Titration of Boric Acid 89 



The first titrations attempted indicated that the two constituent 

 acids in the mixture were apparently neutralized separately. With the 

 four to one molecular ratio used with the polyphenols, the two breaks 

 in the curve occurred very close together so that it was considered 

 desirable to select a different ratio. Therefore an amount of the organic 

 acid was dissolved in the solution to be titrated equivalent to the boric 

 acid contained therein. In determining the course of the neutralization 

 for the two acids a titration was run for each acid alone and for the 

 two together. 



Titrations were made using the following acids: oxalic, malonic, 

 lactic, glycolic, malic and tartaric of the aliphatic series, and gallic and 

 tannic of the aromatic series, assuming that the properties of tannic 

 acid warrant its inclusion in this class. Salicylic acid was tried but 

 its i^ate of solution was so slow that the results seemed to be of no value. 



The results obtained for glycolic acid are shown in figure 2. These 

 curves are representative of those for the other aliphatic acids, except 

 in those cases involving a dibasic acid, where a small upward break 

 occurs when the first hydrogen of this acid is neutralized. There is 

 evident only a small downward displacement of the first portion of the 

 curve for the organic acid when boric acid is present. 



Figure 3 shows the cui'ves for both gallic and tannic acids. It 

 should be noted that the two acids were used on a different basis — 20 

 ml. of boric acid with the gallic acid and 30 with the tannic — and that 

 the curve for boric acid alone is for 30 ml. of the solution. In only 

 one curve — that of boric and gallic — is there a break indicating the pos- 

 sibility of making a satisfactory volumetric determination. In these 

 cases it is evident that the combination of two acids gives a curve dis- 

 tinctly different from that for either one alone. This change in the 

 curve for tannic acid is much greater than the change for any other 

 acid used. 



Discussion. 



A study of the preceding curves brings out several conclusions 

 regarding the complexes formed by boric acid and some polyhydroxy- 

 organic compounds. These conclusions are concerned primarily with 

 the possibility of utilizing these complexes in the volumetric determina- 

 tion of boric acid and may be stated as follows: 



1. In every case the addition of the polyhydroxy-compound pro- 

 duced a downward displacement of the pH curve for boric acid, the 

 degree of displacement varying with the substance used. In general, 

 this may be attributed to the formation of a complex acid which ionizes 

 to a greater extent than does boric acid alone, although in some cases 

 the mere presence of the organic substance might account for the dis- 

 placement produced, even if no complex were formed. While Boes- 

 eken's" conductivity experiments may indicate the existence of a complex, 

 the results of the present investigation show clearly that a complex, 

 capable of being titrated accurately, is not necessarily formed. 



5 Loc. eit. 



