noun
primary salt
Chemistry term for a salt in which not all replaceable hydrogen atoms of the parent acid have been replaced by metal atoms; i.e., an acid salt.
リン酸の第一塩は、酸性塩の一種である。
The primary salt of phosphoric acid is a type of acid salt.
第二塩 (secondary salt) is formed when two replaceable hydrogens are replaced, while 第一塩 has only one replaced.
Compound of 第一 (first, primary) and 塩 (salt), a calque from Western chemistry terminology.