3/14/ EXERCISES DEAL WITH YOUR ASPIRIN SYNTHESIS ph TITRATION / IR ANALYSIS. Cathode reduction. Anode oxidation

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1 // ASPIRIN PURITY BY TITRATIN EXERISES DEAL WIT YUR ASPIRIN SYNTESIS p TITRATIN / IR ANALYSIS Last Update: // : PM SUSB- Synthesis of Aspirin acetic anhydride (AA) salicylic acid (SA) acetylsalicylic acid (ASA) acetic acid G SEAWLVES washed product with cold water - reacted with acetic anhydride acetic acid (miscible in water) product : ASA or SA or ASA SA? Ferric hloride test - eat Test - SUSB- (after Spring Break) Determine the purity of your aspirin product % ASA vs % SA Both ASA and SA are acidic - - mol per mol compound Titration - determine mol of determine mol of? Purpose: To study p variation during an acid/base titration. To use this to determine the purity of your synthesized aspirin oncepts: p Nernst Equation alibration and use of the p meter Molar Mass Effective Molar Mass Titration Stoichiometry Equivalence Point Titration of a mixture Techniques: Weighing by Difference Using a p Meter Graphing IR Spectrum & Analysis Apparatus: p Meter Buret IR Spectrometer A diagram of a battery Anode oxidation electrons are lost at the anode. - + e - flow from to + athode reduction electrons are gained at the cathode. The voltage the cell produces depends on the concentrations of the solutions in each cell. Nerst Equation gives the dependence of voltage on concentration. R if the voltage is known - can be used to calculate the concentration of solutions. In both cases the temperature is a constant. If the concentration in each cell is M and T = K potential at anode. V potential at cathode. V A volt meter attached across the wire would read. V

2 // p and the p METER p Definition p = -log [ + ] p METER: Measures voltage difference between glass and reference electrodes. each in contact with the same aqueous solution Glass Electrodes are selected so that relative voltage depends only on YDRGEN IN concentration, [ + ] ( and temperature ) ther electrodes can measure concentration of other ions. { SUPL- discusses types, use and care of p electrodes } NERNST EQUATIN describes dependence of electrode voltages on concentration and temperature For normal p electrodes,. RT RT E = E -+ p log ln [ + ] N Aa e where: E = VLTAGE e ln x = log DIFFERENE x between reference and indicator electrodes ln x =. ln log E = NSTANT depending x only on TYPES-log of ELETRDES [ + ] = p T is Absolute Temperature R is gas constant N A is Avogadro s number e is absolute value of electron charge Solving for p. RT E = E + p N a e N a e p = ( E - E ). RT Which we can write as: K = K p = ( E - E ) T =. V at K Both slope & intercept depend on T p K p vs E - E p =. ( E E ) K p =. ( E E ) At constant Temperature, p varies linearly with voltage. E - E ALIBRATIN ( STANDARDIZATIN ) Like all measuring devices, must calibrate p METER before using it. Why do we need to be able to measure p? Determine the purity of your aspirin product ow? Adjust Temperature Setting Immerse p electrode into one or more (buffer) solutions of accurately known p. Set alibration ontrol on meter so reading agrees with p of standard buffer % ASA vs % SA Both ASA and SA are weak acids - - mol per mol of substance Titration - titrate our product with a strong base determine mol of determine mol of ASA + SA the manner in which p changes during our titration allows to establish the endpoint or equivalence point

3 // Question : Why can't we make a solution from our product and measure the + concentration directly with the p meter, convert that to moles of + = moles of product? SUSB- Part - Titrate aspirin product with strong base p Titration - change in p establishes endpoint work individually p p ow does p vary with added Na for a weak acid? Titration curve - ml of. M Weak Acid with. M Na pka =. alf Equivalence Point Equivalence Point Buffer Region Volume of. M added (ml) ml of Na Added Note Scale p p Titration urve Full Range - ml End Point Volume of added Na Note Sca ale hange p p Titration urve ~. ml Range.. ml..... Volume of added Na Note Range drop =. ml Buret Rdg SAMPLE TABLE FR p TITRATIN umul Vol p Vol Incr Buret Rdg um Vol p Incr ml ml... drop / ml / ml.... Start using - drop increments about ml before calculated end point.

4 // Start using drop (. ml) increments about ml before calculated end point. ow am I supposed to know what is ml before the calculated end point? SUSB- Part Titrate ASA SA aspirin product using phenolphthalein as an indicator From the results of a normal (phenolphthalein) titration on a weighed sample of your unknown and the weight of sample you are titrating. turns pink at the equivalence point from this aspirin titration, you know how much Na to add to reach the equivalence point can calculate volume of Na needed in p titration (Part ) Phenolphthalein titration:. g requires. ml of Na ow much should I weigh to use ml?. g /. ml = X / ( ) X =. * ( ) /. =.. g I actually weigh out. g p Titration:. g will require:. X. /. =. ml of Na Start drop increments at ~. ADDED volume of Na WAT WE ARE LKING FR Your synthesized aspirin (ASA) most likely contains some unreacted salicylic acid (SA) (Did the Fel test show any violet color?) If the sample is dry, then it can contain only: Molar Mass Aspirin. Salicylic Acid. pk a.. + Fe Titration of these acids individually with Na should allow them to be easily distinguished. From W DES TE STRENGT (pk a ) F AN Last AID AFFET ITS TITRATIN URVE? Semester SA pk a =. ASA pk a =. Titration urves Titration Same curves weight - SA and of ASA SA with and. M ASA Na. M Na p p s depend on pk a s mg ASA alf Equivalence Points Equivalence Points Volumes depend on molar masses Volume of. M Na added (ml) mg SA =..

5 // What about a mixture of the two?. Titrations of Salicylic Acid, Aspirin and Equal Weight Mixture ( ml, mg, with. mol/l Na) End point in acid-base titration is characterized by rapid increase in p This will only occur after both acids are consumed.... mg Aspirin mg Salicylic Acid Total amount of Na is the amount necessary to react with both acids. p. mg Aspirin + mg SA Volume of titrant (ml) Suppose our sample weighs w SA+ASA and contains n SA moles of SA and n ASA moles of ASA From the known number of moles of Na used in the titration, n Na, we can determine n Na = V Na x M Na = n SA + n ASA the sum of the moles of acid in the original sample. But the (known) weight of the original sample was: w SA+ASA = n SA x. + n ASA x. We have two (linear) equations in two unknowns These can be solved simply. The EMM formula (EMM -.) %ASA %ASA= =.(EMM means -.) *. -. mole % is one way of representing the solution using the w SA+ASA w SA+ASA quantity, EMM = = n SA + n ASA n Na * Equations involving EMM apply NLY when EMM is between. and. In normal (indicator only) titrations, can determine (only) total number of available moles of acid in a sample i.e., EMM e.g., if. ml F. M Na is required to react with mg of a sample, the number of moles of acid in the sample is. ml X. mmol / ml =. mmol and sample behaves like an acid of EFFETIVE MLAR MASS (EMM) EMM = mg /. mmol = mg / mmol onsistent with sample being largely ASA (MM = ) with some SA impurity (MM = ) BE SURE T LEAVE ELETRDE IN SLUTIN WEN DING p TITRATIN Improves reliability and saves time STIR TE NTAINER into which you are titrating to obtain good mixing. therwise,,you measure p of only a small, local part of the solution RINSE TE p ELETRDE WIT DISTILLED WATER and DRY IT AREFULLY each time you immerse it into a NEW liquid therwise, p electrode can still see the last solution with which it was in contact

6 // Make abscissa of graph UMULATIVE VLUME of added Na Remember to subtract the initial buret reading Record Na STK SLUTIN NENTRATIN You need it to calculate EMM s p METERS MAY NEED ASINAL REALIBRATIN between users SUMMARY F PREDURE.) Working in ASSIGNED PAIRS, perform NRMAL TITRATINS on samples of: AUTENTI SALIYLI AID (SA) AUTENTI AETYLSALIYLI AID (ASA) You will be given vials of pure SA and ASA.) ompute EMM s for both of the above.) Working ALNE from here on, titrate YUR SAMPLE F ASA with standardized Na, using phenolphthalein as an indicator.) From this, determine EMM and the AMUNT of YUR SAMPLE F ASA needed to consume ml of the same Na solution.) Weigh out your ASA sample and calculate how much Na it will require. Weigh all samples on ANALYTIAL BALANE by DIFFERENE D NT TRANSFER ANY MPUND T BALANE PAN!.) Set up p meter, check calibration.) TITRATE YUR SAMPLE F ASA with standardized Na, using p METER Must use Beaker, Not flask Even though you share p meters, Don t do simultaneous p titrations Datasheet and graph for this exercise may be turned in at the beginning of the next lab Do NT record p titration data on data sheet only in lab notebook STUDENT A Regular titration of synthesized sample p titration of synthesized sample Regular titration of ASA & IR ofsample SEDULE F ATIVITIES W I L E STUDENT B Regular titration of synthesized sample Regular titration of SA & IR of sample p titration of synthesized sample If you have insufficient sample of your own aspirin, see your TA. alculations Regular titration of synthesized ASA Weight of sample. mg Volume of Na. ml onc of Na. EMM =. / (. *.) =. mg What weight is required for ml of Na? ( ) *./. = ( )*. = mg Suppose you actually weigh. mg of sample. Where will the end point be?. *. /. =. ml Na Should begin - drop increments at ~ ml

7 // alculations - ontinued After doing the p titration on your sample, you will have three quantities that let you determine the purity of your synthesized ASA: Your EMM sa for SA and Your EMM asa for ASA (from the regular titrations), Your EMM s for your synthesized aspirin. i You should calculate your purity using these numbers, i.e., (EMM s - EMM sa ) %ASA = EMM asa - EMM sa YUR VALUES REMINDERS ABUT GRAPING (SUPL-) Use rational number of boxes per unit (,,,,, ) for p and volume Arrange graph so that maximum total area of graph paper is utilized Draw a smooth curve through the experimental points Label equivalence (end) point and half-titration point clearly Interpolate values of these points with precision (significant figures) consistent with your plot You should be able to read the ordinate and abscissa to at least the nearest. ml or. p unit You must plot the p vs volume of Na added BY AND not on a computer ne more Test Infrared Analysis of Product What do we expect? p axis does not need to start at Recommend you use cm per p unit for p axis. p Titration urve Volume of added Na = Please download two supplementary pages to SUSB- describing the procedure for determining the IR spectrum of the product. = We will actually do the IR two weeks after the synthesis during SUSB- p Titration of your Synthesized Aspirin

8 // NEXT WEEK Next Week's Exercise uses same p TITRATIN TENIQUE to IDENTIFY AN UNKNWN AID TEST EXERISE READ SUSB- D PRELAB