There's also the pretty good question what the heck this supposed dragon would need a flame breath for. An organic flamethrower doesn't come across as a terribly effective hunting tool, and big carnivores tend not need much beyond their size, power, ferocity and assorted sharp protrusions to resolve arguments with their peers one way or another. For actual defensive weaponry their requirements are minimal, and normally met with evolving thicker skin, scales, mane to protect the throat, and suchlike, and/or making alternate use of a main hunting weapon if appropriate (think of the way some cobras can spit their poison at eyes).

Moreover, while chemistry isn't my strong suit I'd imagine gathering the raw materials and then turning them into usable fuel would be a pretty energy-intensive activity, nevermind now nigh certainly requiring some rather complicated internal organs and "piping" in forms thus far unobserved in nature. Nevermind now that platinum AFAIK isn't a terribly common metal, particularly in even remotely pure (ie. not stuck in ore) forms. And if there was a way to get that stuff to work, and be worth all the trouble, you'd think it was a little more common too, the same way venom glands are. Even natural electricity-based weaponry is found in several widely different animals (ie. electric rays and eels).