Usually, sulfonphtalein dyes (such as Bromocresol green and Bromothymol blue) forms an ion-pair complex with drugs that contain amino group with free electron pair (not involved in a resonance) in acidic medium. Actually, in the acidic environment, the drug that became protonated with a positive charge is attracted toward the acidic dye and form an ion-pair complex. The formed complex is neutral and thus can be easily extracted into an immiscible organic solvent in which the absorbance is measured.

I have made a literature scanning, and all papers with extraction-free methods describe the mechanism as it is in an acidic medium!!! with almost the same phrase, that has been copied and pasted in all articles that introduce an extraction-free method using the mentioned dyes. The phrase is:

" when a solution of BPB/BTB in dichloromethane is added to the drug solution, an intense yellow colored product is produced immediately. This is due to an opening of lactoid ring of the dye and subsequent formation of quinoid group. It is supposed that the two tautomers are present in equilibrium but due to strong acidic nature of the sulfonic acid group, the quinoid body must predominate. Finally, protonated drug forms ion-pair with the anionic dye."

I have attached a photo illustrates the mentioned mechanism.

Now, my question is: how can we call a complex formed between this dye and a drug with nitrogen atom in acetone or chloroform, an ion-pair ? isn't this a charge transfer between the two species? and in this case, a transfer of charge from the nitrogen atom to the dye with high electonegativity. How the drug becomes protonated in acetone, dichloromethane or chloroform?