[constant thinker]

I'm considering trying to purify citric acid from lemon juice. From some obscure source, I remember seeing that lemon juice contains 5-10% citric acid.

I have a few different plans for this. One idea though is a few drops of 98% sulfuric acid to the juice. I'm hoping that because conc. sulfuric acid has such a powerful dehydrating effect that it will cause the citric acid to be precipitated out of the solution. One other thing I'm hoping is that maybe it will also dehydrate the sugars leaving just carbon, then I can dump the left over solid mixture into water, and the filter out the carbon so I'm left with just aqueous citric acid.

Do you guys think this will work? I was also thinking about reducing the citric acid to a citrate salt, then regenerating it with sulfuric acid.

The reason I'm asking before trying this out is I have limited amounts of sulfuric acid in my possesion, about 400ml left.

Also if anyone has a simpler way, I'm open to hearing it.

[Alberto_Kravina]

Hmmm...I don't know if this works, but you could precipitate the citrate anions as calcium citrate (which has the strange property that it is MORE soluble in cold water than in hot water). After having precipitated the calcium citrate you could add a strong acid which protonates the salt to citric acid.

Hope I helped, a little .

[pantone159]

Alberto's plan sounds more promising.  I'm skeptical that your original plan, constant thinker, would work.

BTW - If you don't really care about purifying it, but just want the acid, you can probably find it in grocery stores/health food stores.  That is where I got mine.

[Alberto_Kravina]

BTW - If you don't really care about purifying it, but just want the acid, you can probably find it in grocery stores/health food stores.  That is where I got mine.

Of course you could buy it at the store, but the beauty of constant thinker's project is to make the acid by himself from what nature gave him

It's much more exciting to isolate it by yourself, although I'd opt for Mark's suggestion (because of my lazyness )

[pantone159]

It's much more exciting to isolate it by yourself

I agree, but whether it is worth the trouble depends on what you are really trying to do.

[constant thinker]

Yea, I was skeptical about my plan original plan too. I cooked it up in my mind because I was thinking about dehydration. At first I though about distilling the water off, but I read somewhere that citric acid forms an azeotrope with water at around 50%, which isn't really what I want.

Then again hypothetically at 50%, given that all you had was citric acid and water, that would be a monohydrate I believe.

Alberto_Kravina got my plan right. I want the fun in extracting it from the natural product. I was aware that you can buy it in stores. I think I'll have to purchase some calcium hydroxide for this though.

I took this from wikipedia.

At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form, or as a monohydrate that contains one water molecule for every molecule of citric acid. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating it above 74 °C.

From that wording it sounds almost like if I heat the lemon juice, I'll get my citric acid.

The Ca(OH)₂ method sounds the most promising. Question about it though is, "How will get what's left of the sulfuric acid out?" I have to admit I know nothing about how reprotenation occurs.

[pantone159]

This link looked relevant, although I just skimmed it...

http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/citricacid.txt

[Glaudge]

can you just boil/evaporate the water? and the citric acid would be left behind?

[billnotgatez]

It is nice that in nature compounds are formed which are useful. But, at the same times other compounds are formed so that you have a mixture. It gets more involved due to the possibility that both compounds are soluble in water, which may be the natural solvent to the process or also produced by the process. Even if you evaporate the water you are left with a mixture with the molecules of each compound very commingled. Citric acid has that situation in fruit juice.