On-line generation and hydrolysis of methyl borate for the spectrophotometric determination of boron in soil and plants with azomethine-H

A continuous-flow system for boron determination in soils and plants with spectrophotometric detection using the azometihine-H-boron complex method was developed. In order to avoid the interferences of concomitants present in samples and to increase the sensitivity, the element was separated on-line from the matrix by methyl borate generation. For this purpose, a concentrated sulfuric acid sample solution was combined with methanol in 1:3 ratio which produce enough heating for the esterification reaction without external source. Subsequently, the methyl borate produced was stripped by the addition of a nitrogen flow and separated from the bulk solution in a gas-liquid separator to be then hydrolyzed in an ammonium-phosphate buffer solution (pH 6.8). Finally, the new bulk of phases were separated in a second gas-liquid separator and the liquid phase was combined with azomethine-H to form a boron complex for its detection at 420 nm. The effects of a number of possible interferents, both anionic and cationic were evaluated. The most severe depressions were caused by fluoride and potassium for which a concentration of 100 μg ml⁻¹ caused a 5% depression on the signal. A linear response was obtained between the detection limit of 0.05 μg ml⁻¹ (3σ of the blank) and 50 μg ml⁻¹ of boron. The precision (R.S.D.%) for 10 consecutive readings of the same solution (5.0 μg ml⁻¹ of boron) was 2.6%. Recoveries of boron added to the samples before the extraction process were 94, 97, and 101% for soil, fruit tissue, and leaf tissue, respectively. The developed system was applied to the determination of boron in soil, fruits tissue, and leaves tissue of coffee plantations from different towns of Mérida State, Venezuela.