A good rule of thumb is that if you wonder "is it a war crime to...", the answer is probably yes, and if that question involves chemical weapons, the answer is unquestionably yes.
There isn't much grey area really, the only question is who's going to bring you to trial.
War crimes and Geneva
First, I have to correct a few things about the question, because I have to.
The Geneva Conventions (plural) and associated Protocols (plural) are a series of texts regarding civilians, POWs, the wounded and sick, and more generally people. It gives rights and protections against mistreatment. In a nutshell, combattants can kill each other in the field of battle and that's pretty much it. They don't concern chemical weapons directly.
The Geneva Protocol (singular) is the one that concerns biological and chemical weapons. It is one of a few agreements on the topic. It isn't part of the Geneva Conventions, nor one of the Conventions' Protocols. It's a different thing. International law is funny like that.
War crimes are first and foremost crimes. Perhaps that sounds stupid said like that, but it's true. You have to charge specific people with specific crimes. And then you have to prove it. It's not enough to prove the crime happened, you have to prove the person you're charging did it. You know, like any crime.
Finally, because war crimes are crimes, you need two things: someone to prosecute, and the will to prosecute them. Winners don't prosecute themselves. Winners often don't prosecute losers for a variety of reasons (as part of terms of surrender, because they need someone to build rockets, etc). You can only bring to trial someone you have captured or who surrendered.
It's never Geneva
With that said, neither the Conventions or Protocol matter much here. The Rome Statute (PDF in English), which establishes the International Criminal Court, makes a non-exhaustive list of war crimes for which you can be charged.
Article 8 War Crimes
- For the purpose of this Statute, "war crimes" means:
(b) Other serious violations of the laws and customs applicable in international armed conflict, within the established framework of international law, namely, any of the following acts:
(xvii) Employing poison or poisoned weapons;
(xviii) Employing asphyxiating, poisonous or other gases, and all analogous liquids, materials or devices;
(xx) Employing weapons, projectiles and material and methods of warfare which are of a nature to cause superfluous injury or unnecessary suffering or which are inherently indiscriminate in violation of the international law of armed conflict, provided that such weapons, projectiles and material and methods of warfare are the subject of a comprehensive prohibition and are included in an annex to this Statute, by an amendment in accordance with the relevant provisions set forth in articles 121 and 123;
You can knock yourself out reading the annexes, going through other texts that regulate weapons of war, but really, when it comes down to it, acid burns are horrific and that's a fact. Purposefully causing acid burns should absolutely be consider "superfluous injury" and "unnecessary suffering".
You could maybe argue that this isn't the case, but if you find yourself arguing that it's because you've lost the war. I don't fancy your chances of winning that argument against a bunch of horribly disfigured witnesses.
Just use bullets, like a civilised person.