 Sodium picrate is used as an etchant in metallography to differ preeutectoid ferrite in hypoeutectoid steel from preeutectoid cementite in hypereutectoid steel by etching cementite to a dark colour, whereas not attacking ferrite and thus it remains reflective.Ethers and esters of picric acid are also called picrates.The ether methyl picrate (CAS# 606-35-9) has the formula (O2N)3C6H2OCH3.