DETERMINATION OF ASPIRIN BY INDIRECT TITRATION Legaspi, Maria Susana V., Marquez, Ma. Rica Paulene B., Marzan, Shemaiah Mae T., Nomil, Jan Wyne T. Department of Biology, College of Science, University of the Philippines Baguio Nov. 26, 2013
ABSTRACT
Back titration is determining the concentration of an analyte by reacting it with a known number of moles of excess reagent. The excess reagent is then titrated with a second reagent. The concentration of the analyte in the original solution is then related to the amount of reagent consumed. This experiment aims to evaluate the purity of an aspirin, develop techniques for the proper dilution of a solution to definite volume and how to carry out calculations involving back-titration. Preparation and standardization of 0.1 N HCl solution will be prepared and to be diluted with the standard NaOH solution used from Experiment 3 until a permanent faint pink color is achieved after performing three trials. And an analysis of aspirin will be exercised, solution of hydrolyzed aspirin will be titrated using standardized HCl solution using 2-3 drops of phenolphthalein indicator.
INTRODUCTION Direct Titration is a treatment of a soluble substance contained in a vessel with an appropriate standard solution and the endpoint is being determined visually by an indicator. Indirect Titration, doesn’t involve measuring the substance itself but knowing the quantity of a reagent to be added with a specific reaction and a titration is done to determine the unknown excess. Aspirin, an acetyl derivative of salicylic acid, is a white, crystalline, weakly acidic substance, with a melting point of 136 °C (277 °F), and a boiling point of 140 °C (284 °F), also known as acetylsalicylic acid salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It is derived from two acids, Acetic acid and salicylic acid. It can be hydrolyzed and neutralized with the two components simultaneously. [1]
RESULTS Concentration of NaOH: 0.0927 N = 0.0927 M
Table 1 Volume of HCl Trial 1 Trial 2 Trial 3 25 mL 25 mL 25 mL *Table 1 shows the volume HCl that was titrated for the standardization of HCl Table 2 Volume of NaOH used in HCl Volume Trial 1 Trial 2 Trial 3 of NaOH (in mL) (in mL) (in mL) Initial 1.4 22.9 13.1 Final
22.9
46.2
33.9
Used 21.5 23.3 20.8 *Table 2 shows the volume of NaOH used to titrate 25 mL HCl Mass of Aspirin: 0.1074 g Table 3 Aliquot of Aspirin Trial 1 Trial 2 Trial 3 50 mL 50 mL 50 mL *Table 3 shows the aliquot volume of the ground aspirin to be titrated with the standardized HCl Table 4 Volume of HCl used in Aspirin Volume Trial 1 Trial 2 Trial 3 of HCl (in mL) (in mL) (in mL) Initial 13.1 15.5 17.2 Final 15.5 17.2 20.4
Used 2.4 1.7 3.2 *Table 4 shows the volume of HCl that was used in the titration of the ground aspirin
DISCUSSION The world’s most commonly used painrelieving drug, the Acetylsalicylic acid is generically known as ASA or Aspirin. It is an acetyl derivative of salicylic acid that is a white, crystalline, weakly acidic substance, with melting point 137°C. It is useful in the relief of headache and muscle and joint aches. Aspirin is also effective in reducing fever, inflammation, and swelling and thus has been used for treatment of rheumatoid arthritis, rheumatic fever, and mild infection. HC9H7O4(s) (ASA) is an organic acid (carbon chain) similar to that of acetic acid, HC2H3O2 (l) reacts with strong bases such as sodium hydroxide in the same way. There are several complicating factor that arise from the nature of ASA tablets.[2] The experiment consists mainly of two parts: (1) the preparation and standardization of 0.1 N HCl solution (2) and the determination of ASA in aspirin. In the preparation of the 0.1N HCl, 8.3mL 12 M HCl was mixed with water to generate a 100mL 0.1N HCl. The prepared chemical was then standardized using the NaOH. (The NaOH used was already standardized in the previous experiment) Table 1 and 2 shows the volume of HCl and NaOH used. The standardized HCl was then used in the analysis of the aspirin. In the analysis of the aspirin, 0.1 grams of the aspirin sample was pounded and a 20.00 ml of the standardized NaOH was added to it. The mixture was boiled then for 10-15 minutes. Simmering was done to hydrolyze the aspirin sample. Three trials
were made for the analysis of the aspirin. The mixture was cooled, and was transferred into a 250 ml volumetric flask. The flask was washed several times with 10 ml portions of distilled water and was filled to the mark. Then, the hydrolysed aspirin was titrated with the standardized HCl using phenolphthalein as the indicator. Table 3 and 4 shows the volume of aspirin aliquot and HCl used. To calculate the number of moles of NaOH the formula used was: molarity of NaOH times average volume of NaOH used. The molarity of NaOH is 0.0927 M and its volume for every trial as shown in Table 2. The computed moles per trial were 1.99 x 10-3 mol, 2.16 x 10-3 mol, and 1.93 x 10-3 mol of NaOH respectively. The mole of excess NaOH was computed by multiplying the molarity of HCl by the volume of HCl used. The molarity of HCl was given as 0.1 M and the volume for every trial was given in Table 4. The calculated results were 2.4 x 10-4 mol, 1.7 x 10-4 mol , and 3.2 x 10-4 mol for first, second and third trial respectively. One of the main objectives of this experiment was to determine the concentration of the acetylsalicylic acid in the aspirin tablet. The computed number of moles of the acetyl salicylic acid in the aspirin was 8.92 x 10-4 mol.
CONCLUSION The objectives of this experiment were successfully met and back titration was properly performed and introduced. It is observed that the volume of NaOH used to titrate HCl do not greatly vary, same as with the volume of HCl used to titrate aspirin. The number of moles of Aspirin computed was 8.92 x 10-4 mol.
LITERATURE CITED [1]http://www.pharmacopeia.cn/v29 240/usp29nf24s0_c541.html. Schrör, Karsten (2009). Acetylsalicylic acid. Wiley. ISBN 978-3527-32109-4. Date Removed December 16, 2013
V NaOH = ? weigth of aspirin = 250mg(4) = 1g *Formula: % ASA =
[( [2] RET Lab Lesson Plan. http://www.njit.edu/precollege/docs/. Date Retrieved December 16, 2013 Then then ASA in Analysis. http://survivaltraining.info/Library/. Date Retrieved December 16, 2013 Titration of Synthesized Aspirin: A continuation of the aspirin synthesis lab. http://www.whatcom.ctc.edu/. Date Retrieved December 16, 2013 Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. R. (2004). Titration Curves for Weak Acids. In Fundamentals of Analytical Chemistry 8th Edition (pp. 378-382). Canada: Thomson Learning Inc.
ANSWERS TO QUESTIONS 1. What would be the effect of a more dilute solution of NaOH on the titration? pH values would be lower for the dilute solution than the normal pH value of the standard solution, and, therefore, changes the amount of titrant needed to complete the equivalence point in titration. 2. What volume of 1.0N NaOH is used in the hydrolysis of four 250mg tablets of aspirin (90%pure acetylsalicylic acid) if 10.00ml of 0.05N HCl is consumed in the back titration. *Given: N HCL = 0.05 V HCL = 10.00 ml N NaOH = 1.0
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*10.49ml of NaOH was used to hyrolyzed 1g aspirin; 90% pure ASA.
