Ammonium oxalate, C2H8N2O4 - sometimes written as (NH4)2C2O4 - is an oxalate salt with ammonium (sometimes as a monohydrate). It is a colorless salt under standard conditions and is odorless and non-volatile. It is a constituent of some types of kidney stone. Found also in guano. It must be added to blood to prevent its coagulation in vitro (outside the body).