BENZENE SERIES. AROMATIC COMPOUNDS. 581 



Saccharin is a white, crystalline, odorless powder. It is but sparingly 

 soluble in water, requiring about 250 parts for solution ; this solution is 

 slightly acid and has an extremely sweet taste, which is yet perceptible when 

 saccharin is dissolved in 125,000 parts of water, which shows that it is about 

 500 times sweeter than cane-sugar, a solution of which in 250 parts of water is 

 yet perceptibly sweet. Saccharin is soluble in alcohol and ether, and it is this 

 latter property which is made use of in testing sugar (or other substances in- 

 soluble in ether) for saccharin. The substances are treated with ether, which 

 is filtered off and evaporated, when the saccharin may be recognized by its 

 taste in the residue. 



Saccharin forms very soluble and well-crystallizing salts with the alkalies, 

 which are also intensely sweet ; they are articles of commerce. The sodium 

 salt is known as soluble saccharin or krystallose. Saccharin is known in the 

 British Pharmacopoeia as Glusidum (Gluside), and in commerce as glucusimide, 

 saccharol, saccharinol, saccharinose, agucarine, etc. A number of preparations, 

 such as antidiabetin, contain saccharin. Dulcin or sucrol, another very sweet 

 substance, is para-phenetol-carbamide. 



Phthalic acid, C 6 H 4 QQg Of the aromatic polybasic 



acids, the dibasic acids are the most important. They are called 

 phthalic acids in allusion to the fact that one of them can be obtained 

 from naphthalene. Theoretically, three dibasic acids are possible and 

 all are known. When mixed with lime and distilled they yield 

 benzene. 



Phthalic acid can be obtained by the oxidation of derivatives of 

 benzene containing two side-chain hydrocarbons in the ortho-position, 

 but it is manufactured by oxidizing naphthalene by hot fuming sul- 

 phuric acid with the help of a catalytic agent, as mercury. The sul- 

 phuric acid loses oxygen to the naphthalene and forms sulphur diox- 

 ide, which escapes in great quantities. Enormous quantities of phthalic 

 acid are employed in the manufacture of synthetic indigo. It is a 

 crystalline white substance, readily soluble in hot water, alcohol, and 

 ether. When heated it decomposes, yielding water and phthalic 

 anhydride, which latter sublimes in long needles : 



Phthalic anhydride. 



Iso-phthalic acid (Meta-phthalic acid), C 6 H 4 (COOH) 2 1 : 3, may be obtained by 

 oxidizing benzene derivatives containing two side-chains in the meta-position, 

 and from rosin by oxidation with nitric acid. It is difficultly soluble in water 

 and does not give an anhydride when heated. 



Terephthalic acid (Para-phthalic acid), C 6 H 4 (COOH). 2 l : 4, can be formed by 



