Getting an accurate reading on aspirin's melting point is trickier than you might expect
The literature value sits at 135°C, but if you just load a capillary tube into a standard Melting Point Of Aspirin apparatus and walk away, you will probably get something between 130 and 140 with a broad range. That spread tells you more about your technique than it does about the sample. Most people mess this up by heating too fast. You set the ramp to 5 degrees per minute because that is what the manual suggests, but the thermal mass of the oil bath or aluminum block means the sample inside the capillary is always lagging behind the probe. By the time the thermometer reads 133, the aspirin in the tube has already been sitting at 136 for a while. I once spent two days chasing a depressed melting point on what I was certain was a failed recrystallization, only to realize the hot plate was cycling on and off and my actual heating rate was closer to 12 degrees per minute near the end. The sample was decomposing slightly before it even melted, giving me a range from 128 to 134 instead of the tight 134 to 135 I should have seen. The workaround was simple but not obvious if you are new to this: dial in your heating rate by running a test with a known standard first. Use pure benzoic acid at 122°C or urea at 133°C, run it at your usual setting, and note where the apparatus reading deviates from the true value. Then adjust your voltage or ramp accordingly. After that, your aspirin readings should land within half a degree of the accepted value if the sample is dry and pure.
The practical procedure
Start with a properly dried sample. Aspirin is hygroscopic enough that residual moisture from a recent recrystallization will depress and broaden your melting range. Spread your crystals on a watch glass and leave them in a desiccator overnight, or put them in a drying oven at 40°C for a few hours. Do not bake it hotter than that because acetylsalicylic acid begins to hydrolyze back into salicylic acid and acetic acid when it gets warm and moist, and salicylic acid melts at 159°C, which will throw off your reading in the opposite direction. Pack your capillary tube properly. Tap it gently against the bench or drop it through a long glass tube onto a hard surface until the sample is densely packed at the bottom. A loosely packed column gives poor thermal contact and a wide, unreliable range. The sample height should be about 2 to 3 millimeters. Anything more and the temperature gradient across the column becomes significant. Set your apparatus. If you are using a Thiele tube with oil, heat it slowly at first, then bring the rate down to about 1 to 2 degrees per minute as you approach the expected melting range. With an electric unit, program a slow ramp near 130 and watch carefully. You are looking for the point where the first liquid appears (the onset) and the point where the last solid disappears (the completion). Record both.
Common pitfalls and what they actually mean
A depressed and broad melting range does not automatically mean your aspirin is impure. It could mean your heating rate is still too fast, your sample is wet, or your capillary packing is loose. I have seen all three in the same week. The only way to distinguish between them is to control each variable independently. Dry the sample again. Repack a fresh tube. Slow the heating rate down further. If the range tightens and the onset climbs toward 134, you were dealing with technique, not purity. Another thing nobody warns you about: the type of thermometer matters. Cheap apparatuses come with calibrated-in-error thermometers that can be off by 2 or 3 degrees across the whole range. If you need accurate data, calibrate your thermometer against at least two standards, like naphthalene at 80°C and benzoic acid at 122°C, and apply a correction curve. I stopped trusting uncalibrated thermometers after I got a perfectly sharp 134 to 135 reading on one batch and a 131 to 136 on another that I later verified by HPLC was actually 98.5 percent pure. The apparatus was lying to me.
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When the melting point method is not enough
If you need to know exactly how pure your aspirin is, melting point analysis alone will not give you that answer. It is a yes-or-no test for gross contamination, not a quantification tool. A sample that melts at 134 to 135 could still contain 5 percent salicylic acid and you would not know it from the melting range alone. For actual purity determination, you need HPLC or titration. The melting point is useful as a quick checkpoint during synthesis or recrystallization, not as a final analytical result. Also worth noting: if your aspirin sample has been stored improperly and has started to hydrolyze, you will see a weird dual melting behavior. The acetylsalicylic acid melts around 135 and the liberated salicylic acid around 159, and if the decomposition is significant, your range can look stretched or irregular. In that case, remelting will not help because the compound is changing chemically, not just physically. You would need to prepare a fresh sample. The bottom line is that 135°C is the textbook number, but getting that number from your own apparatus requires attention to drying, packing, heating rate, and thermometer calibration. Skip any of those and your reading will drift, and you will waste time wondering what went wrong with your synthesis when the problem was entirely procedural.
