Analytical Chem istry - DePauw University

322 Analytical Chemistry 2.0(a)(b)thermometerthermometerdistillationadaptordistillationadaptorcondensercondenserdistillation flaskfractionatingcolumnreceiving flaskreceiving flaskdistillation flaskFigure 7.18 Typical experimental set-up for (a) a simple distillation, and (b) a fractional distillation.sublimationchambercrude sampleheatcooling watervacuum inletsublimed analyteFigure 7.19 Typical experimental setupfor a sublimation. The sample isplaced in the sublimation chamber,which may be evacuated. Heating thesample causes the analyte to vaporizeand sublime onto a cooled probe.Modified from Slashme (commons.wikipedia.org).equilibrium. The liquid phase at point C has a lower boiling point thanthe original mixture, and is in equilibrium with the vapor phase at point D.This process of repeated vaporization and condensation gradually separatesthe analyte and interferent.Two experimental set-ups for distillations are shown in Figure 7.18. Thesimple distillation apparatus shown in Figure 7.18a does not produce a veryefficient separation, and is useful only for separating a volatile analyte (orinterferent) from a non-volatile interferent (or analyte), or for an analyteand interferent whose boiling points differ by more than 150 o C. A moreefficient separation is achieved by a fractional distillation (Figure 7.18b).Packing the distillation column with a high surface area material, such asa steel sponge or glass beads, provides more opportunity for the repeatedprocess of vaporization and condensation necessary to effect a completeseparation.When the sample is a solid, sublimation may provide a useful separationof the analyte and the interferent. The sample is heated at a temperatureand pressure below the analyte’s triple point, allowing it to vaporizewithout passing through the liquid state. Condensing the vapor recoversthe purified analyte (Figure 7.19). A useful analytical example of sublimationis the isolation of amino acids from fossil mollusk shells and deep-seasediments. 1313 Glavin, D. P.; Bada, J. L. Anal. Chem. 1998, 70, 3119–3122.