Organic poisons often work through at the biochemical level, e.g. cyanide by interfering with respiration at the cellular level, strychnine by screwing up intersynaptic transmission. There are many molecular substances in foods which offer no nutritional benefit, and must be processed and excreted. Oxalic acid, for example, is excreted in the urine, and its crystals are commonly found in microscopic urinalysis. Too much oxalic acid in the urine will result in kidney or bladder stones. Calcium combines with oxalic acid to form the less soluble salt, calcium oxalate, which is also found in kidney stones.
Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid.
Oxalic acid is also found in potatoes and peas. The can also be produced by heating sodium formate and treating the resulting oxides with sulfuric acid. It can also be obtained by the action of nitric acid on sugar, or of strong alkalis on sawdust. Oxalic acid reduces iron compounds, and is therefore used in metal polishes, stain removers, and writing inks. When it absorbs oxygen, it is converted to the volatile carbon dioxide and to water, and it is used as a bleaching agent, in detergents, and as a mordant in dyeing processes.