DISTILLATION. p B = x B p B

Transcription

1 DISTILLATION Distillatin is the mst imprtant methd fr separatin and purificatin f liquids. This prcess can be simply described as bringing the liquid t a bil in ne vessel, and subsequently cndensing the vaprs in the ther vessel. Thus, a liquid-vapr transitin ccurs during biling, and a vapr-liquid transitin during cling, i.e., cndensatin, will be taking place. If the biling f the liquid and the cndensatin f its vaprs is dne in ne vessel, the prcess will be called reflux. Upn biling, the liquid and vapr phases are in equilibrium: the number f mlecules that are underging a liquid-vapr transitin will be equal t the number f mlecules that are underging a vapr-liquid transitin. Therefre, the vapr pressure f a liquid is simply a measure f the ability f the mlecules t leave the surface f the liquids. The number f mlecules in each phase will depend n the temperature, pressure as well as n the intermlecular interactins in the liquid phase. Fr example, hydrgen bnding can significantly increase the vapr pressure f the liquid, thus making it less vlatile as cmpared t an analgus cmpund that cannt engage in intermlecular hydrgen bnding. In rder t shift that equilibrium, ne f the cmpnents, in this case it will be the vapr phase, shuld be remved frm the system. Quantitatively, fr the ideal slutins, which are devid frm any intermlecular interactins, the relatinship between the rati f the liquids A and B and theirs respective vapr pressures in the mixture is described by Rault s law: p A = x A p A p B = x B p B where, p A and p B are the partial vapr pressures f A and B in the mixture; p A and p B are the vapr pressures f the pure A and B; x A and x B are the mle fractins f A and B in the mixture. Obviusly, x A + x B =. Pictrially the abve crrelatins are shwn in Figure. Mst liquids, especially thse that are structurally similar, wuld bey the behavir f the deal slutins. Hwever, when structurally p A pressure ttal pressure A mle fractin (% cmpsitin) B Figure. p B p A p B dissimilar liquids are in the mixture, strng deviatins frm linearity are t be expected. Thus, fr the ideal slutins, the partial vapr pressure f a cmpnent is a cnstant, which des nt depend n vapr pressures f the ther cmpnents in the mixture. Fundamentally, this can be ratinalized by cnsidering the rates at which the mlecules will be underging liquid-vapr and vapr-liquid transitins: the rate at which mlecules f A leave the liquid phase will be affected by the presence f the mlecules f B, whereas the rate at which A will return t the liquid, i.e., underg vapr-liquid transitin, will nt be hindered by B. It shuld als be nted that the sum f partial pressures f the cmpnents in the mixture, e.g., p A and p B, will cnstitute the ttal vapr pressure: p A + p B = p TOTAL. This is knwn as Daltn s law. The mment the vapr pressure becmes equal t the external pressure will be classified as a biling pint f the liquid. If the external pressure remains the same, a pure liquid will bil at a cnstant temperature r a narrw temperature range. It is imprtant t nte, that the vlatility f the liquid ges up as the ttal pressure decreases. Qualitatively, this is in accrdance with Rault s law, which relates the vapr pressure f a liquid t

2 the pressure f individual cmpnents f the mixture and t the ttal pressure. T apprximately estimate the biling pint at a given pressure, the knwledge f the biling pint at a knwn pressure is required. In general, reducing the external pressure in half will lead t ~ 5 C reductin in the biling pint. Fr example, a liquid with a biling pint f 200 C at the atmspheric pressure (760 mmhg) will bil at abut 85 C at 380 mmhg); this liquid will bil at ~ 70 C at 90 mmhg f pressure. Fr a precise determinatin f the effect f pressure n the biling a pressure-temperature nmgraph shuld be cnsulted (Figure 2). The value f distillatin cmes in separating the mixtures f liquids. Depending n the nature f the mixture that needs t be separated, different distillatin techniques can be used. Figure 2. SIMPLE DISTILLATION Simple distillatin is best used fr liquids with C biling pint range. Abve 50 C many liquids will tend t decmpse (in this case a vacuum distillatin shuld be used, belw), liquids with biling pint belw 40 C are difficult t distill withut significant lsses. Simple distillatin is applicable fr separatin f liquids with biling pints differing by mre than 80 C. The required set-up fr the simple distillatin is shwn in Figure 3. A rund bttm flask (referred t as a distillatin flask since it cntains a liquid that needs t be distilled) is placed int the heating mantle, which usually is filled with sand t prvide gd thermislatin. The distillatin flask is equipped with a biling chip, fllwed by a stillhead. The tp jint f the stillhead (als knwn as 2

3 distillatin head) is cnnected t a thermmeter adapter with the neprene fitting, and then the thermmeter. The psitin f the thermmeter s bulb is crucial fr the crrect determinatin f the temperature (Figure 3). The tp f the thermmeter bulb shuld be aligned with bttm f the side-arm if the stillhead, i.e., the pint where the vapr will start t cndense; placing the bulb abve r belw the arm will lead t lwer r higher biling pint reading, respectively. All the femalet-male cnnectins must be clean and dry t fit tightly; this is dne t avid pssible release f distillate vaprs and the lss f the distillate as well as t prevent the jints frm being stuck tgether. Tefln tape shuld be used t insure a prper seal between the jints. The cndenser is Figure 3. attached t still head n ne end, and t the vacuum adapter n the ther end using keck-clips. The water hses are attached as shwn in the Figure 3: water shuld cme in at the bttm f the cndenser and leave frm the tp utlet. Fr safety reasns, the cndenser is usually clamped in the middle. A receiving flask is then cnnected t the vacuum adapter using a keck-clip. The use f keck-clips and clamps assures that the distillatin set-up is stable thrughut the distillatin. The receiving flask shuld be lwer than the distillatin flask t allw a free flw f the cndensed liquid int the receiving flask. Imprtantly, always place the heating mantle n a labjack. This allws fr quick and cnvenient remval f heat, if necessary t prevent the distilling mixture frm verheating. It is imprtant t keep in mind that the systems shuld always be pen t the atmsphere. Clsed systems shuld never be heated; the pressure build-up f the heated vapr will lead t an explsin. In case f a simple distillatin, the vacuum adapter serves as an pening t the atmsphere. Fr the distillatin f misture sensitive liquids, the vacuum adapter is fitted with a drying tube, which is packed with CaCl 2 r any ther apprpriate drying agent. The drying tube shuld be assembled fresh fr each distillatin since the absrbed misture leads t melting f the drying agent and creates plugs, which are nt air-permeable; thus leading t a clsed system. Als, in cases when the liquid is either vlatile r temperature sensitive, the receiving flask is submersed int an ice bath. A biling chip shuld always be used when a distillatin is carried ut. The biling chip (a.k.a. biling stne) is usually a piece f glass, ceramic, etc. The purpse fr the biling chip is t remve temperature gradient and t assure a smth bil. A mre sphisticated set-up invlves the use f the stirring/heating plate and a stirring bar. Smetimes it is beneficial t knw hw the temperature will influence the vlume f the distilled liquid. A graduated cylinder is used as a receiving flask. The vacuum adapter shuld be placed right at the tp f the cylinder t minimize the ptential lss f the liquid t spillage. Abut 0-5 data pints are required t btain. In general, the distillatin curve has a few distinct phases 3

4 (Figure 4). Distillatin between pints and 2 crrespnds t sme lwer biling impurities, and the crrespnding amunt f liquid shuld be discarded. Once the impurities are remved the temperature starts t stabilize, frm 2 t 3. A pure liquid will be distilling at a very narrw temperature range; this crrespnds t the regin between 3 and 4. At the end f distillatin the temperature tends t rise, simply because the vaprs are much easier distilled vlume, ml verheating than the liquid. Figure 4. Imprtantly, fllwing the abve descriptin, the first fractins f the distillate shuld always be discarded, and a new receiving flask shuld be cnnected t the vacuum adapter. It shuld be kept in mind that the liquid shuld never be distilled t dryness! A small amunt f liquid, usually a dime size, shuld always be left in the distillatin flask. Since vaprs can be heated t much higher temperatures than liquids, verheating and ptential explsin might ccur when the distillatin flaks is heated after the slvent is remved. temperature, C FRACTIONAL DISTILLATION Liquids f cmparable vlatility cannt be sufficiently separated by simple distillatin, i.e., by a single vaprizatin cndensatin sequence. In principle, multiple simple distillatins shuld yield a single cmpund frm the mixture. Hwever, this is a tedius and nn-ecnmical prcess. A better way is t use fractinal distillatin, which prvides a pssibility fr separating liquids that have a biling pint difference f less than 80 C. The set-up fr the fractinal distillatin is quite similar t the simple distillatin, with the nly imprtant exceptin the distillatin flask is cnnected t the stillhead with an additinal clumn (Figure 5). This clumn, knwn as a fractinal clumn, is packed with an inert material t increase the surface area, such as glass beads, metal turnings, etc. In the fractinal clumn the vapr and the cndensate are cnstantly mving in the ppsite directins. The vapr is cndensing n the packing material and returning t the distillatin flask. The cmpsitin f the vapr and liquid will be changing in favr f mre vlatile cmpnent as ne mves up the fractinal clumn frm the distillatin flask t the stillhead. Multiple single vaprizatin-cndensatin cycles are taking place in the packed clumn. The single cycle is usually referred t as a theretical plate. In general, the lnger the clumn and the larger the surface area f the packing material, the mre efficient the separatin between the mixture cmpnents. In ther wrds, the mre theretical plates, the better the clumn. Theretical ratinale fr the fractinal distillatin is shwn in Figure 6. The regin between the lines is a tw-phase regin; the upper curve (vapr cmpsitin) is derived frm Rault s law; the lwer curve defines the biling pint f liquid at a Figure 5. 4

5 given rati f the cmpnents. When a given mixture f A and B, i.e.,, is heated t the temperature T, it will start t bil. At this temperature, the vapr cmpsitin will have a cmpsitin f A and B that is reflected by 2 (nte that and 2 have different mle fractins f A and B). If that vapr pressure is cndensed t a liquid phase, the resulting mixture 3 will have a different biling pint, T2. The vapr cmpsitin at T2 cntains a distinctly different mle fractins f A and B than at T. Ntably, the mle fractin f B in the mixture is prgressively increased as tw biling-cndensatin cycles were carried ut, since the fractin f B, which is the mre vlatile cmpnent, is cnstantly increasing in the vapr phase. Thus, Figure 6 describes a tw-theretical plates separatin prcess. temperature, C vapr cmpsitin T T2 biling pint 2 A x A, mle fractin f A x B, mle fractin f B Figure 6. 3 B VACUUM DISTILLATION Only thse cmpunds that are stable at their biling pint can be distilled at atmspheric pressure. If the cmpund decmpses at elevated temperatures, vacuum distillatin has t be used. Als, if the biling pint f the cmpund is higher than 50 C, vacuum distillatin shuld be used. The set-up fr the vacuum distillatin is virtually identical t either simple r fractinal distillatins, with the nly difference that the vacuum adapter is cnnected t a vacuum pump. Reduced pressures are btained either by water aspiratr pump r by a mechanical vacuum pump. Mst mdern labratries are equipped with central, s called in-huse vacuum. Imprtantly, the distillatin system shuld be placed under vacuum befre the heating is initiated. Vacuum shuld never be applied t ht r refluxing slutins! The biling temperature is strngly dependent n pressure: nce the pressure drps, the biling pint decreases, and the liquid is quickly sucked frm the distillatin flask thrugh the cndenser int the receiving flask, and subsequently int the pump. Smetimes, liquids cntain small amunts f lw biling impurities, which will cause sme faming/bubbling nce the vacuum is applied. In this instance, the vacuum shuld be adjusted. This peratin might be repeated several times, until the surface f the liquid is quiet. Once the distillatin is cmplete, the heating mantle is remved, and the whle set up is allwed t cl. After that, the vacuum shuld be slwly released. Anther distinctin f the vacuum distillatin is that the receiving flask shuld always be a rund bttm flask, and never a flat bttm flask, such as an Erlenmeyer flask. Glassware shuld always be examined fr cracks, since the weak glass will shatter under pressure. AZEOTROPES Mst hmgeneus slutins fllw the behavir f ideal slutins, and thus they fllw Rault s law. Hwever, there are certain cmbinatins f particular liquids that exhibit deviatins frm the ideal slutin: the vapr pressure f these liquids is either higher r lwer than that predicted by Rault s law. This phenmenn implies that the interactins between the cmpnents are causing the deviatin frm ideal slutin behavir. These nn-ideal mixtures are cmmnly called azetrpes. Azetrpes cannt be separated by any types f distillatin, because the liquid and the vapr phases have identical cmpsitins. The biling pint f such mixtures will nt change, until ne f the cmpnents f the azetrpe has been cmpletely remved. 5

6 Depending n the nature f the interactin between the liquid cmpnents f the mixture. In thse instances, when the mixture is destabilized relative t the ideal slutin, the interactin Examples f a lw-biling and a minimum-biling azetrpe maximum-biling azetrpe 2 2 high-biling azetrpes are shwn in Figure 7. In bth cases, A cmpsitin, % B A cmpsitin, % B when the mixture f tw liquids, e.g., A and B, with a given cmpsitin is distilled, the Figure 7. change will nly ccur up t 2. Therefre, the separatin f A and B can nly be achieved befre their rati has reached 2. As lng as bth cmpnents are present in the mixture, the biling will be taking place at a cnstant temperature. Hwever, as sn as ne f the cmpnents is remved, the distillatin will fllw the prfile fr an ideal liquid. Azetrpes are f great practical significance, since they are used fr the separatin f ne cmpund frm a mixture as well as fr the remval f water frm rganic slvents, i.e., drying. Table cntains physical characteristics f several f the binary (mst cmmnly used) and ternary azetrpe mixtures. Ntewrthy, the nature f the cmpnents f the azetrpes varies greatly. Fr example, bth ethanl and tluene frm an azetrpe with water, despite the fact the frmer resembles water, and the latter is cmpletely different frm water, in terms f structural and physical characteristics. temperature, C Table. Azetrpe (b. p., cmpsitin f the b. p. f the C) azetrpe mixture, % azetrpe, C water(00) ethanl(78) water(00) tluene(0) chlrfrm(6) acetne (56) ethanl(78) ethyl acetate (78) ethanl(78) benzene(8) water(00) temperature, C Frm the practical pint f view, azetrpes mixtures can be remved by the use f a special apparatus, such as a Dean-Stark trap, shwn in Figure 8. Fr example, ethanl water mixture bils at a cnstant temperature (78 C) when the water cntent is 4 % by mass. In rder t remve water frm ethanl and thus btain anhydrus ethanl, the azetrpic distillatin with tluene is applied. Ethanl, cntaining water is placed int the distillatin flask equipped with a magnetic stirrer r a biling chip. Upn heating, azetrpe will be remving int the graduated side arm f the Dean-Stark apparatus. The cmpsitin f the mixture in the receiving side arm will nt be the same at different time pints. Once the rati between tluene and water changes frm 80 20, a phase separatin will take place, and the frmatin f tw layers in the side arm receiver will be bserved. Based n the densities f the tw cmpnents, tluene will be the tp layer, and the water will be the bttm layer. Water can be peridically drained frm the receiving arm. It is pssible t measure the amunt f the water that has been present n the riginal liquid. Ntably, due t the txic, cancergenic nature f tluene, anhydrus shuld never be used fr any types f human cnsumptin! Figure 8. water ut water in Dean-Stark clumn heating plate 6

7 BOILING POINT DETERMINATION Biling pint f a liquid is usually determined during its distillatin. Hwever, this requires substantial amunts f liquid. In thse cases, when nly small amunt f liquids are available, the set-up shwn in Figure 9 is used. Abut ml f the liquid is placed int a small test tube, cntaining a biling chip. The test tube is clamped nt a supprt stand, at a distance that allws fr a rapid applicatin and remval f heat, usually with a Bunsen burner. Then the thermmeter is clamped in such a way that the bulb f the thermmeter is abut 2 cm abve the liquid level. Heating f the bttm f the test tube will lead t a reflux f the liquid, thermmemeter and its vaprs will be washing the bulb f the thermmeter. Once the temperature rise stps, the reading f the biling clamps pint can be made. This prcedure shuld be repeated test tube with liquid several times, and average shuld be taken. Care shuld be exercised while heating the test tube; rapid heating can cause the liquid t verheat and splash ut f the test tube, als the test tube might crack and the liquid will leak ut. bunsen burner Due t the pen flame heating, the liquid can catch fire, which will present a serius hazard, which can als lead t a ptential injury. Therefre, the area arund the biling pint Figure 9. set-up shuld be clean and free frm any ptential hazards. REFRACTIVE INDEX In additin t biling pint and density, the refractive index (n) is cnsidered as ne f the main physical prperties f the liquids. It is cnstant at a given temperature, pressure and the wavelength f the applied irradiatin. Usually measurements f the refractive index are dne at 20 C and the D line f the sdium spectrum is used fr irradiatin. Therefre, the refractive indexes are dented as n20d. The refractive index measures the rati f the velcity f liquid thrugh vacuum t the velcity f light thrugh a give liquid. Refractin is a prcess in which light changes directins when it crsses the interface f tw media, i.e., when it leaves ne istrpic medium and enters a different istrpic medium. Light traveling thrugh the liquid will interact with the mlecules, i.e., with the electrns f the bnding and nn-bnding rbitals that cmpsed the liquid, and therefre, its speed shuld be distinct frm that in vacuum, where light travel is nt bscured. Since varius cmpund psses varius structural and functinal features, the interactin f light with distinct cmpunds will be unique. Refractive indexes f mst rganic cmpunds fall in the range between.3 and.7. Water and alchls are at the lwer and, whereas highly light absrbing cmpunds, such as chlrfrm, aniline, tluene, etc at the higher end f the range. The precisin f refractive index determinatin is A B within units. Refractive index is very sensitive t the small amunts f impurities, therefre, all the reading shuld be taken with cautin, when cmparing t the tabulated values. Abbe refractmeter (Figure 0A) is the mst cmmn apparatus that is used fr the determinatin f the refractive index. The prcedure t measure n is fairly straightfrward: the refractmeter prisms (bth upper and lwer prisms) are cleaned (95% ethanl is usually used) and allwed t dry thrughly. A small Figure 0. 7

8 amunt f sample is applied with an eyedrpper nt the lwer prism (cntact f the pipette with the prism shuld be avided, since the prism can get scratched). The upper part f the prism is brught int cntact with the lwer part f the prism, thus the liquid frms a unifrm layer between the prisms. Next, the cntrls wheels are manipulated t bring the light and dark fields int fcus (Figure 0B), with the dark-light interface being psitin at the crssing f the lines in the eyepiece. At this pint, the reading f the refractive index is made. The temperature at which the reading was made shuld be recrded, and the necessary crrectin fr the refractive index, using the equatin given belw, shuld be made: n 20 D = n bs + [(T bs 20)( ] where, n bs and T bs are the bserved refractive index and temperature, respectively. 8

9 PROBLEMS. If a cmpund bils at 00 C at 760 mm, what will be the biling pint at 75 mm? 2. In a distillatin f a mixture f tw cmpunds, the secnd cmpund t cndense int the receiving flask will be the ne with the lwer r higher biling pint? 3. If a liquid is cntaminated by a sluble, yet nnvlatile slid, wuld yu expect the impure liquid t have a higher r lwer biling pint than the pure liquid? 4. If yu want t separate benzene (bp 80 C) frm tluene (bp C), which type f distillatin wuld yu use? Briefly explain yur chice. 5. Give a definitin f a biling pint. 6. Sketch and label the set-up fr a vacuum distillatin. 7. Calculate the mle fractin f each cmpund in a mixture f 2.0 g f cyclhexane and 7.0 g f tluene. 9