944 CITRIC ACID. 



SECTION IV. 



CITRIC ACID AND THE PRODUCTS OF ITS DECOMPOSITION. 



Formula of citrate of silver, 3AgO + C 12 H 5 O n . Of the 

 crystals of citric acid formed on the cooling of a solution 

 saturated at 21 2, 3HO,C 12 H 5 O n + HO, which may be named 

 hydrate A; this hydrate loses no weight and preserves its 

 transparency at 212. It is the type upon which most of the 

 citrates are formed. Of the crystals formed by the spontaneous 

 evaporation of a solution saturated in the cold, the formula is 

 3HO,C 12 H 5 O n +2HO ; of which the two atoms of water of 

 crystallization are lost at 2 1 2. 



Citric acid was discovered by Scheele ; it exists in a consi- 

 derable variety of plants, but is procured only in quantity from 

 the orange and lemon and from gooseberries. The acid juice 

 of these fruits is neutralised by a known weight of carbonate 

 of lime ; the insoluble citrate of lime washed, and then de- 

 composed by a quantity of oil of vitriol equal in weight to the 

 carbonate of lime used, diluted with 5 parts of water. The 

 acid liquid is separated by filtration, from the insoluble sulphate 

 of lime ; and the citric acid crystallized with a slight excess of 

 sulphuric acid present, which is observed to favour the crys- 

 tallization, while it is impeded by citrate of lime in solution. 



Citric acid crystallizes in regular rhomboidal prisms, ter- 

 minated by four faces, has an agreeable acid taste, and is soluble 

 in an equal weight of water forming a thick syrup. A dilute 

 solution of citric acid in water does not keep, but becomes 

 covered with mouldiness. When pure, citric acid dissolves 

 completely in alcohol, without residue, and does not give a 

 precipitate with lime-water. But when a few drops of the acid 

 are added to the latter in excess, the clear liquid obtained 

 becomes turbid when heated, from the deposition of a white 

 basic citrate of lime, 3CaO,C 12 H 5 O n -f CaO,HO, which dissolves 

 in acids without effervescence. Citric acid is commonly dis- 

 tinguished by that property. Hydrate A of citric acid, 

 which contains 4 atoms of water fuses at 266, and loses 

 nothing, although hydrate B, which contains 5 HO, loses 2 HO 

 at 212. When the temperature of 302 is exceeded the acid 

 of both hydrates undergoes decomposition. When 1 part of 



