Spectrophotometric determination of Aspirin content

            Earlier in the semester you titrated a aspirin sample using phenolthalein. The procedure could be considered 'old school' because it used concepts, ideas and equipment that has been around for over 100 years. Even though the results are good, this week the analysis will be similar, but you will use more recent technology, namely that spectrometer that you know how to drive oh-so-well.

            The situation is this, you are a laboratory assistant for the FDA. A new company (Aspirin-by-The-Bucket, Inc.) has submitted their synthesized aspirin to a certain purity. You are tasked with determining the purity and reporting the result to your supervisors. The purity is the ratio of the mass of ASA in the tablet divided by the mass of ASA they SAY is in the tablet. (look on the bottle)

            When aspirin (which is really O-Acetylsalicylic acid hereafter referred to as ASA) is hydrolyzed in a basic solution, it forms the salicylate dianion. If the reaction mixture is then acidified and then mixed with a solution containing the iron (III) ion, an intensely purple complex of tetraaquosalicylatoiron(III) ion. (wavelength of max absorbance = 530 nm) The reactions involved in the three steps are shown below:

            The last reaction proceeds to completion. Since one ASA molecule forms one complex ion, the concentration of the colored ion equals that of the original ASA in the sample.

Experimental Procedure. Sortof.

            Approximately 0.4 g of reagent grade ASA (do you need to know the exact mass?) is placed in a 125 mL flask. Add 10.0 mL of 1.0 M NaOH, and heat until boiling, stirring carefully. Get all of the solution (rinses too!) into a 250 mL flask, dilute with distilled water to the mark.

Take 5.00 mL of this solution and put in into a 50 mL volumetric flask, and dilute to the mark with the FeCl₃-KCl-HCl solution. Get a 'rough and tumble' reading for absorbance of this solution. Based on that absorbance, make up what you think are appropriate other dilutions. Your dilutions will be done using this stock solution and more of the FeCl₃-KCl-HCl solution. Solutions should be made in in test tube using pipets.

            For the unknown aspirin samples (note the plurality of that), you will prepare the samples in a similar fashion.

A picture of the lab:

 The Report: Short, professional memo style. Give a short background of why you chose this procedure. (How did you make the white tablet colored?) Give a brief procedure, a Beer's law plot of known data, sample calculations, and easy to understand results. Don't forget that 'How do I make this a more effective learning experience' paragraph. Make sure you staple the prelab question to the report. You will show it to the instructor when you come to lab.

 Pre-lab Question:

1. On one piece of paper, list the hazards (as found on the MSDS) for ASA, give its molecular formula, and a data sheet for the collection of data. This paper must be shown to the instructor (to get signed) as you enter the lab. No data sheet, no lab for you!

~MEO 03.02.05 15:19