ELABORATION OF CHEMICAL SENSOR SENSITIVE TO BORON BENZILATE
Authors: Fershal M. , Chubirka N. 
Published: May 2023

Over the last decades, the development of simple and selective methods for the determination of organic analytes has been getting attention from analytical chemists. Particularly, the same applies to biologically active substances of natural and artificial origin, precursors, and metabolites of pharmaceuticals[1]. An example of such substance is benzylic acid (2, 2-diphenyl-2-hydroxyacetic acid) (BA), gravimetric reagent for zirconium [2], a product of hydrolysis and a metabolite of the antispasmodic and sedative drugs Pipoxolan, Benactyzine, or Metacin [3]. The complexes of BA have been proposed as corrosion inhibitors for some steels and aluminum alloys [4]. Its derivatives are being studied as new analgesics [5], anticancer drugs [6], and also as deep eutectic solvents [7]. The BA was used in the synthesis of the psychotropic combatant substance 3-quinuclidinyl benzilate (BZ), which causes mental slowing, and hydrolyzes to form BC [8-11]. For BA determination high-performance liquid chromatography and chromatography-mass spectrometry [12-15] are mostly used, hence the development of simple methods for determining benzylic acid is pertinent and relevant.

Boric acid is known to form compounds with diols, α-hydroxy acids, dicarboxylic acids, and α-diketones [16] and that was used in developing potentiometric sensors for the determination of mandelic acid and vitamin C [17, 18]. As BA also forms complexes with boric acid and various metals [19, 20] the H3BO3 has been used in the present study as a reagent for obtaining boric acid benzilate ester - the analytical form of BA, as well as the ionic associate of its anion with tetraoctylammonium(TOA) as the active substance of a potentiometric PVC plasticized sensor sensitive to the mentioned above analytical form. The complex formation has been confirmed using IR spectroscopy. On the IR spectrum of BA, the absorption bands in the region of 3393. 69 cm-1 and 1715. 77 cm-1, which correspond to the vibrations of –ОН and >С=О groups, respectively have been detected [23]. At the same time, in the IR spectrum of the complex, the absorption band of –ОН groups in the region of 3300 - 3400 cm-1 was absent, that indicates their participation in the formation of ether bonds, additionally, the intensive bands of >C=O in the region of 1723. 41 cm-1 and 1742. 05 cm-1 has appeared. The ion pair of tetraoctylammonium cation with synthesized complex ion [B(Benz)2]- has been used as an active substance of the potentiometric sensor. The metrological and operational characteristics of the sensor and the optimal conditions of the analytical form obtaining have been systematically studied. The optimized composition of the sensitive PVC membrane was 0. 1% of IP, 33. 0% of PVC and 66. 9% of o-NFOE. For high yield [B(Benz)(ОН)2]--analytical form preparation the boiling of BA for 15 minutes in the medium of a buffer solution that consists of H3BO3 (sat.) - 0. 1 M NaOH - 0. 1 M H3PO4 with a pH of 5. 5 is needed. In optimal conditions, the slope of the sensor developed is close to the theoretical value of 55. 2 mV/pС, has a detection limit pCmin= 4. 9, and the linearity range 10-2 – 10-4mol/l. The response time of a sensor ranges from 5 to 30 seconds depending on the added quantity. High selectivity of the sensor to metal cations, anions of organic and inorganic acids, as well as polyatomic alcohols that are able to form complex compounds with BA or H3BO3 has been shown. The sensor developed was used for benzylic acid analysis in "Difenin", which can be formed during the synthesis of this drug [27].

It has been shown that the use of potentiometry in combination with the conversion of the analyte into an active form using a cheap reagent - boric acid allows to develop simple methods of the organic analytes determination for which the use of potentiometry is not common. Preparation of the complex boronate ester of benzylic acid in the sample provides the selectivity of the technique to the target analyte.

Keywords: benzylic acid; boron benzilate; potentiometric sensor; ion pair.