INDUSTRIAL AND ENGINEERING CHEMISTRY
,July 15, 1929
tube and distil until the volume in the distilling flask is reduced to about 40 cc., add 50 cc. more of strong hydrochloric acid and again distil until the volume in the distilling flask is reduced to 40 cc. Wash down the condenser, transfer the contents of the receiving flasks to a liter volumetric flask, dilute t o volume, and mix thoroughly. Pipet a 200-cc. aliquot of this solution into a 500-cc. Erlenmeyer flask, almost neutralize with 50 per cent sodium hydroxide solution, complete the neutralization with sodium bicarbonate, and add about 5 grams excess. Titrate with 0.05 N iodine solution, using starch solution as indicator. Add the iodine solution slowly until a permanent blue color is obtained. The equivalent of 1 cc. of 0.05 N iodine is 0.002874 gram arsenic oxide. The arsenic may also be titrated with 0.05 N bromate solution according to method given on page 47 of “Methods of Analysis, A. 0. A. C.”
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reserved for total fluorine estimation. Dilute to 100 cc. with water, add about 0.1 gram of sodium carbonate, heat to boiling, and determine the fluorine as described in the section on mixtures of water-soluble fluorides and arsenic trioxide. Calculate to fluorine, deduct the fluorine equivalent of the bifluorides and silicofluorides present, and calculate the remaining fluorine to sodium fluoride. I n a private communication the author has been informed by C. M. Smith, of Insecticide Control, Food, Drug, and Insecticide Administration, that the method for bifluoride and silicofluoride is unsatisfactory when bifluoride and silica are present together, owing to the reaction between the two to form silicofluoride. These would probably be present in a mixture of commercial sodium bifluoride and commercial sodium fluoride. Results
Table 11-Quantities of Arsenic Trioxide and Sodium Fluoride Found i n Prepared Mixtures ARSENIC TRIOXIDE
Gram 0.4000 0.2000
0.2000 0.1000
Cram 0.4006 0.2000 0.1994 0,0996
SODIUMFLUORIDE
Gram 0.2000 0,2000 0.1000 0.1000
Gram 0.1982 0.1988 0,0989 0.0984
Method for Determination of Sodium Fluoride, Sodium Bifluoride, and Sodium Silicofluoride
Sodium bifluoride and sodium silicofluoride, being acid in reaction, may be titrated with standard carbonate- and silicafree sodium hydroxide solution (2,s). This titration converts both compounds into sodium fluoride, according to the equations: NaHFs NaOH = 2NaF f Hz0 NazSiFe 4NaOH = 6NaF 4-Si(0H)d TOTAL AcIDITY-Dissohe 0.5 to 1 gram of the sample in 25 cc. of cold water in a 100-cc. platinum dish. Titrate with 0.2 N or 0.1 N carbonate- and silica-free sodium hydroxide solution, using phenolphthalein as an indicator and a platinum rod as a stirrer. When the pink color fades with evident sluggishness, heat to boiling and continue the titration to a permanent pink color. This titration is recorded as total acidity, due to bifluorides and silicofluorides. After the titration is complete, transfer the solution to a 200-cc. volumetric flask, dilute to mark, and reserve for determination of total fluorine. BIFLUORIDES-weigh 0.5 gram of the sample into a loo-cc. platinum dish, add 1 gram of solid potassium chloride, and dissolve in 25 cc. of water. Add an equal volume of neutral alcohol and cool in an ice bath to as near 0” C. as possible. Titrate with 0.2 or 0.1 N carbonate- and silica-free sodium hydroxide solution, keeping the platinum dish in the ice bath, using phenolphthalein as indicator, until the red color remains for one minute. To insure accuracy, titrate slowly, so that the temperature of the mixture does not rise appreciably, and avoid excessive dilution by the standard alkali. If more than 15 cc. of standard hydroxide solution are required, repeat the titration, using either a smaller sample or stronger standard sodium hydroxide solution. This titration is calculated to sodium bifluoride. SODIUM SILICoFLuoRIDE-Deduct the equivalent quantity of sodium hydroxide solution due to bifluorides, as determined above, from the titration previously recorded as “total acidity.” The result is the quantity of standard sodium hydroxide solution equivalent to the silicofluoride present. TOTAL FLUORINE-Pipet into a 250-cc. beaker an aliquot containing the equivalent of about 0.25 gram of sodium fluoride from the 200-cc. volumetric flask containing the solution
+
+
Owing to difficulty in obtaining a pure bifluoride, mixtures containing known quantities of ingredients were not used. However, mixtures of insecticides containing these three constituents were obtained in the open market and analyzed in duplicate. The results shown in Table 111 are, therefore, probably not free from the errors indicated by Mr. Smith. Table 111-Analytical Results NaHFz NazSiFs Per cent Per cent 1 Sold as sodium fluoride 0.65 4.12 0.65 4.10 0.25 2 Sold as c. P. sodium silicofluoride 96.63 0.25 96.78 3 Sold as sodium bifluoride 28.87 9.72 28.62 9.81
NaF Per cent 94.22 94.04 0.85 0.79 60.27 60.39
If more than 2 or 3 per cent of silicofluoride is present, the silica must be removed before precipitating the fluorine as calcium fluoride. This is done by the use of ammonium carbonate, as described by Scott in “Standard Methods of Chemical Analysis,” 4th edition, Vol. 1, p. 215, or by Treadwell-Hall in “Analytical Chemistry,” 1st ed., Vol. 11, p. 372. Notes
Other substances sometimes present with fluorine, and interfering with its determination, are calcium and barium compounds and phosphates. If calcium or barium is present, a weighed portion of the sample is fused with sodium-pGassium carbonate fusion mixture, the melt leached out with hot water, and the fluorine determined on the water extract according to the usual methods. Phosphates may be separated with silver nitrate in neutral solution, by methods similar to the one described for the separation of arsenic and fluorides. Scott (“Standard Methods of Chemical Analysis,” 4th ed., Vol. I, p. 216) describes a method that has given good results in this laboratory. Literature Cited Brinton, Sarver. and Stoppel, IND. ENQ.CHEM.,16, 1880 (1923). Dinwiddie, Am. J . Sci., 141 42,421 (1916). Flisik, IND.ENO.CHEM.,17, 307 (1925). Katz, Ckem.- Ztg., 28, 356,387 (1904). Rose, Ann., 62,343 (1849). Thomson, J . ,906. Ckem. I n d . , 18, 432 (1893).
Motion Picture.Showing Manufacture of Alloy Steels “Making It Tough” is the title of an educational motion picture film just released by the U. S. Bureau of Mines, Pittsburgh, Pa., which visualizes the manufacture and uses of alloy steels. The film comprises three reels and was prepared in cooperation with a large alloy steel manufacturing concern. Copies are distributed free of charge to schools, etc., the exhibitor paying transportation charges.
