II. CHEMISTRY 461 



c. Crystal Structure 



Commercial nicotinic acid crystallized from water or ethanol occurs as 

 needle-like crystals or as a crystalline powder.^^ Keenan^^ and Wright and 

 King^- have made extensive studies of the crystal structure of nicotinic 

 acid. When carefully crystalhzed from water, 95% ethanol, or 1:1 mix- 

 tures of water and 95 % ethanol, the crystals appeared either as small 

 rods or as flat, colorless monoclinic plates with {010} as the predominant 

 form and a characteristic edge angle of 114°. All crystals were found to be 

 twinned either about the c axis or across the 100 plane to a greater or lesser 

 degree and exhibited a fibrous structure. Examination under the polarizing 



TABLE II 



Solubility" of Nicotinic Acid, Sodium Nicotinate (Hemihydrate), and 



Nicotinic Acid Hydrochloride at Various Temperatures (from 



Y. M. Slobodin and M. M. Goldman^"") 



Solubility as grams per 100 ml. 



microscope gave the results shown in Table III. The density of nicotinic 

 acid crystals was 1.473 ± 0.002 (g. cm.~^) as determined by a flotation 

 method. ^2 Debye-Scherrer x-ray, oscillation, and Weissenberg photographs 

 were also taken and the results recorded. ^^ 



d. Absorption Spectrum 



Solutions of nicotinic acid exhibit a characteristic absorption spectrum. 

 The absorption spectrum in aqueous solution has been studied by Hunecke,®^ 

 who reported a maximum at 385 A. Hughes et aZ." have made an extensive 



eo" Y. M. Slobodin and M. M. Goldman, Zhur. Priklad. Khim. 21, 859 (1948) [C. A. 



43, 6207 (1949)]. 

 «' G. L. Keenan, J. Assoc. Offic. Agr. Chemists 26, 514 (1943). 

 " W. B. Wright and G. S. D. King, Ada Cryst. 3, 31 (1950). 

 "H. Hunecke, Ber. 60, 1451 (1927). 

 " E. B. Hughes, H. H. G. Jellinek, and B. A. Ambrose, J. Phys. & Colloid Chem. 63, 



414 (1949). 



