Flow-injection determination of boron by using poly(vinyl chloride) membrane-based tetrafluoroborate ion-selective electrode

Tetrafluoroborate (BF₄^-) ion-selective poly(vinyl chloride) (PVC) membrane electrodes based on hydrophobic quaternary ammonium ions are described. Dioctylphthalate (DOP), dioctyladipate, dioctylsebacate and o-nitrophenyl octyl ether were examined as a plasticizer of the PVC membrane. Three quaternary ammonium ions, tridodecylhexa-decylammonium (TDHA), tetradodecylammonium ion and trioctylmethylammonium ion were evaluated as an anion exchanger for the electrode. The BF₄^- ion-selective PVC membrane electrode containing TDHA and DOP shows the best sensitivity to the BF₄^-ion; a linear response range from 10^-1 to 3×10^-6 M with the slope of 58mV/decade. Effects of pH and hydrofluoric acid on the potential of the BF₄^- ion-selective electrode were examined. Interference caused by HF₂^- ion present in solutions with a high concentration of HF was eliminated by adjusting the pH of the sample solution at about 6 adding a disodium hydrogenphosphate solution. The selectivity coefficients of the elec-trode for anions were determined by the mixed solution method. This electrode was ap-plied to monitoring the converting reaction of boron (a Na₂B₄O₇ solution was chosen as the boron sample) into the BF₄^- ion with hydrofluoric acid. The apparent first-orderreaction constant of the above reaction was determined to be 0.039±0.006 (s^-1) at 50°Cin the 1.0 M HF solution. A flow injection system with three-channels was constructed for the determination of boron utilizing the BF₄^- ion-selective electrode detector and a stream of the HF solution. The sample solution containing Na₂B₄O₇ was injected into a carrier stream (water) and merged with a stream of the 1M HF solution and then the B(OH)₄^- ion was converted into the BF₄^- ion in a reaction coil (10m×0.5mm i.d., 80°C). Subsequently, the mixed stream was merged with a mixed solution of 0.5M Na₂HPO₄ and 0.5M Na₂SO₄ in order to neutralize the acidity and to keep the ionic strength of the solution constant. About 95% conversion was achieved within 2 min of residence time in the reaction coil at 80°C. The relationship between the peak height and the logarithmic concentration of Boron. A sampling rate of 20 samples/h was achieved with a relative standard deviation of 1.8 % (0.1 ppm-boron, n= 10). The effect of diverse ions on peak height signal for boron was examined and the interference with Al, P and Si was almost negligibly small but nitrate ion interfered.