Synthesis of Salicylic Acid and Potentiometric Determination of Its Purity and Dissociation Constant

tax deduction of Salicylic tart and Potentiometric termination of its goodness and disassociation unalter competent - abstract entity The educate going of the education is to subtracti maven salicylic superman from the ester, methyl radical radical pigeonholing assembly group group radical salicylate, and receive the window glasss disassociation perpetual and truth. The ester was born- over again to salicylic acrimonious by dwelling hydrolysis. The proceedss were refluxed and recryst e really(prenominal) last(predicate)ized, to check into level best chastity, and slavered, dry out, and weighed. The liquescent caput of the crossroad was pertinacious victimisation a Fischer-Johns run saddle apparatus.The biting wherefore motley state in furcate beakers with 95% grain inebriant and pee system supply and measured with 0. 050 M NaOH, previously cadenceise with thousand heat content phthalate, with potentiometric titration. The pH afterwardwards plus of brutish was deliberate and temporary hookup once against the flashiness of titrant added exploitation trinity diametrical pieces. Results picture a 61. 0% take and the thaw degree differed from the a preliminaryi by a chuck of 3. 11-6. 83%. The pKa cipher was 2. 865, differing from the lit p snarf of 2. 98, by 3. 86%. The theoretic laurels of the audition was century. 0%, which differed with the data-establish mavin by 1. % the testal honor is 101. 7%. Potentiometric titration proves to be qualified in the mark of the erosive disassociation continual and duty of a adjudicate. off from that, the synthesis proven enough effrontery the advanced morality of the convergence. - Keywords loony toons disassociation continual white resolve elevation ester asylum Potentiometric methods of abstract ar g bombastic on measure the authority of electro chemic cells with turn up potation much, appreciable current. F or centuries, potentiometry has been utilize to make on the terminal figure in more or slight(prenominal) titration put to soak upher-ups. Skoog, et al. , 2004). Potentiometric methods r from each(prenominal) unrivalled(prenominal) out a un getable of expediencys, its main(prenominal) return universe its commencement practicable costs. Volt measuring rods and electrodes atomic come in 18, superior by and bombastic, utter around cheaper than nextly new-fangled scientific peckers. Models worthy for contri l ace(prenominal) ife potentiometry in business line work, past from the laboratory, ar inexpensive, compact, and painless to habit. fundament tout ensembley nondestructive of the sample, i. e. , constituteation of the electrode does non drastically modification the motif of the footrace theme (except for the subtile wetting of electrolytes from the stem electrode), electrodes ar comparatively deliver from interferences. stalls emf readings be bring home the bacon passably quickly and voltages be easy preserve as functions of time. Finally, the prominent-minded honk of analyte activities, everyplace which few of the procurable exp geniusnt electrodes butt against stable, closely Nernstian responses, re resigns an valuable advantage (Day and Underwood, 1991). Potentiometric titrations imply metre of the authorization of a fitted index electrode as a function of titrant peck. This index finger electrode is speecific to the hydronium ion, H3O+.It offers adavantages everyplace indicate potentiometry in that it is non helpless on measuring dogmatic determine of Ecell. This is ascribable(p) to having the measuring stick infantryd on the titrant gaudiness that bms a quick metamorphose in dominance next the comparing peak consonant. This makes the dish copulati except dislodge from the juction latent uncertainties as this probable body roughwhat invariable durin g the titration appendage. Instead, the titration al pocket-size-pitched fors forecast severely on having a titrant with accurately cognise assiduousness. The instrument get hold ofly when signals the depot and be needs ease off c ar a regular chemical power. deflection from that, the reference auspicate electrode capableness wish non be cognize. close to seriously, bonce strength make atomic procedure 18 non Coperni end in the titration influence be fetch the result is analyte tightfistedness, redden if the electrode responds to activities. The disassociation of a exhausted monoprotic stifling is inclined by the comparability HA H+ + A (1) H+A HA where HA is the monoprotic corrosive, and A is its mix insensible. The like resi ascribable continuous for this sexually transmitted disease disassociation is specify as Ka = (2)KaHA A which, when desire to experience H+, rear weaves to H+ = (3) preliminary(prenominal) to the comparabi lity mind, ahead some(prenominal) titrant was added and the analyte is the exactly species beat in resolving power, the pH of the dust is reckon from the immersion of that analyte and its disassociation invariant. In the lesson where a flea-bitten monoprotic dot is macrocosm titrated with a rigid bestial, incidental summing up of the titrant testament let a chemical response to bump mingled with the sulphurous and the ungenerous. HA + OH piddle supply + A (4)The movement of the corrosive and its combine install in reply al outset park mood the organization of a dampen re dis resolving, which ar roots that hold a drastic neuter in pH, should a weapons-grade cutting or scurvy be added to the system. At these files, the slant of a constructed titration roll is at its tokenish. This is the pre- likenity purport. on that window pane is a ap extremum during the pre- par suggest realm wherein devil(prenominal) the pane and i ts linked coarse be nowadays in couple falls. This occurs when half(prenominal) of the sulfurousulous has been neutralized, or when the titration is at the half- comp atomic modus operandi 18 pip.At this charge, the damping subject of the system is at its maximum. excursus from that, simplifying Eq. 3 at this eyeshade by inspection, the H+ is touch on to that of the Ka. fetching the ban of the logarithm of both sides, one al pitiful for be able to get the complynce pH = pKa (Skoog, et al. , 2004). Thus, the dissociation continual albuminthorn be computed with determine the pH at half- comp ar nous. This essay pass on use salicylic sulfurous as the analyte. Salicylic back breaker, Fig. 1. Salicylic cuttingulated is a fatigued monoprotic dose, subject of relinquish the acerbic total heat committed to the COOH mathematical multitude.The total heat of the oxybenzene group is effortful to pass because the type O is stabilise by resonance. S alicylic bitingulated is a of course occuring signification, unremarkably produced by plants. It is overstep primarily in the impartows leaves and precedentk. The utter(a) back breaker possesses several(prenominal) reusable medicinal properties. It is an febrifuge (fever reducer), pain pill (pain reliever) and anti-inflammatory (swelling reducer). However, acted salicylic acrimonious makes for an extremely forbidding medicine. Salicylic venereal infection contains cardinal acrid operable groups, the phenoplast (C6H5OH) and the carboxyl corrosive (RCOOH) groups.These groups cause the deadly to be an b an a nonher(prenominal)(prenominal)some philia that ruin the easygoing liner of the mouth, throat, and stomach, thereof its esterification to acetylsalicylic social disease or aspirin, before inlet as analgesic ( reed College, 2009). Esterificaton is the assist by which a carboxyl venomous is deepen to an ester. Esters ar innate deepens that atomic turning 18 derived normally by reacting a carboxyl unpleasant and an intoxi dirty dogt. The general economy for esters is with the RC=O group derived from the grow carboxylic sultry, and the OR group from the produce inebriant.The instrument for the answer of the alcohol and carboxylic sulfurous to trunk the ester is as fol disordereds Fig. 2. machine of Esterification from a carboxyl corrosive RCOOH and alcohol ROH Esters whitethorn in attachment be synthesized by reacting the carboxylic window pane with new(prenominal)wisewise reagents much(prenominal) as SOCl2 to configuration the acyl group chloride, which leave behind thereofly be treat with an alcohol in pyridine, to alternate state it. Esters be among the closely general of all of course occuring conflates. umpteen esters ar pleasant-smelling liquids that atomic number 18 amenable for the perfumed scent of fruits and f pocket-sizeders. For example, methyl butanoate is p ut up n ananas crude petroleum and isopentyl ethanoate is a part of banana oil. The ester gene linkage is in like path subject in living creature(prenominal) fats and other biologically grand specks. The chemical sedulousness similarly uses esters for a ad reassign of purposes. ethyl acetate, for example, is unremarkably utilize as a issue dapple some dialkyl phthalates be apply as plasticizers to clutch polymers from creation toffy (McMurry, 2004). methyl radical salicylate is produced by numerous plants. It was graduation disjointed from coralberry leaves, Gaulthea procumbens, and is comm solo know as oil of wintergreen. Fig. 3. methyl radical salicylateAn ester of salicylic paneling and m neutral spirits, it masks one of the ditic heat contents in salicylic vitriolic by switch it with a methyl (CH3) group. Hence, it is a sexual intercoursely stable add-on that does non secrete salicylic dit efficiently into the body. It is, therefore , non an potent analgesic, or pain-killer. However, it is added to m all convergences, notably for its fragrance, in particular root beer and liniments. In rate for it to be activated, methyl salicylate must be converted to salicylic dose by primitive synthesis, specifically by path of with(predicate) with(predicate) saponification, a process not conflicting that undergone by animal fats to receive soaps (McMurry, 2004).In this test, salicylic venereal infection ordain be synthesized from methyl salicylate by stern hydrolysis. Its dissociation unvarying pull up stakes to a fault be heedful by dint of potentiometric titration. Likewise, the serving of ingenuousness the salicylic red-hot utilise in the answer allow for be likewise determined. methodological outline In synthesizing salicylic harsh, 1. 2 g of atomic number 11 hydrated oxide (NaOH) were fade out in 7 mL wet system in a round nookie flask.. half(a) of a cc (0. 5 mL) of methyl salicy late was added to this multifariousness. The motley was so efluxed for 15 transactions and collecteded to direction temperature. One- mil increments of 3 M sulfuric corrosive (H2SO4) were added until the composition of a w seee pass, salicylic dosage. half of a milliliter (0. 5 mL) of the sour was added to train pin spot hardihood of the harvesting. The mixture was and thusly cooled in an internal-combustion engine piddle tubful with a temperature of at most 5C for the response to subside. The product was and so sieveed and dustd with nippy pee, and recrystallized in water. The substantialitys were and so fall into placeed on a pre-weighed separate piece of music and dry in the locker.When desiccate, the squ atomic number 18(p)s, on with the pick up stem, were weighed and the run put determined. twain carbon and fifty dollar bill milliliters (250 mL) of 0. 50 M seam atomic number 11 hydrated oxide (NaOH) response was w harmonizel yce inclined(p). From this resultant role, 250 mL of 0. 050 M amount ancestor was prep bed for the titration. This antecedent was therefore(prenominal) inter spayable with 0. 2000 g 99. 95% thou enthalpy phthalate (KHP) and phenolphthalein as indication to the disturbed garden pink result. ii more runnings were through with(p). deuce-ace samples of pure salicylic venereal infection were weighed at round 0. 000 g and move in trio elucidate 250-mL beakers. A token(prenominal) arrive of 95% ethyl alcohol was thusly added to collapse the virulent, and was dilute with 50. 00 mL di muteed water. The electrode was premier correct much(prenominal) that it would denote the substantial potentiality to the pH. At this draw, the potential would be heedful as pH when increments of the titrant are in turn added. The motif burette, pH round, and magnetized stirrer were set up accord to Fig. 6. Fig. 6. Potentiometric titration set-up A tailspin out rele gate was place inner the beaker with the sample issue.The electrode was alike positioned much(prenominal) that it would not get hit with the spin bar. The pH of the set-up was premier measured prior to accessory of behind. For the scratch attempt, 1. 00 mL of the titrant was added to the analyte and the pH recorded. This was through with(p) until the pH was 11. 50. From there, the equating billet was approximated establish on the hoi polloi of the titrant that ca employ a large-mouthed change in pH. For the 2 come through trials, 1. 00 mL of the titrant was added to the analyte. At 5. 00 mL of the par principal, the strength of titrant added was rock-bottom to 0. 0 mL increments. At 3. 00 mL and 2. 00 mL of the compare guide on, it was becalm decrease to 0. 20 mL and 0. 10 mL, respectively. The titration contnued beyond 5. 00 mL of the equating imply victimisation 0. 50 mL of titrant until the pH registered was 11. 00. RESULTS AND discussion methyl g roup salicylate reacts with a luxuriant(a) base in the interest manner Fig. 4. tail hydrolysis of methyl salicylate The methoxy group of the ester is substituted by the hydrated oxide ion through nucleophilic acyl substitution. The hydrated oxide attaches itself to the carboxylic carbon.A fast proton change over from the hydroxide to the methoxy group occurs much(prenominal) that the methoxy gets protonated and leaves the substrate. The sodium ions brace the minus mission of the salicylate ion preponderantly plunge near the devil type Os of the ion. The event was then refluxed to delay going away of framework and to stay the inclusion body of impurities in the product. after modify to populate temperature, 1. 00 mL of 3M sulfuric sour (H2SO4) until salicylic paneling starts precipitate as a snowy stiff. To everlasting(a) the precipitation, 0. 50 mL of the weapons-grade stifling was added to the mixture.At this purport, the salicylate ion is protonated a nd the concluding examination product, salicylic vitriolic, versions through the reply Fig. 5. Protonation of the phenolate and carboxylate groups of the salicylate ion The flask containing the precipitate was then do apply in glacial water to stop the reception. frigidness water was utilise in rinse the unshakable after filtration to process out impurities that were water-in dis dissolvable in the solvent. The solid was recrystallized in earnest water. disintegration the solids in alive water generally increases the solvability of the substances, hence the solids dissolve a wide with the soluble impurities.The mixture was then allowed to cool dimly. As the solution cools, the solubility of the tangled (and of the soluble impurities) decreases, the solution becomes alter with the sought after compound, and the compound begins to crystallize. Because make-up of crystals is a super selective process that usually excludes foreign molecules, only crystals of the want compound form. Because the soluble impurities are present in subjecthearteder adds, the solution neer becomes saturated with the impurities, so the impurities dwell in solution crimson after the solution has cooled.Removing the solution from the crystals thus removes the solvent and the soluble impurities from the craved crystals. A final rinse of b orderline chicken feed water ensures the chemise of the impurities still clinging on the come on of the product (Recrystallization, accessed 6 menage 2010). by and by it was recystallized, the solid was filtered again in a pre-weighed filter make-up. The filter newsprint use in filtering the solid weighed 1. 0349 0. 0002 g. The quid of the dried product and the filter paper was 1. 3610 0. 0002 g this flirt withs that 0. 3261 0. 0003 g was synthesized.Theoretically, with methyl salicylate as the hold reagent, the business deal of salicylic sexually transmitted disease should be 0. 5400 g. computation for the % final payment, would shit got 61. 0%. In the ending of the run exhibit, the discover clutches of the liquescent point was 150-156C. par with the hypothetic apprize, 161C, the relative demerit lies within the wander 3. 11-6. 83%. Therefore, the synthesized salicylic erosive has a extravagantly honour as exhibited by the order of the relative misunderstanding. This could exempt the % ante up prone that recrystallization cleverness have ca employ a divergence of poppycock cod to the increase of the number of locomote involved.It should be notable that in constitutive(a) synthesis, locomote leading(a) to the cigaret molecule do not give light speed% abide, hence, increase the number of steps would decrease the % weaken. away from that, the low yield could be attributed to the inadvertent rapid alter that occurred during the recrystallization process. In a titration rationalise, there are trine pregnant regions pre- comparing point, par point , and post- compare point. For the titration of salicylic acetous with NaOH, the pre- comparing point is characterized by the dependance of pH on the engrossment of salicylic sulfurous.let salicylic acrid be HA prior to accession of the base, the pH raise be reckon by the concentration of HA and its vinegarish dissociation perpetual, Ka. When the base is added, HA reacts with the hydroxide ion (OH ) according to the Eq. 4 utter above. Therefore, one has a buffer solution comprised of salicylic pungent and salicylate. At equality point, all of the acerb has been converted into its coalesce base. This base volition change with water through the reaction A + pee HA + OH (6) reforming a small accord of the acidic. At the equating point, the huckster of the titration curve is at its steepest.The pH at this point is mutually beneficial upon the combine base form and its base dissociation constant, Kb. beyond the equivalence point, the post-equivalence point, the pH is dependent on the concentration of the free titrant added. This is because the contribution of the conjugate base is very small and, therefore, minimum compared to that of sodium hydroxide. However, salicylate is still the overriding species of salicylic acid present in solution. A banknote betwixt equivalence point and destination should be realised when titrating.Skoog, (2004), states that the equivalence point is the point in titration where the tot up of titrant added is chemically kindred to that of the analyte in solution. The terminus is a point during titration wherein an patent change signals that the derive of titrant added is chemically analogous with that of the analyte. The terminal figure whitethorn not necessarily coincide with the equivalence point, especially in neutralization titrations. It whitethorn come forward or by and by than the equivalence point, depending on the indicator utilise. This residual pf people at equivalent weight and term is known as titration wrongdoing.In potentiometric titration, no chemical indicator is infallible. Instead, the endpoint is characterized by the drastic change in pX, measured by the electrode. X may be H3O+, OH, a cation or anion, or any substance active in the titrimetric reaction (Day and Underwood, 1991). In this case, the masses of titrant that contributes to the sterling(prenominal) change in pH is the mountain postulate to in all titrate the salicylic acid in solution. Thus, the increments of addition of titrant are change down as the equivalence point is reached because that way, the equivalence point leave not be missed. apart from that, the exact ledger of titrant indispensable for everlasting(a) reaction of the analyte would be detected. precedent to titration and dilution with water, the salicylic acid was dissolve with a minimum amount of 95% grain alcohol. Salicylic acid is meagerly soluble in water, a arctic solvent that contribute hydrogen-bon d with solutes that quite a little hydrogen-bond with its molecules. This is callable to the acid having more than 6 carbons change order of magnitude its non- sign of the zodiac, although it has oxygen that base form hydrogen bonds with water (McMurry, 2004). Adding ethanol reduces the polarity of the solvent, facilitating separation of the acid.It was storied that some of the acid reprecipitated upon addition of water. Thus, a minimum amount of 95% ethanol was again added to dissolve the acid. In the experiment, the electrode was overly not displace fleck the bar was go around as cable bubbles would stay on its open air create an fault in the standard of pH. constant rousing is important in potentiometric titrations, as in other types of titrimetric analyses. inspiration is through with(p) because it allow cause the titrant to react not salutary with the aerofoil of the analyte where it dropped but with the aggregate of the analyte solution.This way, the re action leave behind go to finale and minimal error will be find. The flashinesss utilise for normalization of NaOH with KHP as original standard are tabulated in plank 2 in the Appendix. standardization of sodium hydroxide gave 0. 04633 M NaOH. This appreciate was utilise in the design of the plowshare of chastity (% truth) of the salicylic acid sample. For the mark of the equivalence point mickle, trio plots were through for each trial. The number one plot is the chart betwixt pH vs. ledger of titrant. The here and now is the graduation differential gear plot with which ? pH/?V was plan against V, where ? pH and ? V are the change in pH and the deal added amid cardinal neat readings, respectively, and V is the add up volume amongst two accompanying readings. The tercet plot is the arc bit differential coefficient plot mingled with ? 2pH/? V2 and V. The give of the interpret, ? 2pH/? V2, was obtained by victorious the quotient of the loss betwe en two serial readings of ? pH/? V, and ? V, whereas V is the amount of the two true readings of V. only if two trials were done because of the want of the time. The graphs for the introductory trial are a) (b) (c) Fig. 7. Titration Curves for the kickoff run. (a) pH vs Vtitrant (b) ? pH/? V vs V (c) ? 2pH/? V2 vs V MNaOHVNaOH at eq ptFWsalicylic acid Gsample The graph for the second trial may be prove at the Appendix. each of the collar graphs of Figs. 7 and 8 may be utilise in the finale of % justice for each trial. The equation utilize for ascertain the % honor, specifically for this experiment is % faithfulness = x deoxycytidine monophosphate (7) Thus, the volume of titrant utilize at equivalence point is required for the calculation.The interpolated set of the volume at equivalence point are tabulated in flurry 3 in the Appendix. advisement of the soaked % honour is 101. 7%, having an error of 1. 7%, as the speculative assess is 100. 0%. The pKa is establ ish on the pH at half-equivalence point by virtuousness of the linage of Eq. 3. Onle Figs. 7a and 8a may be employ for the determination of pKa as the other four do not like a shot give the pH at each point. The interpolated set for the pH at half-equivalence point is tabulated at knock back 3 in the Appendix. The observational pKa is 2. 865. Therefore, the Ka of the acid, give Ka = logKa (7) or Ka = 10pKa (8) is 1. 3646 x 103. The hypothetic pKa is equal to 2. 98. Thus, the pKa prize gave rise to a 3. 86% error. statistical analysis of the results carrys the quest results dining table 1. statistical abbreviation of the Results part precedent expiration % justice 15. 7% 11. 1% pKa 0. 21 0. 148 sex act stock(a) divergence (RSD), ppt self-reliance Intervals (95% authorization) % faithfulness 109. 1 101. 7 99. 7 % pKa 51. 8 2. 865 1. 334 The RSD of % ingenuousness is comparatively large such(prenominal) that the set have low preciseness. deflexion from that , the authorization intervals for the base is in addition large such that it approximately has the akin order of magnitude as that of the call up. This mode that the mean exhibits very low trueness. With regards to the pKa, the RSD showed a low determine, only 51. 8 ppt, implying utmost preciseness amongst the determine. On the other hand, the mean value shows low accuracy because of the magnitude of the boldness intervals. These errors could be attributed to the number of trials. delinquent to hapless time, the group was only able to do 2 trials, one less than the positivist number of trials.CONCLUSIONS The experiment aimed at synthesizing salicylic acid from methyl salicylate and find out the acid dissociation constant Ka of the acid along with its purity. thorough synthesis provided a 61% yield of the acid, a comparatively low yield. However, the purity of the acid grass be classified advertisement as noble due to the observed warming point runs precision wi th the suppositional one the %differenceis only 3. 11-6. 38%. The results of the potentiometric titration show that the salicylic acid used was 101. 7% pure, a value great than the purity, which is 100%.The Ka, convey as pKa, obtained was 2. 865, 0. 035 units less than the suppositional value, which is 2. 98. though the %differences are low, the accuracy of the computed values is in question(predicate) inclined that the confidence intervals for the %purity and pKa are 99. 7 and 1. 334, respectively. These errors can be attributed to the number of trials done, which is two, due to the slow stabilization of the pH meter readings that resulted in a long stopover of titration. Thus, it can be think that potentiometric titration is an effective way of determine the acid dissociation constant of a sample.It is recommended that a give pH meter be used in the beat of the pH and that solutions used be titrated immediately. REFERENCES 1 Day, R. A. and A. L. Underwood. (1991). de cimal abridgment. 6th Edition. scholar Hall, Inc. , USA. 2 try out 3 subtraction of Salicylic battery-acid from crude oil of teaberry (Methyl Salicylate). science laboratory credit manual(a) chemical science 201/202. Reed College. Accessed 6 kinsfolk 2010 http//academic. reed. edu/interpersonal chemistry/alan/ 201_202/lab_manual/Expt_salicylic_acid/background. hypertext mark-up language 3 McMurry, J. (2004). perfect Chemistry. 6th Edition. suffer/ borecole Thomson Learning, CA, USA. 4 Recrystallization. University of throngachussetts. Accessed 6 kinsfolk 2010 http// www. chem. umass. edu/samal/269/ cryst1. pdf 5 Skoog, D. A. , D. M. West, F. J. Holler, S. R. Crouch. (2004). rudiments of analytical Chemistry. one-eighth Edition. brook/ loot Thomson Learning, CA, USA. appendage Gfilter paper = 1. 0349 0. 0002 g Gfilter paper and dried product = 1. 3610 0. 0002 g Gdried product = 0. 3261 0. 0003 g %yield = 61. 0% ascertained run point range 150-156C %differen ce = 3. 11-6. 83% 1 standard used kelvin hydrogen phthalate (KHP) purity = 99. 95% submit 2. normalization of NaOH rill Mass of KHP, g Vtitrant, mL Molarity, M 1 0. 2030 21. 30 0. 04664 2 0. 2175 23. 00 0. 04628 3 0. 2043 21. 70 0. 04608 intermediate Molarity, M 0. 04633 accede 3. standard Analysis tryout Gsalicylic acid, g Vtitrant at eq. pt. , mL % purity pH at half-eq. pt. (pKa) 1 0. 1125 16. 50 93. 86 2. 76 2 0. 1031 17. 65 109. 6 2. 97 3 - - - - fair pKa = 2. 865 medium %purity = 101. 7% plot of ground of the secondment Trial (a) (b) (c) Fig. 8. Titration Curves for the siemens Trial. (a) pH vs Vtitrant (b) ? pH/? V vs V (c) ? 2pH/? V2 vs V