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Citric Acid

Citric acid has a few uses in chemistry. One of the main ones is it's use as a catalyst in the HMTD reaction. It's hard to find in solid form now days, due to it's illicit use around heroin. However, it's quite easy to find in it's natural form - lemon juice. From this, it can be isolated in a simple acid base reaction - First by precipitation as it's calcium salt, then acidifying it with sulfuric acid.

3CaCO3(s) + 2C6H8O7(aq) ----> Ca3(C6H5O7)2(s) + 3CO2(g) + 3H2O(l)

Ca3(C6H5O7)2(s) + 3H2SO4(aq) ----> 2C6H8O7(aq) + 3CaSO4(s)

Materials
Equipment
Lemon Juice 600ml beaker
Calcium Carbonate powder evaporation dish
Sulfuric acid blue litmus paper
   

Summary:

Add calcium carbonate to lemon juice until neutral. Separate the precipitated calcium citrate and free it from as much residual lemon juice as possible. Acidify the calcium citrate with dilute sulfuric acid and filter out the precipitated calcium sulfate. Evaporate on low heat and recrystallize mutliple times.

 

Into a 600ml beaker, pour 250ml of your lemon juice. I only give this amount because it's what I used, you can scale it up or down as you like. Just be sure to only fill your reaction vessel no more than half full with the lemon juice. Now, because every lemon juice is different, I can't give any specifics on how much calcium carbonate to use. Add the calcium carbonate in small portions until it registers acid free on blue litmus (or almost acid free - represented by a very faint pink color on litmus). However, it should take around 35-40g of calcium carbonate for every 250ml. Add it at room temperature at first, then once the effervescence slows down, you may begin heating it. The reaction will be rather slow, because calcium carbonate is insoluble in water. The reaction should speed up once you begin heating it - but don't boil it because that's a waste of energy. Take note of how much calcium carbonate you used! Once it is neutral or almost neutral, separate the calcium citrate.

Since filtering out precipitated calcium salts is as painful as amputations, it might be wise to do what I did instead. Fill most of the remaining empty space of your container with distilled water. Tap water can be used, but I recommend against it - the extra minerals might affect your yeild. Once you've added the water, stir it up, then walk away. In a bit of time, you'll have your white precipitate layer at the bottom, and the waste water. Decant the supernatant liquid off, and repeat. Do this 2-3 times until the liquid is colorless (but it can be somewhat cloudy). You now have pure enough calcium citrate.

To calculate the sulfuric acid to use, you must use 0.576ml of 94% sulfuric acid for ever 1g of calcium carbonate you used. Make some extra incase your calculations are wrong. Measure out your sulfuric acid, and dilute it by 10 times it's volume in water (so 10ml of sulfuric acid + 100ml water). This is very exothermic, so be careful. Now, add water to the calcium citrate and stir it around to temporarily form a suspension. Add your sulfuric acid to this slowly. It is better to have an excess of sulfuric acid, but not too much since it's hard to remove. There will be an effervesence as you add the acid, so only add a little bit at a time. You could also do what I did - pour your calcium citrate in a 1 gallon water bottle, and add the sulfuric acid to that. I never had a foam over in a 1 gallon jug, but it depends on how much you fill it. After the addition, continue some form of agitation for 10-20 minutes to ensure a full conversion. Agitation can include magnetic stirring, overhead stirring, manual stirring (stir stick and your hand), a bubbler, or swirling/shaking of the vessel. Once it has been thorougly agitated, now it's your turn to become agitated. Bring it up to a boil, slowly, with lots of stirring. More bubbles will be given off and you'll most likely have to carefully regulate the heat, else you want a foaming mess everywhere. Hold it at this boil for a good 15-20 minutes to aid in conversion. After that time, turn the heat off and let it gradually cool down

Once it is cool enough to touch, it's time to begin filtering the calcium sulfate out. You will now learn why filtering precipitated calcium salts is as painful as amputation. To help speed things along, first filter it through paper towels. That will remove the bulk of the material. Rinse a portion or two of water through the filter. Now, you can filter the minute particles out with double stacked filter paper, etc.