The use of boric acid in the Kjeldahl determination of nitrogen is a variant of the original method widely applied in many laboratories all over the world. Its use is recommended by control organizations such as ISO, IDF, and EPA because it yields reliable and accurate results. However, the chemical principles the method is based on are not detailed in most analytical textbooks, perhaps because the nature of boric acid is not known in depth. To introduce the reader or student to the chemistry of boric acid, this article first describes the differences in acid-base behavior of concentrated and diluted boric acid by simple calculations and experimental pH measurements. Next, polyborate structures and their respective dissociation constants are shown to explain the differences found in previous pH measurements. In the second part of the article, the determination of ammonia with HCl is made in concentrated and diluted boric acid to reproduce the final Kjeldahl titration when two different volumes of ammonia distillate are collected. From the previous information, the apparently paradoxical results of the titration curves are explained in a qualitative manner. The third part consists of a study of systematic errors produced when colored indicators are used in the final Kjeldahl titration in concentrated and diluted boric, respectively.