Why does the melting point of aspirin increase

[Daniel2244]

I used sailcylic acid and acetic anhydride to produce aspirin and preformed recrystalisation to purify the aspirin. I need to evaluate why I have a higher/lower melting point. In this case, the melting point was 143°C whereas the true meling point is 135°C.
I thought one of the reasons my melting point could of elevated is due to impurities from contaminated equipment or the chemicals being in equalibrium causing sailcylic acid to remain in the final product. Sailcylic acid has a higher melting point than aspirin causing the aspirin produced to have a higher MP.
However, not sure if this would increase/decrease and I need to know what could of caused the MP to increase.

 

[DrClaude]

What I remember from my days at the university was that a lower melting point meant impurities and a higher melting point meant you had the wrong substance!

It is not clear from your post if you synthesized the aspirin or are testing a commercial medicine. If it is the former, how did you proceed? If it is the latter, did you take any steps to isolate the acetylsalicylic acid from the rest of the tablet?

 

[Daniel2244]

What I remember from my days at the university was that a lower melting point meant impurities and a higher melting point meant you had the wrong substance!

It is not clear from your post if you synthesized the aspirin or are testing a commercial medicine. If it is the former, how did you proceed? If it is the latter, did you take any steps to isolate the acetylsalicylic acid from the rest of the tablet?

sailcylic and acetic anhydride was used to synthesize aspirin and then to purify crystalisation was carried out

 

[DrClaude]

Then I would guess that there is a lot of salicylic acid left. If you can go back to the lab, you should try to redo the purification. Did you do any other analysis of the final product, such as IR or NMR spectra?

 

[Borek]

If what you have 'produced' is mostly non-reacted salicylic acid (contaminated with some aspirin) then yes, you can see a higher MP.

Edit: DrC beat me

 

[Daniel2244]

If what you have 'produced' is mostly non-reacted salicylic acid (contaminated with some aspirin) then yes, you can see a higher MP.

Edit: DrC beat me

Then I would guess that there is a lot of salicylic acid left. If you can go back to the lab, you should try to redo the purification. Did you do any other analysis of the final product, such as IR or NMR spectra?

Thanks for your help. Just wanted to make sure I was on the right lines

 

[Daniel2244]

If what you have 'produced' is mostly non-reacted salicylic acid (contaminated with some aspirin) then yes, you can see a higher MP.

Edit: DrC beat me

is there any other factors which could of elevated my melting point?

 

[Borek]

Every other factor that I can think about will produce lower MP, not a higher one.

That is, assuming the MP apparatus and operator are in working order

 

[Daniel2244]

Every other factor that I can think about will produce lower MP, not a higher one.

That is, assuming the MP apparatus and operator are in working order

ahaha thanks for the help

 

[DrClaude]

is there any other factors which could of elevated my melting point?

A wrong reading of temperature can sometimes be due to a too high temperature gradient (the temperature goes up too fast such that when the substance is completely melted, the temperature measured is actually above the actual melting point). But in your case, the difference is so big that I would say that only another substance with a higher Tmelt can explain it.

By the way, did yid you measure a single melting point or a range of temperatures?

 

[Daniel2244]

A wrong reading of temperature can sometimes be due to a too high temperature gradient (the temperature goes up too fast such that when the substance is completely melted, the temperature measured is actually above the actual melting point). But in your case, the difference is so big that I would say that only another substance with a higher Tmelt can explain it.

By the way, did yid you measure a single melting point or a range of temperatures?

I repeated it 3 times, I got 140 145 145 deg C

 

[DrClaude]

I repeated it 3 times, I got 140 145 145 deg C

What I meant by range is that one records the temperature at the onset of melting and at the point where the entire solid has melted. A wide range is usually indicative of a mixture.

 

[Daniel2244]

What I meant by range is that one records the temperature at the onset of melting and at the point where the entire solid has melted. A wide range is usually indicative of a mixture.

I'm still not sure what you're saying. Also, I know it's off topic but is acetylsalicylic acid purity testest by IR spectro?

 

[DrClaude]

I'm still not sure what you're saying.
Also, I know it's off topic but is acetylsalicylic acid purity testest by IR spectro?

I was thinking that you could see the presence of acetylsalicylic acid by looking at the OH band in the IR spectrum. You could also confirm the presence of aspirin by the signature of the CH₃ group.

 

[Daniel2244]

I was thinking that you could see the presence of acetylsalicylic acid by looking at the OH band in the IR spectrum. You could also confirm the presence of aspirin by the signature of the CH₃ group.

Thanks

 

[mjc123]

More obvious than the CH₃ group is the aliphatic ester C=O stretch at ca. 1740 cm^-1. This is absent in salicylic acid; the aromatic acid/ester C=O band is < 1700. Salicylic acid has a phenolic OH band at ca. 3300 cm^-1, absent in aspirin. (The acidic OH is broad and weak.)

 

[DrClaude]

More obvious than the CH₃ group is the aliphatic ester C=O stretch at ca. 1740 cm^-1. This is absent in salicylic acid; the aromatic acid/ester C=O band is < 1700. Salicylic acid has a phenolic OH band at ca. 3300 cm^-1, absent in aspirin. (The acidic OH is broad and weak.)

Thanks for the clarification. It's been a couple of decades since I did any of this *feeling old*.

 

[Mark Harder]

is there any other factors which could of elevated my melting point?

If you used a large excess of acetic anhydride, might you have produced the mixed anhydride of acetic and acetyl-salicylic acids? What was the ratio of the anhydride to salicylate?

 

[Baluncore]

How is the temperature measured?
When was it last calibrated or checked?
Do you have another temperature sensor you can substitute or use in parallel?

 

[zbikraw]

The most interesting question is the yield of your recrystallization (recrystallization means you dissolve and crystallise the crystalline product of reaction, not solid residue after volatiles evaporation). Small yield makes possible you crystallize impurity of low solubility. Just measure IR spectra and compare result with literature data.
In principle, it is possible to obtain higher melting point through presence of impurities. Inside phase diagrams one can meet "compounds" or "complexes" with Mp much higher than pure components. They must be accompanied by Mp minima on both sides (eutectics) and the composition of each characteristic point differs by tens of percent from another. In orgenic substances such a situation is rather rarity, I do not expect it in simple salicylates. When you purify acetylsalicylic acid to more than 90%, you are on side of phase diagram and can expect simple eutectic behaviour of your sample. When you can prove that your measuring set is proper one and measurement is well performed, increased Mp is worthy of further investigation and publishing of results.
Best regards.