Toluidine is a weak organic base. We will use this property to protonate it with

Question

Toluidine is a weak organic base. We will use this property to protonate it with 3M HCl and form a hydrochloride salt to move it from the organic to the aqueous phase then recover it by neutralizing the aqueous layer. The amine is soluble in the organic phase, the salt is soluble in the aqueous phase. At acidic pH, it will exist in the hydrochloride salt form, but once the pH is neutral the equilibrium will be shifted to favor the amine. Benzoic acid has a pKa of 4.5. At high pH the base will deprotonate benzoic acid forming a sodium benzoate salt. The sodium benzoate salt is soluble in the water.

What pH is required to obtain 99% amine?

Explanation / Answer

pH = pka + log [conc of salt / sonc of organic base]

since we see that 99% of the amine should be obtained

amine conc = 0.99*3M

and the remaining will be salt 0.01*3M

pH = 4.5 + log (0.01/0.99)

= 2.5043

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