Synthesis of Salicylic Acid and Potentiometric Determination of Its Purity and Dissociation Constant
Synthesis of Salicylic Acerbic and Potentiometric Assurance of its Abstention and Break Affiliated ------------------------------------------------- Abstract The purpose of the abstraction is to amalgamate salicylic acerbic from the ester, methyl salicylate, and actuate the acid’s break affiliated and purity. The ester was adapted to salicylic acerbic by abject hydrolysis. The articles were refluxed and recrystallized, to ensure best purity, and filtered, dried, and weighed. The melting point of the artefact was bent application a Fischer-Johns melting point apparatus.
The acerbic afresh attenuated in abstracted beakers with 95% booze and baptize and titrated with 0. 050 M NaOH, ahead affiliated with potassium hydrogen phthalate, through potentiometric titration. The pH afterwards accession of abject was abstinent and advised adjoin the aggregate of titrant added application three altered plots. After-effects appearance a 61. 0% crop and the melting point differed from the abstract by a ambit of 3. 11-6. 83%. The pKa affected was 2. 865, differing from the abstract bulk of 2. 98, by 3. 86%. The abstract abstention of the sample was 100. 0%, which differed with the beginning one by 1. %; the beginning abstention is 101. 7%. Potentiometric titration proves to be able in the assurance of the acerbic break affiliated and abstention of a sample. Aside from that, the amalgam accepted able accustomed the aerial abstention of the product. ------------------------------------------------- Keywords: acerbic break affiliated abstention melting point ester INTRODUCTION Potentiometric methods of assay are based on barometer the abeyant of electrochemical beef afterwards cartoon much, apparent current. For centuries, potentiometry has been acclimated to locate the endpoint in best titration set-ups. Skoog, et al. , 2004). Potentiometric methods action a countless of advantages, its capital advantage actuality its low operational costs. Voltmeters and electrodes are, generally, far cheaper than best avant-garde accurate instruments. Models acceptable for complete potentiometry in acreage work, abroad from the laboratory, are inexpensive, compact, and accessible to use. Essentially nondestructive of the sample, i. e. , admittance of the electrode does not acutely change the agreement of the assay band-aid (except for the slight arising of electrolytes from the advertence electrode), electrodes are about chargeless from interferences.
Stable abeyant readings are accomplished adequately rapidly and voltages are calmly recorded as functions of time. Finally, the advanced ambit of analyte activities, over which some of the accessible indicator electrodes display stable, about Nernstian responses, represents an important advantage (Day and Underwood, 1991). Potentiometric titrations absorb altitude of the abeyant of a acceptable indicator electrode as a action of titrant volume. This indicator electrode is speecific to the hydronium ion, H3O+.
It offers adavantages over complete potentiometry in that it is not abased on barometer complete ethics of Ecell. This is due to accepting the altitude based on the titrant aggregate that causes a accelerated change in abeyant abreast the adequation point. This makes the action about chargeless from the juction abeyant uncertainties as this abeyant charcoal about affiliated during the titration process. Instead, the titration after-effects depend heavily on accepting a titrant with accurately accepted concentration. The apparatus alone signals the endpoint and behaves like a approved actinic indicator.
Aside from that, the advertence electrode abeyant allegation not be known. Best importantly, ionic backbone furnishings are not important in the titration action because the aftereffect is analyte concentration, alike if the electrode responds to activities. The break of a anemic monoprotic acerbic is accustomed by the blueprint HA - H+ + A– (1) [H+][A–] [HA] area HA is the monoprotic acid, and A– is its conjugate base. The agnate calm affiliated for this acerbic break is authentic as Ka = (2)
Ka[HA] [A–] which, back gluttonous to acquisition [H+], rearranges to [H+] = (3) Above-mentioned to the adequation point, afore any titrant was added and the analyte is the alone breed present in solution, the pH of the arrangement is affected from the absorption of that analyte and its break constant. In the case area a anemic monoprotic acerbic is actuality titrated with a able base, after accession of the titrant will account a acknowledgment to action amid the acerbic and the base. HA + OH– > H2O + A– (4)
The attendance of the acerbic and its conjugate abject in band-aid will account the accumulation of a absorber solution, which are solutions that abide a desperate change in pH, should a able acerbic or abject be added to the system. At these points, the abruptness of a complete titration ambit is at its minimum. This is the pre-equivalence point. There is a point during the pre-equivalence point arena wherein both the acerbic and its conjugate abject are present in according amounts. This occurs back bisected of the acerbic has been neutralized, or back the titration is at the half-equivalence point.
At this point, the buffering accommodation of the arrangement is at its maximum. Aside from that, simplifying Eq. 3 at this point by inspection, the [H+] is according to that of the Ka. Demography the abrogating of the logarithm of both sides, one will be able to get the affiliation pH = pKa (Skoog, et al. , 2004). Thus, the break affiliated may be computed through free the pH at half-equivalence point. This agreement will use salicylic acerbic as the analyte. Salicylic acid, Fig. 1. Salicylic acerbic is a anemic monoprotic acid, able of absolution the acerb hydrogen affiliated to the –COOH group.
The hydrogen of the phenol accumulation is adamantine to absolution because the oxygen is counterbalanced by resonance. Salicylic acerbic is a artlessly occuring substance, usually produced by plants. It is begin mainly in the willow's leaves and bark. The authentic acerbic possesses several advantageous alleviative properties. It is an antipyretic (fever reducer), analgesic (pain reliever) and anti-inflammatory (swelling reducer). However, authentic salicylic acerbic makes for an acutely abhorrent medicine. Salicylic acerbic contains two acerb anatomic groups, the phenolic (C6H5OH) and the carboxylic acerbic (RCOOH) groups.
These groups account the acerbic to be an acerbic actuality that burns the aerial lining of the mouth, throat, and stomach, appropriately its esterification to acetylsalicylic acerbic or aspirin, afore assimilation as analgesic (Reed College, 2009). Esterificaton is the action by which a carboxylic acerbic is adapted to an ester. Esters are amoebic compounds that are acquired usually by reacting a carboxylic acerbic and an alcohol. The accepted blueprint for esters is with the RC=O accumulation acquired from the ancestor carboxylic acid, and the –OR’ accumulation from the ancestor alcohol.
The apparatus for the acknowledgment of the booze and carboxylic acerbic to anatomy the ester is as follows: Fig. 2. Apparatus of Esterification from a Carboxylic acerbic RCOOH and booze R’OH Esters may additionally be actinic by reacting the carboxylic acerbic with alternative reagents such as SOCl2 to anatomy the acyl chloride, which will afresh be advised with an booze in pyridine, to esterify it. Esters are amid the best boundless of all artlessly occuring compounds. Abounding esters are pleasant-smelling liquids that are amenable for the ambrosial odor of fruits and flowers. For example, methyl butanoate is begin n pineapple oil and isopentyl acetate is a basic of assistant oil. The ester bond is additionally present in beastly fats and alternative biologically important molecules. The actinic industry additionally uses esters for a array of purposes. Ethyl acetate, for example, is frequently acclimated as a bread-and-butter while abounding dialkyl phthalates are acclimated as plasticizers to accumulate polymers from actuality breakable (McMurry, 2004). Methyl salicylate is produced by abounding plants. It was aboriginal abandoned from wintergreen leaves, Gaulthea procumbens, and is frequently accepted as oil of wintergreen. Fig. 3. Methyl salicylate
An ester of salicylic acerbic and methanol, it masks one of the acerb hydrogens in salicylic acerbic by replacing it with a methyl (CH3–) group. Hence, it is a about apathetic admixture that does not absolution salicylic acerbic calmly into the body. It is, therefore, not an able analgesic, or pain-killer. However, it is added to abounding products, conspicuously for its fragrance, abnormally basis beer and liniments. In adjustment for it to be activated, methyl salicylate allegation be adapted to salicylic acerbic by amoebic synthesis, accurately through saponification, a action not clashing that undergone by beastly fats to become soaps (McMurry, 2004).
In this experiment, salicylic acerbic will be actinic from methyl salicylate by abject hydrolysis. Its break affiliated will additionally be abstinent through potentiometric titration. Likewise, the allotment of abstention the salicylic acerbic acclimated in the acknowledgment will be additionally determined. METHODOLOGY In synthesizing salicylic acid, 1. 2 g of sodium hydroxide (NaOH) were attenuated in 7 mL baptize in a annular basal flask.. Bisected of a milliliter (0. 5 mL) of methyl salicylate was added to this mixture. The admixture was afresh efluxed for 15 account and cooled to allowance temperature. One-milliliter increments of 3 M sulfuric acerbic (H2SO4) were added until the accumulation of a white precipitate, salicylic acid. Bisected of a milliliter (0. 5 mL) of the acerbic was added to ensure complete precipitation of the product. The admixture was afresh cooled in an ice baptize ablution with a temperature of at best 5°C for the acknowledgment to subside. The artefact was afresh filtered and rinsed with algid water, and recrystallized in water. The debris were afresh filtered on a pre-weighed clarify cardboard and air-dried in the locker.
When dried, the solids, forth with the clarify paper, were advised and the melting point determined. Two hundred and fifty milliliters (250 mL) of 0. 50 M banal sodium hydroxide (NaOH) band-aid was afresh prepared. From this solution, 250 mL of 0. 050 M accepted band-aid was able for the titration. This band-aid was afresh affiliated with 0. 2000 g 99. 95% potassium hydrogen phthalate (KHP) and phenolphthalein as indicator to the anemic blush endpoint. Two added trials were done. Three samples of authentic salicylic acerbic were advised at about 0. 000 g and placed in three abstracted 250-mL beakers. A minimum bulk of 95% booze was afresh added to deliquesce the acid, and was adulterated with 50. 00 mL distilled water. The electrode was aboriginal calibrated such that it would chronicle the developed abeyant to the pH. At this point, the abeyant would be abstinent as pH back increments of the titrant are successively added. The abject burette, pH meter, and alluring stirrer were set up according to Fig. 6. Fig. 6. Potentiometric titration accoutrement A circuit bar was abode central the alembic with the sample solution.
The electrode was additionally positioned such that it would not get hit with the circuit bar. The pH of the accoutrement was aboriginal abstinent above-mentioned to accession of base. For the aboriginal trial, 1. 00 mL of the titrant was added to the analyte and the pH recorded. This was done until the pH was 11. 50. From there, the adequation point was approximated based on the aggregate of the titrant that acquired a ample change in pH. For the two afterwards trials, 1. 00 mL of the titrant was added to the analyte. At ±5. 00 mL of the adequation point, the aggregate of titrant added was bargain to 0. 0 mL increments. At ±3. 00 mL and ±2. 00 mL of the adequation point, it was added bargain to 0. 20 mL and 0. 10 mL, respectively. The titration contnued aloft 5. 00 mL of the adequation point application 0. 50 mL of titrant until the pH registered was 11. 00. RESULTS AND DISCUSSION Methyl salicylate reacts with a able abject in the afterward manner: Fig. 4. Abject hydrolysis of methyl salicylate The methoxy accumulation of the ester is commissioned by the hydroxide ion through nucleophilic acyl substitution. The hydroxide attaches itself to the carboxylic carbon.
A fast proton alteration from the hydroxide to the methoxy accumulation occurs such that the methoxy gets protonated and leaves the substrate. The sodium ions balance the abrogating allegation of the salicylate ion predominantly begin abreast the two oxygens of the ion. The band-aid was afresh refluxed to anticipate accident of actual and to anticipate the admittance of algae in the product. Afterwards cooling to allowance temperature, 1. 00 mL of 3M sulfuric acerbic (H2SO4) until salicylic acerbic starts bottomward as a white solid. To complete the precipitation, 0. 50 mL of the able acerbic was added to the mixture.
At this point, the salicylate ion is protonated and the final product, salicylic acid, forms through the reaction: Fig. 5. Protonation of the phenolate and carboxylate groups of the salicylate ion The alembic absolute the accelerate was afresh abolished in algid baptize to stop the reaction. Algid baptize was acclimated in rinsing the solid afterwards filtration to ablution out algae that were baffling in the solvent. The solid was recrystallized in hot water. Dissolving the debris in hot baptize about increases the solubility of the substances, appropriately the debris deliquesce forth with the acrid impurities.
The admixture was afresh accustomed to air-conditioned slowly. As the band-aid cools, the solubility of the admixture (and of the acrid impurities) decreases, the band-aid becomes saturated with the adapted compound, and the admixture begins to crystallize. Because accumulation of crystals is a awful careful action that usually excludes adopted molecules, alone crystals of the adapted admixture form. Because the acrid algae are present in abate amounts, the band-aid never becomes saturated with the impurities, so the algae abide in band-aid alike afterwards the band-aid has cooled.
Removing the band-aid from the crystals appropriately removes the bread-and-butter and the acrid algae from the adapted crystals. A final bathe of minimum ice baptize ensures the displacement of the algae still adhering on the apparent of the artefact (“Recrystallization,” accessed 6 Sept 2010). Afterwards it was recystallized, the solid was filtered afresh in a pre-weighed clarify paper. The clarify cardboard acclimated in clarification the solid advised 1. 0349 ± 0. 0002 g. The accumulation of the broiled artefact and the clarify cardboard was 1. 3610 ± 0. 0002 g; this agency that 0. 3261 ± 0. 0003 g was synthesized.
Theoretically, with methyl salicylate as the attached reagent, the accumulation of salicylic acerbic should be 0. 5400 g. Computing for the %yield, would accord 61. 0%. In the assurance of the melting point, the empiric ambit of the melting point was 150-156°C. Comparing with the abstract value, 161°C, the about absurdity lies aural the ambit 3. 11-6. 83%. Therefore, the actinic salicylic acerbic has a aerial abstention as apparent by the consequence of the about error. This could absolve the %yield accustomed that recrystallization ability accept acquired a accident of actual due to the access of the cardinal of accomplish involved.
It should be acclaimed that in amoebic synthesis, accomplish arch to the ambition atom do not accord 100% yield, hence, accretion the cardinal of accomplish would abatement the %yield. Aside from that, the low crop could be attributed to the adventitious accelerated cooling that occurred during the recrystallization process. In a titration curve, there are three important regions: pre-equivalence point, adequation point, and post-equivalence point. For the titration of salicylic acerbic with NaOH, the pre-equivalence point is characterized by the assurance of pH on the absorption of salicylic acid.
Let salicylic acerbic be HA; above-mentioned to accession of the base, the pH can be affected by the absorption of HA and its acerbic break constant, Ka. Back the abject is added, HA reacts with the hydroxide ion (OH– ) according to the Eq. 4 declared above. Therefore, one has a absorber band-aid comprised of salicylic acerbic and salicylate. At adequation point, all of the acerbic has been adapted into its conjugate base. This abject will hydrolyze with baptize through the acknowledgment A– + H2O > HA + OH– (6) reforming a baby allocation of the acid. At the adequation point, the abruptness of the titration ambit is at its steepest.
The pH at this point is abased aloft the conjugate abject formed and its abject break constant, Kb. Aloft the adequation point, the post-equivalence point, the pH is abased on the absorption of the balance titrant added. This is because the accession of the conjugate abject is actual baby and, therefore, negligible compared to that of sodium hydroxide. However, salicylate is still the ascendant breed of salicylic acerbic present in solution. A acumen amid adequation point and endpoint should be accustomed back titrating.
Skoog, (2004), states that the adequation point is the point in titration area the bulk of titrant added is chemically agnate to that of the analyte in solution. The endpoint is a point during titration wherein an appreciable change signals that the bulk of titrant added is chemically agnate with that of the analyte. The endpoint may not necessarily accompany with the adequation point, abnormally in abatement titrations. It may appear beforehand or after than the adequation point, depending on the indicator used. This aberration pf aggregate at agnate and endpoint is accepted as titration error.
In potentiometric titration, no actinic indicator is required. Instead, the endpoint is characterized by the desperate change in pX, abstinent by the electrode. X may be H3O+, OH–, a cation or anion, or any actuality accommodating in the titrimetric acknowledgment (Day and Underwood, 1991). In this case, the aggregate of titrant that contributes to the greatest change in pH is the aggregate appropriate to absolutely titrate the salicylic acerbic in solution. Thus, the increments of accession of titrant are narrowed bottomward as the adequation point is accomplished because that way, the adequation point will not be missed.
Aside from that, the exact aggregate of titrant appropriate for complete acknowledgment of the analyte would be detected. Above-mentioned to titration and concoction with water, the salicylic acerbic was attenuated with a basal bulk of 95% ethanol. Salicylic acerbic is sparingly acrid in water, a arctic bread-and-butter that can hydrogen-bond with solutes that can hydrogen-bond with its molecules. This is due to the acerbic accepting added than 6 carbons accretion its non-polarity, although it has oxygen that can anatomy hydrogen bonds with baptize (McMurry, 2004). Adding booze reduces the polarity of the solvent, facilitating dissolution of the acid.
It was acclaimed that some of the acerbic reprecipitated aloft accession of water. Thus, a minimum bulk of 95% booze was afresh added to deliquesce the acid. In the experiment, the electrode was additionally not bargain while the bar was spinning as air bubbles would attach on its apparent causing an absurdity in the altitude of pH. Affiliated active is important in potentiometric titrations, as in alternative types of titrimetric analyses. Active is done because it will account the titrant to acknowledge not aloof with the apparent of the analyte area it alone but with the absoluteness of the analyte solution.
This way, the acknowledgment will go to achievement and basal absurdity will be observed. The volumes acclimated for acclimation of NaOH with KHP as primary accepted are archival in Table 2 in the Appendix. Acclimation of sodium hydroxide gave 0. 04633 M NaOH. This bulk was acclimated in the assurance of the allotment of abstention (%purity) of the salicylic acerbic sample. For the assurance of the adequation point volume, three plots were done for anniversary trial. The aboriginal artifice is the blueprint amid pH vs. aggregate of titrant. The additional is the aboriginal acquired artifice with which ? pH/?
V was advised adjoin V’, area ? pH and ? V are the change in pH and the aggregate added amid two after readings, respectively, and V’ is the boilerplate aggregate amid two after readings. The third artifice is the additional acquired artifice amid ? 2pH/? V2 and V”. The ordinate of the graph, ? 2pH/? V2, was acquired by demography the caliber of the aberration amid two after readings of ? pH/? V, and ? V, admitting V” is the boilerplate of the two after readings of V’. Alone two trials were done because of the blemish of the time. The graphs for the aboriginal balloon are: a) (b) (c) Fig. 7. Titration Curves for the Aboriginal Trial. (a) pH vs Vtitrant; (b) ? pH/? V vs V’; (c) ? 2pH/? V2 vs V” MNaOHVNaOH at eq ptFWsalicylic acerbic Gsample The blueprint for the additional balloon may be begin at the Appendix. Either of the three graphs of Figs. 7 and 8 may be acclimated in the assurance of %purity for anniversary trial. The blueprint acclimated for free the %purity, accurately for this agreement is %purity = x 100 (7) Thus, the aggregate of titrant acclimated at adequation point is appropriate for the calculation.
The amid ethics of the aggregate at adequation point are archival in Table 3 in the Appendix. Adding of the beggarly %purity is 101. 7%, accepting an absurdity of 1. 7%, as the abstract bulk is 100. 0%. The pKa is based on the pH at half-equivalence point by advantage of the ancestry of Eq. 3. Onle Figs. 7a and 8a may be acclimated for the assurance of pKa as the alternative four do not anon accord the pH at anniversary point. The amid ethics for the pH at half-equivalence point is archival at Table 3 in the Appendix. The beginning pKa is 2. 865. Therefore, the Ka of the acid, accustomed Ka = –log[Ka] (7) or Ka = 10–pKa (8) is 1. 3646 x 10–3. The abstract pKa is according to 2. 98. Thus, the pKa bulk gave acceleration to a 3. 86% error. Statistical assay of the after-effects shows the afterward results: Table 1. Statistical Assay of the After-effects | Range| Accepted Deviation| %purity| 15. 7%| 11. 1%| pKa| 0. 21| 0. 148| | About Accepted Deviation (RSD), ppt| Aplomb Intervals (95% confidence)| %purity| 109. 1| 101. 7 ± 99. 7 %| pKa| 51. 8| 2. 865 ± 1. 334| The RSD of %purity is about ample such that the ethics accept low precision.
Aside from that, the aplomb intervals for the beggarly is additionally ample such that it about has the aforementioned adjustment of consequence as that of the mean. This agency that the beggarly exhibits actual low accuracy. With commendations to the pKa, the RSD showed a low value, alone 51. 8 ppt, implying aerial attention amidst the values. On the alternative hand, the beggarly bulk shows low accurateness because of the consequence of the aplomb intervals. These errors could be attributed to the cardinal of trials. Due to bare time, the accumulation was alone able to do 2 trials, one beneath than the assigned cardinal of trials.
CONCLUSIONS The agreement aimed at synthesizing salicylic acerbic from methyl salicylate and free the acerbic break affiliated Ka of the acerbic forth with its purity. Amoebic amalgam provided a 61% crop of the acid, a about low yield. However, the abstention of the acerbic can be classified as aerial due to the empiric melting point range’s attention with the abstract one; the %differenceis alone 3. 11-6. 38%. The after-effects of the potentiometric titration appearance that the salicylic acerbic acclimated was 101. 7% pure, a bulk greater than the purity, which is 100%.
The Ka, bidding as pKa, acquired was 2. 865, 0. 035 units beneath than the abstract value, which is 2. 98. Though the %differences are low, the accurateness of the computed ethics is ambiguous accustomed that the aplomb intervals for the %purity and pKa are ± 99. 7 and ± 1. 334, respectively. These errors can be attributed to the cardinal of trials done, which is two, due to the apathetic stabilization of the pH beat readings that resulted in a continued aeon of titration. Thus, it can be assured that potentiometric titration is an able way of free the acerbic break affiliated of a sample.
It is recommended that a bigger pH beat be acclimated in the altitude of the pH and that solutions acclimated be titrated immediately. REFERENCES  Day, R. A. and A. L. Underwood. (1991). Quantitative Analysis. Sixth Edition. Prentice Hall, Inc. , USA.  “Experiment 3: Amalgam of Salicylic Acerbic from Oil of Wintergreen (Methyl Salicylate). ” Class Advertence Manual Chemistry 201/202. Reed College. Accessed 6 September 2010 ;http://academic. reed. edu/chemistry/alan/ 201_202/lab_manual/Expt_salicylic_acid/background. html;  McMurry, J. (2004). Amoebic Chemistry. Sixth Edition.
Brooks/Cole – Thomson Learning, CA, USA.  ”Recrystallization. ” University of Massachussetts. Accessed 6 September 2010 ;http:// www. chem. umass. edu/~samal/269/ cryst1. pdf;  Skoog, D. A. , D. M. West, F. J. Holler, S. R. Crouch. (2004). Fundamentals of Analytical Chemistry. Eighth Edition. Brooks/Cole – Thomson Learning, CA, USA. APPENDIX Gfilter cardboard = 1. 0349 ± 0. 0002 g Gfilter cardboard and broiled artefact = 1. 3610 ± 0. 0002 g Gdried artefact = 0. 3261 ± 0. 0003 g %yield = 61. 0% Empiric melting point range: 150-156°C %difference = 3. 11-6. 83% 1° accepted used;: potassium hydrogen phthalate (KHP) abstention = 99. 95% Table 2. Acclimation of NaOH Trial| Accumulation 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| Boilerplate Molarity, M| 0. 04633| Table 3. Sample Assay Trial| 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| -| -| -| -| Boilerplate pKa = 2. 865 Boilerplate %purity = 101. 7% Artifice of the Additional Trial: (a) (b) (c) Fig. 8. Titration Curves for the Additional Trial. (a) pH vs Vtitrant; (b) ? pH/? V vs V’; (c) ? 2pH/? V2 vs V”
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