Citric acid is an organic acid which has three carboxyl groups: C3H4OH(CO2H)3.
Also, sometimes you'll see this written as "2-hydroxy-1,2,3-propanetricarboxylic acid". Or not ... I shouldn't be making assumptions about your profession, I suppose.

A whitish crystalline powder, it has a melting point of 153°C but has an annoying tendency to decompose when heated. Highly soluble, and relatively non-toxic, it is slightly more acidic than acetic acid (when in an aqueous solution, of course). As an intermediate product of metabolism, it can be found in many plants and animals.

Say you want to get some citric acid for yourself. What's the best way to make it? You can extract it from citrus fruit juices by the use of lime (think calcium carbonate, not little green fruits that cover half the earth) and sulphuric acid; however, it is much more efficient to harvest it as one of the waste materials of the mold Aspergillus niger.

Now that you've got it, what do you do with it? Here are some reasonable ideas:

In particular, "sour salt", or calcium citrate, which is made with citric acid, is used with foods quite often. As a preservative, it is used when making sausages and in canning; as an antioxidant, it can keep fruits from discoloring. Also, this salt is often used as a flavorant in foods which require a sour-yet-sweet taste, such as lemon cheesecake or cold beet borscht.

Approximately two thirds of the worldwide "crop" of citric acid goes into foods or drinks, with the remainder being relatively evenly split between pharmaceuticals, detergents, and industrial applications.

If you still haven't decided what to do with it, I might point out here that it usually sells for around $1.50/kg, making it twice as cheap as three-dollar crack. Don't get any funny ideas, though.

This acid is also cited as the catalyst in invisible ink; really, though, the reverse is true. The citric acid in the ink actually prevents the oxidation of the other ingredients (such as little bits of lemon); once the ink dries, the citric acid actually keeps the lemon bits from being turned brown by the air. However, if heat is applied to the ink, then the citric acid decomposes, leaving the little heated lemon bits vulnerable to oxidation.*

If you want to try this, gather the following: a piece of paper, some lemon juice, a toothpick, and a heat source. Using the toothpick, write a message on the paper with the lemon juice. After the juice dries, hold the paper up to the heat source, taking care not to let anything unexpectedly burst into flames. The message should slowly appear, after the heat breaks down the citric acid.

Pretty cool, huh?

* No, the decomposed citric acid itself does not cause the brown color. And no, the acid does not cause the cellulose bonds in the paper to burn at a lower temperature. Thanks to the Yale and Scientific American websites for this information, as well as Jerboa Kolinowski.

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