Oxalic Acid
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Oxalic Acid is a dicarboxylic acid which is a colorless, crystalline, organic compound that dissolves in water making a colorless Solution. Oxalic Acid is a reducing agent and the base for Oxalates.

Calcium Oxalate is the most common element in Kidney Stones. Ingestion of Oxalic Acid, even via skin contact, can be dangerous, especially for individuals with Kidney Disease, joint pain, Primary Hyperoxaluria or Enteric Hyperoxaluria. Oxalic Acid also chemically blocks Iron and Calcium absorption by the body.

The lowest lethal dose on record is only .02 ounces per 2.21 pounds of body weight (600 mg per kg). For a sensitive, 150 pound person (68 kg) that could be as little as 1.36 ounces (38 g) of Oxalic Acid as a lethal dose. So be aware, but not ALARMED; at 150 pounds you would have to eat 5 pounds (2.26 kg) of Raw Parsley to accumulate a potentially lethal dose in a single serving. It would take about 5.7 pounds (2.59 kg) of Raw Chives to be fatal. Of course, these numbers are averages, based on the most sensitive case on record, no testing that we know of has been done on accumulation, etc. Basically, don’t even play around the edges of a fatal dose with something as mildly toxic but potentially fatal as Oxalic Acid.

Oxalic Acid's main use is for cleaning or bleaching, especially for the removal of rust. The Bar Keepers Friend is a cleanser made with Oxalic Acid.

The good news is that Oxalic Acid is reduced by SteamingBoiling or Cooking.  Plants containing Oxalic Acid should not be cooked in an un-coated Copper or Aluminum Pot or Pan because the Oxalic Acid will react with the metal ions and turn the foods brown (in a bad way).  Anodized or Teflon coated Aluminum Pots or Pans will work fine.

Unless you want a pot full of Oxalic Acid, just make sure to toss out the cooking water when you are done.

Varieties

Bacteria produce Oxalates from the Oxidation of Carbohydrates and Oxalic Acid occurs naturally in relatively high quantities in unripe Starfruit, leaves and stems of members of the Spinach Family (Beet GreensChardQuinoa Leaves & Stems), Buckwheat, AmaranthCassavaAlmondsSorrelChivesParsley (the highest concentration), Purslane and Rhubarb Stalks & Leaves. 

Oxalic Acid is also present, to a lesser extent in the following vegetables: AsparagusSnap BeansBroccoliBrussels SproutsCabbageCarrotCauliflowerCeleryChicoryCollardsCorianderSweet CornCucumbersEggplantEndiveGarlicKaleLettuceOkraOnionParsleyParsnipPeasBell PeppersPotatoesRadishSquashSweet PotatoesTomatoesTurnipsTurnip Greens, and Watercress.