Help with working out causticity

[Jackeh]

Wasn't sure where I should post this.

Okay, using the structure of some molecules you can work out their approximate pH. For example, sodium hydroxide:

NaOH -> Na+ + OH-

5g of NaOH is (5/40) = 0.125 mol. Dissolve this in a dm^3 and you have a solution of NaOH of concentration 0.125M.

Because the above dissociation is 1:1, the concentration of OH- ions is also 0.125M.

[OH-] = 0.125
pOH = -log(0.125)
.:. pH = 14 + log(0.125) = ~13.1

This works for something like NaOH or HCl or H2SO4, you can clearly see what ions dissociate, but what about compounds like the following?

How do you know which parts dissociate and thus how can you work out the pH from the structure? Ethylphenidate is apparently extremely caustic, yet I don't see any hydroxyl groups and I was confused about the hydrogen on the piperidine ring, does it dissociate? If not, then how else does this substance have such a reputation for nasal damage?

 

[Bob Loblaw]

~~~~~> The forum formerly known as ADD

 

[23536]

How do you know which parts dissociate and thus how can you work out the pH from the structure? Ethylphenidate is apparently extremely caustic, yet I don't see any hydroxyl groups and I was confused about the hydrogen on the piperidine ring, does it dissociate? If not, then how else does this substance have such a reputation for nasal damage?

It's probably "caustic" because of its biological properties, not its chemical properties.

To compare, capsaicin is an irritant despite not being a strong acid or base. It simply fits into the right type of receptor.

 

[sekio]

I think it's because most people are idiots who shove anything up their nose without checking the pH first.

You can only make guesses at the pKa of compounds, you have to measure it empirically if you want to get a figure on what pH it is gonna be. Also to consider, is that most people don't have ethylphenidate freebase, but rather the HCl salt.