[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Precipitation or coprecipitation from solutions is

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Precipitation or coprecipitation from solutions is widely used to concentrate elements before determining them by XRF analysis. Various coprecipitation agents (collectors) have been proposed in the literature. Hy- droxides of iron, aluminium, indium (III) or zirconium, diethyl- or pyrrolidine- dithiocarbamates are often used as collectors for heavy metals. Sometimes cocrystallization on 1-(2-pyridilazo)-2-naphtol, or coprecipitation on com- plicated collectors, such as, e.g., on indium (III) oxiginate in the pres- ence of tannin, are used. Separating phosphate and arsenate ions from aqueous solutions as low-soluble quinidinium phosphoro-(arseno-)molybdate [38] or bromides as AgBr [39] in sea water are examples of using precipitation as a preconcentration method for subsequent XRF analysis. In addition to paper filters and membranes, nuclear (Nuclepore) and glass fibre filters are used to collect precipitates from solutions. The concentrates obtained are separated from the filters and compacted or placed between two Mylar films in a special holder [40]. Concentration by means of precipitation or coprecipitation can be used to analyse many different materials, but this procedure is labour intensive and
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6 Specimen Preparation 417 difficult to automate. Separation of the precipitate from the filter may also result in an underestimation of the concentrations. A procedure such as cake compaction (pressing, pelletizing) is a good way to produce a specimen with a smooth surface [13]. A method to analyse for low concentrations in solution is to preconcentrate with ion exchange resins or adsorbents and then elute and evaporate onto filter paper discs. If elements of a low concentration are to be determined in a solid, the solid can be put into solution and the solution then passed through an ion exchange resin column for adsorption and subsequent analysis. Preconcentration factors of up to 4 × 10 4 can be achieved by using about 100 mg of a suitable ion-exchange resin [41]. An ion-exchange resin can also be used as a sample support, either by pelletizing the exchange resin or by using ion-exchange filter papers. This method is useful for the determination of metals in very dilute solutions. The procedure involves shaking the solution with a known amount of resin for several minutes, followed by filtering, drying, and pelletizing. By this method, e.g., 0.1 ppm of Au in a solution can be determined within less than 15 min [42]. Another possible method involves concentration of a litre of water on a cation-exchange resin column and the direct analysis of the resin by energy-dispersive XRF. This method can be used to measure trace metal contaminations of a few parts per billion in drinking water. Anion-exchange filters can be used to concentrate or selectively remove a certain component. An example is the determination of Cr(III) and Cr(VI) in an acid medium: an anion-exchange filter paper will collect Cr(VI), while Cr(III) passes through. Subsequently, toxic Cr(VI) can be directly analysed on the filter paper [43].
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  • Spring '14
  • MichaelDudley

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