Hi.
I wanna discuss with you the difference between Solubility and Ionization in the solution.
For example,
Salicylic acid (SA) has very low solubility in water. However, it is a strong acid, which means that the H can be ionized from -COOH in the SA molecule.
Therefore, I'm confused that the difference between Solubility and Ionization in the solution environment.
Can you discuss this?
Thank you so much.
Dear Kim Soonho ,
salicylic acid or 2-Hydroxybenzoic acid, is largely use in plant and cell culture.
Just by inspecting the structure it is clear that hydrolyse will play a significative role. In acid medium it may hydolize loosing water. As it has one hydroxyl group in the structure, it affects the solubility.
Their solubility in water is very low, once water has a pKw of 14, so 10-7 in H+ and OH-. However is highly soluble in ethanol (polar), about 50 mg/mL.
It is a weak acid, pKa1 of 2.97 and pka2 of 13.74. Therefore the ka1 = 1.07x10-3.
The dissociation may increase by increasing the pH, for a direct acid and base neutralisation of the carboxyl group.
By reducing the pH the -OH group (from the hydroxy part of the hydroxybenzoic acid) reacts dehydrating the acid.
In both cases, the solubility would be affected.
Nevertheless, its low solubility will be affected by acid and base neutralisation, solvation processes and hydrolyses, depending on the medium.
I hope it helps somehow.
Best regards,
WNM
Solubility of a given substance in a given solvent is the concentration of that substance when the solution is saturated. It is the highest concentration that substance can attain in that solvent at that temperature.
Ionization (not to be confused with dissociation) is a behaviour shown by some substances, such that.a fraction of the molecules of these substances, by interaction with those of the solvent, give origin to ions .
Solubility and ionization are not related, although the degree of ionization (that is, the fraction of molecules of the substance that undergo ionization) usually depends on the concentration of the substance.. Btw, salicylic acid is not a strong acid; it is weak, as any carboxylic acid.
The ionized form of an acid or base is more soluble than the free acid or free base. It might not be enough to overcome the hydrophobicity of the molecule, therefore logP (D) is more relevant to assess the solubility of the molecule. Sometimes it is needed more than one ionizable group to increase solubility as well as functional groups with hydrogen acceptor or donor capabilities (amides, esters).
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