Solubility of Salicylic Acid: Acid vs Base Form

[pisluca99]

TL;DR Summary

Brief clarification on the solubility of drugs based on pH.

The intrinsic solubility of salicylic acid is 2 g / L.
Suppose to put it in a buffered solution at pH = pKa = 4.76. In this situation, the total solubility increases and [acid] = [ coniugate base]. If this solubility is reached in solution, in what form does the drug precipitate? in the form of salicylic acid or salicylate?

 

[Borek]

and [acid] = [ coniugate base].

Not sure what you mean by that, please elaborate.

If this solubility is reached in solution, in what form does the drug precipitate? in the form of salicylic acid or salicylate?

I don't think there is an universal answer to that. Depends on what is the solubility of the salicylate itself. This is not different from calculations done using Ksp for competing weakly soluble salts.

 

[pisluca99]

Not sure what you mean by that, please elaborate.

Sorry, I meant that pKa of salicylic acid Is about 3, so when pH = pKa, [salicylic acid]=[salicylate]. I got confused with pKa of acetic acid.

I don't think there is an universal answer to that. Depends on what is the solubility of the salicylate itself. This is not different from calculations done using Ksp for competing weakly soluble salts.

Can you make an example please?
Essentially, when pH Is < 1 (in this case), Stot=S0 of the acid (intrinsic solubility), so we can see precipitation of salicylic acid if its concentration reaches S0. Indeed, when pH Is > 5, Stot = S0 of the base (intrinsic solubility), so we can see precipitation of salicylate. But when 1<pH<5 we cannot neglect one of the two forms, so if we reach Stot= [acid] + [base] in solution, which form precipitates? I'm a bit confused.

 

[Borek]

Starting from "solubility product" you can write conditions for the precipitate to show as

Ksp1 = [SalH] (for the acid, and obviously [SalH] is a function of the total concentration and pH)

Ksp2 = [Sal^-][Na⁺] (for the salt)

For a given pH and total concentrations of salicylate and Na+ you can always calculate whether one of the conditions is meet. If so - you know what starts to precipitate.

If both are met - something started to precipitate earlier, on your way to reach the conditions (and preparation of the solution with given parameters is actually impossible, unless we get into non-equilibrium conditions).

 

[pisluca99]

ok, so both species can potentially precipitate, once the pH is fixed. Thank you.