4 - Experimental Techniques

8/2/2019 4 - Experimental Techniques

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The importance of separation. To isolate the compound because sample may be tocomplex to be analyzed directly. Removal of only interferingcompounds/ interferents(chemical species that causes asystematic eror by enhancing the analytical signal or thebackground.

Principles of separation

Complete

separation and Partial Separation.

Extraction Techniques

Solid-Liquid extraction (SLE) : Like the brewing of tea,the lower MW polar cmpds dissolve in hot water areremoved from hiugh MW water insoluble cellulose,proteinand lipid materials.

Liquid-Liquidextraction(LLE) : commonmethod used in orglab to separate the

desired cmpd andunwanted material(work up of a rxnbefore purification). It is based on the difference indistribution/solubility of solute in two immiscible liquids(hexane/water) . The distribution of the two immisciblephases in eqm is governed by the Nernst distribution law. :A(aq) ↔ A(org)

Ideal Extraction Solvent used in LLE needs to havegood extracting where it has low solubility inwater;moderate volatility; Hi-Stability; Lo-Reactivity; Lo-flammability; Lo-Toxicity; Inexpensive & readily available inreq purity.-Less Dense than water = Diethyl ether, ethyl acetate,

hydrocarbons-More Dense than water = Dichloromethane

Gibbs free energy (measure of max amt of useful workfrom a system)(state function) G = H-TS. For a change occuring at constant T & P; ∆G= Σn∆G(pdt)-Σn∆G(rxts)=[Σn∆H(pdt) –Σn∆H(rxts)]–T.[ΣnS(pdt)–ΣnS(rxts) ]=∆H – T ∆S

Reaction Quotient, Q = [products]m/[reactants]n

Eqm constant, k = [products]meqm/[reactants]neqmIf [Q < K]→[Q/K < 1] →[ln Q/K < 0] → the rxn go right (∆G <0) fwd is Spon.If [Q > K]→[Q/K > 1] →[ln Q/K > 0] → the rxn go Left (∆G >0) fwd nonSponIf [Q = K]→[Q/K = 1] →[ln Q/K = 0] → the rxn @ eqm (∆G =0) ∆G = RT ln Q/K = RT ln Q – RT ln K ; ∆G° = RT ln 1 – RT ln K = 0 (Q=K) → ∆Go = -RT ln K ; K = exp(-∆G0/RT) ∆G = RT lnQ – RT lnK = ∆G0 + RT InQ = ∆G0 + RTIn{aA(org)/aA(aq)} = 0 (eqm) ∆Go = -RT ln {aA(org)/aA(aq)} ; K D = exp(-∆G0/RT) ={aA(org)/aA(aq)}ratio of activities for A in the two phases will be constant and independent of Aat any given temp

*Distribution ratio is affected by side dissociations.Purpose of K D is to find most efficient way toextract/separate; and also to calculate [remaining analyte]in the (aq)soln after certain no. extractions.Fraction extracted in the org phase if FA and that thefraction remaining in (aq)phase(G) after 1 cycle of extraction

Extraction involving Acid-Base EquilibriaWhen a weak monoprotic acid HA is ectracted from water toorg solvent :1)Dissociation of weak monoprotic acid HA in (aq)phaseonly 2)Partition of HA in [aq] and [org] phase

D in terms of fraction of HA in (aq)phase.When extraction of HA is done into an [org]solvent, it ismore effective at a low pH where D increases. Extraction of Base is more effective at hi-pH. Opposite applies forextraction into [aq]solvent.

Extracting Inorganic species (Metal Ion)Mn+

(aq) + nHL(org) ↔ MLn (org) + nH+(aq)

Metal ion in [aq]phase can be extracted to the [org]solventwhen it is complexed with a ligand or chelates (attach tometal via >1 atom)1. Mn+: Free metal ion will be found in [aq] phase only

2. HL: Ligand (HL) is partitioned between [aq] and org phase3. MLn : Metal complex formed will be partitioned btwn [aq]&[org] phases4. H+: Dissociation of ligand occurs in [aq] phase only



Separations of Metal Ions byextraction The separation is

better if the pH1/2of the 2 metal ionsdiffer by 3 pH units(1000 x) or more.Otherwise the quantitative separation cannot be achieved.When the extraction curves of the 2 metal ions areseparated by less than 3 pH units, it is necessary to maskone of the ions to maintain selectivity and quantitativeseparation of the desired metal over the other.Masking: to prevent one species from interfering in theanalysis of another. Masking can be done by adding anotherchelating agent capable of binding to the unwanted ion(s) =masking agent. Since the unwanted ion(s) now is ‘invisible’,it will not be extracted into organic layer. The maskingagent has to selective to the undesired ion, and be morestable than the metal chelating agent complex. It must also

be charged and water soluble to stay in the [aq]phase~contrast to the extracting agent (into org)

Small Summary

Parameters to consider in LLE

1) Choice of Solvent : Solvent differs in physical ppts(polarity;density) and should be able to remove matrixinterferent

2) Treament of Emulsions ( due to mixing atinterfacial surface) :↑[surfactant] or ↑mutual solubility or ↓Surface tension =↑emulsionWays to get rid of it, Time&patience to separate; if waterone of the solvent, add c.NaCl to dehydrate org layer ordetergent to remove org layer; if total Vol<13ml,centrifudge; Gravity filtration via cotton

3) Stripping and Backwashing

Kinetics Of Extraction : Rate of formation of complexmay be slow. When mass transfer of not very rapid, it is

possible to separate using difference in speed of formationof complexes. This is mostly possible in extraction of orgniac species(metal ion) using chelating agents.

Acidifying the medium : Relatively volatile analyte maybe lost my evaporation. To overcome this problem, addition



of acid may be requires to promote the salt formation sothat the analyte will remain in the aqueous phase.

Batchwise single stage extraction : Suitable if F>0.99or DRv>100; usually perfoemd in small scale chemistry labsusing a separating funnel. Successive batch extarction toimprove fraction extraction.

Multistage countercurrent continuous process :Suitable if fraction extracted is low, small DRv. Used for raremetal extraction in dustry. Many extraction stages arerequired. Solvent distileld from extract-colelction flask,contacted with aqueous phase and resutrned to flask in acontinuous manner.

Soxlet Extraction (Continuous)

Solid-Liquid Extraction where analyte is extractonfrom a solid medium.Only required where the desired compoundhas a limited solubility in a solvent and theimpurity is insoluble in that solvent. Choosesolvent that will extract the analyte only and

repeat extraction due to the limitedsolubility. If the solubility os the desiredcompound and the impurities aresignificantly different, then simple filtrationcan be used.1) Heating solvent in round bottomed flask,vapors form and condense.

2) Liquid enter the Soxhlet chamber and immersion of thepermeable cellulose extraction thimble that holds the solidsample occurs.3) The solvent extracts the compounds of interest andleaves the solid mass behind. (Indication: Solvent maychange color)4) As the solvent level rises, the solution is forced throughthe small inner tube and the chamber is flushed due to asiphoning effect.5) The solvent resutrs to the RBF taking the extractedcompounds with it. 6) After many cycles the desiredcompound is concentrated in the RBF. The extractingsolvent is removed via rotary evaporator, yielf the extractedcompound.

Limitations of LLE : Large volumes of solvent isconsumed and has to be disposed, labor intensiveand slow.

Solid-phase Extraction (SPE)/ Solid-liquid Extraction :the analyte is extracted from liquid medium has it is flowedthrough the solid extracting phase (sorbent). This uses waylesser amount of solvent as compared to LLE and can beautomated and provides faster extraction time whenpressurerised so liquid is pushed/pulled through the solvent.Successful SPE : Rapid sorption, high and reproducible%solutes taken up by sorbent. and that solutes must beeasily and completely removed later. The Sorption-desoription process must be reversible and sosorbent is hydrophobic and oleophillicAproaches : analyte of interest retained in sorbentcartridge or the other way around.Before sorbent cartridge is used, it has to be conditioned( remove any adsorbed material using a range of solvents)Elution is started with weak solvent, impurities having weakinteractions with the sorbent will be aluted out. Now thesolven str/polarity can be increase to elute out solutes thatare moderately retained by sorbent ( bit by bit approach)Types of interaction in SPE : Intermolecular hydrogenbonding between polar analytes & un/modified silicasorbents. Analyte may/may not interact with solid phasesorbent via IMF; ionic, or even reacting with it.

Solid-phase micro extraction (SPME) : Instead of liquidpassing through the sorbent, the sorbent immersed intoliquid sample, A fused(amorphous non crystalline) silicafiber coated with a non volatile solid polymer adsorbent thinfilm used to extract organic analytes from aq samples. Thisis a SAMPLE prep extraction technique and is generally usedfor an analyte collection for determination by gaschromatograpy(GC). It allows the combi of analyte prep andpre-concentration in a single step.Approaches : Extraction of volatile / non-volativeextraction Extraction is complete when the [analyte]reaches distribution eqm between sample matrix & fusedsilica fibre absorbent thin film coating. To speed up :agitate

Adsorption-DesorptionSPME is used for analyte prep and pre[] for GC. Followngadsorption, the analyte is usually thermally desorbeddirectly in the injection port of a GC for analysis. The mostcommon adsorbent thin film is Polydimethylsiloxane(PDMS). Various polymeric coatings for various types of analytes are commercially available.SPME Advantage : Solventless extraction; integratesextration, analyte preparation & pre[] in a single step; Fastsimple and easy to automate; great potential for field app(on site sampling). When properly stored, the loaded firbrecan be transported to the lab and analysed days laterwithout significant loss of analytes. Fibre is reusable.SPME Limitations : Fibre is fragile; loss of analyte duringtransfer to GC injection port.

Other extraction techniques.1) Extraction using super critical fluid (CO2) :•

Diffusion constant is in between gas and liquid phases.• Density of SCF is close to that of liquid.• Good solvating power(like liquid) so analyte can be non

volatile.• Viscosity, minimal surface tension(like gas).• High diffusity, rapid mass tranfer and can penetrate

better the pores



• Extraction is fast and high efficiency

• The extracted material is easily recovered by simplydepressurizing, allowing the SCF to return to gas phaseand evaporate.

Common SCF : CO2 , modifier can be added to adjust thepolarity of SCF which improves the extraction efficiency of analyte. However, CO2 is not a good medium for extractionof highly polar compounds as it is non polarExtraction using SCF is done to facilitate the extraction of organic material from solid samples.

Decaffeination Process (due to health issues of caffeine)• Causes aroma and flavor losses.

• SCF CO2 has been used to extract the aroma and flavorbefore decaffing and is returned to the coffee beans/tealeaves therafter• (1)Swelling raw beans with water (2) extrating caffeinefro beans with organic solvent (3) steam stripping toremove all residual solvent from the beans (4) drying thedecaff-ed beans.

**Many solvent have been used : Benzene, methylene

chloride..

Microwave assisted Extraction (MAE)

• When microwave radiation is applied to gas it willabsorb the energy and change the mass rapid transfer rate.In liquids and solids, the molecules do not rotate freely andhas to be heated.

• Heating process is due to the ionicconduction(movement of ions generating heat via friction)and dipolar polarization( reorientation of dipoles undermicrowave). ↑dipole moment ↑vigorous oscillation• Up to 40 samples simultaneously extracted and havecontactless temp/pressure control. The system uses 90%less solvent than Soxhlet (environmentally friendlytechnique)

• Solid sample placed into vessel w extracting solvent

and heated up to 10000C in 2min. Pressure-relief lid willrelease vapor if it exceeds set limit.• Application : Microwave Decomposition/pressuredigestion of oragnic substances @↑T↑P, leaving behindonly the inorg components. If process is done in(l):_______________ (g):______________

Advantages of MAE : ↑process economics (↑extractionyield/purity); ↓energy costs/time; ↑products (↑extractselectivity ↓heat degradation, potential for uniqueproducts) Environmentally Responsible (↓green house gasemission/pollutants ↓solvent use↓toxicityLimitations of MAE : Potential explosion hazard; Higherinstrument costs; limited to small scale extraction.

Ultrasonic extraction : Uses altrasonic vibration toensure contact between sample and extractin solvent. It isfast but extraction efficiency is not as high as some othertechniques.

Ion Exchange : Ions held on an ion exchange areexchanged for ions in a solution brougth into contact with

the resin. Ion exchange resins are high molecular weightpolymers that contain lalrge number of ionic functiongroups. They are porous and are essentially insoluble solids.

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4 - Experimental Techniques