# Titration of aspirin

I have performed an experiment to test how temperature affects how fast aspirin dissolves by titrating solutions at various time intervals. According to this website, aspirin reacts with water as such:

$$\ce{C9H8O4(aq) + H2O(aq) -> C9H7O4-(aq) + H3O+(l)}$$

However, I've also read that aspirin undergoes hydrolysis with water, according to this site, where Aspirin (acetylsalicylic acid) hydrolyses to produce 2-hydroxybenzoic acid and ethanoic acid.

So, I'm confused — when I'm dissolving the aspirin tablets in water, am I just making acetylsalicylic ions and hydronium ions which I can then titrate to find the concentration, or am I making two other acids as per the second site suggests? I have all my titre values, I just need to calculate the concentration of the aspirin the solutions, and I'm now confused about what chemical I'm actually finding the concentration of, e.g.

$$\ce{NaOH + HCl -> NaCl + H2O}$$

Here, sodium hydroxide reacts with hydrochloric acid. In my experiment, does the $$\ce{NaOH}$$ react with acetylsalicylic acid, or with 2-hydroxybenzoic acid and ethanoic acid?

## 1 Answer

I am afraid, direct acid-base titration is not the right way to analyze aspirin exactly because of the you stated - hydrolysis. The rule number no of any titration is that there should be no side reaction and it should go to completion almost instantly. The direct titration of aspirin is problematic because hydrolyzes pretty fast to salicylic acid- an unwanted side reaction which may or may not go to completion.

In order to circumvent this, analytical chemists do a back-titration. You add an excess of base, heat the mixture so that neutralization as well as hydrolysis are complete. The remaining base is titrated. The rest of the calculations are normally done.

• Thanks for your response. This is a time-critical project that I can't repeat, but I can mention flaws and improvements. Would the results of the experiment be completely invalidated? I calculated the concentrations using a 1:1 ratio and the trend is still obvious - the heated aspirin mixture was ultimately much more acidic than the cooled mixture after stirring for the same amount of time. Do you think most of the acid I titrated would be aqueous aspirin, or would it be salicylic acid?Your help is much appreciated. – Caspar B Jun 10 '19 at 8:41
• I am afraid your results will be overestimated because more base is consumed (neutralization +hydrolysis). If you heated, it would be mostly salicylic acid. – M. Farooq Jun 10 '19 at 14:32