EXPERIMENT 5 : DETERMINATION OF ACETYLSALICYLIC ACID IN ASPIRIN USING BACK TITRATION METHOD.

OBJECTIVETo determine the amount of acetylsalicylic acid in aspirin tablet.

APPARATUSWeighing boatFilter FunnelPipette (25mL)BuretteDropperVolumetric flask (250mL)Retort stand and white tile3 Conical flasks (250mL)

CHEMICAL REAGENTSAspirin tablet1.0 M NaOH solution0.1 M HCl solutionPhenol red indicator

PROCEDUREA. Hydrolysis of Aspirin1. One aspirin tablet is weight accurately in a weighing boat and this tablet is transferred

into a 250 mL conical flask containing 25.0 mL of distilled water.2. 25.0 mL of 1.0 M NaOH is pipette to the conical flask and this mixture is heated until

boiling. The mixture is allowed to boil for about 10 minutes in order to hydrolyse the aspirin.

3. The solution is cooled the solution is transferred quantitatively into a 250 mL volumetric flask and the solution is diluted to the mark with distilled water.

B. Back Titration1. 25.0 mL of the aspirin solution is pipette into a 250 mL conical flask and then 2-3

drops of phenol red indicator is added.2. The aspirin solution is titrate steadily in the 250 mL conical flask against 0.1 M HCl

solution from a burette and the flask in kept swirling until there a colour change. The reading at the end point of the titration is recorded.

3. The titration is repeated until student obtain consistent readings and titration data is recorded in Table 5.0 as shown in datasheet for experiment 5.

C. Blank Titration1. 25.0 mL of 1.0 M NaOH is diluted with distilled water in a 250 mL volumetric flask.2. 25.0 mL of the diluted NaOH solution is pipette into a 250 mL conical flask. Then, 2-

3 drops of phenol red indicator is added.3. The NaOH solution is titrate steadily in the conical flask with 0.1 M HCl solution

from the burette and the flask is kept swirling until there is a colour change. The reading at the end point of the titration is recorded.

4. The titration is repeated until student obtain consistent readings and the titration data is recorded in Table 5.1 as shown in datasheet for experiment 5.

RESULTS

DATASHEET FOR EXPERIMENT 5

Weight of aspirin tablet = 0.7541 gCommercial name of the tablet is Ispirine.Manufacturer of the tablet is Reckitt Benckiser Health Care (UK) LTD. Date of manufacturer is November 2009.

1. To determine the excess amount of acetylsalicylic acid after hydrolysis of aspirin tablet.Molarity of NaOH solution = 1.0 MVolume of hydrolysed solution = 25.0 mL

Table 5.0 : Back titration of excess NaOH with HCl solution Titration number

Burette reading1 2 3 4

Final reading 24.40 24.50 24.20 24.40Initial reading 0.00 0.00 0.00 0.00Volume of HCl used (mL) 24.40 24.50 24.20 24.40

Average volume of HCl used = 24.375 mL

2. To determine the volume of HCl required to react with NaOH solution without any aspirin sample.

Table 5.1 : Blank titration of NaOH solution with HCl

Titration number

Burette reading1 2 3 4

Final reading 30.10 30.30 30.20 30.30Initial reading 0.00 0.00 0.00 0.00Volume of HCl used (mL) 30.10 30.30 30.20 30.20

Average volume of HCl used = 30.225 mL

Reaction : NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

Calculation :1. Calculate the no. of moles of NaOH which actually reacting with the acetylsalicylic

acid during the hydrolysis. Since 1 mol H+ ions react with 1 mol OH- ions, the no. of moles of NaOH reacting with acetylsalicylic acid in aspirin during hydrolysis, = no. of mol of NaOH blank titration - no. of mol of NaOH back titration

= (MHCl ×VHCl )(blank titration) - (MHCl × VHCl)(back titration)

= ( 0.10 × 0.030225) – (0.10 × 0.024375)

= 5.85 × 10-4mol of HCl

2. From equation 1, the mole ratio of NaOH reacting with acetylsalicylic acid is given as no. of moles of NaOH = 2 (no. of moles of acetylsalicylic acid).Calculate the no. of moles of acetylsalicylic acid hydrolysed.

MaVaMbVb =

ab

CH3COO-C6H4-COOH = 1 NaOH 2CH3COO-C6H4-COOH = 1 5.85 × 10-4 2CH3COO-C6H4-COOH = 2.925 × 10-4 mol

3. Calculate the mass of acetylsalicylic acid. Mass = mol × RMM = 2.925×10-4 × 180 =0.05265 g

4. Calculate the % (w/w) of acetylsalicylic acid present in your aspirin tablet. = 0.05265 × 100 0.74150 =7.10%

DISCUSSIONThe apparatus is set up to determine the amount of acetylsalicylic acid in aspirin tablet. The aspirin tablet is weigh accurately and the weight of aspirin is 0.7541 g. The molarity of NaOH used is 1.0 M. The average volume of HCl used in back titration is 24.375 mL and the average volume of HCl used in blank titration is 30.225 mL. The conical flask must be keep swirling while titrate until the colour change from purple to yellowish. The parallex error must be avoid to achieve the accurate reading. The eyes must be perpendicular to the burette reading.

QUESTIONS1. Tabulate the class result of the % (w/w) of acetylsalicylic acid present in aspirin tablet

analysed and determine the confidence limit of the mean of the % (w/w) of acetylsalicylic acid at 95% confidence level.

Student Result(%w/w)1 5.032 6.483 7.084 8.25

Mean,x = 5.03+6.48+7.08+8.25 4 = 26.84 4 = 6.71

CL,µ = x ± ts

√N = 6.71 ± (3.182) (1.16) √4 = 6.71 ± 1.84

2. Comment on the overall result of % (w/w) of acetylsalicylic acid obtained.-The result obtained is different because have the error like systematic error and random error

-The result also different because have affected by different mass of aspirin tablet.

CONCLUSIONSThe experimentally obtained value of % (w/w) of acetylsalicylic acid in an aspirin tablet is 7.10% . The confidence limit of the mean of the % (w/w) of acetylsalicylic acid at 95% confidence level is 6.71 ± 1.84.

REFFERENCES

Handout Chapter 1- Introduction : Analytical Chemistry Handout Chapter 2 – Evaluation of Experimental Data