vin, a, 1 Gibbs, Williams, Galajikian: Methyl Salicylate IV 7 



centrations of which may be represented by the symbols opposite 

 them. 



Free base (NaOH) n 



Free ester (C 6 H 4 OHCOOCH,) e 



Sodium methyl salicylate (CsILONaCOOCHa) m 



Mono-sodium salicylate (C 8 H 4 OHCOONa) s 



Di-sodium salicylate (CeH^ONaCOONa) d 



Let a = the total concentration of sodium atoms and i = the 

 total concentration of radicals containing the benzene grouping. 

 Let x = the concentration of sodium titrated as combined when 

 Congo red is employed as indicator. Then the following rela- 

 tions are true. 



en 



-=r (1) 



m 



ns 



— =r'=0.03 (Goldschmidt) (2) 



d 



x=s+d (3) 



a — x = m + n + d (4) 



i = m + e + s + d. (5) 



Equation (1) represents the hydrolytic equilibrium of sodium 

 methyl salicylate. Equation (2) represents the equilibrium for 

 di-sodium salicylate, and Goldschmidt's average experimental 

 value of 0.03 is used as the hydrolytic constant. Equations (3), 

 (4), and (5) follow from the fact that, with most indicators 

 such as Congo red, the sodium in the phenol position is titrated 

 as free. 



Solving the above five simultaneous equations, 



_ 0.03 e (a-i+e) ,„* 



r (e+0.03) (i-x-e)' l ; 



a can be determined by previous titration of the sodium hydrox- 

 ide solution, i by the colorimetric method for salicylic acid after 

 complete saponification, and x by titration at any time when the 

 system has attained equilibrium, disregarding the progress of 

 the saponification. For determining e, two methods are applic- 

 able: The first, by using an excess of the ester in which case e 

 will be equal to the maximum solubility, about 0.005 molar; and 

 the second, according to Farmer and Warth, 13 by means of its 

 partition between water and an immiscible solvent. 



a Trans. Chem. Soc. London (1901), 79, 863; ibid. (1904), 85, 1713. 



