§ 148. FRÄNGULIC ACID, ALIZARIN, ETC. 133 



FranguUc acid can be obtained as an orange-red powder, con- 

 sisting of small acicular (? hexagonal) crystals, which melt at 

 255° and are not dichroic. At a temperature of 18° 1 cc. of 

 glacial acetic acid dissolves 0*00235 gram, 1 cc. of 96 per cent, 

 spirit 18 gram. The solutions of this substance in aqueous or 

 alcoholic alkalies are also of a fine red colour, but prove to be 

 somewhat different from those of chrysophanic acid when ex- 

 amined sjDectroscopically. Keussler made the following observa- 

 tions with aqueous solutions in caustic potash, under the conditions 

 mentioned in § 22 : 



Diminished Undiminished Diminished No colours 



Intensity. Intensity. Intensity. observable. 



Chrysophanic acid . 0°— 1-3^ 13°— 3i" 34°— 38° 48° to end. 



Trangulic acid . . 0° — 18° 18°— 38° From 38° to end gradual dimini- 



tion of intensity to complete dark- 

 ness. 



Compare Plate I., 1 and 2.i 



Alizarin forms orange-red prisms, which are also almost 

 insoluble in cold water, but soluble in alcohol, ether, benzene, and 

 aqueous alkalies. They melt at 215°, and can be sublimed Avith- 

 out decomposition. Its alkaline solutions are violet, and yield 

 j)urple precipitates with salts of calcium, barium and lead. Vogel ^ 

 states that the absori^tion-spectrum of a solution of alizarin in 

 dilute alcoholic potash shows two dark 1)ands, one of Avhich is 

 exactly divided by the line d, whilst the other begins a little 

 before D, and may be traced some distance past that line. (Com- 

 pare Plate I., 3.) An alcoholic solution of alizarin, after addition 

 of ammonia, shows an absorption spectrum with a single ill- 

 defined band in the green between D and F. (Plate I., 4). 



Purpurin shows under the last-named conditions two ill-defined 

 absorption bands to the right and left of E (Plate I., 5), whilst 

 an alcoholic solution made alkaline with potash absorbs dark 

 blue powerfully, and shows two very deep bands between F and 

 E, and E and I), and one weak one at d. (Plate I, G.) The 

 difference between the spectra of alizarin and purpurin is so great 

 that an admixture of 1 per cent, of the latter can be detected with 

 facility in the former. The direct detection of small quantities of 

 alizarin in purpurin is, however, impossible, but, according to 



^ For f rangulin and fi-anguHc acid, see also Faust, Archiv d. Pharm, clxxxvii. 

 8, 1869 (Pharm. Journ. and Trans. [3], iii. 1033). 



- 'Prakt. Spektralanalyse,' Nördlingen, Beck, 1877 ; and Ber. d. d. chem. 

 Oes. X. 157. See also ibid. 175 and 550 (Journ. Chem. Soc. vol. xxxii.). 



