to make inert, corrosion-resistant nebulizer capillaries. An alloy of 75% Pt and 25% Ir is not soluble in laboratory mineral acids, including freshly prepared aqua regia. Interestingly, the old international meter bar is made of a Pt/Ir alloy.

Some Ir Chemistry — A Brief Summary

• Name — Iridium comes from the Greek word iris (rainbow).
• Oxidation States — 0, +1, +2, +3, +4, +5, +6
• The most stable oxidation states are +3 and +4.
• Inorganic Ventures Ir CRMs are all in the +4 oxidation state.
• CRMs manufactured by Inorganic Ventures that are in HCl contain Ir as the IrCl₆^2- complex ion.
• Reactions — The fixed alkali hydroxides or carbonates precipitate from boiling solutions of IrCl₃ or IrCl₄ as dark blue Ir(OH)₄, insoluble in all acids except HCl. Potassium nitrite, added to hot solutions of iridium salts, gives first a yellow color and finally a yellow precipitate insoluble in H₂O or acids. H₂S reduces Ir⁴⁺ to Ir³⁺ and then precipitates Ir₂S₃ (brown), which is soluble in alkali sulfides. Sulfurous acid precipitates Ir⁰ (black) from hot solutions. Potassium and ammonium chloride form, respectively, the dark-colored, slightly soluble K₂IrCl₆ and (NH₄)₂IrCl₆. Metallic Zn precipitates Ir⁰ from hot solutions. Formic acid effects the same result. (From: Qualitative Chemical Analysis by R.K. McAlpine and B.A. Soule, D. Van Nostrand Company, 1933.)
• Metals — Freshly precipitated Ir⁰ is soluble in aqua regia. Unfortunately, the ignited metal is insoluble in all acids. Pt/Ir alloys up to 10% Ir can be dissolved in aqua regia, however, very slowly. For higher Ir contents, the metal is mixed with NaCl and heated to 800 °C, and Cl₂ gas is passed over the mixture, forming the double chloride salt, which dissolves in water. If the Pt and Ir are alloyed with lead (the Pb is added at at least 10 times its weight) and dissolved in HNO₃, the Pb and Pt dissolve, leaving insoluble Ir.
• Oxides — Two oxides of Ir, which have been well identified, are Ir₂O₃ and IrO₂. These oxides are not soluble in acids. See ref (1) below for methods.

The Handling and Use of Ir CRMs — A Summary

Location: Group 9, Period 6 Atomic Weight: 192.22 Coordination Number: 6 Chemical Form in Solution: IrCl₆^2-

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.

Ir Containing Samples (Preparation & Solution): Metal (elevated temperature with aqua regia or HCl/Cl₂ {gas}); Ores (HF/H₂SO₄ digestion followed by aqua regia digestion); Platinum scrap (aqua regia digestion).

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

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 subsampling, see Sampling and Subsampling.

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