I accidentally inhaled a little acid gas from heating diammonium phosphate (see Material observations.) It was just a whiff, but it set me to thinking.
It occurs to me that, because it polymerizes above 200° instead of simply boiling like a decent liquid should, hot phosphoric acid with some impurities might outgas a bunch of muriatic (-114.22°) or vitriol (over 300°), which is a potential inhalation hazard from heating it (something I hadn’t thought to consider hazardous); electrolysis of ammonium compounds could suffer the same problem. (Brown & Whitt 1952 say the aqueous azeotrope boils at 840° at 92.7%.)
I wonder if the Potash Corp.’s helpful booklet mentions this? Well, it says you should use rubber-lined (latex or chlorobutyl) steel, PE, polyester, or (if you can keep Cl content below 15 ppm, so don’t dilute it with chlorinated water!) 316L for the stuff. They especially warn you away from carbon steel, and they warn that over 3000° F (1650°) it decomposes into phosphorus oxides. They have reassuring words about toxicity: LD₅₀ of 1530 mg/kg, slight dermal toxicity, but it can cause acid burns, especially in eyes. Also on p. 24 they have a reassuring graph of “vapor composition over boiling solutions”, with <1% H₃PO₄ up to 300° (“at which point the acid strength is about 103%”, i.e., some of it has become pyrophosphoric acid), crossing 10% around 500°, and continuing up to about 57% at 800°. It never mentions the risk of protonating other substances to create volatile acid gases.