Oxalic acid, also called, clover extract, is an organic compound. It is present in large quantities in rhubarb, but is also in black tea, spinach, beetroot, and nuts.

Oxalic acid is poisonous in high doses because it binds vital nutrients like calcium, magnesium, and iron. In doing so, it reduces the availability of these minerals to the body and, therefore, fosters illnesses such as osteoporosis and anaemia. Frequent consumption of foods containing oxalic acid can, for some people, also lead to kidney stones.

Oxalic acid occurs in both cocoa and chocolate. A quarter bar of milk chocolate contains approximately 50mg of oxalic acid, the equivalent to an 850mg portion of spinach. As long as you enjoy chocolate moderately, there is no need to worry about it.