Samples Containing Palladium

General Information

Occurrence — Palladium (Pd) has an abundance of 0.0006 ppm in the lithosphere and is the third most abundant Pt group metal, being only slightly more abundant than Ir, Rh, and Os. It is found with the other platinum and some base metals as the pure metal, and as intermetallic compounds of sulfur and arsenic. A very important source of Pd has more recently been from the recycling of used catalysts.

Uses — Pd is a hard, silvery white metal, ductile and malleable, conducting electricity about 1/8th as well as Ag. Pd softens at temperatures distinctly below the melting point; hence, it can be welded easily. Pd sponge absorbs many times (1000 to 3000 times) its volume in H₂, forming the hydride, and it retains most of the gas even at 100 °C. Pd is used as a catalyst in oil refining, for coating electronics, and in dental work.

Chemical Properties — Pd can be in the 0, +2, and +4 oxidation states, with the +2 being the most common. Inorganic Ventures produces single-element CRMs of Pd in the +2 oxidation state in dilute HNO₃ and HCl that can be mixed with HCl, HNO₃, H₂SO₄, HF, and H₃PO₄. Although Pd is classed with the platinum metals, its reaction with acids is different. Pd metal can be dissolved in pure HNO₃, unlike the other PMs, but if the Pd is 5-9's pure, it will not dissolve unless a small amount of an impurity, such as HCl, is added. Avoid basic media. Pd is easily reduced to the metal by aldehydes/ketones. Pd is stable with most metals and inorganic anions in acidic media. 2 ppb Pd is stable for 1 day in 1% HNO₃/LDPE container. 10 ppb is stable for 3 days in 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 1-5% HNO₃/LDPE container.

At Inorganic Ventures — Inorganic Ventures uses metallic Pd as the starting material, with a documented purity that is ~5-9's. The pure metal is dissolved in electronic-grade HNO₃ for nitric acid matrices, and high-purity PdCl₂ is used 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 Pd CRMs:

Location: Group 10, Period 5 Atomic Weight: 106.42 Coordination Number: 6 Chemical Form in Solution (IV's HNO₃ Matrix / HCl Matrix): Pd(H₂O)₆²⁺ / PdCl₄^2- Oxidation States: 2+ and 4+; Pd does not form the 3+; IV's CRMs are 2+. Chemical Properties: Both the 2+ and 4+ form the chlorides; PdO is stable, but PdO₂ is unstable and decomposes in water when heated; the 2+ (most common by far) is stable in air-saturated solutions, begins forming Pd(OH)₂ at a pH >-0.3; stability to hydrolysis is greatly increased through complexation with acetylacetone (stable up to pH = 10), chloride (stable up to pH 3.5), cyanide (stable up to pH = 12), and EDTA (stable up to pH = 12).

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, HNO₃, H₂SO₄, HF, and H₃PO₄. Avoid basic media. Stable with most metals and inorganic anions in acidic media. Avoid contact with water-soluble organics, such as aldehydes, since Pd²⁺ is easily reduced.

Stability: 2-100 ppb levels. 2 ppb Pd is stable for 1 day in 1% HNO₃/LDPE container. 10 ppb is stable for 3 days 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 1-5% HNO₃/LDPE container.

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

Sampling and Handling: In developing a sampling approach, it is helpful to have a ballpark idea of the concentration of Pd in the sample. There are a wide variety of sample matrices containing a wide range of concentrations. Pd, like the other PGMs, occurs in nature in the form of rare, discrete, and inhomogeneously distributed mineral species having a high financial value. A primary Pd-containing ore is braggite [(Pt, Pd, Ni)S].

The most common materials to be analyzed for Pd are ores, geological materials, alloys, silver and gold bullion, refined Ag and Au, and jeweler's sweeps. Typically, ore samples will require sophisticated sampling methodology, where large samples are collected and reduced to a laboratory-sized sample using specialized grinding and crushing equipment.

Pd contamination risk is low. There are no special precautions beyond the usual to be considered.

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 — Palladium metal is the easiest and most readily dissolved Pt Group metal. Dissolved palladium does have a tendency to reduce to the metal by trace organic impurities and other mild reducing agents. Therefore, extra care is suggested, such as the use of glass and quartz beakers, crucibles, etc., and avoidance of any organic solvents, including common laboratory ketones, aldehydes, and alcohols. The metal can be dissolved in concentrated HNO₃ (high-purity metal requires heat and the addition of 1 drop of concentrated HCl). If the presence of HCl is of no concern, then the metal will readily dissolve when treated with aqua regia (1:3 v/v concentrated HNO₃:HCl).

Alloys — Palladium is often found with the other Pt Group metals, and can be dissolved along with the other metals using aqua regia. Pd/Ag alloys can be dissolved in HNO₃.

Oxides, Minerals and Ores

Oxides — The oxides, hydroxides, and carbonates are soluble in dilute acids. Dilute HNO₃ and/or HCl are most popular.

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: Pd is very easily reduced to the metal in organic matrices. Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge, as a preliminary decomposition step, can be expedient for samples containing Pd. 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 Pd 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 Pd can be digested with nitric/perchloric acid. Only use trace metals grade acids, due to contamination issues. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples