Abstract
The simultaneous determination of fluoride and chloride in calcium halophosphate phosphors was studied by a direct potentiometry by using fluoride and chloride ion-selective electrodes.
In order to isolate the halide components from a phosphor, it was dissolved and steam distilled in the presence of phosphoric acid. The phosphor sample could be easily dissolved during the distillation. Test solutions for potential measurements were prepared from the distillates by adjusting the pH and ionic strength to appropriate values.
Two different buffer solutions were tested to minimize the errors in the measurements of the electrode potential. One was a modified constant ionic strength buffer solution prescribed by Elving {Anal. Chem., 28, 1179 (1958)}, the other was a modified TISAB based on Frant's prescription {Anal. Chem., 40, 1169 (1968)}. The results showed that the latter was better than the former in the electrode response and reproducibility of the electrode potentials. The linearity of the calibration curve for the fluoride ion was affected by the conditioning of the ion-selective electrode. The procedure established is as follows: A 300 mg sample of calcium halophosphate is put into a distillation flask and a small amount of water is added to disperse the sample. Then 30 ml of phosphoric acid is added. After assemblage of the apparatus, the halides are steam distilled at 145150°C. The distillate is stored in a graduated cylinder, in which about 10 ml of water is placed beforehand. The 150 ml of the distillate obtained is transferred into a 200 ml volumetric flask, and 40 ml of modified TISAB (97 g of sodium nitrate, 17 g of glacial acetic acid, 116 g of sodium acetate and 0.3 g of sodium citrate/1 liter) is added and diluted to 200 ml, the pH and ionic strength are adjusted to 5 and 0.4, respectively.
The test solution is stirred magnetically with a rotating teflon encapsulated bar. The fluoride and chloride electrode potentials, vs. a silver-silver chloride reference electrode with a saturated potassium nitrate salt bridge, are measured simultaneously after 15 minutes by using an electrode switch. Both fluoride and chloride can be determined by the ordinary calibration curve method. Approximately 100 minutes were required to determine the halides by the above mentioned procedure. Average recoveries for the synthetic fluoride and chloride samples were 100.0% and 100.3%, respectively. The relative standard deviations were 2% at the 3% fluoride level and 1.2% at the 0.3% chloride level in the phosphor samples.
