Analytical Chemistry of Fluorine and Fluorine-Containing Compounds

CHAPTER 3 Analytical Chemistry of Fluorine and Fluorine-containing Compounds BY PHILIP J. EL VINO University of Michigan CHARLES A. HORTON Carbide & Carbon Chemicals Company, Plant Oak Ridge, Tennessee AND HOBART H. WILLARD University of Michigan Page Introduction 53 I. Sampling of Fluorine-containing Materials 54 II. Analysis of Gaseous Samples 55 A. Determination of Fluorine 55 B. Determination of Hydrogen Fluoride 56 C. Determination of Other Gaseous and Volatile Inorganic Fluorides 57 D. Determination of Volatile Organic Fluorides 58 E. Analysis of Fluorine Gas 59 F. Analysis of Electrolytic Cell Gases 61 1. Gas Analysis 62 2. Molecular Weight Determination 65 3. Fluorocarbon Analysis 67 III. Separation and Isolation of Fluorine 67 A. Decomposition, Dissolution, and Other Preliminary Treatment of In organic Materials 68 1. Ashing Procedures 68 2. Fusion Procedures 71 3. Evaporation Procedures 73 B. Decomposition of Fluorocarbons and Organic Compounds 74 1. Oxidation Methods 76 2. Reduction Methods 78 3. Methods Involving Alkaline Fusion 81 4. Methods Involving Reaction with Silicon Dioxide 81 5. Hydrolytic Methods 83 C. Isolation of Fluoride by Volatilization 83 1. Distillation As Fluorosilicic Acid 83 2. Pyrohydrolysis: Evolution As Hydrofluoric Acid 89 3. Miscellaneous Volatilization Methods 90 51 52 PHILIP J. ELVING, CHARLES A. HORTON AND HOBART H. WILLARD Page IV. Qualitative Detection and Identification of Fluorine 90 A. Fluoride Ion and Fluorine-containing Compounds 92 1. Etching and Hanging Drop Tests 92 2. Bleaching of Zirconium-alizarin and Similar Lakes 94 3. Miscellaneous Color and Fluorescence Tests 96 4. Precipitation Tests 97 5. Microscopic Tests 98 6. Miscellaneous Tests 99 B. Fluorine in Organic Compounds 100 C. Fluorine in Gaseous Samples 102 V. Determination of Fluoride Ion 102 A. Precipitation and Gravimetric Determination 102 1. As Lead Chlorofluoride 102 2. As Calcium Fluoride ! 104 3. As Rare Earth Metal and Other Metal Fluorides 108 4. Miscellaneous Gravimetric Methods 109 B. Titrimetric Methods: Precipitation and Complexation 110 1. Thorium Titration HO 2. Zirconium Titration 117 3. Titration with Iron (III) and Aluminum (III) 118 4. Titration with Cerium (III) and Rare Earth Metal Ions 119 5. Indirect Titration of Lead Chlorofluoride and Calcium Fluoride 121 C. Titrimetric Methods: Neutralization 123 1. Reactions Involving Fluorosilicic Acid and Potassium Fluorosilicate 123 2. Miscellaneous Neutralization Titrations 127 D. Electrometric Methods 128 1. Potentiometric Titration 128 2. Conductometric Titration 131 3. Amperometric Titration 132 4. High-frequency Oscillator Titration 133 5. Polarography 134 E. Photometric Methods 134 1. Colorimetric Methods Involving Bleaching 134 2. Miscellaneous Colorimetric Methods 147 3. Fluorometric Methods 149 4. Nephelometric Methods 150 5. Emission Spectroscopy 152 F. Miscellaneous Methods 155 1. Enzymatic Activity Inhibition 155 2. Etching and Wettability of Glass 156 3. Catalytic Activity 157 VI. Fluorine Compounds 157 A. Spectrophotometric Technics 157 1. Infrared Absorption and Raman Scattering 157 2. Ultraviolet Absorption 159 B. X-Ray Diffraction Patterns 16° C. Miscellaneous Methods Based on Measurement of Physical Properties. 161 D. Analysis of Fluorocarbons 163 1. Decomposition to Obtain Fluoride Ion 163 ANALYTICAL CHEMISTRY OF FLUORINE 53 Page 2. Determination of Constituents Other Than Fluorine 164 3. Determination of Fluorocarbons As Compounds 168 E. Assay and Analysis of Hydrofluoric Acid and Hydrogen Fluoride 170 VII. Determination of Fluorine in Specific Materials 171 A. Biological Samples 172 1. Plants 172 2. Animals 173 B. Fertilizers, Phosphates, and Phosphate Rocks 173 C. Foods and Beverages 174 D. Rocks, Minerals, and Ores 174 E. Water 175 F. Air 176 G. Miscellaneous 176 Bibliography 177 Introduction The analytical chemistry of fluorine is very different from that of the other halogens. This is emphasized by the differences in comparative solubilities of metallic salts, e.g., silver fluoride is very soluble while cal cium fluoride is sparingly soluble. As might be expected, the analytical chemistry of fluorine is dominated by those properties of fluorine which differentiate it from the halogens and other elements. An important factor in the detection, separation, and determination of fluorine is the volatility of silicon tetrafluoride and its ready formation in the presence of dehydrating acids. The unique reaction between hydrofluoric acid, and silica and silicates, resulting in the property of hydrofluoric acid of etching glass, is also of prime importance in the detec tion and determination of fluorine. Other factors of importance in the development of analytical methods for fluorine include the stable com plex species formed by fluoride with aluminum, iron, thorium, titanium, and zirconium ; the volatility of many fluorine-containing compounds such as the fluorides of boron, hydrogen, and silicon; and the usually amor phous or gelatinous nature of the comparatively few insoluble inorganic fluorine compounds. Fluorine is usually best separated from other possibly interfering ele ments by steam distillation as fluorosilicic acid from a perchloric or sul furic acid solution. In many cases the sample must be ashed or subjected to a fusion process as a necessary preliminary to the distillation step. The recovery of fluoride ion from organic compounds is usually difficult, owing to the great stability of the carbon to fluorine bond when additional fluorine or chlorine atoms are linked to the same carbon atom. The decomposition of the organic compound may require such drastic attack as high temperature fusion with an alkali metal. 54 PHILIP J. ELVING, CHARLES A. HORTON AND HOBART H. WILLARD The analytical chemistry of fluorine has received tremendous impetus in recent years as a consequence, to cite only a few causes, of the demands of the atomic energy program for both inorganic and organic fluorine- containing compounds, the role of fluorine in dental caries with the conse quent demand for the determination of minute amounts of fluorine in water, and the importance of fluorine in insecticides and in agricultural raw materials and products in general. A moderate number of reviews of the analytical chemistry of fluorine or of special subdivisions of it have appeared. The latter are usually referred to in the appropriate section of the discussion. The most compre hensive reviews which have appeared recently are probably those of Hernler and Pfeningberger in 1938 (H58), Wilson in 1944 (W63), Rinck in 1948 (R37), Kurtenacker in 1949 and 1951 (K73 and K74), Element in 1950 (K39), the report published in 1950 on the analytical chemistry of the Manhattan Project of World War II (B122, M28), and McKenna in 1951 (M27). Other reviews and bibliographies are given in references B3, B15, E14, E21, F66, F67, F68, F72, G33, M28, M67, M100, N18, Rl, and W27. I. Sampling of Fluorine-containing Materials The technics and precautions necessary in sampling various types of fluorine-containing materials can be readily located from the references given in Section VII for the different specific classes of substances as well as in other sections, e.g., electrolytic cell gases in Section II-F. The main factor to be normally considered in the sampling, handling, and preliminary treatment of fluorine-containing substances is the possi ble loss of fluorine through the formation of hydrofluoric acid, which would volatilize as HF or react with any silica or silicate present, such as the glass of the container, to form volatile silicon tetrafluoride. The volatility of boron and other fluorides is another possible source of fluorine loss. In any preliminary treatment involving heating or evaporation of an acidic solution, the possibility of HF loss should be considered. In the preliminary separation of the R203 group by precipitation in ammoniacal solution fluorine may be lost if calcium is present due to the coprecipita- tion of calcium fluoride (F73). The drying of the sample itself may result in some fluorine loss if the water can react with the sample at the drying temperature to form volatile fluorides. Chapman, Marvin, and Tyree (C47) evaporated at 200° mixed hydro fluoric and perchloric acid solutions containing compounds of some thirty-seven different elements. Losses due to volatilization of the fluorides of the elements were found in varying degree for boron, silicon, ANALYTICAL CHEMISTRY OF FLUORINE 55 germanium, arsenic, antimony, chromium, selenium, manganese, and rhenium. No loss was observed for compounds of the metals of the first two groups: lanthanum, cerium, titanium, thorium, tin, lead, vanadium, bismuth, molybdenum, tungsten, uranium, iron, cobalt, and nickel. II. Analysis of Gaseous Samples A. DETERMINATION OF FLUORINE The assay and analysis of fluorine gas as well as the analysis of fluorine-rich gaseous mixtures are discussed in subsequent subsections of the present section which deal with the analysis of fluorine gas and of the cell gases produced in the electrochemical process for making fluoro carbons. The detection of fluorine in gaseous samples is considered in Section IV. The detection and determination of fluorine and fluorine compounds in air is discussed in Section VII-F. Air samples are generally taken so as to separate particulate fluorides from the gaseous fluorides. Generally, electrostatic precipitators are used to remove the solids and the gases are absorbed in caustic solution using an impinger or gas scrubber (A18,

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