I've put some undissolved ninhydrin in a test tube and added about 0,5 mL of conc. KOH(about 5 M). The colour changed to dark blue. I tried the same thing with ninhydrin(aq) and nothing happened.

When I added H2O2 to the blue solution, CO2 started bubbling from the solution( I know it is CO2 because I tested it with Ca(OH)2 (aq) and it turned cloudy white, and after adding HCl turned clear again).

Is it possible that H2O2 oxidized that blue compound to phthalic acid?

What would be the structure of that compound?

And can anyone propose a mechanism?

Can I ask why you are doing this? What is the purpose?

I wondered if I could do an Aldol condensation of ninhydrin and acetone. I accidently put too much KOH in acetone and when I added ninhydrin in, the blue colour appeared. I tried it again with just ninhydrin in base and got the same result. When I grinded some ninhydrin(s) with KOH(s) in a mortar the mixture turned dark blue and some liquid appeared. I tried putting some oxalic acid in the blue solution and there was no change in colour, but when I added H2O2(30%) in another sample of blue solution CO2 formed.

 

So I could say that the purpose of this experiment is satisfying my curiosity

 

I did try googling it, but all I got were posts related to the reaction of ninhydrin with aminoacids