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Quantitative Acid-Base Titration Curve Report

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Updated: Mar 9th, 2022

Despite consuming double the recommended dose of internet aspirin there was no remedy for headache and ulcerous stomach problems started. There is a doubt about the authenticity of internet aspirin, whether it has any acetylsalicylic acid and antacid adjunct or none.

Looking into the conditions it is hypothesized that internet aspirin lacked any acetylsalicylic acid and antacid ingredient.

There is a semi-quantitative acid-base titration to evaluate the amount of acetylsalicylic acid in pre-weighed internet aspirin (placebo) considering being an acid (pK towards the acidic side) titration with NaOH would neutralize and end-point can be determined using phenolphthalein indicator subsequently by pH meter. For control purposes, a genuine aspirin tablet (Bayer Inc) with a prescribed amount of acetylsalicylic acid would be titrated in parallel.

First, visibly the coating onto the tablets would determine whether antacid adjuvant is present or not. Both control and placebo tablets would be ground in mortar and pestle. About 0.5 g is then dissolved in 25 ml of methanol in two beakers, and 2-3 drops of phenolphthalein indicator are added. Individually these solutions are titrated with 0.1 M NaOH solution prepared by adding 0.4 g dry NaOH pellets in 1 liter distilled water. NaOH solution is to be filled in a burette fixed to a stand until zero mark. Swirling the beakers, NaOH solution from burette is added drop by drop to the control and placebo aspirin solutions. Nearing the endpoint pink color starts to fade and then disappears leaving turbid and colorless solutions in the beakers. The volumes of NaOH solution required to attain the endpoint in each case are then recorded. It may be necessary to measure pH at the endpoint using a pH meter, and if it is pH 7.0 or a little above, we may conclude that the endpoint is attained.

Addition of phenolphthalein indicator did not produce typical dark pink color representing acidic pH, and this means at neutral pH the placebo did not deprotonate. In other words whatever the substance is the pK value is not lower than the solution pH. Weak acids like acetylsalicylic acid have pK lower than 7.0 and hence deprotonate at neutral pH. Eventually, the release of protons changes the solution pH acidic. Further, just adding a few drops of NaOH (~0.2 ml) was sufficient to attain the end-point. If the material is not acetylsalicylic acid then deprotonation is not achieved and there would be no dark pink color from phenolphthalein, and insignificant alkali is sufficient to bring back neutral pH. The placebo had negligible acetylsalicylic acid. On the other hand, in control deep pink color of phenolphthalein appeared, and upon titration, it started to fade after 13.25 ml of 0.1 M NaOH was added. Verification of the pH at the endpoint revealed that the solution was neutral (pH = 7.0). A positive result of titration with alkali suggests the presence of acetylsalicylic acid in control tablets.

Before calculation, it is necessary to know how many mol of NaOH in 0.1 M solution would neutralize several mol of acetylsalicylic acid. In titration 2 mol of OH- would be needed to titrate one mole of acetylsalicylic acid (one for acid-base and the other for hydrolytic titration). For determining the mass of acetylsalicylic acid in 500 mg placebo and control the following calculations will be assumed to be done:

  • Placebo – 0.2 ml of 0.1 M NaOH equals to (0.2 x 0.1)/1000 = 0.00002 g mol

Given- 2 mol NaOH titrate 1 mol acetylsalicylic acid

Therefore 0.00002 g mol titrate 0.00001 g mol acetylsalicylic acid

Given- Mass = g mol x molecular mass (188 for acetylsalicylic acid)

= 0.00001 g x 188 = 0.00188 g or 1.8 mg in 500 mg tablet

  • Control – 13.25 ml of 0.1 M NaOH equals to (13.25 x 0.1)/1000 = 0.001325 g mol

0.001325 g mol titrate 0.0006625 g mol acetylsalicylic acid

Mass = 0.0006625 g x 188 = 0.12455 g or 124.55 mg in 500 mg tablet

The method is semi-quantitative and the quantitative method is back titration in which an additional step of titration of unutilized NaOH with HCl is to be included. It is possible that due to incomplete hydrolysis the theoretical 2:1 molar ratio of NaOH: Acetylsalicylic acid is not attained. Acetylsalicylic acid is not fairly water-soluble and as titration proceeds, the solution becomes aqueous which brings out a possibility of precipitation resulting in turbidity that causes errors in endpoint determination. Placebo may contain substances like starch and other adulterated powders interfering with solubility and titration. Sometimes aspirin intrigue may also contain sodium acetate salt a reactant used in the synthesis of acetylsalicylic acid. This may self take part in titration as it is an acid and can give misjudgment about the quantity of acetylsalicylic acid.

It is concluded that there is the negligible presence of the effective drug acetylsalicylic acid in placebo internet aspirin (only 1.4%) as compared to what is found in the control Bayer’s aspirin.

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