A laboratory experiment based on the titration of boric acid with strong base in the presence of d-mannitol is described. Boric acid is a very weak acid and direct titration with NaOH is not possible. An auxiliary reagent that contributes to the release of protons in a known stoichiometry facilitates the acid-base titration. Students obtain the potentiometric titration curves of boric acid with standard NaOH in the absence and in the presence of different quantities of mannitol. The results are used for the determination of boric acid concentration and for the discussion of the possibility of performing the titration with a visual end-point determination, including the influence of the quantity of mannitol in solution. The experiment was developed for an introductory analytical chemistry course, but may be extended to an advanced analytical course with the inclusion of a more detailed theoretical treatment. The experiment may be adapted to general chemistry course as an example of a nontrivial titration. (Contains 4 figures.)