Determination of Fluorocarbons As Compounds

The identification, characterization, and determination of specific fluorocarbons is a matter that presents considerable analytical difficulty.

Infrared absorption spectroscopy has proved to be the most useful in dealing with mixtures of fluorocarbons, especially of those containing isomeric species which boil at about the same temperature and, of course, have the same elemental composition. Libraries of the spectra of various fluorocarbons have been prepared by various industrial concerns and

governmental installations; such information is slowly being made available.

Examination of the infrared spectra of about seventy fluorohaloear-bons has shown that the frequency range of the fundamental C—F vibra­

tion runs between 1080 and 1115 c m . -1

(9.3 to 9.0 ě); in multifluoro, multiearbon atom molecules, the range is somewhat greater (E4).

Infrared and Raman spectra of various fluorine-containing ethylenes and other hydrocarbons have been examined from the viewpoint of the assignment of frequencies. Further discussion of absorption spectro­

photometry technics will be found in the section on photometric methods for determining fluorine.

Some mass spectra (cracking patterns) of fluorine-containing com­

pounds have been accumulated in industrial laboratories, but, unfor­

tunately, little of note from the analytical viewpoint has yet been published.

Traces of fluorocarbons in air can be detected by aspirating the air through a platinum tube, packed with platinum and heated to 950°, and then over a moist piece of thorium-alizarin or zirconium-alizarin im­

pregnated paper (K29). On reaction with water, the pyrolysis fragments form H F , which causes the paper to change color. The sensitivity of the technic is 1 p.p.m. of C8^{F i}6 by weight in air in less than 1 minute, or 100 p.p.m. in 10 seconds.

The use of one of the Freons, izns?/ra-difluorotetrachloroethane, as solvent in the cryoscopic determination of the molecular weights of fluoro­

carbons has been suggested; the molecular freezing point depression constant was 37.6 ± 0.4 (B47, L41). Few data were, unfortunately, pre­

sented. A modified Victor Meyer method has also been used in determin­

ing the molecular weight of fluorocarbons and gave results correct to within 1 or 2 % (B18). The Bratton and Lochte modification of the Victor Meyer vaporimetric method gives good results for the microscale molecu­

lar weight determination on liquid organic compounds containing fluorine; the Rast micro cryoscopic procedure, using s//m-tetrachlorodi-fluoroethane as a substitute for camphor, is suitable for solids (J11 ).

Benzotrifluoride has also been suggested as a cryoscopic solvent for organic compounds containing fluorine (H13) ; the freezing point constant is 4.90 ± 0.05.

The preferred method of determining the molecular weights of fluoro­

carbons and fluorocarbon derivatives is by vapor density measurement.

The Victor Meyer method is much less satisfactory than the use of gas density balances. Methods based on the latter are particularly useful and precise as a result of the high density of fluorocarbon vapors. Suitable vapor density balances have been described by Simons and coworkers for

the determination of the molecular weight of compounds boiling below 100° (S70) and for that of compounds boiling above 100° (S75); the precision of measurement is ± 1 % or better.

Fowler, Burford, and coworkers (F53, F54) have described the follow­

ing analytical procedures used to characterize pure fluorocarbon mate­

rial : (a) molecular weight determination by Victor Meyer vapor density method (accuracy of ± 2 %) ; (b) molecular weight determination of high-boiling compounds by the freezing-point depression method using unsym-difluorotetrachloroethane (B47) ; (c) refractometric method as an indirect check on the molecular weight determination by comparing the experi­

mentally determined specific refractivity with that calculated on the basis of an assumed formula; (d) determination of fluorine by decomposi­

tion with metallic potassium (E20) and modified thorium titration of the fluoride ion.

E. ASSAY AND ANALYSIS OF HYDROFLUORIC ACID AND HYDROGEN FLUORIDE

The manufacturing Chemists Association has recommended a set of methods for the analysis of anhydrous hydrofluoric acid, i.e., liquid hydrogen fluoride (S53). The precautions and care necessary in the sampling procedures are discussed in considerable detail. The design and use of sampling cylinders and sample weighing tubes are explained. In addition to the description of sampling technics, the following analytical procedures are given: (a) sulfur dioxide, by reaction with an excess of a standard iodine solution (iodate-iodide solution) and back-titration of the excess iodine with thiosulfate; (b) total acidity, by reaction with stand­

ard sodium hydroxide; (c) hydrofluorosilicic [fluorosilicic] acid, by neu­

tralization to phenolphthalein with standard sodium hydroxide, first in the cold and then continuing at about 100°; the volume used in the hot titration is equivalent to the silica ; (d) sulfuric acid, in which sulfuric and fluorosulfonic acids are calculated as sulfuric acid on the basis of the alkali consumption of the residue obtained on evaporation on the steam bath;

(e) water computed as the difference from 100% by subtracting the assay and impurities. The procedures involve many critical details which must be taken into account in order to obtain trustworthy results. These pro­

cedures are based in large part on a study of the sampling and analysis of anhydrous hydrogen fluoride published by Swinehart and Flisik (S131).

Cook and Findlater (C78) have also described a procedure for the analysis of anhydrous hydrogen fluoride which is claimed to be suitable for rapid routine work and to have experimental hazards reduced to a minimum. The experimental simplification is largely due to the use of glacial acetic acid as a diluent for the anhydrous hydrogen fluoride

samples. Sulfur dioxide is determined by reaction with excess standard iodine solution and back-titration with thiosulfate with a correction for the H₂S present; hydrogen sulfide by treatment with cadmium acetate solution and measurement of the precipitated sulfide by the usual iodine-thiosulfate method; and total sulfur by treatment with bromine water and gravimetric measurement as barium sulfate. Total sulfur less the sulfur due to S 0₂ and H₂S equals the sulfur due to sulfuric and fluoro­

sulfonic acids. Silica is determined on a moist residue by dissolution in boric acid solution, addition of ammonium molybdate, and photometric measurement. Water is determined by Karl Fischer reagent in a cyclo-hexene-pyridine solution of the sample. The details, basis, and time requirements of the different determinations are given.

The specifications of the American Chemical Society for reagent chemicals include a set for hydrofluoric acid (48 to 51 % HF) (W34, pages 172-74). Hydrogen fluoride and fluorosilicic acids are determined on the basis of titrations at 0° and 80° with sodium hydroxide. Tests for maxi­

mum permissible amounts of chloride (silver chloride turbidity) phos­

phate (molybdenum blue color), sulfate (barium sulfate turbidity), heavy metals (sulfide precipitation), and iron (thiocyanate color) are made on individual residues. Sulfite limit is ascertained by an iodine test. Tests for reagent grade sodium fluoride are also given (W34, pages 332-33).

Similar procedures for the assay and testing of hydrofluoric acid are given in the U. S. Pharmacopeia (P40).

Laszlo (L25) has recently described a method for the simultaneous determination of hydrofluoric and fluorosilicic acids which is based on the precipitation of K₃A1F₆ (derived from the HF) and K₂S i F₆ and determina­

tion of the aluminum. McKee and Hamilton (M26) have described a pro­

cedure for the control of hydrofluoric-nitric acid stainless steel pickling baths, which includes the determination of total acidity, iron, nitrate, and fluoride. The latter in the range of 1 to 3 g. per 100 ml. is determined by a Willard-Winter distillation and thorium titration.