Determining Purity of Organic Compounds

Question

You have one gram of crystalline material that you believe to be hippuric acid. You want to know if it is pure, and the NMR instrument that you would otherwise always use is broken. How would you determine with absolute certainty that it is pure?

Answer 1

Hello! There are multiple ways to figure out if a sample is pure.

The easiest would be checking the melting point. A melting point apparatus can be used, which will allow you to determine when the sample begins to melt. The melting point for hippuric acid is 191.5 degrees Celcius. If the sample melts within 2-3 degrees of that number, then it is likely pure. In addition, if there is a small range (less than 5 degrees or so) from when the sample begins to melt and is fully melted, it is purer than if it has a larger range.

Furthermore, IR can be used to determine if the sample is pure. If each peak matches the standard hippuric acid peaks, then it is pure. However, if there is even one peak that does not match, it could be an impurity showing up.

Lastly, high-performance liquid chromatography will give you purity. The instrument will boil the sample, and it will be carried through a column. The peaks depend on intermolecular forces, and if there are more peaks than hippuric acid should have, it will tell you if it is pure or not.