Inorganic Synthesis

Boric acid preparation

Boric acid is a weak mineral acid that is a water soluble solid at room temperature. It occurs in nature as a mineral sassolite in volcanic fumaroles and hot springs [1]. It crystallizes in triclinic crystal system. Boric acid is also present in seawater [2]. Borate ions present in seawater contribute in absorbtion of low-frequency sound waves. Pressure exerted from low-frequency soundwaves passing the sea water cause borate ions [B(OH)4]- to lose an -OH group and convert them into molecules of boric acid [B(OH)3]. In the process, the molecules absorb energy from passing sound waves [3]. The compound is prepared upon treatment of borax (Na2B4O7·10H2O) with hydrochloric acid. It acts both as a Lewis acid forming [B(OH)4] anion and as a weak Bronsted acid with pKa ranging from 9.24 (pKa1) to 13.30 (pKa3).

Na2B4O7·10H2O + 2 HCl →4 B(OH)3 + 2 NaCl + 5 H2O

Boric acid forms many monomeric esters that are readily hydrolyzed in water. A reaction of formation is shifted to the right by using an excess of an alcohol and removement of water by azeotropic distillation with hydrocarbon solvents [4]. In the case when azeotrope is formed between the alcohol and the boric acid ester (e.g. trimethyl borate/methanol) the method is complicated. Trimethyl borate B(OCH3)3 is an ester that is prepared by a reaction of boric acid with methyl alcohol in the presence of strong dehydrating agent such as sulfuric acid. The compound has a melting point of -29 °C and it burns with a green flame. It is used as a precursor to a strong reducing agent – sodium borohydride (4 NaH + B(OCH3)3 → NaBH4 + 3 NaOCH3).

H3BO3 + 3 CH3OH → B(OCH3)3 + 3 H2O

Procedure I (boric acid preparation):
Borax powder (38.14g, 100 mmol) was dissolved in distilled water (50 mL). It was poured into excess of HCl solution (10 M, 25 mL). After the reaction ended up, the solution was placed into an ice-bath and recrystallized. The recrystallization was repeated several times to get a rid of the side product (NaCl). Yield: 95,66%.


Boric acid [photo: Juraj Kmotorka]

Procedure II (Trimethyl borate preparation):
Boric acid (2.54 g, 41 mmol) was dissolved in methyl alcohol (20 mL, 15.84 g, 495 mmol ) and two drops of 96% sulfuric acid was added. A mixture was stirred for ten minutes and then poured into a crucible. The mixture containing a product – trimethyl borate – was ignited and burnt in the air.

trimethyl borate

Trimethyl borate [photo: Michal Hegedus]


[1] Mineral Data publishing. Retrieved from:
[2] Ingri N.  Acta Chem. Scand., 16, 439 (1962).
[4] Brotherton, R. J., Weber, C. J., Guibert, C. R. and Little, J. L. 2000. Boron Compounds. Ullmann’s Encyclopedia of Industrial Chemistry.