What connects a titration flask, rust, spinach, and kidney stones?
Oxalate.
Oxalic acid dihydrate is a classic analytical reagent, while oxalate also interacts strongly with metal ions. With iron, that chemistry contributes to cleaning and rust removal. With calcium, it forms poorly soluble calcium oxalate.
Plants make microscopic calcium oxalate crystals, and the same salt is the principal mineral component of many kidney stones.
One two-carbon unit connects all four.
Global Chemical Database