I recently successfully managed to make concentrated sulfuric acid by electrolysis of copper sulfate. I start from a 0.3 M solution of copper sulfate, and turn it into dilute sulfuric acid (0.3 M) with electrolysis. The resulting product is very clean, as all the copper sulfate is consumed, and I use electrodes that are inert with respect to sulfuric acid (copper for the cathode, and lead-dioxide for the anode). Then I boil the water off in order to concentrate the acid. All the steps are shown in my video:

https://vadcpa.com/Val/Project-SulfuricAcid.php

In the video, I achieve a concentration of 83%, but I successfully achieved a concentration of 92% with another batch which I didn't catch on video. In principle, I could boil the acid longer in order to reach the azeotropic concentration of 98.5%. But the problem I am having is that, as I approach a temperature 310 Celcius, some white fumes appear, and I am not sure whether they are due to some impurities that I couldn't get rid of, or if it is the sulfuric acid which starts to decompose.

Any idea on what's going on?