Determanation of PF₆^- in Environmental Water by Omim₆Mo₇O₂₄ Carbon Paste Ion Selective Electrode

A self-madeOmim₆Mo₇O₂₄ carbon paste ion selective electrode (ISE) was used to determine the content of PF₆^- in water. The solid ionic liquidOmim₆Mo₇O₂₄ was prepared by 1-octyl-3-methylimidazolium tetrafluoroborate and ammonium molybdate, and characterization results from FT-IR and XRD showed that solid ionic liquidOmim₆Mo₇O₂₄ was successfully prepared. The TG analysis showed that the ionic liquid had good thermal stability in the range of 28-280℃. After mixing the solid ionic liquidOmim₆Mo₇O₂₄ powder with graphite powder with the mass ratio of 1:9, the above mixed powder was mixed with dimethyl silicone oil with the mass ratio of 6:1 to prepare carbon paste ISE. The content of PF₆^- in water was determined at 30℃ with carbon paste ISE as indicator electrode and Ag/AgCl electrode as reference electrode in the mixed solution composed of working buffer solution (containing boric acid, citric acid and sodium phosphate, pH 9) and Na₂EDTA solution with volume ratio of 1:1. The results showed that the linearity relationship between the negative logarithm of PF₆^- concentration and carbon paste ISE potential response was kept in the range of 1.0×10^-8-1.0×10^-1mol·L^-1, with detection limit of 5.4×10^-9mol·L^-1. The recovery of PF₆^- obtained was more than 95.0%, and the RSD (n=5) of the determined values was less than 2.5%. Three groups of PF₆^- spiked blind samples were analyzed by the proposed method and ion chromatography from reference, obtaining a consistent result. When using this method to analyze the environmental water, due to the easy hydrolysis of PF₆^-, no residue of PF₆^- was detected. Therefore, this method was used to investigate the hydrolysis process of PF₆^- in water with a normal acidity range (pH 6-8), finding that the half-life of PF₆^- was in the range of 0.426-0.444 h, and the acidity changes of conventional water did not affect the hydrolysis of PF₆^-.