4 P-AMINOBENZOIC ACID 



soluble in benzene and carbon disulfide; and insoluble in petroleum 

 ether.i". is 



The pH of a water solution of PABA at 25° has been reported^ "^ to be 

 3.79 with the hydrogen electrode, and 3.63 with the antimony electrode. 

 The latter value agrees more closely with the calculated value of 3.60. 



The acid and base dissociation constants of PABA have been determined 

 by a number of investigators^^ "^^ to be of the order of i^o = 1.2 X 10~^ and 

 Ki = 1.5 X 10-12. 



PABA in its isoelectric state in aqueous solution exists in the neutral 

 rather than the zwitterionic form. Evidence for this behavior was first 

 presented by Harris^^ • ^^ in his classic papers on the formol titration of 

 various amino acids. A different approach to the problem was made by 

 Klotz and Gruen.^^ By calculating the ionization constants of PABA and 

 of its methyl and ethyl esters from the ultraviolet absorption spectra of the 

 compounds, these authors reached the same conclusion as did Harris. It is 

 interesting to note that sulfanilamide, too, exhibits this behavior at its 

 isoelectric point. 



The ultraviolet spectrum of PABA is characterized by two bands, one 

 of high extinction at long wavelengths, the other of low extinction at short 

 wavelengths. For the primary band Kumler-^ reports a molar extinction 

 coefficient, in 95 % alcohol, of 17,400 at the wavelength of maximum ab- 

 sorption, 288 m^x, and an extinction coefficient of 8220, for the secondary 

 band, at 220 m/x. Kern and his associates'^ found the wavelength of peak 

 absorption to be 288 m^u in 95 % alcohol or 99 % isopropanol, with extinction 

 coefficients ranging, for several experiments, from 15,760 to 16,250 and 

 from 18,400 to 18,900 for the respective solvents. In aqueous solution, 

 values for Xmax of 266 m^,^^ 271 mju,^^ and 284 m/u-^ have been reported, 

 with extinction coefficients of the order of 14,000. 



i"" Beilstein's Handbuch der organischen Chemie, 4th ed., Vol. 14, p. 418. Springer, 



Berlin, 1931. 

 '5 C. J. Kern, T. Antoshkiw, and M. R. Maiese, Anal. Chcm. 20, 919 (1948). 

 i« F. Fenwick and E. Oilman, /. Biol. Chevi. 84, 605 (1929). 

 1' J. N. Pring, Trans. Faraday Soc. 19, 705 (1924). 



18 F. Arnall, /. Chem. Soc. 117, 835 (1920). 



19 A. Albert and R. Goldacre, Nature 149, 245 (1942). 



20 L. Michaelis and M. Mizutani, Z. physik. Chem. 116, 135 (1925). 



21 K. Winkelblech, Z. physik. Chem. 36, 546 (1901). 



22 R. llolmberg, Z. physik. Chem. 62, 726 (1908). 



23 L. J. Harris, Biochem. J. 24, 1080 (1930). 



2' T. W. Birch and L. J. Harris, Biochem. J. 24, 1086 (1930). 



26 I. M. Klotz and D. M. Gruen, /. Am. Chem. Soc. 67, 843 (1945). 



2«W. D. Kumler, ./. A?n. Chem. Soc. 68, 1184 (1946). 



2' E. R. Riegel and K. W. Buchwald, J. Am. Cheyn. Soc. 51, 484 (1929). 



2« L. i:)()ul) and J. !\T. Vund('nl)ell, ./". .4///. Chcm. Soc. 69. 2714 (1947). 



