Determination of fluorine in fused salt by decoloration photometry in hydrogen peroxide-titanium(IV)-fluoride system

In sulfuric acid medium at pH3.2-3.5,Ti(Ⅳ) reacted with fluorinion to form H2TiF6,resulting in the decoloration of yellow complex(TiO·H2O2)SO4 formed by Ti(Ⅳ) and H2O2.In addition,the decoloration degree was linear to the content of fluorinion.In view of the above,a decoloration photometry for the determination of fluorine content in fused salt was proposed.The sample was fused with mixed flux(zinc oxide,sodium carbonate and quartz sand).After leaching with water and filtration,proper amount of solution was sampled.Titanium dioxide solution was added,and the pH was adjusted with sulfuric acid.Then,the hydrogen peroxide solution was added.After reaction for 15 min,the absorbance was measured at 410 nm.When the fluorinion amount was in the range of 0.5-6 μg/mL,the calibration curve showed linearity,with correlation coefficient of r=0.999 6.The detection limit of method was 0.35 μg/mL.For 0.5 μg/mL fluorinion,5-fold of SO42-,10-fold of Na+,and 20-fold of K+ and Cl-had no interference.Ca2+,Fe3+ and Mg2+ in fused salt sample had been precipitated and separated after fusion,so they did not interfere in the determination.The fluorine content in two fused salts from magnesium electrolysis plant of sponge titanium factory was determined by the proposed method.The results were consistent with those obtained by fluorine ion-selective electrode method and polarography.The relative standard deviation(RSD,n=5) was 2.2 % and 4.2 %,respectively.