lec07.solvent_select

lec07.solvent_select - Solvent Selection for Separation...

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Solvent Selection for Separation
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SOLVENT SELECTION FOR SEPARATION Processes Requiring Solvents Extraction Partition Fractional Crystallization Extractive Distillation Liquid Chromatography Gas-Liquid Chromatography
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SOLVENT SELECTION FOR SEPARATION “Peripheral” Properties of the Solvent Factors that usually don’t affect efficiency of the separation but are of interest Synder defines a solvent “as either a pure compound or a mixture of pure solvents.” Binary and ternary solvent mixtures afford a wider range of solvents to choose than pure solvents
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PERIPHERAL PROPERTIES Boiling Point Properties Easily evaporated or removed bp 10-50 C higher than the temperature of separation Minimize accidental evaporation Diethyl ether Volatile samples – fractional distillation to remove solvent or sample Boiling pt. (bp) Normally select a solvent with bp above that of the operation. Which distillation technique would you not want the above property?
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PERIPHERAL PROPERTIES Viscosity Low viscosity solvents preferable (General rule) Liquid chromatography = poorer separation Low viscosity coincides with low bp Exceptions: very polar solvents (alcohols) & compact molecules (cyclalkanes, aromatics, CCl 4 ) Low viscosity enhances diffusion which speed separation Viscosity of a solvent mixture is usually intermediate between those of the pure solvents, ie binary mixture A & B
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Take home – it is possible to use a viscous solvent when in a mixture Peripheral Properties Viscosity of Mixtures η =( η a ) xa ( η b ) xb
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PERIPHERAL PROPERTIES Viscosity of Water-Organic Solvent Mixtures
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PERIPHERAL PROPERTIES UV Cutoff -Solvent may interfere with detection Appendix A shows minimum UV cut off for solvents Solvent Properties Affecting Detection What solvents might be poor choices for use with UV detection?
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PERIPHERAL PROPERTIES Refractive Index Maximize differences in refractive index between sample and solvent (Appendix A) Solvent Properties Affecting Detection Note the relatively small differences What does this mean in relation to detection?
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PERIPHERAL PROPERTIES Specific Element Content Common Gas Chromatography Detectors: Method Element Solvent Electron Capture Cl Chloroform Flame Thermionic N Acetonitrile Flame Photometric S Dimethyl Sulfoxide Solvent Properties Affecting Detection
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SOLVENT MISCIBILITY CHART Peripheral Properties Appendix C in your notes
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Toxicity Flammability Reactivity Cost Disposal PERIPHERAL PROPERTIES
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FACTORS AFFECTING SOLUBILITY AND SEPARATION If the two solvents are immiscible, they can be shaken together and they will separate. If an analyte represented as ‘x’ is placed in one of the solvents before mixing, where will the analyte be after mixing?
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Cx,a, Cx,b, concentration of solute x in solvents A & B; R, gas constant (1.99 cal/ o
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lec07.solvent_select - Solvent Selection for Separation...

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