Determination of Fluorine

Abstract

H E determination of fluorine in most materials requires T its separation from other constituents of the sample.For this separation volatilization (or distillation) methods, being relatively rapid, have been favored whenever the fluorine-bearing constituents were readily soluble in acid, whereas fusion with alkali compounds with subsequent separation of the fluorine by rather cumbersome precipitation methods has been the rule with acid-insoluble materials. The chief carrier of fluorine in natural phosphates is apatite ( l a ) , and calcium fluoride has been identified in a few instances as a constituent of phosphate rock. Fortunately, both compounds can be treated successfully by distillation methods. On the other hand, fusion with alkali fluxes is not very effective in decomposing apatite (2%').The volatilization method (65, 27) used prior to about 5 years ago was not entirely satisfactory. One of the principal difficulties arose from the interfering effect of gelatinous zirconium salt from the indicator used for titrating very small amounts of fluorine. Because of the greater sensitivity of the simplified indicator to changes in the pH of the solution during titration, it was not satisfactory for the titration of several milligrams of fluorine in a slightly buffered solution (20), such as was used a t that time. Later, the advent of the use of a buffer solution (16) to control the pH during titration made the addition of a zirconium salt to the indicator unnecessary for the titration of the larger quantities of fluorine. The almost constant attention required to maintain the temperature of distillation within the permissible range by the frequent additions of small quantities of water to the distilling flask was overcome by means of a form of steam distillation (24).With the foregoing improvements the Willard and Winter method has been used in this laboratory on a wide variety of natural phosphates and phosphatic products. A number of silica, which renders the method inapplicable to materials that contain acid-decomposable silicates (23). Furthermore, the method of Reynolds and Jacob @a), involving fusion and acid extraction of the sample with subsequent precipitation of the fluorine as lead chlorofluoride, was very tedious and gave low results on some types of phosphate rock, such as Tennessee blue rock and Florida waste-pond phosphates.The publication of a volumetric method by Willard and Winter (28) in 1933 marks a great forward step in the analysis of fluorine-containing materials. I n simplicity of requisite a p p a r a t u s , ease of manipulation, speed, and quality of results, this is far superior to any other known method for the analysis of a wide variety of materials.I n the original procedure fluorine is (1) separated from the sample (fused with alkali carbonate if the fluorine compounds are not decomposed by acid) by distillation with sulfuric acid or perchloric acid and (2) determined in the neutralized distillate, after the addition of an equal volume of ethyl alcohol, b y t i t r a t i o n with ...

Titrations with Photoelectric Titrimeter

“…Later Armstrong (1) and Rowley and Churchill (21) both found the color change sharper in aqueous than in 50% alcoholic solution wdth a pH from 2.9 to 3.1. Reynolds and Hill (19) confirmed these indicator results and found that the concentration of indicator, the fluoride concentration, and the temperature also affected the titration. They also reported a small blank in aqueous solution which increased with the volume.…”

“…of fluoride solution. This is about twice that used by Reynolds and Hill (19) and Matuszak and Brown (14)• The use of 10 drops necessitated the diminishing of the light intensity to prevent fluctuations of the galvanometer and 20 drops broadened the maximum in the curve of dial readings versus volume and caused the results to be farther removed from the stoichiometric values, as also found by Reynolds and Hill.…”

Interaction of Fluoride Ions and Ground Glass

“…In the determination of the fluorine content of dilute solutions, an evaporation may be necessary, either to decrease the volume and facilitate separation of fluorides, or to bring the concentration within the range of the colorimetric or volumetric method that is to be used. Reynolds and Hill (7) were probably the first to report loss of small amounts of fluoride ions during the evaporation of slightly alkaline solutions. A solution alkaline to phenolphthalein was evaporated in glassware; it was noted that, when an original volume of 150 ml.…”

Radiometric Determination of Inorganic Fluoride

“…Sample containing least amount of titrant would have pH as much as 0.2 unit higher. 6 Plotted in Figure 3 6 pH of titrant, 2.72. However, the data follow a straight line which has a different slope from the ideal line.…”

“…Sample containing least amount of titrant would have pH as much as 0.2 unit higher. 6 Plotted as circles in Figure 4. * Plotted as squares in Figure 4.…”