Preparation of citric acid from lemon juice

To Prepare Citric Acid, add 4.5 ounces chalk by degrees to 4 pints lemon juice,
heated, and mix; set aside so that the precipitate can settle; afterwards pour
off the fluid. Wash the precipitated calcium citrate frequently with warm water,
then pour upon it 27.5 fluid ounces diluted sulphuric acid and 2 pints distilled
water, and boil for 15 minutes. Filter and evaporate the filtered solution with
a gentle heat, and set it asideso that crystals may form. To obtain the crystals
pure, dissolve them in water a second and third time, filter each solution,
evaporate, and set it aside to crystallize.

In conducting this process some care is necessary to ensure success. The chalk
used should be dry, and in fine powder, andbe added to the juice till its
perfectly neutralized, and the quantity consumed must be exactly noted. The
precipitated calcium citrate of time should be well washed with water, and the
sulphuric acid diluted with 6 or 8 times its weight of water, poured upon it
while still warm, and throughly mixed with it. The agitation must be
occasionally renewed for 8 or 10 hours, when the dilute citric acid must be
poured off, and the precipitate of calcium sulphate thoroughly washed with warm
water, and the washings added to the dilute acid. The latter must then be poured
off from the impurities that may have been deposited, and boiled unitil it
aquires a specific gravity of 1.13, when the process must be continued at a
lower temperature until a pellicle (crusty film) appeares upon the surface.

This part of the process requires great attention and judgment, as, if not
properly done, the whole batch may be carbonized abd spoiled. At this point the
evaporation must be stopped, and the concentrated solution emptied into warm and
clean crystallizing vessels in a dry room. At the end of 4 days the crystals
will be ready to be romoved, when they must be well drained, redissolved in as
little water as possible, and, after being allowed to stand for a few hours to
deposit impurities, again evaporated and crystallized.

When the process has been well managed, the acid of the second crystallization
will usually be sufficiently pure; but if this be not the case, a third, or even
a fourth crystallization must be done. The mother liquors from several pans are
collected together, and, by evaporation, yield a second or third crop of
crystals obtained by evaporation as before. Citric acid crystals are obtained
with great ease, but in some cases, where all the calcium citrate has not
undergone decomposition by the acid, a little of that salt is taken up by the
free citric acid, and materially obstructs the crystallization. This is best
avoided by exactly measuring the quantity of the sulphuric acid to that of the
calcium, 49 parts by weight of sulphuric acid to 50 parts chalk. In practice it
is found that a very slight excess of acid is prefered to leaving any calcium
citrate undecomposed.

2 gallons of lemon juice yields about 1 pound of crystals (about 5%).

Tests for the purity of citric acid: When pure, it does not yield a crystalline
precipitate when added in excess to a solution of potassium corbonate, such a
precipitate indicates the presence of tartaric acid. It is entirely soluble in
water, and what is thrown down by lead acetate from this solution is entirely
soluble in dilute nitric acid. No potassium salts (besides tartate) yeilds a
precipitate with the aqueous solution. It is entirely decomposed by heat. Added
sparingly to cold lime water it doesn't render it turbid.
