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Now i am extracting citric acid from lemon juice and i have a problem in adding sulphuric acid to calcium citrate because i don't know how to calculate the proper amount sulphuric acid to add to calcium citrate. Could anyone show me how to calculate the amount of sulphuric acid? And if anyone of you know the procedure for this extraction; please share me. Thank you!

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When you add calcium hydroxide to the sodium citrate solution, you end up forming tricalcium dicitrate

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In order to neutralize 1 mol of tricalcium dicitrate and form 2 mol of citric acid you need 3 mol of H2SO4 (each mol of tricalcium dicitrate have 6 mol of deprotonated oxygens and each mol of sulphuric acid provides 2 mol of H+).

So the balanced equation should be:

$$\ce{Ca3(C6H5O7)2 + 3H2SO4 <=> 2C7H8O7 + 3CaSO4-}$$

So, the proportion of tricalcium dicitrate to sulphuric acid is 1:3.

To fully neutralize the citrate to citric acid, for each mol of citrate you need 3 mol of sulphuric acid, or mathematically:

$$\ce{3n_{\ce{Ca3(C6H5O7)2}} = n_{\ce{H2SO4}}}$$

You know the molecular weight of both and the mass of tricalcium dicitrate that you have, now you can calculate how much sulphuric acid you need from the equation above.

There are several videos on youtube showing how to extract citric acid from fruits, I particularly like this one: https://www.youtube.com/watch?v=FMtayizdFiw

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