http://growingtaste.com/oxalicacid.shtml
What Oxalic Acid Is
Oxalic acid is, of course, a chemical substance. At high concentrations, it is a dangerous poison, but such immediately toxic levels are not found in foodstuffs but rather in manufactures, such as some bleaches, some anti-rust products, and some metal cleaners (among other things). It is also a naturally occurring component of plants, and is found in relatively high levels in dark-green leafy foods (relatively high, though, is just that).
In the human body, ingested oxalic acid is not (so far as is known today) a useful nutrient; so, like all such unneeded components of diet, it is processed by the body to a convenient form and that byproduct is then excreted—in this case, in the urine. In the course of being processed by the body, oxalic acid combines with other substances to form various salts, called oxalates; usually, those salts are in solution (like salt in salt water or sugar in coffee), but if their concentration is high enough some may precipitate out in crystalline form. Such tiny crystals of these salts can be irritating to human tissue, especially to the stomach, the kidneys, and the bladder. It is commonly believed that oxalates contribute to the formation of kidney and bladder stones; one common nutrient with which oxalic acid combines is calcium, making the salt calcium oxalate, and calcium oxalate is found in kidney stones.
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