free acid
Chemistry term for an acid not combined with other substances; used in technical contexts.
この溶液には遊離酸が多く含まれている。
This solution contains a lot of free acid.
Compound of 遊離 (yūri, 'free, unbound') and 酸 (san, 'acid'). The exact historical derivation is uncertain; the term is a modern technical coinage.