Oxalates

Oxalates are various salts of oxalic acid, a waste product of plant metabolism found in a number of foods, notably spinach, chard, beets, amaranth, and rhubarb. The sodium and potassium salts are soluble, while the calcium salts are insoluble and form crystals that irritate the mouth and digestive system. Soluble oxalates can combine with calcium in the human kidney to form painful kidney stones. In very large doses—a few grams— oxalic acid is corrosive and can be fatal.