Interference of Phosphorus in the Determination of Fluorine H. V. CHURCHILL, R. W. BRIDGES, A N D R. J. ROWLEY, Aluminum Research Laboratories, New Kensington, Pa.
S
p. p. m. of fluorine, phosphate was added as phos horic acid in amount equal to that present in the original ash. Eoncentration of the distillate followed by distillation, using perchloric acid, gave a titration indicating less than 1 p. p. m. of fluorine. In this distillate phosphate and sulfate ions could not be detected.
I N C E the discovery that fluorine is a causal factor of the dental defect known as mottled enamel on teeth, the literature of analytical chemistry has abounded with papers describing methods for its determination in small amountsthat is, up to a few parts per million. More recently the increased use of fluorides as insecticides has stimulated interest i n such methods and, since the U. s. Food and Drug Administration placed the maximum allowable limit for fluorine content of food a t 0.01 grain per pound (1.4 p. p. m,), the need for an accurate method to determine it in the range of 1 t o 2 parts per million is paramount.
Satisfactory recovery of added fluorine was made by the double distillation. It, therefore, seems necessary to make a double Willard-Winter distillation of the fluorine from the ash of foods, for satisfactory determination of fluorine. The first is made with sulfuric acid rather than perchloric acid to eliminate hazard, as some carbonaceous matter is usually present. The second should be made with perchloric acid a t 135’ C., which gives a distillate free from phosphate and sulfate. Fluorine in a number of food products, mostly purchased in retail stores, was determined b y the above double-distillation procedure *and titration with thorium nitrate, with the following results: Less than 0.2 p. p. m . of fluorine was indicated in dried beans, wheat bran, crushed oats, peanuts, mackerel (boned), carrots, turnips, sweet potato, Irish potato, canned corn, beef, mutton, and pork. Less than 1.0 p. p. m. of fluorine was indicated in baking powder C, pure wheat germ, alfalfa, wheat, wheat straw, yellow corn, and whole rice. More than 1.0 p. p. m . of fluorine was found as follows:
Workers in this field have used various procedures for the determination of fluorine. Churchill @),when investigating the occurrence of fluorine in some waters of the United States, determined fluorine by a modified Fairchild method and verified the results by means of the spectrograph. Willard and Winter (7) developed a method for the determination of fluorine by distilIing it with perchloric or sulfuric acid and titrating it with thorium nitrate, using the zirconium sodium alizarin sulfonate lake as an indicator. Armstrong (1) improved this method by using alizarin sulfonate only as the indicator. Thompson and Taylor (6) developed a colorimetric method using the zirconium alizarin lake for the determination of fluorine in sea water. Sanchis (4) modified the rocedure for the determination of fluorine in natural waters. %inter (8) used it for the determination of fluorine in alfalfa and grass, and Wichmann and Dahle (6) developed an improved titanium bleach method (Steiger Merwin reaction) and determined the most suitable pH for color development and the effect of many interferin ions and various solution concentrations. Hoskins and Ferris 73) made a study of the thorium nitrate titration, determining the most suitable pH a t which it should be made and the minimum concentration at which the ions of halogens. NO*, C104,808, AsOa, SOa, AsOa, and PO4 interfere.
Mackerel (with bones) Salt pork Baking powder A B&kingpowder B Commercial wheat germ A Commercial wheat germ B Cottonseed meal
It is well known that phosphates interfere with the determination of fluorine by the above colorimetric and thorium nitrate titration methods, but it generally has been considered t h a t the Willard-Winter distillation made a complete separation of fluorine from phosphate. However, in determining fluorine in some foods having a high phosphorus content, by the thorium nitrate titration of a distillate made from the ash of the food, results for quorine were obtained which varied from “not detected” to 11 parts per million. Investigation showed that such varying results obtained on navy beans were caused by titration of phosphate carried over into the distillate. When the first distillate was made alkaline with sodium hydroxide t o phenolphthalein, evaporated t o small volume, and redistilled, satisfactory results for fluorine were obtained which indicate the fluorine content to be less than 1 part per million. It was evident that some variable factor caused the phosphate contamination. It seemed plausible that the contamination was caused b y the presence of unburned carbonaceous matter or t h a t possibly during ashing phosphate was reduced t o a form which is readily carried over in the distillate. This is indicated by the following experimental analysis of iiavy beans : Thorium nitrate titration of a distillate using sulfuric acid on the ashed sample indicated 7.2 p. p. m. of fluorine. In this distillate phosphate was present in appreciable amount, sulfate a trace. When the distillate from sulfuric acid was concentrated and redistilled, using perchloric acid, less than 1 p. p. m. of fluorine was indicated. Phosphate and sulfate were not detected in this distillate. To another distillate from sulfuric acid, which indicated 7.2
3.9
1.1 220.0 19.0 1.7 4.0 12.0
Corn germ 16.0 English Breakfast tea 66.0 Gun Powder tea 67.0 Oolong tea 41.0 Canned salmon 4.5 Canned sardines 7.3
Literature Cited (1) Armstrong, W. D., J . Am. Chem. Soc., 55, 1741-2 (1933). (2) Churchill, H.V., SND. ENQ.CHEM., 23, 996 (1931). (3) Hoskins, W.M.,and Ferris, C. A., Ibid., Anal. Ed.,8, 6-9 (1938). (4)Sanchis, J. M., Ibi& 6, 134-6 (1934). (5) Thompson, T. G., and Taylor, H. J., Ibid., 5, 87-9 (1933). (6) Wichmann, J. H., and Dahle, D., J . Assoc. Agr. Chem., 18, 612 (1933). (7) Willard, H. H., and Winter, 0. B., IND.ENO.CHEW.,Anal. Ed., 5, 7-10 (1933). (8) Winter, 0. B., J. Assoe. Ayr. Chena., 19,362-5 (1936). R B C ~ X WFebruary D 6, 1937.
Correction In the article entitled “Physical and Chemical Properties of Petroleum Fractions. I. Behavior in Dilute Benzene Solutions” [IxD.EKG.CHEW,Anal. Ed., 8, 324 (1936)I by Harry T. Rail and Harold M. Smith, C in the equations in Tahle I should be multiplied by 100. Thus, the equation for A-I, Wet, should be
K
= 64.98
- 0.16 C X
100
I n the second article of the series, “11. Relations between Molecular Weight and Concentration in Dilute Solution” [Ibid., 8, 436 (1936)], S for oil 0-1 in C2H4Br2should be 11,600.0 instead of 1160.0.
HARRY T.RALL 222
