What can affect purity of aspirin?

Firstly place 100 to 150 mg of aspirin into a 125 ml conical flask. Next proceed to mix in 15 ml of 95% ethanol solution and add 2 drops of phenolphthalein indicator. Then use the titration method to mix this solution against a standard solution of sodium hydroxide from a burette. Using the value obtained from the titration calculate the molarity of the aspirin. Then calculate the ratio of the observed molarity of aspirin with its theoretical molarity and finally multiply this ratio with 100 to obtain the percentage purity of the aspirin sample.