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 it 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 word 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 RhCl₆³⁻ complex ion.
• CRMs manufactured by Inorganic Ventures that are in HNO₃ contain Rh as the Rh(H₂O)₆³⁺ complex ion.
• Reactions — Hydroxides precipitate Rh(III) solutions as the hydroxide, which is yellow. This precipitate dissolves in excess NH₄OH. With Rh(III), the sulfide, iodide, phosphate, and borate (as borax) form precipitates. Cyanide forms the soluble Rh(CN)₆³⁻. Metallic Zn or Mg will form a fine black precipitate of Rh⁰.
• Metals — Rh⁰ 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 °C. RhO₂ (black) is formed when Rh⁰ 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: RhCl₆³⁻

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/Cl₂ [gas]); Ores (HF/H₂SO₄ digestion followed by aqua regia digestion); Platinum scrap (aqua regia digestion).

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

Overview

Ashing in analytical chemistry is defined as the heating of a substance to leave only noncombustible ash, which is analyzed for its elemental composition.

Ashing Techniques

The sample preparation techniques incorporating some form of "ashing" are as follows:

Dry Ashing

Dry ashing is usually performed by placing the sample in an open inert vessel and destroying the combustible (organic) portion of the sample by thermal decomposition using a muffle furnace. Typical ashing temperatures are 450 to 550 °C. Magnesium nitrate is commonly used as an ashing aid. Charring the sample prior to muffling is preferred. Charring is accomplished using an open flame.

Sulfated Ashing

Sulfated ashing involves treatment of the sample after charring, using an open flame, with sulfuric acid (the char is wetted using the minimum amount of sulfuric acid and then brought to dryness before muffling), and then placing in a muffle furnace.

Wet Ashing

Wet ashing is treatment of the sample with a moderate amount of sulfuric acid before charring. Charring is performed using an open flame. Liquid samples tend to foam. After the excess sulfuric acid is driven off, the sample is muffled as above.

Low-Temperature Ashing

Low-temperature ashing involves treatment of the sample at ~120 °C using activated (singlet state) oxygen.

Closed System Ashing

Closed system ashing involves thermal decomposition in oxygen in a closed system, such as a Schöniger flask or an oxygen Parr bomb.

Advantages of Ashing

Ashing techniques are understandably used only for samples containing a significant amount of combustible or organic material as the matrix. With this in mind, let's look at the major advantages of ashing:

• The ability to decompose large sample sizes.
• The need for little or no reagents.
• The technique is relatively safe.
• The ability to prepare samples containing volatile combustion elements, such as sulfur, fluorine, and chlorine (the Schöniger oxygen flask combustion technique is very popular in this case).
• The technique lends itself to mass production.

The technique of graphite furnace atomic absorption spectrometry (GFAA) incorporates sample ashing as part of an automatic measurement cycle. Trace analysts have learned a great deal about the loss of volatile components during ashing, due to the ease with which the analyte signals can be compared to different ashing temperatures, times, addition of ashing aids, and other conditions. Typically, graphite carbon is the container material that the sample comes in contact with when using GFAA.

Disadvantages of Ashing

The trace analyst should be very familiar with their sample type before performing an ash. Some of the problems that have been encountered are listed as follows:

• Losses due to retention in the ashing container.
• Losses due to volatilization.
• Contamination from the ashing container.
• Contamination from the muffle furnace.
• Physical loss of "low-density" ashes when the muffle door is opened (air currents).
• Difficulty in dissolving certain metal oxides.
• Formation of toxic gases in poorly ventilated areas (note that all charring should take place in a hood, and the muffle furnace must have a hood canopy for proper ventilation).

Common Problems

If the sample type is unknown (with respect to the matrix), then a % ash, EDXRF scan, IR scan, and C, H, and N analysis will provide sufficient information in most cases to make informed decisions. The following are common problems that can be averted with one or more of the above preliminary analyses:

• Protect your Pt⁰ ware by looking for P (high levels will attack and attach to the Pt⁰) and elements that alloy with Pt⁰, which include the precious metals, Cu, and Hg.
• When using "silica" containing crucibles (porcelain, Vycor, quartz, glass, and fused silica), look for elements that form basic oxides, such as the alkaline earth elements. Na is commonly found, and its oxide will form (unless the char is sulfated) and attack the silica.
• Look for volatile elements (Cd, B, Hg, Pb, Se, Zn, As, Sn, Sb, S, and halogens), especially if moderate to large amounts of F or Cl are present.
• Si is a common element that is typically determined by dissolution of an ash performed in Pt⁰. Methyl silicones are widely used and very common. If Si is present as a silicone oil, then it will be partially lost as hexamethylcyclotrisiloxane and hexamethyldisiloxane.
• Retention and physical loss of analyte(s). The use of high-purity Mg(NO₃)₂ as an ashing aid will help prevent losses of "low-density" ashes, and will help in preventing retention losses.
• Difficult-to-dissolve oxides. Use as low an ashing temperature as possible (400 to 550 °C maximum). Look for Ti, Zr, Nb, Hf, Ta, W, Ni, Co, Fe, Cr, Sb, and Mo. The type of crucible material will determine the treatment that the ash can undergo. Pt⁰ is not attacked by HF, which will dissolve several of the above oxides.
• Loss due to reduction to the metal. Look for easily reduced elements, such as Cu and the precious metals. Use the appropriate crucible material to allow for the necessary dissolution reagents for the metal.

Examples of Ashing Procedures

Consult the elemental profiles found in our Interactive Periodic Table for additional ashing procedures.

General Dry Ashing Procedure

This procedure is used for a wide variety of sample types, which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products, and synthetic organic research materials. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.

Procedure: Dry ashing procedures are typically and preferentially performed in Pt⁰ crucibles. Glassy carbon can be used, but some attack may occur. Nickel and iron can also be used, but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. Due to highly toxic fumes, the crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types, such as polyethylene. As soon as fumes cease to evolve, the sample is placed in a muffle furnace at 450-500 °C for one hour, or until all of the carbon has been oxidized.

General Sulfated Ashing Procedure

This procedure is used for a wide variety of sample types, which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products, and synthetic organic research materials. The sulfated ash is used over dry ashing when the analyst needs to fix a material as the sulfate, to prevent volatilization; otherwise, it has no real advantages over dry ashing. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.

Procedure: Sulfated ashing procedures are typically and preferentially performed in Pt crucibles. Glassy carbon can be used, but some attack may occur. Nickel and iron can also be used, but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. The crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood due to highly toxic fumes. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types, such as polyethylene. As soon as fumes cease to evolve, wet the char with concentrated sulfuric acid. Typically, a few drops are required. Continue to heat the sample until the white, dense sulfur trioxide fumes cease to evolve. The sample is placed in a muffle furnace at 450-500 °C for one hour, or until all of the carbon has been oxidized.

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)