Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques
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ANALYTICAL SCIENCES JULY 2002, VOL. 18 737 2002 © The Japan Society for Analytical Chemistry Reviews Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques Maria BALCERZAK Department of Analytical Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Recent advances in digestion methods used in the analysis of precious metal samples by spectrometric techniques are reviewed. The applicability of a fire assay, a wet acid treatment, chlorination and alkaline oxidizing fusion to a quantitative recovery of metals from various materials is discussed. Data on the precious metal contents obtained by using particular digestion methods as well as UV-VIS spectrophotometry, atomic absorption spectrometry, atomic emission spectrometry and inductively coupled plasma mass spectrometry in the examination of various samples are tabulated. (Received November 26, 2001; Accepted April 4, 2002) 1 Introduction 737 5 Oxidizing Fusion 746 2 Fire Assay 738 6 Conclusions 748 3 Wet Acid Treatment 739 7 References 748 4 Chlorination 741 sensitivity, selectivity and reliability have been carried out. 1 Introduction Spectrometric techniques, ultra-violet visible (UV-VIS) spectrophotometry, atomic absorption spectrometry (both flame The members of the platinum group metals (PGM) (ruthenium, (FAAS) and graphite furnace (GFAAS) techniques), inductively rhodium, palladium, osmium, iridium and platinum) and gold coupled plasma combined with atomic emission spectrometry are called “precious” or “noble” metals. These names have (ICP-AES) or mass spectrometry (ICP-MS), are widely applied roots in the unique physical and chemical properties of the in the analysis of a variety of samples containing noble metals metals, owing to their low abundance and high economic value. over a large range of concentrations.3–14 Precious metals are colored and lustrous, exceptionally stable, UV-VIS spectrophotometry was historically the first hard, malleable, electrically resistant and inert to chemical instrumental technique used for the quantification of small attacks. amounts of noble metals in various materials. The technique Nobility and catalytic activity are unique properties of requires a quantitative conversion of the analytes into stable precious metals that result in their wide applications, e.g. as complexes that can act as the basis of spectrophotometric catalysts in various chemical processes, in electrical and measurements. The high chemical similarity of noble metals, electronic industries, and in jewellery.1 Growing demand for resulting in the formation of complexes of similar compositions rhodium, palladium and platinum has resulted from and properties, limits the direct application of UV-VIS autocatalysts production. Since 1978, platinum complexes, spectrophotometry in the analysis of multi-component samples. cisplatin and second-generation compounds, have been applied The developed methods are usually combined with separation in chemotherapy as effective anticancer drugs.2 The procedures.15 Recent generations of ultra violet-visible investigation of ruthenium complexes as an alternative to spectrophotometers that can operate in the derivative mode or platinum cancer inhibitors has recently become a subject of allow direct numerical processing of the absorption spectra of extensive studies. the examined mixtures have substantially extended the The large variety and complexity of examined materials, wide possibilities of spectrophotometric methods. Derivative concentration ranges to be determined (from ng g–1 and sub-ng spectrophotometry is a unique technique that allows the g–1 in geological, environmental and clinical samples to % elimination of the separation steps that are generally required levels in some industrial products), high inertness of noble prior to the detection of the analytes by classical metals towards many chemical reagents and high chemical spectrophotometric methods.16 Success in the application of similarities of numerous complexes formed make the choice of derivative spectrophotometry requires, however, the the analytical methodology for their determination in the sample simultaneous conversion of the metals into stable complexes. of interest a challenge. Extensive studies on the methods for the This can cause problems when examining multi-component determination of the metals in various matrices with satisfactory precious metal samples. The detection limits (DLs) offered by UV-VIS † E-mail: [email protected] spectrophotometry make the technique applicable to the 738 ANALYTICAL SCIENCES JULY 2002, VOL. 18 determination of relatively higher concentrations of the metals (SiO2), lead oxide as a collector (PbO) and a reducing agent (µg g–1 levels) as compared with the other spectrometric (flour, starch).3,17–23 It results in the extraction of noble metals techniques, i.e. AAS, ICP-AES and ICP-MS. The latter into a metallic lead button produced by a reaction of PbO with a techniques can be applied to the determination of trace amounts reducing agent under fusion. The matrix elements react with (ng g–1 and sub-ng g–1) of the metals. They have been widely the flux component to form a slag that is subsequently used for the determination of precious metals in a large variety discarded. The lead button is placed on a cupel prepared from of samples, such as ores, rocks, industrial products, waste bone ash or magnetite. Upon heating at a temperature of about solutions and solids, as well as clinical, biological and 800 – 850˚C in an oxidizing atmosphere, the lead oxide and the environmental materials. ICP-MS is particularly suited to the non-noble metal oxides are produced and absorbed into the determination of metals in various materials owing to the cupel where upon a bead of precious metals remains. The excellent detection limits (low pg levels), wide dynamic range, precious metal bead is treated with acids (HCl, HNO3) to extract possibility of accurate multi-element analysis and unique the analytes. capability of measuring isotopic ratios. The collection of precious metals from large samples (10 – 50 The direct application of spectrometric techniques to the g) of a complex matrix into a relatively small bead of simple detection of metals in complex matrices is limited due to metal alloys is the main advantage of the fire assay procedure. numerous interferences, both mutual and those from associated Success in the quantitative recovery of precious metals requires, base metals. Interference effects substantially arise in the however, an experienced and skilled assayer to optimize both analysis of non-homogeneous materials when the examination the flux composition and the fusion conditions. High amounts of large samples is required. Much consideration is to be taken of salts introduced to the sample are disadvantages of the fire for the elimination of interferences, particularly those from assay procedures. They may provide high procedural blanks common elements present in examined samples, when using the and difficulties in the direct analysis of the obtained solution by AAS detection technique. The interfering effects of matrix instrumental techniques. Another drawback of the classical fire elements may cause problems in the direct determination of assay using a lead collector is that it does not provide for an precious metals by the ICP-AES technique. Base elements, e.g. efficient recovery of all noble metals. Quantitative collection iron, copper, nickel, chromium, titanium, manganese and can be achieved for gold, silver, platinum and palladium. The vanadium, interfere with the ICP-AES signals of precious technique has found limited applications to the determination of metals.11 Numerous interferences limit the direct application of ruthenium, osmium, rhodium and iridium in complex matrices. ICP-MS to the quantification of metals in complex materials. The recovery of these metals is seriously affected by the flux Interferences from ions and polyatomic groups as well as from composition and experimental conditions. Serious losses of particular analytes can occur.4 ruthenium during the FA procedure were observed.24 Low concentrations of precious metals, particularly in Significant retention of the metal by the slag and the cupel geological and environmental samples, may require occurred. Losses of osmium due to retention by the slag or the preconcentration prior to detection. Volatilization, solvent volatilization of OsO4 formed during the cupellation were also 25 extraction, coprecipitation, sorption and chromatographic reported. Losses of ruthenium in the form of RuO4 during the methods are applied to separate precious metals from associated fusion and cuppelation were negligible. base metals, as well as particular analytes from their mixtures, Modifications to the lead fire assay procedure using silver or and to preconcentrate them to the levels detected by the used gold as collectors for the isolation of PGM from rocks, ores and instrumental technique. An effective combination of the minerals have been proposed.26–28 Combining the technique digestion procedure with the separation and detection steps with GFAAS allows the determination of gold, palladium, determines the reliability of the results. platinum and rhodium26 and of platinum, palladium, rhodium The problem of choosing the procedure for the digestion of and iridium27 down to ng g–1 levels. An alkaline cyanide the examined samples remains a fundamental one in spite of a solution has been used to dissolve