D. Harvey - Modern Analytical Chemistry (794078), страница 63
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The importance of sulfideas a precipitating reagent for separations is due to two factors: most metal ions, except for the alkaline earths and alkaline metals, form insoluble sulfides; and the solubilities of these metal sulfides show a substantial variation. Since the concentrationof S2– is pH-dependent, control of pH was used to determine which metal ionswould precipitate.
For example, in Fresenius’s gravimetric procedure for the determination of Ni in ore samples (see Figure 1.1 in Chapter 1 for a schematic diagramof this procedure), sulfide is used three times as a means of separating Co2+ andNi2+ from Cu2+ and, to a lesser extent from Pb2+.7F.5 Separations Based on a Partitioning Between PhasesThe most important class of separation techniques is based on the selective partitioning of the analyte or interferent between two immiscible phases.
When a phasecontaining a solute, S, is brought into contact with a second phase, the solute partitions itself between the two phases.Sphase 1t Sphase 27.18The equilibrium constant for reaction 7.18KD =[S phase 2 ][S phase 1]is called the distribution constant, or partition coefficient. If KD is sufficiently large,then the solute will move from phase 1 to phase 2. The solute will remain in phase 1,partition coefficientAn equilibrium constant describing thedistribution of a solute between twophases; only one form of the solute is usedin defining the partition coefficient (KD).1400-CH07 9/8/99 4:04 PM Page 212212Modern Analytical ChemistryextractionThe process by which a solute istransferred from one phase to a newphase.however, if the partition coefficient is sufficiently small. If a phase containing twosolutes is brought into contact with a second phase, and KD is favorable for only oneof the solutes, then a separation of the solutes may be possible.
The physical states ofthe two phases are identified when describing the separation process, with the phasecontaining the sample listed first. For example, when the sample is in a liquid phaseand the second phase is a solid, the separation involves liquid–solid partitioning.Extraction Between Two Phases When the sample is initially present in one of thephases, the separation is known as an extraction. In a simple extraction the sampleis extracted one or more times with portions of the second phase. Simple extractions are particularly useful for separations in which only one component has a favorable distribution ratio. Several important separation techniques are based onsimple extractions, including liquid–liquid, liquid–solid, solid–liquid, and gas–solidextractions.Phase 2Phase 1Figure 7.15Separatory funnel for use in a liquid–liquidextraction.Syringe needleLiquid–Liquid Extractions Liquid–liquid extractions are usually accomplishedwith a separatory funnel (Figure 7.15).
The two liquids are placed in the separatory funnel and shaken to increase the surface area between the phases. When theextraction is complete, the liquids are allowed to separate, with the denser phasesettling to the bottom of the separatory funnel. Liquid–liquid extractions alsomay be carried out in the sample container by adding the extracting solvent whenthe sample is collected. Pesticides in water, for example, may be preserved forlonger periods by extracting into a small volume of hexane added to the sample inthe field. Liquid–liquid microextractions, in which the extracting phase is a 1-µLdrop suspended from a microsyringe (Figure 7.16) also have been described.16 Because of its importance, a more thorough discussion of liquid–liquid extraction isgiven in Section 7G.Solid-Phase Extractions In a solid-phase extraction the sample is passed through acartridge containing solid particulates that serve as the adsorbent material.
For liquid samples the solid adsorbent is isolated in either a disk cartridge or a column(Figure 7.17). The choice of adsorbent is determined by the properties of the speciesbeing retained and the matrix in which it is found. Representative solid adsorbentsDirection ofsolvent flowMicroliter dropof extractionsolventDirection ofsolvent flowPorous retainerFigure 7.16Schematic of a liquid–liquid microextractionshowing syringe needle with attached 1-µLdroplet.Solid adsorbent(a)Porous retainerFigure 7.17Solid-phase extraction cartridges: (a) disk cartridge;(b) column cartridge.(b)1400-CH07 9/8/99 4:04 PM Page 213Chapter 7 Obtaining and Preparing Samples for Analysisare listed in Table 7.8.
For example, sedatives, such as secobarbital and phenobarbital, can be isolated from serum by a solid-phase extraction using a C-18 solid adsorbent. Typically a 500-µL sample of serum is passed through the cartridge, with thesedatives being retained by a liquid–solid extraction. The cartridge is then washedwith distilled water to remove any residual traces of the serum’s matrix. Finally, theretained sedatives are eluted from the cartridge by a solid–liquid extraction using500 µL of acetone. For many analyses, solid–phase extractions are replacingliquid–liquid extractions due to their ease of use, faster extraction times, decreasedvolumes of solvent, and their superior ability to concentrate the analytes.
The lastadvantage is discussed in more detail in the final section of this chapter.Solid-phase microextractions also have been developed. In one approach, afused silica fiber is placed inside a syringe needle. The fiber, which is coated with athin organic film, such as poly(dimethyl siloxane), is lowered into the sample by depressing a plunger and exposed to the sample for a predetermined time.
The fiber isthen withdrawn into the needle and transferred to a gas chromatograph foranalysis.17In gas–solid extractions the sample is passed through a container packed with asolid adsorbent. One example of the application of gas–solid extraction is in theanalysis of organic compounds for carbon and hydrogen. The sample is combustedin a flowing stream of O2, and the gaseous combustion products are passed througha series of solid-phase adsorbents that remove the CO2 and H2O.Continuous Extractions An extraction is still feasible even when the component ofinterest has an unfavorable partition coefficient, provided that all other componentsin the sample have significantly smaller partition coefficients.
Because the partitionTable 7.8Selected Adsorbents for Solid-Phase Extractionof Liquid SamplesAdsorbentSurface StructureProperties and Usessilica—O—Si—O—Si—O—||HOOHalumina—O—Al—O—Al—O—||HOOH• retains low-to-moderate polarity speciesfrom organic matrices• fat-soluble vitamins, steroids• retains hydrophilic species from organicmatricescyanopropyl—C3H6CNdiol—CH2—CH2—||OH OHoctadecyl (C-18)—C18H37octyl (C-8)styrenedivinylbenzene—C8H17• retains wide variety of species fromaqueous and organic matrices• pesticides, hydrophobic peptides• retains wide variety of species fromaqueous and organic matrices• proteins, peptides, fungicides• retains hydrophobic species fromaqueous matrices• caffeine, sedatives, polyaromatichydrocarbons, carbohydrates, pesticides• similar to C-18• wide variety of organic species fromaqueous matrices• polyaromatic hydrocarbons2131400-CH07 9/8/99 4:04 PM Page 214214Modern Analytical ChemistryWater-cooledcondenserExtractionthimbleUpperreservoirSampleReturn tubeLowerreservoirFigure 7.18Schematic diagram of a Soxhlet extractor.purge and trapA technique for separating volatileanalytes from liquid samples in whichthe analytes are subsequently trapped ona solid adsorbent.coefficient is unfavorable, a simple extraction will not be quantitative.
Instead, theextraction is accomplished by continuously passing the extracting phase throughthe sample until a quantitative extraction is achieved.Many continuous extractions involving solid samples are carried out with aSoxhlet extractor (Figure 7.18). The extracting solvent is placed in the lower reservoir and heated to its boiling point. Solvent in the vapor phase moves upwardthrough the tube on the left side of the apparatus to the condenser where it condenses back to the liquid state.
The solvent then passes through the sample, whichis held in a porous cellulose filter thimble, collecting in the upper reservoir. Whenthe volume of solvent in the upper reservoir reaches the upper bend of the returntube, the solvent and any extracted components are siphoned back to the lowerreservoir.
Over time, the concentration of the extracted component in the lowerreservoir increases.Soxhlet extractions have been replaced in some applications by microwaveassisted extractions.18 The process is the same as that described earlier for microwavedigestion. The sample is placed in a sealed digestion vessel along with the liquid extraction phase, and a microwave oven is used to heat the extraction mixture. Usinga sealed digestion vessel allows the extraction to take place at a higher temperatureand pressure, thereby reducing the amount of time needed for a quantitative extraction. In a Soxhlet extraction the temperature is limited by the solvent’s boiling pointat atmospheric pressure.
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