Question

How can a sample's melting point indicate the presence of an organic impurity?

Answer 1

The best way to answer this, is with an example.

Say that you are trying to measure the m.p. of an solid (an acid for example) and you know that the acid has a reported m.p. of 125 °C. and let's suppose that the acid you are trying to measure, you didn't recristallize it, you just got separated from an unknown sample. When you measured the m.p. you realize that the m.p, was 104 °C instead of the reported 120 °C. What happen here? This could be either another compoudnd or the compound has some impurities within.

The melting point is the temperature which a solid changes it's phase from solid to liquid. This change is very associated with it's purity. A minimun impurity in a sample, will cause that the m.p. of the solid will go down and down.

You can see the impurity as another compound B. and when it's with another compound (In this case the acid A), you are actually mixing the two of them, you are not having a pure compound of A, but instead a mix of A and B, so depending of the quantities of B, the melting point will be lower, because it's like you are taking an average of both m.p. of both compounds, that's why the m.p. is lower.

To avoid this, is highly reccomended that you recristallize the compound you are measuring so you can be sure that you are not going to have impurities (or at least, almost nothing) and the m.p. will be equal or closer to the reported one.

Hope this helps