Hello. I have been using nitric acid concentrated using ultrasonication bath. By that, you can increase the carboxylic group (saturated).

Thank you for replying. Btw, for how long, i tried to do the oxidation by using vial in which HNO3 concentration is about 42%, in vial glass bottle, i heat it T 70C, but the number of carboxylic group was not significanty increased. Thank you. I will try to use the ultrasonication bath

if you want to do it this way, maybe you can do heat reflux treatment using nitric acid as the oxidative reagent? Octa Octa