Noble Metals, Analytical Chemistry of

Abstract

Analytical methods for the determination of noble (precious) metals: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt) and gold (Au) are presented in this article. Discovery, natural occurrence and main applications of the metals are described. Physical and chemical properties of noble metals are summarized. The nobility and catalytic activity of precious metals are the main properties that allow their use in a wide variety of applications, e.g. as catalysts in various chemical processes, as autocatalysts, in the electrical and electronic industry and in jewellery. Recent applications of some platinumcompounds (cisplatin and its derivatives) as anticancer drugs are important. The large variety of complex matrices, wide analytical concentration range (from sub-ppb to >99.99%), low reactivity towards single chemical reagents, great chemical similarities (especially between the pairs Ru and Os, Rh and Ir, Pt and Pd), complexity of platinum group metals (PGMs) species in solutions and rates of reaction make the accurate determination of noble metals a difficult analytical problem. The use of direct instrumental methods is restricted owing to interferences caused by matrix elements and low analyte concentrations. Sampling, sample decomposition, separation and preconcentration are critical steps in the majority of analytical procedures used. The choice of the digestion procedure used depends on the nature of the sample matrix and the analyte concentration. Fire assay (lead, iron, copper, nickel, tin or nickel sulfide as collectors), oxidizing fusion, acids treatment and chlorination are used to digest various materials. Precipitation, solvent extraction and chromatographic methods (ion-exchange and chelating resins, capillary electrophoresis) are applied to separate noble metals from associated base metals and to separate the individual precious metals. Preliminary isolation of ruthenium and osmium from the other noble and base metals, as well as from each other, by distillation or extraction in the form of RuO4 and OsO4, is often applied. Spectrophotometric methods using the complexes with inorganic and organic reagents can be applied to the determination of precious metals at ppm levels. Atomic absorption spectroscopy (AAS) (flame and graphite furnace) is well suited to the determination of Au, Pd, Rh and Pt (ppm and ppb levels, respectively). Ultratraces (ppb and sub-ppb levels) of noble metals can be determined in a large number of complex matrices by inductively coupled plasma mass spectrometry (ICPMS) with or without separation and preconcentration steps. A wide range of PGM concentrations, from percentage to ppm levels, can be determined by X-ray fluorescence (XRF) directly in solid samples or after pretreatment procedures (fire assay, coprecipitation, chromatographic preconcentration). Nuclear techniques (mainly neutron activation) are favored for the determination of low (ppb and sub-ppb) levels of precious metals (high sensitivities for Au, Ir, Pd and Os) in geological samples. Voltammetric measurements in combination with sample preparation, including the transformation of noble metals into electrochemically active compounds, provide high sensitivity (ppm and ppb levels) for the determination. The use of standard reference materials is essential to check the accuracy of the results obtained by various analytical techniques.