Chapter 30 - Chapter 30 Introduction to analytical...

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Chapter 30 Introduction to analytical Separations An important part of most analyses is dealing with foreign species that either attenuate the signal from the analyte or produce a signal that is indistinguishable from that of the analyte. A substance that effects an analytical signal is called an interference or an interferent . Several general methods are used for dealing with interferences in an analysis; i) masking, ii) chemical or electrolytic precipitation, iii) distillation, iv) solvent extraction, v) ion exchange, vi) chromatography, and vii) electrophoresis.
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MASKING In masking, a reagent is added to the solution of the sample to immobilize, or chemically bind, the interferent as a complex that no longer contributes to or attenuates the signal from the analyte. A masking agent must not affect the behavior of the analyte significantly.
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PRECIPITATION AND FILTRATION Precipitation, in which the analyte or an interferent is removed from a solution selectively as an insoluble species, is one of the oldest methods for dealing with interferences in an analytical procedure. SEPARATING SPECIES BY DISTILLATION Distillation is widely used to separate volatile analytes from nonvolatile interferents.
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SEPARATING SOLUTES BY EXTRACTION The extent to which solutes, both inorganic and organic, distribute themselves between two immiscible liquids differs enormously, and these differences have been used for decades to accomplish separations of chemical species.
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Principles The partiton of a solute between two immiscible phases in an equilibrium phenomenon that is governed by the distribution law . If the solute species A is allowed to distribute itself between water and an organic phase, the resulting equilibrium may be written as A aq A org The ratio of activities for A in the two phases will be constant and independent of the total quantity of A; that is, at any given temperature, ( 29 ( 29 [ ] [ ] K a a A A A o r g A a q o r g a q = ≈
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…continued… The equilibrium constant K is known as the distribution constant . The concentration of A remaining in an aqueous solution after I extractions with an organic solvent is given by the equation where, [A] i is the concentration of A remaining in the aqueous solution after extracting V aq mL of the solution having an original concentration of [A] o with I portions of the organic solvent, each with a volume of V org . [ ] [ ] A V V K V A i a q o r g a q o = + i
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SEPARATING IONS BY ION EXCHANGE Ion exchange is a process by which ions held on a porous, essentially insoluble solid are exchanged for ions in a solution that is brought in contact with the solid. The ion-exchange properties of clays and zeolites have been recognized and studied since the late nineteenth century. Synthetic ion-exchange resins were first produced in 1935 and have since found widespread application in water softening, water deionization, solution purification, and ion separation.
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Ion-Exchange Resins Synthetic ion-exchange resins are high-molecular-weight
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Chapter 30 - Chapter 30 Introduction to analytical...

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