How to measure aspirin concentration by UV-vis spectroscopy or HPLC?

Question

Want to measure concentration of aspirin or 5-aminosalicylic acid with either UV-vis or HPLC so need a method or any information that is relevant

Answer 1

By UV:

Weigh about a sample of reagent grade 5-aminoacetylsalicylic acid into a flask and add 10 mL of 1 M NaOH. Heat the mixture gently in a boiling water bath for fifteen minutes. Wash down the sides of the flask with a small amount of distilled water. Allow the solution to cool to room temperature.
Transfer the solution to a 250 mL volumetric flask. Rinse the flask several times with distilled water and add the washes to the volumetric flask. Fill the volumetric flask to the mark with distilled water. This solution is the stock solution. Determine the Molarity of this solution.

Prepare 4 dilutions of the stock solution. Fill to the mark with 0.02 M iron (III) chloride solution pH 1.5. Run each of these samples in the UV-vis spectrometer. Label each solution clearly and record the concentrations. Determine the molar concentration of each dilution. Sketch a graph of Absorbance vs. Concentration.

Weigh about 0.4 g of your synthesized aspirin sample into a flask. Add 10 mL of 1 M NaOH solution and heat the solution in a boiling water bath for fifteen minutes. Cool and transfer the solution to a 250 mL volumetric flask, rinsing the Erlenmeyer flask several times with distilled water. Transfer all of the washes to the 250 mL volumetric flask. Fill to the mark with distilled water. Pipet a 5 mL sample into a 100 mL volumetric flask and fill to the mark with the 0.02 M iron solution. Transfer to a flask and label your synthesized aspirin sample.
Run your sample on the UV and use your graph to determine its concentration.

Hope this helps =)

Answer 2

Annie's method is pretty good - apart from one thing. If you are using aspirin that you have synthesized yourself, then remember that it is likely to contain some unreacted salycilic acid, so when you hydrolyse the "aspirin" sample and then read the "salicylic acid" concentration from the calibration graph, what you are actually measuring is the TOTAL salycilic acid concentration : the amount from the hydrolysis of your aspirin sample plus the amount of salycilic acid present as a contaminant! That's why the answer is often greater than 100%. You can work out the original amount of aspirin by calculation after this.
Good luck!

Answer 3

By Quantification.
Firstly you must have a series of known concentration solution to determine the concentration of unknown solution. Solutions with known concentration will diffuse across the chromatographic paper and will give series of peaks that will compare to your unknown sample. Then calculate the area under each peak A=1/2bxh and plot the data on the calibration graph by putting concentration of standards on x- axis and peak area on the y-axis. After you fit the best line to determine concentration.

A relationship between absorbance peak area and concentration of aspirin can be determined by Beer-Lambert Law.

To start your experiment you must have
water bath that must be heated from 30-60C
PH meter
0.01M sodium carbonate solution
0.01M salicylic acid
Buffers NaOH KH2PO4 at pH 7.0

Answer 4

Prepare soln with acidic buffer and dry.