Everyone interested can find a finger with a smaller acid burn on wikipedias nitric acid article.
There should be some videos youtube.
http://images.google.de/imgres?imgurl=https%3A%2F%2Fi.ytimg.com%2Fvi%2FoipksRhISfM%2Fmaxresdefault.jpg&imgrefurl=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DoipksRhISfM&h=1080&w=1920&tbnid=zyyoLoh2IUnPPM%3A&docid=SXkJfC5QJG1LOM&ei=Wow_V9avBIeVsgHkja7wCg&tbm=isch&client=firefox-b&iact=rc&uact=3&dur=1696&page=1&start=0&ndsp=17&ved=0ahUKEwjWxOXa1unMAhWHiiwKHeSGC64QMwgeKAAwAA&bih=627&biw=1366
Thats without nitric acid, but acid burns are usually slow! to flesh and very likely extremely painful.
So that's probably one of the sickest ways to 'execute' someone. More like torturing to death.
P.S.: of course you can easily find real and fake acid burns on google.