[Determination of boric acid and silicic acid in mineral water by nonsuppressed ion chromatography].

Abstract

Boron and silicon are widely distributed in nature; in water, these compounds typically present in the forms of boric acid and silicic acid, respectively. The maximum allowable levels of silicic acid and boric acid in water are stipulated in relevant national and industry standards, such as GB 8538-2022. Quality changes in water, which are of great significance in water-quality evaluations, can be understood in terms of its silicic acid and boric acid contents. Boric acid content is usually determined by ion exclusion chromatography, whereas silicic acid content is usually determined by postcolumn derivatization. Therefore, traditional methods cannot achieve the simultaneous determination of silicic acid and boric acid contents in water. Modern ion chromatography has been widely used in the detection of ionic compounds, such as anions, cations, organic acids, organic amines, amino acids, and sugars. Boric (pK_a=9.24) and silicic (pK_a=9.77) acids are weak acids that dissociate into ionic states under alkaline conditions. Although these compounds cannot be tested using suppressed ion chromatography, they can be retained on ion chromatography columns. In this study, a method based on nonsuppressed conductance detection was established for the simultaneous determination of boric acid and silicic acid in water. The contents of boric acid and silicic acid were detected by nonsuppressed ion chromatography using a Dionex IonPac^TM AS20 analytical column. The chromatographic conditions were as follows: flow rate, 1.0 mL/min; column temperature, 30 ℃; eluent, 6 mmol/L sodium hydroxide solution and 60 mmol/L mannitol; and sample injection volume, 50 μL. The effective separation of silicic acid and boric acid was achieved within 8 min. SiO₃^2- and boric acid demonstrated good linear relationships in the concentration ranges of 0.25-100 and 0.5-100 mg/L (correlation coefficients, 0.9999), respectively. The method detection (MDL) and quantification (MQL) limits were 0.078 and 0.26 mg/L for SiO₃^2-, and the MDL and MQL limits were 0.18 and 0.60 mg/L for boric acid. The average recoveries of boric acid and SiO₃^2- (n=6) were 97.3%-105.3%. Moreover, the relative standard deviations were less than 0.9% for boric acid at four spiked levels and less than 0.30% for SiO₃^2- at three spiked levels. Thus, the method meets detection requirements. The pretreatment method is very simple, and the sample can be directly injected through a 0.22 μm water filtration membrane and into the column. The boric acid and silicic acid contents in nine mineral drinking water samples were determined under the optimized analytical conditions. Boric acid was not detected in these nine samples, but silicic acid was detected in six samples. The silicic acid contents detected were between 18.70 and 62.08 mg/L, which was consistent with the concentration ranges marked on the manufacturers' packaging. The proposed method can be used for the determination of boric acid and silicic acid in mineral drinking water and laboratory water, and provides a reference for the simultaneous detection of boric acid and silicic acid in ultrapure water used in the semiconductor industry.