亚 (yà) in chemistry names the lower oxidation state:
硫酸铁 = ferric (Fe³⁺),
硫酸亚铁 = ferrous (Fe²⁺). Same pattern as
亚硝酸钠 (sodium nitrite) vs
硝酸钠 (sodium nitrate). On medicine labels, the full form
硫酸亚铁片 (ferrous sulfate tablet) is sold as an over-the-counter iron supplement.