Lab Manual Chm 260

EXPERTMENT2 W.YISIBLE DETERMINATION OF AN TJNKNOWN CONCENTRATION OF KMnOr SOLUTION TMORY/ BACKGROIJND All absorbancc spectrophotometers coatain a light source, a sample comparheot, aud a detector. Many speckophotometers a[5s sontzin one or more monochromators, a device used to separate light into its component wavelengtls. The Spectronic 20 operates in the visible range only and uses a priim as its monochromator. Speckophotometer that rleasure in the UV ana visibte iegion are of fwo general tlpes: scenning aud diode-array. A diode array qpeckophotometer imparts light of all wavelengths ts thg 5amPle at onc time. In contrasl 3 5sanning UV-Vis spectophoiometer coatains a monocbromator, usually consis';rg of. holographic gratings, wavelengths to be sequentially imparted io the sample. - which allows light of iadividual Spectroscopy involves the observations of absorption or emission of electomap.etic radiation resulting from kansitions of atoms or molecules from one etrergy level to auother. Wdn a molecule at its "ground state" (the state of lowest energy) absorbs some g4re, the molecule is said to "o"rgitf rmdergo a "bansition" to a higber energy state. The higher energy state is refered to as an ..excited state"- A molecule can only absorb energJ i1 16s input energy exacfly matches a molecular bansition from one energy level to aaother. Examples of some traasitions that can occur in a molecule include bond vibratiou or rotation around a bond due to interaction with infi=ared energy (IR spectoscopy), or changes in tle elcctonic skucture of a molecule due to iateractions witfr v;siUrc oi ultaviolet radiation (Visible and UV Speckoscopy). The transitioas betweeu ground and excited safs5 r rirhin molecules are a firoction of the types of stuctures that are found within a molecule aud the eavironments that these structures are located u.ifhin a giveu moldcule- So if we know something about tle absorption of energy in a given molecule, then we can predict the types of structures or fiinctioual groups that we might have within a molecule. Many orgaaic aud biological'molecules have Fansitisus fhat occur betwem energ-y levels of electronic states of atoms or molecules. Therefore, our focus will be ou the visible and ul#viot"t ."gio"s J tl" electromaggetic spectum iu this lab. The visible and ultaviotet region of isterest is fouia between 170 and 800 nm; though ttre most useful region for experimental use is between 250 and 700 nm. Principles of IfV Spectroscopy UV-Vis qpectoscopy is based on the selective absorption of electomapetic radiation in the 1g0-7g0 lm wavelength range. tfV-Vis radiation hac sumcient eoergy to cause furritio* in bouding electons (as opposed to atomic inner shell or valence electons) and thus, is correlated best with the behavior of bo''ds and fuactional groups in the analyte. Absorption in the W-Vis range is mainJy a study of molecules and their electronic transitions. in bond formation eJ 1s rrnshared, outer electons that are ]ocalized about electouegative atoms (such as oxygen, the halogens, sulfirr and nitoge,a) can be promoted to a higher energy molecular orbital. Each of these molecular o6ital represents i aifferent .r.rgy level as FJechons particrpating directly shown below. 10 :--v.!-- urvy--e ---L -1 -::::ij:::: lanthaaides and actinides as well as inorganic complexes or charge transfer complexes- 11 t!----. Transitions with e < 103 are howev'er considered to be of low intensity. carboxyl groups. bonding t' Bondlng Bondlng Most IJV-Vis spectra involve n---tc* and r-r" Eansitions. r'_>E* transitions are generally more intense fhan a--+76*- Transitions to a zc* orbital requires the preseuce ofan unsaturated firnctional group (chromophord to supply the z* orbitals. --r-bE ^L^^::-+ t) 1--.vuo. TT\f. i -i --^1-r-r:- ^^- L- ^Lh.vsvAr . and nitro groups are the best absorbers in the W-Vis rrnge. Each firnctioual group has a wavel*gth associated with an absorption maximum. Organic molecules with conjugated double bonds. that can be used for qualitative ideutification in arr unknown sample- t* a--- Electronic iransitions C6HpCH:CH2 177 13000 lr+tc* CsHnC=C-CH3 178 i0000 1t---r'* o II 186 CH3CCII3O n-+g* -1000 o il cH3cor{ 204 4l rr--+rE* CH3NO2 280 ZZ \-+tt* CHrN:NCHs 339 5 B---+n* Molar absorptivities (e) of up to 10s are suitable for use in IIV-Vis absorbauce speckoscopy measurements.Antlbondlng Antibonding 1e lru lz.\_I:.:-^l c---:------:jv-. . carbouyl groups. Radiation in the 200-700 Em raoge brings about these transitions makieg molecules with cbromophores convenient for analysis using a LIV-Yis spectrophotometer. Transition Mdals" Spectra from taasitiou eleuent ions arisc &om the 3d and 4d electons.06. L. &I. O. isCI rt.. lo I v3" 6 v 4 o 300 >r .r0 o$8 l.Rr*""s and as such the spectra are comparatively sharp. o. i. often in thc visible (solutions are brighfly coloured).0O 400 500 ?N 600 8@ Lanthanides and adinide* LIV-Vis qpectra of lanthanide ions arise 6o* 4lsrbitals and 5f orbitals for ttre acrinides..- I . o 400 500 t50() 700 8rx.o4 o-oE. o. and oniy weakly influencedby ligands and solvents.These specka are quite broad. l po a JSO 9 €s *Eo c{8. o 5 C' o = 20 f I I I I 15 r0 5 ot: 300 400 /--\\ 500 \\ 600 Qf+ 8m 700 o..{#. These are rather well-screened from outside i. and are significantly affected by ligands aad solveots.$sfi s60 (so t2 ?flo . and Beer's Law can be used to determine the concentations of a mixhrre of species. Using Beeds Law. and is expressed in u1its L molr cm-r wheu c (concentration) is expressed in molarity and b (pah ligh of lighi) 1a @. a device for isolating specific wavelengtb.+oo \:. LIV-Vis spectra can be used presence of absorbing fuuctional $oups or chromophores. Absorption is proportional to the path length b through a saurple solution and the eonces.rate result will be obtaiued by using a source able to produce inteuse radiation at a siagle wavelengtb- UV-Yisible Sp echophoto meters Speckophotometers are made uP of stable source of radiaot energy. 13 to a to detect for the ."LiUty Changes in the refractive index of the solutior. Beer's law is only valid for monochromatic radiation. caa undergo absorption processes where the eleckon jrrrnps from an orbital mostly centred ou the ligaud to au orbital Eostly centered ou the metal ion (the opposite c2n occur.Absorbau. g is tbe molar absorytiity.laorgan'is somFlexcs made up of metal ions with surrouuding ligands. a ransparent sample container. and chemical reactioo wiil also cause error in applyng geert taw to a sarnple. n. or charge transfer complexes. In concentrated solutioas (> 0.Thus.m 6so 700 IfV speckoscoPy can be used to quatify the amouut of au absorbing material present. and a sipal processor aad readout. Bee/s Law is limited in that it applies ouly to dilute solutions.ces are additive.01 M) particles iateract altering the .kation c of an absorbing species according to Beer's Law: A: ebc The proportionality constant in Beer's Law. a radiation detector which coavers ta$mitted radiation usable sipal. 12000 >r roooo 8000 6000 -a o +50 5{)0 550 650 Fc(phen)l* {ooo 2000 o 400 450 550 600 650 700 25000 20000 Stcrch-t] 15o00 toooo 500() o 400 450 soo 550 60(} Wavelength. the absortance of a sample cau be related to its concentration.I fnorganic complaa."alysfs to absort a certain wavelength of radiation which causes uon linear deviations from Beels Law. Finally. but less frequeutly)- 5000 4000 FcscN"+ 3000 2000 IOO0 o . as long as they are not interacting.-the most acc. To plot the calibration curve of potassium permaagaaate.m infra-red region. * Wavel en gth Selectors : Filters and monochromators. and used to detero. and located in relatively flat region of the specta so tlat absorbance will be high and coustznt in a narrow r:rnge around the chosen wavelengtb. A staDdard curve relating absorbance to concenhation can be developed ficr any 66'r. At around :iO nm tne inskument switches its radiation source to a tuagsteu filament. Glass and plastic can be used in the visible reglotr as well. pbotodiode. wavelength range and subility for detectable ard reproducible results. The radiaat euergy emiued from a heated tungsten filament approaches that of a black body and thus.The optimal waveleirgth should ensure good absortance of the analyte aud low absorbance by other spccies in the solution. Sample containers: $amPle containers (cells or cuvettes) must be coastucted of a material that is transpiareat to radiation in the wavelength raage of interest.tIV-visible speckoscopy is a valid.The analysis should be done at a wavelength with naximum absorptioa. Detectors: Phototube. and used with the appropriate standard curve.found. 2. photodiode array- OBJECTTYES t. The eleckical excitation of deuteri.ine the concentration of sanples containing tie srme compouud.'m at low pressure results in a continuous spectum of emitteil radiatioa from 160 nrn to the beginning of the visible (375 nm). 5imple and cost effective method for determiaing the coacentration of absorbing species if applied to pure compounds. APPARATUS Beaker Burette Glass rod Volumetric flask 100 mL D'opper CHEMICALS Potassinm p ermanganate (KtvInO4) Distilled water t4 . Many t-IV-Vis speckophotometers r6e a deuterium t:mf for the UV range and switch to a tungsten filament lamp at 350 nm for the visibie range. photomultiplier tube (pMT). Coutainers of quarE or fused silica are necessitry ft: YV raugq and can be used into 700-3000 .. This waveleugth will allow valid absorption measuremcnts to be made on samples that contain mixhrres of materials- *"iy" Sources: A radiation sonrce for spectroscopy must generate a beam with sufficient power. To determine the maximum waveleugth of potassium permanganate. To determine the concentration of an unknown56lution of potas5inm pennangaaate. is temperature depeudent. 3. Stopper the flask and shake several times to homogenize the solution.T. Stopper aad shake the flask Attd distilled water to the oark. Iu this experiment. I:r order to 'clea. Dissolve the solid with a few ''.PROCEDURE During this laboratory expcrimcnt you will rnake a series of dilutions. using a medicine dropper io aaa t}re iast few &ops. B.00 to 20-00 rDl of the 'stock' KlrllaOa solution and dilute with distiiled water iu a 100 'nI. 6. distilled water. C. add a srnall amount of lhe 'new' solution using a medicine &opper and rinse &e sides of the cuvette with the solution. and then onlv fill with the sample before tqking the absorbance reading- 15 .of $stil&k+Ier.voluaetric flask.using 10 mr .r) Take the absorbance reading of each of your samples in successioo. 2. Label the bealer as '100 ppm': 4. Repeat Step 4. Pipet between 5. Transfer iato abeaker a:rd label as it as '5 ppm'. an iaitial peroxanganate stock solution is prepared and the solutions to be measured are diluted &om a dilutiou of the stock. Pour the 'stock' solutiou iuto a beaker. generate a Beer's law plot for trGdnOa and determine the concentration of tbe 'nlcnsvm solution. Record ffis rc3ding. respectively.00 mr of the 'stock' solution and dilute with distilled water in a 100 'nTflaslc 5. alt of your data must be taken on the same the sqme time If you need to change instruments you must stort over and take all of the datafor the spedrum again CAATTON instnamen$ - d Remember that your blark is always the s^*e as the solvmt. 7- Label the beakers as'10 ppm'. Each spectrophotometer wili need one cuvette for the blarh and each student will need one cuvette for the sample. y6lrrmeEic Pipet 5. The permanganate iou absorbs at 534 om aud it is at this wavelength mat we will determine the absorbance values for the solutions. in this case. on a wgjgbigeBper. 15 mL and 20 mT stock solution and tansfer into small beakers. and thea take the absortancc of the dilutions at L*. '15 ppm' and'20 ppm'.Ouce tbe absorbaner values are taken.Rinse the cuvette three seoarate ti"'es in this wav. A- Preparation of the KI\{nO4 Standard Solutions l. Weigh accurately 0O1 g KlvInOa. Determination of Absorption Maximnm (L. Traasfer into a beaker aad labelas it as 'I]nknowu'.n' the cuvette so that a sample is at the appropriate concentration. 3. starting with the least Yhen using spedrophotometers. Usiug a funnel tarsfer the solid to a@ 2. to the nearest 'ng. Preparatior of the Unknown 1. J* \qt) rv method water '!a5sline. let your lab instructor check the results' rt may be that you need to re-make one or more of the diluted solutions. set . click on ttre 'GARY'. \' . The most corumon error in this experiment is not measuriag the vol.l 1 . click on Cary icoq then key ia maximum wavelengtb tr*. l?t\^1." . u' .6\qtr . Select cycle mode ifmore than I cycle is r-equired >- 5. cliik -+ Fast on 'Baserine' icon aud check ..mes accurately. Select Replicate : 3 4. To save the method. \6r 7. 17^ X( t 4.. '.. Go to the f. 3. cwette solution with distilled the'BLANK' 13.storage on @ompt At 10. select maximum peak In the Auto Store icon. Click oa the conceukation icon 2. Rinse aud retura your cuvettes at the end gf the experiment.v the 'BLANK' cuvette and put the . X--_ nY ' . Select "scan control speed" 6. Go to Sehrp.i" il ' *' ^\ "-[$' t 4l \fi . 's * .# * . D. then key in the required start and stop scan wavelength (Dna) q) (b) (c) Y-mode (rnia:0 aad max: l) 'X-mode: 800-200 nm Bearn 6qds:Dual Beam ^{v- *I*l \ \eCY 4:. 12. click on'standard' icon.r Fr:action.sAMpLE. Click on 'Scar. u. 3' ' (fV icon Go to setup. check the 'calibrate During Run' fimction 16 f' . Remove solution) cuvette solution and crick Start).Baselile correction . Fill the'BLANK./ e''+ At the peak table option.. 8.re and save the scan 11. Click 'Start' icon to start the Deasurement- E.once this series of absorbauce readings has beeu recorded. Select Cary'Win 2.r" -'d /rn\* 9. cuvette (you may use ttre 20 ppm Determina6on of the Unknowu Concentration 1- iU f $? 14. Put r. - do Operation of the {fv-Vls Spectrophotometer Instramenb Vadan/ Cary 50 W_Vis Speckophotometer Operating Instructions 1. RINSE with distilled water only not use a brush or soap on thesc oryou risk scratching thcm. this will help you. Notes on Graphing in Excel Input da4: r' . Y-axis data in the next columl Higbiight the data (and not the headers for the data) aud click on the INSERT tab at the top of the screen from the meuu select ' SCATTER with markers but no lines.Click 'Start' icon to start the conceukatiou measurements . The graph should have a descriptive title that includes the particular Soft Drink you aaal'yzed and the concentrations at which all of tne absorbances were obtaiaed. r .. To Open Excel: . through the Excel adjust the axes appropriately. This shouldproduce tle appropriate graph.laf{g (units). tle Graph If you are unfamiliar with the latest version of Excel. Go to the Report icon. The axes should bave approp. click on Microsoft OfEce . saphintfirnctions. t7 . click on START in the lower left corner and . In the Auto Store icon.d separarely.5. and the rf value should appear on your graph. Once the graph is made. right click ou the trendline (not on a marker) and select . . You will submit this graph as part oJyour rqort.ate. Remove the 'BLANK' cuvette andput the . 1.. ' . click on Microsoft Office Excel 2007 toopea Excel o 2.l I -t GRA?EING USING MICROSOF'T EXCEL I I + Use the computer and open Excel and create a graph of absorbance vs. coocenkation (in nnits of ppm or m€ll) on the computer. -'t(-". Make sure that the box containing the equatiou aud the R2 value does not interfere with aa.. set 'storage on (hompt at Start)' 7 10. To save thc method ciick 'File 11.Equation. 12. Select the Fit Type (Linear Dircct) ' Go to sample icon. select the number o1'ths sarnples and key in unknowq 8.SAMpLE' cuvette 13. Set the calibration standard unit (mg/L) and the number of tle st"nd. key in operator name and the commsrl 9. ' ' X-axis data in the left-most col ..ft6prtling. right click on one of the marters.. This should produce a skaight line that may or may not go through all of the points on your graphNext. . Both the equation of the [iue.n. Put the -+ Save Method As -+ Ok 'BLANK' cuvette aad click 'Zero.ard srmples 6. and select . click on Prograos . The Leclude the trendli:re and the To Make R' li''e should fill the graph value.. ) rvaverengti. 3.' . Show ouly 400_700 nm. Discuss the sigaificance of the corelation coefficient for the graph you obtaiaed_ 4. Discuss the principal of operation of the of each component. ' 3. Next in the top tab.to format X-axis: start at 400 not at 0 nm.reading the graph.:13"_:T. REPORT l' You-will record ail observations and tabulate your data.t"i a" of thereference you used 2":":l:^:r$wavetengthi(nctudethename.. Attach the print suf slfeined from the LIV-Vis instrument into your lab report aloug with your Microsoft Excel spread sheet. -T 2' at maximum absorption. to get the answ*. QUESTTONS I' 2' why is glass not a suitabie cen material for use state one advantage of using the analysis. page it-. wheu you saye the wortbook.-. select SAVE As Excsl 97-2003 and it should be compatibre on most computers.Y:'.. lrv-vis in w spectroscopy? speckophotometer compared to a Spectonic 20 for this t8 . 2.d add Absorbance (no uuits) to the y-axis Rjght click on Series I and delete iL Click on Axes . you sho.ld always maximize your graph space appropriately. . . Then use Microsoft Excel to construct the Beer's Law plot to determine the unknown concentation of KMuOr. ToSAVE: ' If you do NoT have Excel 2007..r"d utie. L* for the Kirdnoa solutioa? what i . uv-vis instumeat you used and include the fuuction PRE-LABORATORY QTTESTIONS 1' Show how you will PiePare a 5 ppm solution fr-om a 100 ppm KMno4 stock solution using a 100 volumekic flask Briefly aescribe the procedure.The box caa be moved around using the mouse to select it and drag . SelectAxis Tifle and add Wavelength (nm) to the X_axis select Axis Title a. select LAyOI-ff Select: Chart Title and write iu "visible Absorption Speckum of (Braad aud Flavor). it. F4uu .INKNOWN CONCENTRATION OF KMnO4 SOLUTION Nam6 .1 Solution Standard I Concentration @pm) ao Standard 2 1q Standard 3 Iu Standard 4 4 0-qq1 D-q-9 $ o ..DATASHEET EXPERTMENT 2 UV-YISIBLE DETERMINATION OF AN T.= Se+-gtf Show the sample calculation for ttre preparation of standard 3: r rn kn .)\L{ L.2s1 i'Jll\- MassofKMnOo: 0'S\ Coaceatration gf r nv . Lectursr's signature.r^ . 19 . \-l-o a Unknown Absorbance 916.11.wArrr -Apn r* Date Student ID Group: : : Table 2. 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