2
$\begingroup$

I was planning to extract citric acid from lemon juice by reacting the lemon juice with calcium chloride hoping it would react to form calcium citrate and precipitate according to the reaction $$\ce{2 C6H8O7(aq) + 3 CaCl2(aq) -> Ca3(C6H5O7)2(s) + 6 HCl(aq)}$$ However I was not sure that the reaction would occur. I know that the calcium citrate should precipitate out and drive the reaction forward, however it seems that the $\ce{HCl}$ generated reaction would then react with any calcium citrate bringing back to calcium chloride and citric acid. Which reaction would move to completion or would the two reactions reach an equilibrium. If this will not work I plan to just use calcium hydroxide instead by reacting calcium chloride with sodium hydroxide.

$\endgroup$
2
$\begingroup$

How do you know that calcium citrate will precipitate out and drive the reaction forward? In fact, it will not. Why in the world would it? Because calcium citrate is poorly soluble (which is true)? Well, that's not enough. Calcium carbonate or calcium phosphate are even more insoluble, yet even they will not precipitate in similar conditions (i.e., in acidic media). That's the problem with the salts of weak acids. (Yes, we may consider phosphoric acid weak, if we are talking of its third proton.) You have a solution with plenty of calcium and citrate (or phosphate, or carbonate), but most of that citrate (or phosphate, or carbonate) is not in the fully deprotonated form, so the solubility limit is not reached, and the precipitate can't appear.

Also, the very word "extract" does not quite apply here anyway.

Also, what was your plan to regenerate citric acid from calcium citrate afterwards?

$\endgroup$
  • 1
    $\begingroup$ Well I didn't vote for deletion but I feel that at the moment it reads more like a comment than a proper answer, especially seeing as it throws a lot of questions back at the OP. $\endgroup$ – bon Oct 10 '15 at 10:24
  • $\begingroup$ I guess it does. Well, let's add some supporting info. $\endgroup$ – Ivan Neretin Oct 10 '15 at 10:35
5
$\begingroup$

This procedure is done in our laboratory class, but first you have to add sodium hydroxide until the lemon juice is somewhat alkaline. Then add the calcium chloride (use $1~\mathrm{ml}$ for every $2~\mathrm{ml}$ of the lemon juice + $\ce{NaOH}$ solution).
I think the calcium chloride would first react with the hydroxide, producing calcium hydroxide, which would then react with citric acid to form calcium citrate.

$$\ce{2 C6H8O7 + 3 Ca(OH)2 -> Ca3(C6H5O7)2.4H2O + 2H2O}$$

Filter out the salt (precipitate) formed, weigh it, then you would have to add sulfuric acid afterwards to react with the calcium citrate. This would then form citric acid (soluble) and a precipitate which is calcium sulfate.

$$\ce{Ca3(C6H5O7)2.4H2O + 3H2SO4 -> 2C6H8O7 + 3CaSO4.2H2O + 2H2O}$$

You would then have to concentrate the filtrate to obtain the citric acid crystals.

$\endgroup$
  • $\begingroup$ Hi and welcome to chemistry.stackexchange.com. Feel free to take a tour of the site. I improved the formatting of your post by using MathJax. You can learn more about it in the help center or by visiting this meta-post. Please note that there is a difference between uppercase O (the letter) and 0 (the number) and between a subscript 5 (the number) and lowercase s (the letter). I was astounded at $\ce{C6Hs07}$ ;) $\endgroup$ – Jan Nov 4 '15 at 19:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.