Para-aminophenol

Para-aminophenol is very similar to, though less powerful than metol in its photographic action. Because of high solubility of its alkali-metal salts, it has been used in the manufacture of very concentrated developers such as Rodinal, which, for use, are diluted with 10–80, even 100 parts of water.

Edmund W. Lowe, “What You Want to Know About Developers Fine Grain and Otherwise”, p. 53.