uisite quantity of dilute acid. The end pomts are stable and reproducible under the prescribed conditions. Organic sulfides, disulfides, and thiocyanates do not interfere. Interference is caused by ions such as thiosulfate, thiocyanate, sulfide, cyanide, and others which react with copper ions. Sulfite ion does not interfere. Thioglycolic acid, like other mercaptan compounds, can be estimated by direct titration with iodine (6, 8, 1 2 ) or silver nitrate (1, 3, 4, 6). The present cupric method appears to be relatively more simple and rapid. No indicator is needed and a single titration requires only 2 to 5 minutes. Besides, cupric sulfate can serve as a primary standard. Experiments a t different time intervals revealed that 0.05 to 0.2N aqueous
solutions of thioglycolic acid are stable under ordinary conditions of temperature and day light for 2 weeks. ACKNOWLEDGMENT
The authon are indebted to Lucy W. Pickett and Philip W. West for research facilities. One of them (S.B.S.) acknowledges the award of a Special Skinner Fellowship. Appreciation is also due to Evans Chemetics Inc., New York, for a gift sample of thioglycolic aeid. LITERATURE CITED
J. Mitchell, et al., eds., Vol. I, p. 343, Interscience, New York, 1953. (3) . . Karchmer, J. H., ANAL. CHEM.29, 425 (1957). . (4) Kolthoff, I. M., Harris, W. E., IND. ENQ.CHEM.,ANAL.ED. 18, 161 (1946). (5) Gamer, .H., H., J. Asso; Assoc. Oflc. Agr. Chemists 35, 285 (1952). (6) . . Leisv, F. A., ANAL. CHEM.26. 1607 (1954j.’ (7) Siggia, S. “Quantitative Organic Analysis Via Functional Groups,” p. 88, Wiley, New York, 1949. (8) Stephen, W. I., Mfg. C h k t 26, 450 (1955). (9) Stone, K.
G., “Determination of Organic Compounds,” p. 188, McGrawHill, New York, 1956. (10) Turk, E., Reid, E. E., IND.ENQ. CHEM.,ANAL.ED. 17, 713 (1945). (11) Walker, G. T., Mfg. C h i s f 24, 376 429 (1953).
(1) Cecil, R., McPhee, J. R., B i o c h a . J. 59,234 (1955). (2) Dal Nogare, S., in “Organic Analysis,”
RECEIVEDfor review April 14, 1959. Accepted July 28, 1959.
Determination of Fluorine as Lithium Fluoride EARLE R. CALEY and GERALD R. KAHLE’ McPherson Chemical laboratory, The Ohio State University, Columbus Fluorine present as fluoride in aqueous solution is quantitatively precipitated by the addition of an equal volume of a 3% solution of lithium chloride in 95% ethyl alcohol. The precipitated lithium fluoride is easily filtered off and washed in a filter crucible. After drying for an hour at 110” C., it is weighed. No special apparatus is needed and accurate results are obtained.
V
objections have been raised against the gravimetric methods advocated for the exact determination of fluorine in large proportion as fluoride ion. One is the difficulty of filtering and washing such gelatinous precipitates as calcium fluoride or thorium fluoride (2). Another is the serious interference of many commonly associated anions-i.e., sulfate, when barium, calcium, lead, or thorium ions are used to precipitate the fluoride (1). By using lithium ion as the precipitant, a precipitate is obtained that is easy to filter and wash, and the interference from associated anions is less. ARIOUS
was found to be very suitable. Unless the concentration of ethyl alcohol is at least 50% by volume, precipitation is not quantitative in the presence of lithium ion in the low concentration needed to produce precipitates with desirable physical properties. Higher concentrations of ethyl alcohol are also undesirable both from the standpoint of producing precipitates with desirable physical properties and the probability of increased interference from the precipitation of various salts. Because of the appreciable solubility of lithium fluoride in 50% ethyl alcohol solutions containing lithium ion in low concentration, the volume of the solution must be restricted in order to obtain quantitative precipitation. This is evident from the results of some preliminary experiments shown in Table I, using a similar procedure to the one in the text. Ethyl alcohol is added slowly in the form of ‘a dilute solution of lithium chloride in 95% ethyl alcohol to the warm aqueous solution containing the fluoride. No other way .of Table 1.
CONDlTlONS FOR PRECIPITATION
Precipitation is far from complete in aqueous solution regardless of the concentration of the added lithium ion. The addition of some miscible organic solvent is necessary, and ethyl alcohol Present address, Phillips Petroleum Co., Bartlesville, Okla. 1
1880
ANALYTICAL CHEMISTRY
IO, Ohio
Need for Restricting Volume of Solution
Final Volume, M1.
F Taken, Mg.
F Found, Mg.
Error, Mg.
200 100 50 50 30 30
100.0 100.0 100.0 50.0
95.2 98.5 100.2 49.7 50 :1 24.9
-4.8 -1.5 +0.2 -0.3 +0.1 -0.1
50.0
25.0
combining the components produces a precipitate that may be filtered and washed without difliculty. RECOMMENDED PROCEDURE
Adjust the neutral fluoride solution containing from 20 to 200 mg. of fluorine to a volume of from 15 to 40 ml., depending on the probable amount of fluorine present. Heat to 70” C. and add slowly with stirring an equal volume of a solution of lithium chloride, 30 grams per liter, in 95% ethyl alcohol. Allow the mixture to cool to room temperature and stand until the supernatant liquid is clear. Filter the mixture through a weighed porcelain filter crucible of medium porosity using a rubber-tipped stirring rod as a transfer aid. After all the liquid has passed through the crucible, wash the precipitate with 50% ethyl alcohol saturated with lithium fluoride until the washings give a negative test for chloride with silver nitrate test solution. After the final portion of wash solution has p a d through the crucible, wash the precipitate with 5 ml. of 95% alcohol. Dry the crucible and precipitate a t 110” C. for 1 hour. Multiply the weight of the precipitate by 0.7325 to obtain the weight of fluorine. Use of Glassware. I n the preliminary experiments precipitations were first made in polyethylene beakers because glass or porcelain vessels were attacked by fluoride, preventing accurate determinations. However, polyethylene beakers, were found t o be unsatisfactory for accurate determinations by this method because the precipitate hati a tendency to adhere to the walls of auch beakers, and ita close similarity to polyethylene in color and
degree of translucency makes its complete removal from such a beaker difficult to ascertain. Trial determinations with beakers of borosilicate glass gave good results, and the possibility of precipitation in glass was further investigated. Eight samples of pure sodium fluoride, each containing 100.0 mg. of fluorine, were placed in 100-ml. beakers of borosilicate glass and dissolved in 25 ml. of water. Two of the solutions were heated to 70” C. and precipitated immediately by the recommended procedure, two were allowed to stand for 2‘ hours before precipitation, two for 4 hours, and the other two for 6 hours. The results of the determinations, shown in Table 11, indicated no significant error from the use of glass beakers. Test Determinations. As reagent grade sodium fluoride is not sufficiently pure to serve as an exact standard for a critical test of the accuracy of the method, a sample of the very pure salt had to be specially prepared. Two methods were used, one of which yielded a purer product. In this method a thin slurry of reagent grade sodium bicarbonate and distilled water was formed in a large polyethylene centrifuge tube, and an ample excess of reagent grade hydrofluoric acid was slowly added. The mixture was allowed to stand with frequent stirring for half an hour to ensure complete conversion of the sodium bicarbonate to sodium fluoride and sodium acid fluoride. Then the supernatant liquid was removed by centrifuging, and the solids were washed with dilute hydrofluoric acid several times by centrifuging and decanting. The residual solids were transferred to a platinum dish and heated to 1000° C. in an electric mufle furnace. After cooling in a desiccator, the fused cake of sodium fluoride was removed from the dish and pulverized in an agate mortar. The pulverized salt was then returned to the dish and heated again a t 500” C. for 4 hours. An assay of this final product, by treatment with an excess of concentrakd sulfuric acid in a platinum crucible followed by evaporation and ignition, gave weights of anhydrous sodium sulfate that were close to the theoretical. The small observed differences were apparently due to accidental errors of manipulation and weighing. Carefully weighed samples of this pure sodium fluoride were then dissolved in the proper volumes of water and their fluorine content determined by the recommended procedure. As may be seen from Table 111, the wsults were eatisfactory. The slightly high resulta for the largest quantity of fluorine may be ascribed to coprecipitstion of sodium ion. Attempts to determine quantities of fluorine as small as IO mg. yielded results that were too low to be considered satisfactory. However, B reduction in scale of the procedure with precipitation in a sufficiently small total
Table
II.
Satisfactory Results from the Use of Glass Beakers
Time of Standing, Hours
Weight of LiF,
0 2 4 6
Error,
Mg.
Weight of F, Mg.
136.5 136.8 136.7 136.6 136.9 136.5 136.7 136.3
100.0 100.2 100.1 100.0 100.3 100.0 100.1 99.8
0.0 +0.2 +0.1 0.0 +0.3 0.0 +O.l -0.2
Table 111.
Mg.
even sninll quantities of sulfntc interfere seriously, and the results of individual determinations are much less consistent. As the amount of coprecipitation increases with the amount of fluorine being determined, the interference is caused by adsorption and not by mrrr precipitation of sotliuni sulfate or lithium sulfate due to reduction of solubility in the alcoholic medium. So study was made of thc interfcrence of other anions, but it swms probablc on the basis of the greater solubility of their lithium salts that many of them should interfprv lcsss seriously with this
Results of Test Determinations
F Taken,
LiF Found,
F Found,
Mg.
Mg.
Mg.
200.0
273 4 273 6 273 3 136.6 136.8 136.5 68.2 68.2 68.3 34.0 34.1 34.2
200 2 200 4 200 2 100.0 100.2 100.0 50.0 50.0 50.0 24.9 25.0 25.1
100.0 50.0 25.0
volume of solution should allow 10 mg. or even less to be determined accurately. Extension of the range much above the recommended upper limit of 200 mg. does not seem to be feasible because the amount of lithium chloride that can be added under the necessary restrictions of concentration and the volume of reagent are insufficient to precipitate quantitatively much more than 200 mg. of fluorine. Interference of Sulfate. Sulfate anions seemed especially worthy of investigation as they interfere so seriously with some of the other gravimetric methods for fluorine t h a t they cannot be present in more than trace amounts. A considerable number of samples made up of pure sodium fluoride and pure sodium sulfate were dissolved and analyzed by the recommended procedure. The most significant results are listed in Table IV. Each average result is based on three determinations. The extent of the interference is not very serious with small quantities of both sulfate and fluoride, but, as illustrated by the results for 25 mg. of fluorine, it is so regular that rather satisfactory results could be obtained by the application of a correction factor. With larger quantities of fluoride
Error, Mg. +o 2 +o 4 +o 2
Average
Error, Mg +0.3 $0.1
0.0 +0.2 0.0 0.0 0.0 0.0 -0.1 0.0 +o. 1
Table IV.
NsSO, Present, Mg. 5.0 10.0 20.0 28.0 35.0 50.0 5.0 10 0 20.0 28.0 35.0 50.0
0.0 0.0
Interference of Sulfate
F Average F Average Taken, Found, Error, Mg. Mg. Mg. 25.0 25.0 25.0 25.0 25.0 25.0 100.0 100.0 100.0 100.0 100.0 100.0
25.0 25.0 25.1 25.2 25.3 25 6 100.1 100.5 100.5 100.9 101.6 101.7
0.0 0.0
+o. 1
+0.2 +0.3 +O 6
+o. 1 f0.5 $0.5
t0.9 f1.6
+1.7
method than with the methods in which barium, calcium, lead, or thorium ions are used to precipitate fluoride. LITERATURE CITED
(1) Hillebrand, W. F., Luridell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 741, Wiley, New York, 1953. (2) Treadwell, F. P.,,,Hall, W. T., “Analytical Chemistry, Vol. II, 9th ed., p. 397, Wiley, New York, 1942.
RECEIVEDfor review June 5, 1958. Accepted June 15, 1959. hfaterial taken from a thesis for the M.S. degree submitted by Gerald R. Kahle t o the Ohio State University, 1953.
VOL. 31, NO. 11 NOVEMBER 1959
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