Titration of Boric Acid 85 



THE ELECTROMETRIC TITRATION OF BORIC ACID IN 



THE PRESENCE OF POLYPHENOLS AND 



ORGANIC ACIDS. 



M. G. Mellon and V. N. Morris, Purdue University. 



It has been recognized for some time that boric acid is too weak 

 to be titrated alone. The addition of certain polyhydroxy-organic com- 

 pounds, such as mannitol, permits of an accurate titration of this acid. 

 In a previous article by the authors/ there was published the results 

 of titrations of boric acid in the presence of a number of polyalcohols 

 and sugars. The present work, an extension of the previous investiga- 

 tion, presents the results of titrations in the presence of two other types 

 of polyhydroxy-organic compounds; namely, polyphenols and hydroxy- 

 acids. 



Although it has never been definitely established, the usual assump- 

 tion is that the organic substance unites with the boric acid to form a 

 complex acid, which is ionized to a much greater extent than is the boric 

 acid alone. Consequently, the complex can be titrated with a standard 

 solution of a base, using phenolphthalein as the indicator. 



As in the previous investigation, the titrations were made with an 

 clectrometric apparatus. By this means it is possible to obtain the 

 hydrogen ion concentration, or, what is more generally desired, the pH 

 value after each addition of basic solution. Plotting the pH curve, with 

 milliliters of base as abscissas and pH values as ordinates, provides an 

 easy means of following the course of the neuti'alization. 



Previous Work. — The bearing of previous work on the present in- 

 vestigation was discussed, in general, in the authors' earlier paper. 

 Specifically, it should be mentioned that it was Magnanini' who made 

 the first observations that various hydroxyacids increase the electrical 

 conductivity of boric acid to a marked degree. In recent years Boeseken^ 

 and his students have studied the efi'ect of a large number of poly- 

 hydroxy compounds, including polyphenols and hydroxyacids. He con- 

 cluded that a complex acid, much stronger than the components, is 

 formed, and that, for the formation of such a complex, two OH groups 

 must be situated on the same side of the adjacent carbon atoms to 

 which they are attached. When such a configuration does not exist, 

 as with the beta and gamma polyhydroxy arrangements, a negligible 

 or slightly negative effect on the conductivity was observed. 



Present Work. — It has been the general practice to add either man- 

 nitol or glycerol to boric acid before making a titration of the 

 latter. The authors have pointed out in their previous article that 

 invert sugar and manna are more economical than either mannitol or 

 glycerol for this purpose. The work of Manganini and Boeseken indi- 

 cated that there were still other substances, among the polyphenols and 



ilnd. Eng. Chem., IG. 123 (1924). 



= Gazetta, 21, 9 (1891) ; Z. Phys. Chem., 11, 281 (1893). 



3Rec. trav. chim.. AO, 553, 578 (1921) ; 37, 130, 165, 178 (1918) ; 35, 211 (1916) ; 

 36, 167 (1917) ; Ber., U6, 2612 (1913). 



"Proc. Ind. Acad. Sci., vol. 33, 1923 (1924)." 



