Why is the melting point of p-toluenesulfonic acid monohydrate higher than that of anhydrous p-toluenesulfonic acid?

Compare for example the melting point of benzenesulfonic acid hydrate, which is lower than for anhydrous benzenesulfonic acid.


2 Answers 2


The hydrated forms might be an azeotrope which means there's a content of water present that is difficult to remove via distillation techniques which may be how the P-toluene sulfonic acid was prepared.

The anhydrous form would require further processing.

The moisture present in the hydrate forms may affect the melting point if it were acting as an azeotrope with the P-toluene sulfonic acid, also it wouldn't be as high purity as the anhydrous form which would display the true melting point.

  • 2
    $\begingroup$ Hello, and welcome to Chem.SE! This answer is a bit short, so preferably it should be expanded on or turned into a comment. $\endgroup$ Commented May 23, 2015 at 17:40
  • $\begingroup$ Sorry its my first time on this site It said add comment on my phone I wasn't trying to say anyones wrong or anything just some input thanks for the feedback :) $\endgroup$
    – Technetium
    Commented May 23, 2015 at 17:49
  • $\begingroup$ Sorry, what's the relationship between melting point and azeotrope? If it likes what you said, why is the melting point of benzenesulfonic acid hydrate lower than for anhydrous benzenesulfonic acid? $\endgroup$
    – Henry
    Commented May 24, 2015 at 16:09
  • $\begingroup$ when a compound is sublimed with water present the compound can melt and sublime with water at a lower temperature that it's true melting point. This is why compounds with melting points higher than water can be distilled with water. I learnt this in distillation theory. I'm guessing your using a melting point apparatus but I think the same thing could happen subliming a hydrate salt in a melting point apparatus. Correct me if I'm wrong I'm only meaning to comment :) $\endgroup$
    – Technetium
    Commented May 25, 2015 at 5:20
  • $\begingroup$ I think it depends on the the compound whether the hydrate makes the m.p higher or lower. So the azeotrope is the hydrate salt which is the compound and water which is different to the anhydrous form possibly because it's an azeotrope mixture which won't sublime at its true melting point. $\endgroup$
    – Technetium
    Commented May 25, 2015 at 5:27

The solubility of p-toluenesulfonic acid is $670 \mathrm{g}.\mathrm{L}^{-1}$ so the solution which contains this acid and water is an azeotrop. Then the melting temperature depend if you keep pressure or temperature constant in the laboratory.

If you take a Handbook I have not here, I think you'll find a lot of interesting things about that.

  • $\begingroup$ How do the solubility of chemical influence whether it is an azeotrop and its melting point? $\endgroup$
    – Henry
    Commented May 24, 2015 at 16:12
  • $\begingroup$ If there are not miscible you have not an azeotrop but an heteroazeotrop. The solubility is just to justify you have an azeotrop. $\endgroup$
    – ParaH2
    Commented May 25, 2015 at 2:22
  • $\begingroup$ This is what I mean by a similar thing happening as what occurs when distilling. A heteroazeotrope is probly it. $\endgroup$
    – Technetium
    Commented May 25, 2015 at 5:32
  • $\begingroup$ This applies to miscible and immiscable substances. $\endgroup$
    – Technetium
    Commented May 25, 2015 at 8:17

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