Metal analysis precipitation methods

Analytical Methods. Molybdenum contents in ore concentrates and technical oxide are most accurately deterrnined gravimetricaHy by precipitating lead molybdate. Molybdenum content is usually not determined on pure compounds or metal. Instead, spectrographic methods are used to measure impurity elements that must be controlled. Carbon and oxygen in metal products are measured by standard gas analysis methods. 

Although the co-precipitation method was initially conceived with large volume extraction coupled with flame atomic absorption analysis in mind, its utihty may prove to lie in other areas. In particular, the introduction of improved fiameless atomizers will allow the determination of trace metals using much smaller sample volumes. We have combined pre-concentration by this method on 100 ml samples with use of a Perkin-Elmer HGA-2100 graphite furnace for the analysis of copper. 

However, the presence of metal in pharmaceuticals, even in trace amounts, is a form of contaminant. For example, metallic ions may act as a catalyst in oxidation that may be detected in drug products. High-sensitivity methods and techniques for metal analysis are essential for quality control. The classical method of detecting metal contamination is the heavy metal testing described in the USP. This method is a precipitation test with H2S. The limit is observed by comparing with the standard solution. 

O. 8 and 1.0) were prepared by Co-precipitation method. Characterisation was done by X-ray phase analysis and DRS studies and by measurements of surface area and electrical conductivity. The amount of metallic copper and monovalent copper were estimated by reversible adsorption of CO respectively. Hydrogenation of nitrobenzene to aniline was carried out at 250°C in a fixed bed flow type reactor. Comparison of hydrogenation activity with CO adsorption data show that monovalent copper is more active than metallic copper for the hydrogenation of nitrobenzene. 

Probably the most widely used procedure for trace metal analysis of seawater over the last 20 years has been preconcentration followed by ETAAS because of the wide availability, good sensitivity, and large range of elements that can be measured by this method. Preconcentration procedures have in general been one of three types. First, co-precipitation, with iron(III) hydroxide or cobalt pyrrolidinedithio-carbamate, being the most widely used co-precipit-ants. Second, cornplexation followed by solvent extraction with a number of different complexants, of which dithiocarbamates, 8-quinolinol, and dithi-zone are particularly popular, and with extraction into a range of solvents. The third approach is extraction on to a chelating column (usually Chelex-100 but recently also other complexants such... 

The amount of iron and manganese in an alloy can be determined by precipitating the metals with 8-hydroxyquinoline, C9H7NO. After weighing the mixed precipitate, the precipitate is dissolved and the amount of 8-hydroxyquinoline determined by another method. In a typical analysis, a 127.3-mg sample of an alloy containing iron, manganese, and other metals was dissolved in acid and... 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Chemical analysis methods maybe used for assay of silver alloys containing no interfering base metals. Nitric acid dissolution of the silver and precipitation as AgCl, or the Gay-Lussac-VoUiard titration methods are used iaterchangeably for the higher concentrations of silver. These procedures have been described (4). 

Bromo-2-pyridyla2o)-5-diethylamiQophenol (5-Br-PADAP) is a very sensitive reagent for certain metals and methods for cobalt have been developed (23). Nitroso-naphthol is an effective precipitant for cobalt(III) and is used in its gravimetric determination (24,25). Atomic absorption spectroscopy (26,27), x-ray fluorescence, polarography, and atomic emission spectroscopy are specific and sensitive methods for trace level cobalt analysis (see... 

These methods may prove useful in the qualitative analysis of organic compounds, once the selectivities of the precipitants are understood. The metallic oxides suffer from the disadvantage of producing a precipitate which is difficult to filter, while calcite and zirconium phosphates produce relatively well-mannered precipitates. Even when the efficiencies of collection of various model compounds in seawater is known, the immense variety of organic compounds in seawater will keep this technique largely qualitative. 

Three methods for trace metal preconcentration were examined liquid-liquid extraction aided by a chelating agent, concentration on a synthetic chelating resin and reductive precipitation with NaBTLt. The latter method gave 1000-fold preconcentration factors with total recovery of Pb and other elements17. Preconcentration of nanogram amounts of lead can be carried out with a resin incorporating quinolin-8-ol (3)18. Enhancement factors of 50-100 can be achieved by such preconcentration procedures followed by determination in a FLA (flow injection analysis) system limits of detection are a few pg Pb/L19. 

Among the most important indirect methods of analysis which employ redox reactions are the bromination procedures for the determination of aromatic amines, phenols, and other compounds which undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The metal-oxine complex can then be determined by bromine substitution. 

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