462 



NIACIN 



study of the ultraviolet absorption spectrum of nicotinic acid at pH values 

 from 1.28 to 13.0 (Fig. 1). The concentration of nicotinic acid used in de- 

 riving the curves shown in Fig. 1 was 0.0004 M, and the ionic strength 

 was the same at each pH. It will be noted that the molecular extinction 

 coefficient varied with pH, although the absorption maximum was at the 

 same wavelength, 2615 A., in each instance. There was a straight-lme rela- 

 tionship between extinction coefficients and concentration of nicotinic acid 

 (Beer's law, Fig. 2). The validity of Beer's law was also confirmed using 

 nicotinic acid solutions at pH 1.28 and pH 13. 



TABLE III 

 Optical Properties of Nicotinic Acid for Sodium Light (from W. B. Wright 



AND G. S. D. King62) 



e. Dissociation Constants 



From the data depicted in Fig. 1, Hughes et al.^^ were able to calculate 

 dissociation constants for nicotinic acid. The spectra fell into two groups, 

 each having two isobestic points indicating two equilibria. The first two iso- 

 bestic points were at wavelengths of 2495 A. and 2695 A. and were associated 

 with pH values from 1.28 to 4.08. They belong to the dissociation equilib- 

 rium of nicotinic acid when acting as a base. 



-COOH 



+ B.2O 



H+ 



-COOH 



+ OH- 



