Samples Containing Platinum

General Information

Occurrence — Platinum comes from the Spanish word platina (little silver). Platinum (Pt) has an abundance of 0.003 ppm in the lithosphere and is the most abundant Pt group metal, being only slightly more abundant than Ru. It is found with the other platinum group metals in alluvial deposits, or in nickel, copper, or iron sulfide ores.

Uses — Pt is a gray, malleable metal. It has a variety of uses, and consequently a variety of sample types for analysis, namely:

1. Geological samples, such as sulfide ores of Ni, Cu, Fe, rocks, minerals, and soils.
2. Catalysts (auto exhaust, nitric acid production, petroleum refining)
3. Electronic components (wires, alloys, electrodes)
4. Jewelry
5. Chemotherapy (biological materials)
6. Pharmaceutical preparations

Chemical Properties — Pt can have oxidation states of 0, +2, +4, +5, and +6. Pt does not form the +3 oxidation state. The +4 and +2 states are common, with the +4 being the most common for Pt. The most important compounds for Pt are the chlorides (PtCl₄^2-, PtCl₆^2-). Inorganic Ventures produces single-element CRMs of Pt in the +4 oxidation state, primarily as PtCl₆^2-, in dilute HNO₃ and/or HCl that can be mixed with most other mineral acids. Avoid basic media, due to precipitation of black/brown Pt(OH)₄ from PtCl₆^2-.

Pt is reduced to the metal by formic acid, stannous chloride, and metallic Zn, Mg, Fe, Al, and Cu. Treatment of PtCl₆^2- solutions with H₂S will form PtS₂, which is soluble in aqua regia. Pt is stable with most metals and inorganic anions in acidic media. Solutions of PtCl₆^2- can be diluted in HNO₃, where 2-10 ppb Pt is stable for 2 months (1% HNO₃/LDPE container); 100 ppb is stable for 5 months in 1% HNO₃/LDPE container. 1-10,000 ppm solutions are chemically stable for years in 10% HCl/LDPE container.

Fusion of H₂PtCl₆ with NaNO₃ at 500 °C yields Pt(OH)₄, i.e., oxidation to a higher oxidation state does not occur.

At Inorganic Ventures — Inorganic Ventures uses PtCl₄ as the starting material, with a documented purity that is ~4-9's+. The pure starting material is dissolved in electronic-grade HNO₃ for nitric acid matrices, or HCl for HCl matrices. Impurities for each lot are measured using ICP-MS and ICP-OES and are reported on the Certificate of Analysis. Following is some information about the handling and use of IV's Pt CRMs:

Location: Group 10, Period 6 Atomic Weight: 195.08 Coordination Number: 6 Chemical Form in Solution: PtCl₆^2-

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: Stable in HCl and HNO₃, as the chloride complex. Avoid basic media. Stable with most metals and inorganic anions in acidic media.

Stability of ICP-MS/OES Calibration Mixtures (the following stability is for 70-element mixtures made using IV CRMs, where the other elements are at equivalent concentrations; trace amounts of fluoride and chloride are present): 2-10 ppb Pt is stable for 2 months in 1% HNO₃/LDPE container. 100 ppb is stable for 5 months in 1% HNO₃/LDPE container. 1-10,000 ppm solutions chemically stable for years in 10% HCl/LDPE container.

Pt Containing Samples (Preparation & Solution): Metal (aqua regia); Oxides (soluble in HCl); Ores (dissolve in HCl/HNO₃).

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

Sampling and Handling: Pt contamination risk is low. There are no special precautions beyond the usual to be considered, unless your laboratory handles a wide variety of sample types. In developing a sampling approach, it is helpful to have a ballpark idea of the concentration of Pt in the sample. Naturally occurring Pt requires relatively large samples, collected using detailed sampling schemes with sample reduction according to statistically and experimentally sound methodology. The many and diverse uses of Pt result in a wide variety of sample types and concentrations, ranging from metallic alloys to organic carbon-bonded (σ and π C to Pt bonds) Pt compounds. Pt, like the other Pt Group metals, is rare, and therefore environmental contamination is not of particular concern, but great care should be exercised in keeping sample handling areas for sample types low in Pt free of sample handling/preparation activities for samples containing high Pt levels. Catalysts, ores, geological materials, alloys, and jeweler's sweeps are examples of samples high in Pt and require sophisticated sampling methodology, where large samples are collected and reduced to a laboratory-sized sample using specialized grinding and crushing equipment. The aforementioned should not, in any way, occupy laboratory space or use the same glassware as that used for biological samples, where sub-ng quantities of Pt are the rule.

For more on sample contamination risks, see chapters 8, 9, and 10 of the Inorganic Ventures Trace Analysis Guide.

For general information on sampling and subsampling, see Sampling and Subsampling.

Sample Preparations of the Metal and Alloys

Metal — Platinum metal is not affected by air or H₂O at any temperature. At red heat, Pt forms PtCl₄ with Cl₂, which is widely used with the Pt group metals for synthesis of the chloride salts. Pt metal is not attacked by HNO₃ or dilute H₂SO₄, but hot, concentrated H₂SO₄ very slowly forms Pt(SO₄)₂. Aqua regia, HCl plus HClO₃, and HCl + H₂O₂ all dissolve Pt metal, forming H₂PtCl₆. Pt is not as reactive as Pd, but it is the second easiest/most readily dissolved Pt Group metal.

Pt can be fused with alkali nitrates, hydroxides, and peroxides, forming Pt(OH)₄, which is soluble in dilute HCl. In an attempt to remove contamination of Ni and/or Fe and/or Ag as contaminants during the preparation, Inorganic Ventures has worked on a NaOH/NaNO₃ fusion method using a glassy carbon crucible. Please check with Inorganic Ventures for the current status of this method.

Alloys — Platinum is often found with the other Pt Group metals, in addition to a variety of other metals. Fusion of the sample with Na₂O₂, followed by dissolution of the melt with dilute HCl, is often used, or by performing a fire assay with nickel sulfide collection, followed by dissolution of the bead in aqua regia. Pt/Ag, Cu, Pb alloys can be dissolved in HNO₃, where the Ag, Cu, Pb, and other impurities are dissolved along with much of the Pt. The remaining residue, after washing, is Pt and Au, which are dissolved in aqua regia.

Oxides, Minerals and Ores

Oxides — The oxides and hydroxides are soluble in dilute HCl.

Ores and Minerals — There are many techniques that have been reported. The following link to "Sample Preparation Methods for the Platinum Group Metals and Gold Compatible with AAS, ICP-OES and ICP-MS Measurement" is recommended. This is an excellent review paper citing 153 references discussing platinum group metal (PGM) + gold (Au) sample preparation methods, and one that is easily obtained using this link:

Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques

Balcerzak, M., Analytical Sciences, Vol. 18, July 2002, pp. 737-750

This review compares the following sample preparation methods that are commonly used for the PGMs + Au:

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.

Organic Matrices: Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge, as a preliminary decomposition step, can be expedient for samples containing Pt. If ashing is used, it is suggested to keep the temperature low (400 to 450 °C max) and to use an ashing aid, such as high-purity sodium carbonate. Acid digestions using nitric, perchloric, and sulfuric acids are also suggested. Wet ashing is suggested for oil and petroleum products, using sulfuric acid in combination with magnesium nitrate as an ashing aid. Do not use ashing temperatures above 450 °C. If sulfuric acid is added to the petroleum sample, then heat on a hot plate slowly until foaming stops and a char is produced, i.e., wet ashing is very time-consuming, but it is a common practice in the petroleum industry. For more on ashing, please see the following paper: Ashing

Although Pt is not listed in the scope for EPA Methods 3050A and 3050B (Open Vessel Acid Digestion) and 3051 and 3052 (Microwave Assisted Acid Digestion), it is suggested that these methods should be explored for environmental samples (sediments, sludges, soils, and oils).

Samples containing mid to low ppm levels of Pt can be digested with nitric/perchloric acid. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples