A procedure for the micro determination of fluorine

Abstract

A method of the micro determination of fluorine based on the bleaching effect of the fluorine ion towards the thorium lake of the anthraquinone dyes sodium alizarin sulphonate and alizarin Bordeaux is described. This procedure is not concerned with the determination of more than 100 micrograms of fluorine per 200 ml. distillate. 1. Neutralization of the acid distillate is avoided, the total acidity in both standards and samples being controlled equivalent to 2 ml. 0.05 N acid (hydrochloric or perchloric acid) per 43 ml. volumes. The pH of the medium is 2.74. 2. The need for controlling the indicator concentration at a constant throughout the series is emphasized by the variations observed in the titer of the thorium solution, at varying indicator concentrations. 3. A sodium fluoride of satisfactory purity may be prepared by the method of Reynolds and Hill (1939). 4. The use of a standard sodium fluoride solution for the determination of 20 micrograms of fluorine as silico fluoride may be accomplished at a pH 2.74 without the use of a buffer. 5. The use of sulphuric acid as final volatilizing agent cannot be recommended. Even under the most favourable conditions plus errors are caused due to the presence of the sulphate ion. 6. Blank distillates collected from perchloric acid exhibit a bleaching effect towards the alizarin compound. This bleaching effect is not due to fluorine, as added fluorine results in a decrease of this plus error. Neither is this apparent fluorine value due to partially volatilized perchloric acid. The magnitude of the apparent fluorine value is not influenced by treating the perchloric acid at 160°C. prior to distillation. 7. The introduction of the sodium ion is manifested by plus errors when the concentration of sodium chloride reaches 4.095 x 10⁻³ grams per 43 ml. in the absence of fluorine and 2.925 x 10⁻³ grams per 43 ml. in the presence of 10 micrograms of fluorine. The corresponding concentrations for sodium perchlorate are 3.063 x 10⁻³ and 1.839 x 10⁻³ grams per 43 ml. respectively. 8. The evaporation of the alkaline distillate in glass leads to a minus error of variable size. Quantitative recoveries are observed when evaporation is effected in platinum. 9. A method for the complete ashing of blood at low temperatures is described and the subsequent fluorine determination on the ash may be duplicated with a maximum error of 5 per cent . A low recovery of added fluorine to whole blood was encountered, presumably clue to the formation of acid resisting iron-fluorine complexes on ashing. Blood iron converted to ferric sulphate has no inhibitory effect on the volatilization of added fluorine.