Linda T. Hannett Fernandez and Frederick H. Klappmeier Russell Sage College Troy, New York 12180
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An Experiment for hdrumental Analysis The determination of aspirin by ultraviolel absorption
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T h e ohjectives of this experiment are t o provide a simple which will illustrate abspectrometric analytical sorption in the ultraviolet region by what is essentially a clear, colorless solution and to make use of a sample suhstance of a more practical nature than the usual labo&ry unknown. The method ~ r e s e n t e d h e ~ e w a s dine a~ senior ~ l ~ ~research ~~ project. It utilizes the fact t h a t aspirin can be hydrolyzed t o salicvlic acid hv base in aqueous solution. T h e calibration curvk is constr"cted from standard solutions prepared from pure salicylic acid. T h e acetylsalicylic acid in the unknwm, a commercial asoirin tahlet. is hvdrolvzed to salicvlic acid in a 0.1 N NaOH solution, diluted, a n d t h e absorbance of t h e solution measured a t 297 nm. T h e amount of salicvlic acid present in t h e solution is obtained from the calibration curve, and t h e weieht of acetvlsalicvlic acid in the tablet is then calculated &ing the appropriate stoichiometric factor. The hydrolysis of acetylsalicylic acid proceeds rapidly and quantitatively in 0.1 N NaOH at room temperature. (Hydrolysisis tooxlow to be useful in solutionswith concentrationsof 0.01 N NaOH or less.) Furthermore. Beer's Law is obeyed by salicylicacid in 0.001 N NaOH over a conce&ration range of 5.00-50.0 m d l with an average molar absorptivity of 3.55 X lo3 Ilcm mole, relative standard deviation of 3%.The exact base concentration is not critical since both the wavelength of maximum absorbance at 297 nm and the absorptivity of salicylic acid are constant in solutions with pH between 6 and 12. The determination of acetylsalicylic acid in commercial aspirin tablets by this method can be expected to give recoveries of aspirin within the limits of the U.S.P. standards for such tablets (95-105% of label value).' Student Experimental Procedure Preparation of Standards Accurately weigh (to 0.1 mg) 0.1 g of pure salicylic acid and transfer to a 100-ml volumetric flask. Bring to volume with distilled water. (Gentle heating may be necessary to complete dissolution.) Calculate the concentration of salicylic acid in mgfl and label the flask Stock # 1. Into five, 100-ml volumetric flasks, pipet 1,2,3,4, and 5 ml of the
266 I JwrMl of Chemical Education
Stock #1 of salicylic acid. To each of the flasks add 1 ml of 0.1 N NaOH and bring to volume with distilled water. Calculate the concentration of salicylicacid in each standard. Absorotion Soectrum and Calibration Curve Using one centimeter quartz cells, obtain the absorption spectrum of the 3 ml dilution of salicylic acid standard with distilled water as the reference blank, and ascertain the wavelength of maximum absorbance. Measure the absorbance of each of the standards versus the water l,lank, st the wavelength of maximum ahwrhanre. Construct theralihratimcurve 11splotting absorbance arafunctranufsalicylic acid concentratiun. ~
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of~cetylsalicyljc ~ ~ ~ j id ~id in a ~ commercial t j ~ ~
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Crush one 5 grain, commercial aspirin tablet with a mortar and pestle and dissolve the aspirin component of the powdery material by adding 20 ml of 0.1 N NaOIf. Remove the insoluble filler material by filtering the solution into a 250-ml volumetric flask. Rinse the mortar and pestle with six successive 10-mlportions of 0.1 N NaOH and also filter these into the 250-ml volumetric flask. Bring the solution to volume with additional0.1 N NaOH and shake well for about 2 min. Pioet a 2-ml aliauot of this solution intoa 100-ml volumetric flask andbring this to Golume withdistilled water. Measure the absorbance of this solution at the same wavelength used for the absorbance of the salicylic acid standards. Use the calibration curve to find the concentration of salicylic acid present in this solution from the measured absorbance. Convert from the concentration of salicylic acid (SA) to the concentration of acetylsalicylicacid (ASA)using the followine stoichiometric factor c~nrentrstionof SA X lormula weight of ASA formula weight of SA Then calculate the weight of acetylsnlicylir acid present in the aspirin tablet by conridering the dilutions involved. Compare the result to the content specified on the label and compute the percent recovery. The recovery ahould be within the raneeof the U.S.1'. standardsfor tablet content, 95-105% '"The United States Pharmaeopeia," Eighteenth revision, Mack Printing Company, Easton, Pa., 1970, p. 54.
