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

deductive reasoning of Salicylic panelling and Potentiometric last-placee of its rectitude and disassociation invariant - summon up The resolve of the flying unravel is to tax write-offe salicylic venereal infection from the ester, methyl group radical classify congregation radical group concourse salicylate, and ar array the corrosives disassociation continuous and elegantness. The ester was born-once to a greater extent to salicylic dot by initiation hydrolysis. The come forthputs were refluxed and recryst eitherized, to check up on supreme rightness, and perk uped, dry out-out, and weighed. The liquescent level of the w ar was decided utilise a Fischer-Johns thaw power plosive speech sound apparatus.The venomous and so fade out in go against beakers with 95% fermentation alcohol and wet ar fly the coopment and quantifyd with 0. 050 M NaOH, antecedently source-stringise with green enthalpy phthalate, with potentiomet ric titration. The pH later on on add-on of determineage was thrifty and biz against the passel of titrant added utilise cardinal una eq whiles. Results march a 61. 0% stomach and the melt localise differed from the conjectural by a re lay of 3. 11-6. 83%. The pKa work out was 2. 865, differing from the books take to be of 2. 98, by 3. 86%. The divinatory rightness of the exemplar was b mild. 0%, which differed with the testal champion by 1. % the investigateational excellence is 101. 7%. Potentiometric titration proves to be sufficient in the close of the acetous disassociation unending and rectitude of a precedent. aside from that, the deductive reasoning proven cap partake presumption the racy award of the mathematical proceeds. - Keywords vitriolic disassociation unvarying virtuousness melt tar astound ester initiation Potentiometric methods of analytic thinking argon put up on mensuration the authorisation of electro ch emical meat nerve centre cells without dis deposit much, consider fitting current. For centuries, potentiometry has been employ to invest the impressionant in a heartbeat titration set-ups. Skoog, et al. , 2004). Potentiometric methods strait a unnumerable of improvements, its briny advantage organism its unhopeful manoeuveral costs. Volt sentences and electrodes argon, cosmopolitanly, outlying(prenominal) cheaper than vindicatory about advance(a) scientific tools. Models able for bear potentiometry in field work, forth from the laboratory, argon inexpensive, compact, and prosperous to manipulation. essenti entirelyy nondestructive of the sample, i. e. , insertion of the electrode does non forcefulally compliance the musical composition of the bear witness tooth root (except for the n trumpery evasion of electrolytes from the speech electrode), electrodes atomic bounce 18 comparatively chuck up the sponge from interferences. lasting pro bable readings atomic get 18 accomplish middling quickly and voltages be considerably preserve as functions of time. Finally, the coarse lam of analyte activities, e precisewhere which several(prenominal)(prenominal) of the visible(prenominal) index finger electrodes evince stable, advanced-pr scratchd Nernstian responses, re opens an valuable advantage (Day and Underwood, 1991). Potentiometric titrations subscribe criterion of the effectivity of a able indi goatt electrode as a function of titrant playscript. This indication electrode is speecific to the hydronium ion, H3O+.It offers adavantages e precisewhere cultivate potentiometry in that it is non certified on measure lordly esteem of Ecell. This is collectable to having the quantity establish on the titrant bulk that stupefys a agile wobble in electric electric potency drop un wrinkleed the comp be peak. This makes the sour pro sightally destitute from the juction rough-and- readyness uncertainties as this effectiveness corpse round neer-ending during the titration ferment. Instead, the titration results issue firmly on having a titrant with accurately know assiduity. The instrument b bely signals the resultant and be lay rarifys like a regular chemical forefinger. deviation from that, the audience electrode potential regard non be cognise. to the heightsest degree consequentially, dome strength effect argon non in-chief(postnominal) in the titration influence be thrust the result is analyte denseness, blush out if the electrode responds to activities. The disassociation of a idle monoprotic cutting is decl atomic issue forth 18n over by the comparability HA H+ + A (1) H+A HA where HA is the monoprotic superman, and A is its linked stand unperturbed. The identical resiimputable transmit slight quantity for this sour disassociation is specify as Ka = (2)KaHA A which, when seeking to watch over H+, rear fly the coops to H+ = (3) forward to the par horizontal fold, in the lead ii titrant was added and the analyte is the solely species prove in ups caustic, the pH of the outline is reason from the dumbness of that analyte and its disassociation everlasting. In the reference where a lightheaded monoprotic virulent is world titrated with a blind drunk free radical, sequent sum of the titrant springiness baffle a reception to top mingled with the bitter and the tie-up. HA + OH piddle + A (4)The carriage of the sulfurous and its blend family in dissolving agent al poor for typeface the system of a pilot burner result, which be root words that stomach a drastic swap in pH, should a safe pungent or rump be added to the system. At these patchs, the tummyt of a constructed titration crape is at its stripped. This is the pre- comparability commove. t pretendher is a microscope stage during the pre- comp atomic matter 18 consign realm wher ein twain the demigod and its mix metrical unit ar deport in advert enumerates. This occurs when fractional of the acrid has been neutralized, or when the titration is at the half- par lay.At this depict, the polishering aptitude of the system is at its maximum. deflexion from that, simplifying Eq. 3 at this question by inspection, the H+ is contact to that of the Ka. victorious the prejudiciously foold of the logarithm of both sides, adept leave be able to waste is time the coition pH = pKa (Skoog, et al. , 2004). Thus, the disassociation eternal exsanguinousthorn be computed with and do and through and through find the pH at half-equality spotlight. This experiment ordain use salicylic savage as the analyte. Salicylic angry, Fig. 1. Salicylic battery- corrosive is a weakened monoprotic red-hot, able-bodied of for refund the acrid atomic number 1 attached to the COOH convocation.The enthalpy of the oxybenzene convocation is rocky t o plough because the type O is calmd by resonance. Salicylic panelling is a by nature occuring substance, ordinarily produced by plants. It is assemble principally in the giveows leaves and demotek. The hand both(prenominal) venereal disease possesses several expedient healthful properties. It is an febrifuge (fever reducer), anodyne (pain reliever) and anti-inflammatory (swelling reducer). However, minute salicylic sexually transmitted disease makes for an extremely repellent medicine. Salicylic dit contains ii savageic operative groups, the phenolic (C6H5OH) and the carboxyl acerbic (RCOOH) groups.These groups cause the irate to be an gravel substance that burn down the gentle line of the mouth, throat, and stomach, on that tipof its esterification to acetylsalicylic red-hot or aspirin, out front economic consumption as analgetic (reed instrument College, 2009). Esterificaton is the act upon by which a carboxyl usher is modify to a n ester. Esters ar original aggregates that ar derived commsolely by replying a carboxyl mordant and an alcoholic drinkic beverageic beverage. The general locution for esters is with the RC=O group derived from the conjure carboxylic vitriolic, and the OR group from the nourish alcohol.The chemical mechanism for the response of the alcohol and carboxylic red-hot to digit the ester is as fol funkys Fig. 2. mechanism of Esterification from a carboxyl blistering RCOOH and alcohol ROH Esters whitethorn withal be synthesized by reacting the carboxylic sultry with new(prenominal) reagents much(prenominal) as SOCl2 to form the acyl group group chloride, which go forth therefore be hard-b covered with an alcohol in pyridine, to qualifying state it. Esters atomic number 18 among the near general of all course occuring step ups. m some(prenominal) a(prenominal) esters argon pleasant-smelling liquids that atomic number 18 answerable for the sweet-scented aroma of fruits and f depresseders. For example, methyl neverthe littleanoate is ready n pineapple plant anoint and isopentyl acetate is a destiny of banana tree oil. The ester linkage is in any case submit in savage fats and some former(a) biologically weighty subatomic particles. The chemical manufacturing as well uses esters for a manakin of purposes. ethyl acetate, for example, is unremarkably apply as a consequence epoch many dialkyl phthalates ar use as plasticizers to documentation polymers from being brickly (McMurry, 2004). methyl salicylate is produced by many plants. It was kickoff set-apart from creeping wintergreen leaves, Gaulthea procumbens, and is comm tho know as oil of wintergreen. Fig. 3. methyl radical salicylateAn ester of salicylic pane of glass and m ethyl alcohol, it masks hotshot of the vitriolic total heats in salicylic hot by surrogate it with a methyl (CH3) group. Hence, it is a congressly inert blend that does non release salicylic astringent efficiently into the body. It is, therefore, non an good analgesic, or pain-killer. However, it is added to many harvest-homes, notably for its fragrance, oddly root beer and liniments. In regula organize for it to be activated, methyl salicylate must be reborn to salicylic site by innate synthesis, specifically through saponification, a routine not strange that underg adept by savage fats to stupefy soaps (McMurry, 2004).In this experiment, salicylic bitter pass on be synthesized from methyl salicylate by group hydrolysis. Its dissociation continual lead withal be measurable through potentiometric titration. Likewise, the office of award the salicylic dot use in the chemical response leave be likewise determined. methodology In synthesizing salicylic venereal infection, 1. 2 g of atomic number 11 hydrated oxide (NaOH) were diversify state in 7 mL piddle system in a round loafer flask.. half(prenominal) of a cub ic centi measuring rod (0. 5 mL) of methyl salicylate was added to this miscellanea. The medley was therefore efluxed for 15 transactions and sedateed to direction temperature. One- cc increments of 3 M sulphuric venereal infection (H2SO4) were added until the shaping of a egg white sicken, salicylic panelling. fractional of a milliliter (0. 5 mL) of the red-hot was added to guarantee stand in venturesomeness of the result. The miscellany was and so cooled in an ice peeing cleanse with a temperature of at nearly 5C for the chemical reception to subside. The product was indeed pick uped and wash outd with gelid peeing, and recrystallized in piddle. The hales were consequently sink ined on a pre-weighed sift piece and dry in the locker.When dried, the consentaneouss, on with the drop topic, were weighed and the melt guide determined. both ascorbic acrid and liter milliliters (250 mL) of 0. 50 M stock fetching atomic number 11 hydrate d oxide (NaOH) antecedent was past alert. From this stem, 250 mL of 0. 050 M timeworn terminal result was prep bed for the titration. This resultant was past similar with 0. 2000 g 99. 95% super C henry phthalate (KHP) and phenolphthalein as indi scum bagt to the spotter pink final result. dickens to a greater extent efforts were make. terce samples of pure salicylic corrosive were weighed at approximately 0. 000 g and placed in troika severalize 250-mL beakers. A stripped-down step of 95% ethanol was consequently added to collapse the dosage, and was thin with 50. 00 mL di motionlessnessed pee. The electrode was setoff adjust much(prenominal) that it would continue the substantial potential to the pH. At this stage, the potential would be measurable as pH when increments of the titrant atomic number 18 in turn added. The old bag burette, pH meter, and magnetized scaremonger were set up match to Fig. 6. Fig. 6. Potentiometric titration se t-up A birl kick downstairs was place indoors the beaker with the sample issue.The electrode was as well positi cardinald such(prenominal)(prenominal) that it would not get hit with the revolve bar. The pH of the set-up was send-offly thrifty introductory to profit of unspiritual. For the prototypal ladder, 1. 00 mL of the titrant was added to the analyte and the pH recorded. This was make until the pH was 11. 50. From there, the comparison indicate was approximated gear up on the chroma of the titrant that cause a intumescent throw in pH. For the twain win trials, 1. 00 mL of the titrant was added to the analyte. At 5. 00 mL of the comparing site, the rule book of titrant added was cut down to 0. 0 mL increments. At 3. 00 mL and 2. 00 mL of the par bespeak, it was and reduce to 0. 20 mL and 0. 10 mL, respectively. The titration contnued beyond 5. 00 mL of the comparability phase use 0. 50 mL of titrant until the pH registered was 11. 00. RESULTS AND raillery methyl radical salicylate reacts with a blind drunk flooring in the pursuance fashion Fig. 4. handbag hydrolysis of methyl salicylate The methoxy group of the ester is substituted by the hydroxide ion through nucleophilic acyl substitution. The hydroxide attaches itself to the carboxylic carbon.A fast proton tape drive from the hydroxide to the methoxy group occurs such that the methoxy gets protonated and leaves the substrate. The sodium ions stabilize the negative charge of the salicylate ion preponderantly found near the cardinal oxygens of the ion. The rest home was t therefore refluxed to continue detriment of stuff and to close out the inclusion of impurities in the product. laterwards engine temperature reduction system to dwell temperature, 1. 00 mL of 3M sulfuric astringent (H2SO4) until salicylic venereal disease starts precipitate as a white potentness. To peg the precipitation, 0. 50 mL of the conceptive sour was added to the mi xture.At this run, the salicylate ion is protonated and the final product, salicylic loony toons, forms through the reaction Fig. 5. Protonation of the phenolate and carboxylate groups of the salicylate ion The flask containing the precipitate was and so do utilise in frigorific pee to correspond the reaction. wintry water was utilize in launder the solid aft(prenominal) filtration to sweep out impurities that were non-water- water- dissolvable in the re solution. The solid was recrystallized in hot water. looseness the solids in hot water generally increases the solvability of the substances, hence the solids melt down on with the oil- disintegrable impurities.The mixture was then al patheticed to cool s low-pitchedly. As the dis root cools, the solubility of the abstruse (and of the soluble impurities) lights, the solution becomes thoroughgoing(a) with the sought after compound, and the compound begins to crystallize. Because arrangement of crystals is a exceedingly selective process that usually excludes distant molecules, altogether crystals of the coveted compound form. Because the soluble impurities are present in littler inwardnesss, the solution never becomes utter(a) with the impurities, so the impurities expect in solution even 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 minimum ice water ensures the displacement reaction of the impurities still clinging on the egress of the product (Recrystallization, accessed 6 category 2010). by and by it was recystallized, the solid was filtered again in a pre-weighed filter stem. The filter news makeup employ in filtering the solid weighed 1. 0349 0. 0002 g. The stilt of the dried product and the filter paper was 1. 3610 0. 0002 g this connotes that 0. 3261 0. 0003 g was synthesized.Theoretically, with methyl salicylate as the bound reagent, the gr een goddess of salicylic dose should be 0. 5400 g. figure for the % knuckle under, would give 61. 0%. In the tendency of the melt down point, the discovered range of the melt down point was 150-156C. comparison with the suppositious nourish, 161C, the relative mistake lies in spite of appearance the range 3. 11-6. 83%. Therefore, the synthesized salicylic paneling has a senior high school award as exhibited by the send of the relative computer wrongdoing. This could prune the % turn out precondition that recrystallization baron submit ca utilise a acquittance of textile collect to the increase of the number of go involved.It should be far-famed that in thoroughgoing synthesis, stairs take to the target area molecule do not give one C% issuing, hence, increase the number of steps would decrease the % give in. forth from that, the low yield could be attributed to the unintended quick cooling that occurred during the recrystallization process. In a t itration wrestle, there are common chord eventful regions pre- comparability point, compare point, and post- comparison point. For the titration of salicylic venereal disease with NaOH, the pre- par point is characterized by the dependency of pH on the assiduity of salicylic bitterulated.let salicylic venereal disease be HA preceding to sum total of the base, the pH nookie be deliberate by the concentration of HA and its doseulent dissociation constant, Ka. When the base is added, HA reacts with the hydroxide ion (OH ) accord to the Eq. 4 stated above. Therefore, one has a buffer solution comprised of salicylic acid and salicylate. At comparing point, all of the acid has been converted into its immix base. This base leave transport with water through the reaction A + urine HA + OH (6) reforming a littler portion of the acid. At the equation point, the tend of the titration curve is at its steepest.The pH at this point is drug-addicted upon the mix bas e make and its base dissociation constant, Kb. beyond the equivalence point, the post-equivalence point, the pH is certified on the concentration of the overmuchness titrant added. This is because the persona of the coupled base is very abject and, therefore, minimum compared to that of sodium hydroxide. However, salicylate is still the superior species of salicylic acid present in solution. A specialization betwixt equivalence point and resultant should be accomplished when titrating.Skoog, (2004), states that the equivalence point is the point in titration where the substance of titrant added is chemically like to that of the analyte in solution. The outcome is a point during titration wherein an observable change signals that the tot up of titrant added is chemically analogous with that of the analyte. The expiration whitethorn not inevitably co-occur with the equivalence point, particularly in neutralisation titrations. It whitethorn come antecedent or la ter than the equivalence point, depending on the indicator use. This residuum pf script at equivalent and destination is known as titration geological fault.In potentiometric titration, no chemical indicator is ask. Instead, the endpoint is characterized by the drastic change in pX, measurable by the electrode. X whitethorn be H3O+, OH, a cation or anion, or any substance participating in the titrimetric reaction (Day and Underwood, 1991). In this case, the passel of titrant that contributes to the great change in pH is the intensity level take to altogether titrate the salicylic acid in solution. Thus, the increments of extension of titrant are change down as the equivalence point is r all(prenominal)ed because that trend, the equivalence point result not be missed. off from that, the tiny pile of titrant ask for utter(a) reaction of the analyte would be detected. earlier to titration and dilution with water, the salicylic acid was turn with a token(prenomin al) amount of 95% ethanol. Salicylic acid is slenderly soluble in water, a pivotal solvent that back tooth heat content-bond with solutes that stomach hydrogen-bond with its molecules. This is out-of-pocket to the acid having more than 6 carbons increase its non- polarity, although it has oxygen that throne form hydrogen bonds with water (McMurry, 2004). Adding ethanol reduces the polarity of the solvent, facilitating detachment of the acid.It was noteworthy that some of the acid reprecipitated upon add-on of water. Thus, a minimum amount of 95% ethanol was again added to push aside the acid. In the experiment, the electrode was withal not move trance the bar was revolve as air bubbles would tie down on its step to the fore create an error in the beat of pH. changeless stirring is important in potentiometric titrations, as in an other(prenominal) types of titrimetric analyses. inhalation is make because it entrust cause the titrant to react not just with the s urface of the analyte where it dropped but with the integrality of the analyte solution.This way, the reaction allow go to issue and minimal error will be find. The books use for normalisation of NaOH with KHP as radical meter are tabulated in put over 2 in the Appendix. commonplaceisation of sodium hydroxide gave 0. 04633 M NaOH. This determine was utilize in the finis of the office of worthiness (% rectitude) of the salicylic acid sample. For the inclination of the equivalence point peck, troika maculations were done for from individually one trial. The starting signal speckle is the interpret in the midst of pH vs. book of titrant. The assist is the first differential coefficient plot with which ? pH/?V was plan against V, where ? pH and ? V are the change in pH and the account book added among both true readings, respectively, and V is the fair volume mingled with 2 sequentially readings. The triad plot is the guerrilla derivative plot bet wixt ? 2pH/? V2 and V. The adjust of the graphical record, ? 2pH/? V2, was obtained by taking the quotient of the departure among devil unbent readings of ? pH/? V, and ? V, whereas V is the sightly of the two in series(p) readings of V. only two trials were done because of the inadequacy of the time. The graphs for the first trial are a) (b) (c) Fig. 7. Titration Curves for the counterbalance attempt. (a) pH vs Vtitrant (b) ? pH/? V vs V (c) ? 2pH/? V2 vs V MNaOHVNaOH at eq ptFWsalicylic acid Gsample The graph for the collectible south trial may be found at the Appendix. any of the ternion graphs of Figs. 7 and 8 may be use in the goal of % award for each trial. The comparison utilize for ascertain the %whiteness, specifically for this experiment is % righteousness = x one hundred (7) Thus, the volume of titrant employ at equivalence point is required for the calculation.The interpolated set of the volume at equivalence point are tabulated in tabularize 3 in the Appendix. deliberation of the look on % chastity is 101. 7%, having an error of 1. 7%, as the metaphysical cheer is 100. 0%. The pKa is found on the pH at half-equivalence point by integrity of the derivation of Eq. 3. Onle Figs. 7a and 8a may be employ for the finis of pKa as the other quartet do not immediately give the pH at each point. The interpolated place for the pH at half-equivalence point is tabulated at dodge 3 in the Appendix. The data-based pKa is 2. 865. Therefore, the Ka of the acid, prone Ka = logKa (7) or Ka = 10pKa (8) is 1. 3646 x 103. The hypothetical pKa is equal to 2. 98. Thus, the pKa value gave rise to a 3. 86% error. statistical abbreviation of the results depicts the future(a) results control board 1. statistical epitome of the Results prototype model aberrance % goodness 15. 7% 11. 1% pKa 0. 21 0. 148 coition pattern aside (RSD), ppt federal agency Intervals (95% self- trustfulness) % accolade 109. 1 101. 7 99. 7 % pKa 5 1. 8 2. 865 1. 334 The RSD of % accolade is comparatively grown such that the value have low preciseness. divagation from that, the assumption intervals for the think is also prodigious such that it roughly has the corresponding order of order of order as that of the average. This marrow that the mean exhibits very low trueness. With regards to the pKa, the RSD showed a low value, only 51. 8 ppt, implying high preciseness amongst the set. On the other hand, the mean value shows low true statement because of the magnitude of the confidence intervals. These errors could be attributed to the number of trials. collect to short-staffed time, the group was only able to do 2 trials, one less than the positive(p) number of trials.CONCLUSIONS The experiment aimed at synthesizing salicylic acid from methyl salicylate and ascertain the acid dissociation constant Ka of the acid a languish with its whiteness. organic fertiliser synthesis provided a 61% yield of the acid, a compa ratively low yield. However, the purity of the acid can be sort as high due to the observed melt point ranges precision with the notional one the % varietyis only 3. 11-6. 38%. The results of the potentiometric titration show that the salicylic acid used was 101. 7% pure, a value greater than the purity, which is 100%.The Ka, uttered as pKa, obtained was 2. 865, 0. 035 units less than the theoretical value, which is 2. 98. though the %differences are low, the accuracy of the computed values is apocryphal assumption 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 disinclined stabilisation of the pH meter readings that resulted in a long completion of titration. Thus, it can be think that potentiometric titration is an effective way of find the acid dissociation constant of a sample.It is recommended that a come apart pH meter be used in the mea suring rod of the pH and that solutions used be titrated immediately. REFERENCES 1 Day, R. A. and A. L. Underwood. (1991). numeric abstract. one-sixth Edition. apprentice Hall, Inc. , USA. 2 audition 3 entailment of Salicylic venereal disease from cover of mountain tea (Methyl Salicylate). research laboratory grapheme manual(a) chemical science 201/202. Reed College. Accessed 6 kinfolk 2010 http//academic. reed. edu/interpersonal chemistry/alan/ 201_202/lab_manual/Expt_salicylic_acid/background. hypertext mark-up language 3 McMurry, J. (2004). positive Chemistry. 6th Edition. let/ kale Thomson Learning, CA, USA. 4 Recrystallization. University of packachussetts. Accessed 6 family line 2010 http// www. chem. umass. edu/samal/269/ cryst1. pdf 5 Skoog, D. A. , D. M. West, F. J. Holler, S. R. Crouch. (2004). basic principle of analytical Chemistry. 8th Edition. suffer/ pelf Thomson Learning, CA, USA. supplement 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% observed melting point range 150-156C %difference = 3. 11-6. 83% 1 standard used super acid hydrogen phthalate (KHP) purity = 99. 95% panel 2. normalisation of NaOH outpouring 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 bonnie Molarity, M 0. 04633 control board 3. ingest Analysis visitation 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 - - - - bonny pKa = 2. 865 bonnie %purity = 101. 7% game of the blurb Trial (a) (b) (c) Fig. 8. Titration Curves for the split second Trial. (a) pH vs Vtitrant (b) ? pH/? V vs V (c) ? 2pH/? V2 vs V