The Acid–Base Titration of a Very Weak Acid: Boric Acid

Abstract

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.

Boric acid-catalyzed multi-component reaction for efficient synthesis of 4H-isoxazol-5-ones in aqueous medium

“…Boric acid, H 3 BO 3 , is a white solid moderately soluble in water, and is considered a very weak acid, with a presented ionization constant of about pKa = 9.2 [17]. It is a very stable compound and relatively benign to humans [18].…”

Method transfer assessment for boric acid assays according to different pharmacopoeias' monographs

“…It was evident that the measured pH values of solutions were in good accordance with the calculated pH values for the different initial concentrations of H 3 BO 3 . The best agreement was obtained for the highest investigated initial concentration ≈0.48 M. Celeste et al reported that for enough large ratio of mannitol (mannitol/boric acid ≥ 10), the values for pKa* (conditional acidity constant) were in the range 4-5 applying potentiometric titrations of boric acid (c ≈ 0.05 M) with the standard solution of 0.1 M NaOH (Celeste et al 2012). Zhang 4 pH-metric analyses of the solutions prepared for the titrations-original/altered procedures a Calculated pH values obtained by using constant Ka ≈ 1.5 × 10 -4 (of formed stronger complex acid by addition of mannitol to boric acid solution) (Mendham et al 2000), b Amount ratio of H 3 BO 3 (mol): polyol (mol), c original procedure of corresponding pharmacopoeia, d alteration et al proposed the usage of minimum pH value method and the other method of most quantity of consuming alkali for measurement of complex acid (boric acid-mannitol) dissociation constant (Zhang et al 2009).…”

Recovery of boron from unacidified salt lake brine by solvent extraction with 2,2,4-trimethyl-1,3-pentanediol

“…Metal ions concentration of aqueous phase was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (ICAP6500 DUO; America Thermo Scientific). The concentrations of boric acid in aqueous phase were determined by titration [27] , but the trace amounts of boric acid were measured by ICP-AES. The boron in the organic phase was stripped into the aqueous for analysis, so the accuracy of the analysis can be verified by the mass balance of the boron in the two phases.…”