Formic acid has a pKa of 3.7, so that will not be enough. The lowest pKa of citric acid is 2.9, so that could be a better choice to force acesulfame in its undissociated form.

However, checking the pKa of acesulfame on the web, I found 2.0 (see https://pubchem.ncbi.nlm.nih.gov/compound/acesulfame#section=Dissociation-Constants ) . Using diluted HCl might work, but then you have to precipitate the acesulfame from an aqeous solution. May be with acetone or ethanol, check the solubilities in these solvents.

Quynh - I found a detailed description of the procedure for converting acesulfame potassium into acesulfame. It is published in a paper entitled "Polymorphism in acesulfame sweetener: Structure-property and stability relationships of bending and brittle crystals" which was published in Chemical Communications 2010, 46, 2562-3564. The Electronic Supplementary Information (ESI) is available free of charge (see attached).

Neutralization of Acesulfame potassium (Ace K)

It was obtained from Sigma Aldrich at a stated purity of 99% and no attempt was made at further purification. Ace K (5 g) was dissolved in water (5 mL), neutralized with concentrated HCl (5 mL) and achieved the highly acidic solution of ~ pH =2 and extracted with ethyl acetate (15 mL). Up on routine work up afforded a white solid of salt free acesulfame. It was crystallized from EtOAc by slow evaporation at the ambient conditions. Needle crystals were yielded. Mp: 122- 124 °C.

Dear Jos Wielders,

Thank you so much for your information.

I am so sorry for my mistakes. You are right. I will try this approach.

Thank you again.

Khoa.