Submitted by A. F. Holleman
Checked by H. T. Clarke and H. J. Bean.
1. Procedure
In a
copper or iron kettle of 4-l. capacity is placed a solution of
200 g. (1.33 moles) of d-tartaric acid and
700 g. of sodium hydroxide in 1400 cc. of water. A
12-l. flask through which cold water is run is placed in the mouth of the kettle in order to prevent loss of water vapor, and the mixture is boiled gently over an open flame for four hours. The solution is now transferred to a 12-l. flask or
crock and partially neutralized with
1400 cc. of commercial hydrochloric acid (sp. gr. 1.19). To the still alkaline solution is now added just enough
sodium sulfide to precipitate all the
iron or
copper which has been dissolved from the kettle (Note
1). The filtered solution is then just acidified with
hydrochloric acid, boiled to expel all
hydrogen sulfide, and made very faintly alkaline to
phenolphthalein with
sodium hydroxide solution. To the hot solution is then added a concentrated solution of
300 g. of anhydrous calcium chloride which causes an immediate precipitation of
calcium dl-tartrate and
meso-tartrate.
The mixture is allowed to stand for one week (Note
2) and the precipitate filtered, washed with cold water until free of chlorides, and dried at 40–50°. The yield is
246–315 g. The product is found on analysis to consist principally of the tetrahydrate,
C4H4O6Ca·4H2O (Note
3). It is suspended in 800 cc. of water and treated with the calculated quantity
(0.4 g. for every 1.0 g.) of concentrated (95–96 per cent) sulfuric acid. The mixture is warmed on the
steam bath, with occasional shaking, or mechanical stirring, for thirty to forty hours (Note
4), filtered hot, and the
calcium sulfate washed well with hot distilled water.
The united filtrate is evaporated on the steam bath until the volume amounts to 200 cc., at which point crystals should have already begun to separate from the hot solution. After standing at room temperature for twenty-four hours or longer, the crystals are filtered by suction as free from mother liquor as possible and recrystallized from an equal weight of distilled water (Note
5). The filtrate from this recrystallization is evaporated on the steam bath and the second crop of
dl-tartaric acid filtered and recrystallized as before. The yield is
65–75 g. (
29–33.4 per cent of the theoretical amount).
2. Notes
1. If
iron is not removed at this point it separates with the
calcium salt and contaminates the final product. If desired,
copper can be removed at a later stage by passing
hydrogen sulfide through the acid solution.
2. The
calcium salts, in particular that of
meso-tartaric acid, separate very slowly, and the yield is appreciably lower if the mixture is allowed to stand only eighteen hours.
3. The
calcium salt of the principal product,
dl-tartaric acid, crystallizes with four molecules of water; the secondary product,
meso-tartaric acid, forms a
calcium salt which crystallizes with three molecules of water. The amount of
sulfuric acid actually required may readily be calculated from the percentage of
calcium found on analysis in the regular way; or it may be estimated by igniting a sample, and titrating the residue with standard acid.
4. This long treatment is necessary since, on the one hand, the
calcium salts tend to be protected by the
calcium sulfate which crystallizes on them, and, on the other hand, it is important to allow all the free
sulfuric acid to react.
5. A very complete separation of the
calcium sulfate may be effected by adding to the concentrated solution
1000 cc. of alcohol, allowing the mixture to stand for twenty-four hours, and filtering. The alcohol is then distilled from the filtrate and the procedure followed as indicated above.
3. Discussion
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