When one thinks of elemental transmuters from mythology or comic books, one thinks of single touch by the transmuter to a large, multi-elemental structure (like a human body) that causes cascading changes across a wide variety of elements, resulting in a complete conversion to a totally new material, targeted by the transmuter, irregardless of whatever the source materials used to be.
However, the more "practical" view of an element transmuter is that he/she is only able to flip a few ounces of a single element into a different element, based upon familiarity with both the source material and the final result. This enforces severe limits to what the operator can do, and the scope of the consequences. A Bachelor's Degree in Chemistry would be extremely useful to such an operator.
The details of the character in question are irrelevant at this point, other than he/she was researching the Biological Transmutation Theory of Corentin Louis Kervran (see here, here, here, and here), notably the transmution of sodium to potassium within the human body.
Sodium and potassium are essential electrolytes, a dietary requirement for us humans. Sodium helps regulate the amount of water that's in and around your cells. A 50 kg person would contain around 200 grams of sodium chloride – around 40 teaspoons. It plays a key role in many functions, from the quality of blood to transmission of nerve signals. Potassium protects the heart and arteries, and may even prevent cardiovascular disease. The total quantity of potassium in the human body lies somewhere between 110 and 140 grams, and while most of it lies in the red blood cells and brain tissue, its final value depends upon muscle mass. It serves mainly as a nerve stimulus, in muscle contractions, blood pressure regulation, and protein dissolution. But, sodium and potassium can only operate within a narrow range of values. Outside of that range, they both turns into nasty, caustic agents.
It was during the aforementioned research that the character discovered that he/she could replicate transmution of sodium to potassium on a macro scale (again, the mechanics are irrelevant at this point)...which induces sudden, severe, simultaneous Hyponatremia (sodium starvation) and Hyperkalemia (potassium poisoning) – bringing about rapid brain swelling, paralysis, and/or heart failure in the subject.
Note: while this process could be extremely useful in a life-or-death fight, it's rather mundane in nature for someone with superpowers.
So, here's my question: could this process lead to other, somewhat more spectacular methods of offense/defense?
For instance, potassium reacts rapidly and intensely with water (an exothermal reaction which heats it to such an extent that it burns a purple flame), forming both a colorless basic potassium hydroxide solution, and hydrogen gas (which reacts strongly with oxygen and ignites). Sodium also reacts quickly with water, to produce sodium hydroxide and hydrogen gas. And since 60% of the human body is water...could this volatile reaction lead to something approximating Spontaneous Human Combustion?