which
hydrolyses to dichlor-hydroxy iso-butyric acid. Potassium cyanide reacts
with this acid to form the corresponding dinitrile, which is converted
by hydrochloric acid into citric acid. This series of operations proves
the constitution of the acid. A. Haller and C.A. Held synthesized the
acid from ethyl chlor-acetoacetate (from chlorine and acetoacetic ester)
by heating with potassium cyanide and saponifying the resulting nitrile.
The acetone dicarboxylic acid, CO(CH2CO2H)2, so obtained
combines with hydrocyanic acid, and this product yields citric acid on
hydrolysis.

Citric acid has an agreeable sour taste. It is soluble in 3/4ths of its
weight of cold, and in half its weight of boiling water, and dissolves
in alcohol, but not in ether. At 150 deg.C. it melts, and on the continued
application of heat boils, giving off its water of crystallization. At
175 deg. C. it is resolved into water and aconitic acid, C6H6O6, a
substance found in _Equisetum fluviatile_, monks-hood and other plants.
A higher temperature decomposes this body into carbon dioxide and
itaconic acid, C5H6C4, which, again, by the expulsion of a
molecule of water, yields citraconic anhydride, C5H4O3. Citric
acid digested at a temperature below 40 deg.C. with concentrated sulphuric
acid gives off carbon monoxide and forms acetone dicarboxylic acid. With
fused potash it forms potassium oxalate and acetate. It is a strong
acid, and dissolved in water decomposes carbonates and attacks iron and
zinc.

The citrates are a numerous class of salts, the most soluble of which
are those of the alkaline metals; the citrates of the alkaline earth
metals are insoluble. Citric acid, being tribasic, forms either acid
monometallic, acid dimetallic or neutral trimetallic salts; thus, mono-,
di- and tri-potassium and sodium citrates are known. On warming citric
acid with an excess of lime-water a precipitate of calcium citrate is
obtained which is redissolved as the liquid cools.

The impurities occasionally present in commercial citric acid are salts
of potassium and sodium, traces of iron, lead and copper derived from
the vessels used for its evaporation and crystallization, and free
sulphuric, tartaric and even oxalic acid. Tartaric acid, which is
sometimes present in large quantities as an adulterant in commercial
citric acid, may be detected in the presence of the latter, by the
production of a precipitate of acid potassium tartrate when potassium
acetate is add