488
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table
II.
Determination of Silicon Tetrafluoride in Synthetic Hydro. carbon Samples si^,
Hydrocarbon Alkylation effluent C.P.
ieobutane C.P. propane 0
Added
Electrophotometer SiFi found Difference
%
%
%
0.0026a 0.0066 0.0018
0.0021 0.0062 0.0021 0.0150 0,0052 0.0006 0.0021
-0.0005
0.0144
0.0054 0.0007 0.0021
Neaeler Tubea SiFd found Differenoe
- 0.0004
+0.0003 +0.0006 -0.0002 -0.0001 0.0000
% 0.0024 0.0065 0.0020 0.013 0.0050 0.0006 0.0018
% -0.0002 - 0.0001 f0.0002 -0.001 -0.0004 -0.0001 -0.0003
Determined gravimetrically.
Vol. 17, No. 8
Synthetic samples similar in composition to samples obtained from a hydrofluoric acid alkylation unit were prepafed by mixing known volumes of standard isobutane-silicon tetrafluoride mixtures with known weights of C.P. hydrocarbons in metal sample bombs. The results of colorimetric determinations of the silicon tetrafluoride contents of these samples are shown in Table 11. Experiments showed that rinsing the interior of the sample bomb with sodium bicarbonate solution, after vaporization of volatile materials, is necessary for complete recovery of silicon e tetrafluoride. Although the reagents for the present method are available substantially free from silica, blank determinations should be made.
PRECISION AND ACCURACY
LITERATURE CITED
Silicon ktrbfluoride was prepared by the action of concentrated sulfuric acid on a mixture of sodium fluosilicate and ground glass. Standard mixtures with dry gaseous C.P. isobutane were prepared and were stored over mercury in a gas buret, and the silicon tetrafluoride contents were determined acidimetrically ( 2 ) . Known volumes of the standard mixtures were contacted with sodium bicarbonate solution, and the silicon tetrafluoride wa+s determined colorimetrically. In some of the determinations, known amounts of sodium fluoride were added to determine the effect of fluoride. As much as 50 mg. of sodium fluoride (equivalent to 24 mg. of hydrofluoride acid) did not interfere. The data, which are given in Table I, indicate a precision of t0.1 mg. for results obtained with Nessler tubes and.a precision of *0.05 mg. for results obtained with the Electrophotometer.
(1) Case, 0.P.,
[email protected].,ANAL.ED.,16, 309-11 (1944). (2) Furman, N. H., “Scott’s Standard Methods of Chemical Analysis”, 5th ed., Vol. 1, p. 410,New York, D.Van Nostrand Co., 1939. (3) Jolles, A., and Neurath, F., 2. angew. Chem., 11, 315-16 (1898) (4) Knudson, H. W., Juday, C., and Meloche, V. W., IND. ENG CHEM.,ANAL.ED., 12, 270-3 (1940). (5) Robinson, R. J., and Spoor, H. J., Ibid., 8, 455-7 (1936). (6) Schrenk, W. T., and Ode, W. H., Ibid., 1, 201-2 (1929). (7) Snell, F. D., and Snell,C. T., “Colorimetric Methods of Analysis”, 2nd ed., Vol. 1, pp. 517-20, New York, D.Van Nostrand Co., 1936. (8) Swank H.W., and Mellon, W.G., IND.EN(;.C H E ~A . ,N ~ LED., . 6, 348-50 (1934).
Rapid Determination of Alumina in Titanium Pigments IRVIN BAKER
AND
GEORGE MARTIN
Chemical Laboratory, Norfolk Navy Yard, Portsmouth,
The determination of alumina in titanium pigments by alkali fusion and subsequent precipitation and titration of the aluminum as the quinolate reduces the time required from 3 or 4 days by the carbonate fusion to 1.5 hours. Greater accuracy is obtained and no special apparatus i s required. The method provides an excellent rapid routine procedure of analysis.
IN
T H E manufacture of chalk-resistant titanium dioxide pigments, approximately 1% of alumina and 1% of antimony oxide are added. Small quantities of alkaline or alkaline earth salts and silica may be present as impurities. Numerous procedures for the separation of titanium and aluminum have been devised (1, 3, 4, 6 , 7, 8). However, these methods deal chiefly with the separation of small amounts of titanium, whereas most titanium pigments used in paints are composed of approximately 98% titanium dioxide. Since the pigment is heat-treated, the aluminum oxide is not readily soluble and cannot be extracted in weak acid solution. Solution of the pigment in ammonium sulfate-sulfuric acid mixture followed by precipitation of the titanium results in considerable error due to the gelatinous nature of the titanate precipitate. The Navy Department specification (8) gives a method for the determination of alumina which has been found unsatisfactory as a routine procedure of analysis. Approximately 3 to 4 days are needed for an analysis. The method requires a preliminary separation of titanium dioxide by a sodium carbonate fusion followed by leaching of the aluminum oxide in water. This procedure tends to give low results, since, according to Weiss and Kaiser (9), the extraction of aluminum from a sodium carbonate fusion of titanium dioxide followed by 24-hour leaching will not remove all the aluminum and a second 24-hour leach on the residue is required. After leaching, the precipitate is filtered and must be washed many times, bemuse of
Va.
the difficulty of extracting the small uantity of aluminum from the large titanium precipitate. Folowing the filtration, the silica is removed by a 24-hour dehydration from a hydrochloric acid solution. This is a lengthy procedure, requiring one or possibly two 24-hour dehydration periods to remove the silica completely. After removal ,of the silica, the aluminum is precipitated in ammoniacal solution, dissolved in hydrochloric acid, and finally precipitated as the quinolate. Before final weighing, the precipitate must be dried for 1 hour a t 155” C. or overnight a t 110’ C. The excessive time required in the sodium carbonata fusion method is evidently due to the time consumed in the complete extraction of all the aluminum from the sodium carbonate fusion and for the precipitation of silica. The former can be reduced from 2 days to approximately 20 minutes by replacing the carbonate fusion with fused alkali as recommended by Koenig (6). The extreme solubility of the aluminum in the alkali solution results in rapid extraction. The need for separa;tion of the silica can be eliminated by dissolving the aluminum quinolate precipitate and titrating rather than by weighing the quinolate precipitate. Since silica does not form a complex with 8-hydroxyquinoline under the conditions of the experiment, its separation is unnecessary. Because the quantity of silica is small, no difficult filtration problem is caused by the gelatinous nature of the silica precipitate. REAGENTS
SODIUM HYDROXIDE. A
ide is required.
C.P.
aluminum-free sodium hydrox-
8-HYDROXYQUINOLINE k!OLUTION. Make a paste Of 2.5 grams of 8-hydroxyquinoline with ‘5 ml. of glacial acetic acid, stir until dissolved, and add 95 ml. of water. POTASSIUM BROMIDE, 20%. Dissolve 20 grams of C.P. potassium bromide in water and dilute to 100 ml. POTASIUM IODIDE, 20%. Dissolve 20 grams of C.P. potassium iodide in water and dilute to 100 ml. with water.
A N A L Y T I C A L EDITION
August, 1945
STARCH SOLUTION. Triturate 2 grams of soluble starch and 10 mg. of mercuric iodide with a little water and add the swpension slowly to 1 liter of boiling water. Continue boiling unQl the solution is clear, cool, and transfer to a glass-stoppered bottle. POTASSIUM BROMATE SOLUTION, 0.1 N . Dissolve 2.783 grama of C.P. potassium bromate previously dried a t 150’ C., and dilute to 1 liter. Standardize by any satisfactory analytical procedure. SODIUM THIOSULFATE SOLUTION, 0.1 N . Dissolve 25 grams of sodium thiosulfate pentahydrate in 1 liter of freshly boiled, cooled water and add 0.1 gram of sodium carbonate to the solution. Allow to stand for a day and standardize with 0.1 N potassium bromate solution. Standardize solution frequently.
489 Table
I. Determination of Alumina AlsOa Found
Samplee
All08 Present
-4lkali fusion method
Carbonate fusion method
%
%
%
Samples contained approximately 98% Ti02 and 1% SbtO8.
PROCEDURE
Weigh accurately a sample containing 10 to 20 m , of alumina, add approximately 10 times the sample weight of C.P.sodium hydroxide in a 100-ml. capacity pure nickel crucible, and heat, gently a t first to avoid spattering. Fuse a t dull redness 5 to 10 minutes. Cool and place crucible in a 400-ml. beaker containing 200 ml. of cold water. Heat slowly to boiling and boil gently for 10 minutes to extract the aluminum. Remove the crucible, allow precipitates to settle and decant through filter paper. Boil the residue in the beaker with 50 ml. of 5% sodium hydroxide solution for several minutes and transfer the precipitate to the filter paper. Wash thoroughly with 50 ml. of hot 5% sodium hydroxide. Make the filtrate acid with concentrated hydrochloric acid (sp. gr. 1.19), add 5 grams of ammonium chloride per 100 ml. of solution, and bring to boiling. Neutralize the solution with ammonium hydroxide until it is distinctly yellow to methyl red indicator. Boil 2 to 3 minutes. Allow the precipitate to settle, filter, and wash with hot 5% ammonium chloride solution. Dissolve the precipitate with 150 ml. of hot 4 N hydrochloric acid into a 400-ml. beaker and wash the filter paper thoroughly with hot 0.5N hydrochloric acid solution. Neutralize the filtrate with ammonium hydroxide to distinct yellow with methyl red and then make slightly acid with hydrochloric acid. Add with vigorous stirring in the following order: 15 ml. of a 3% hydrogen peroxide solution and 10 ml. of a 2,5708-hydroxyquinoline reagent, and neutralize with ammonium hydroxide. Add 5 ml. of ammonium hydroxide (sp. gr. 0.90) in excess for every 100 ml. of solution. Heat to 70 C. and stir a t this temperature 15 minutes. Remove from heat, allow to settle and cool. Filter and wash thoroughly with dilute ammonium hydroxide solution to remove excess oxine. Dissolve the precipitate with 75 ml. of hot 4 N hydrochloric acid, receiving the filtrate in the same beaker in which the quinolate was precipitated, and wash the filter paper thoroughly with hot 0.5 N hydrochloric acid solution until the washings are colorless. Add to the filtrate 5 ml. of 207, potassium bromide solution and several drops of a 0.1% solution of the sodium salt of methyl red. Titrate slowly with 0.1 N potassium bromate solution, swirling constantly until the color changes from red to orange or orange-yellow, and add 1 ml. in excess. Allow the solution to stand several minutes. Add 5 ml. of a 20% potassium iodide solution and 3 ml. of starch solution and titrate with 0.1 N sodium thiosulfate solution to the disappearance of the blue color. Calculate to A1208:
-
( N KBrOa X ml. of KBrOa N NaAOa X ml. of r\’anSzOs)X 0.00425 X 100 grams of sample % A1201 All filtrations are made with No. 589 S. 8: S. Blue Ribbon paper or equal, using platinum cone and suction. Iron crucibles are unsatisfactory, since iron will be carried into the filtrate during the fusion. Iron is not separated in the procedure, but its concentration, if present, will be small and can be determined colorimetrically and subtracted from the alumina found.
-
DISCUSSION
Antimony present in the sample and any nickel from the fusion in the nickel crucible do not interfere Sith the determination, since they are not precipitated by 8-hydroxyquinoline in an ammoniacal solution. Iron interferes, but it is absent or present in negligible quantities in titanium pigments; its presence is not permissible because of its deleterious effect on the color of the pigment. Any small amount of iron can be determined colorimetrically on a fresh sample and deducted from the aluminum. Titanium remaining in the sodium hydroxide solution after fusion will not be precipitated with the aluminum quinolate because of the addition of hydrogen peroxide. Alkaline earth salts are
Table II.
Determination of Alumina in Titanium Pigments
AhOi Found
Semple No.
Alkali fusion method
%
%
131716
0.95
0.90
131727
0.93
0.94 1.04
Carbonate fusion method
0.84 0.87
0.95
1.06 1.07
0.87 1.01
132628
1.03 1.00 1.01
0.94
132736
0.89 0.90
0.88 0.82
136241
0.87
0.93
0.90 0.87
0.99 1.01
0.84 0.84 0.77 0.75
separated from the aluminum by precipitation of the aluminum in ammoniacal solution, the alkaline earth salts remaining in solution. Table I lists the results of the determination of aluminum in synthetic mixtures of C.P. titanium dioxide, antimony oxide, and aluminum oxide. A maximum deviation of 0.04% was obtained for the proposed procedure (19 against 0.1% for the sodium carbonate fusion (2). I n the latter case, the results are lower than the theoretical. Table I1 shows results obtained in the determination of aluminum in commercial titanium pigments by the two procedures. As in Table I, the results using the specification method are lower than those obtained by the proposed method. In the proposed method, a maximum deviation of 0.06% was found as agaimt 0.14% for the carbonate fusion method. ACKNOWLEDGMENT
The authors wish to thank, R. Stevens Gibbs, principal chemist, Sorfolk Navy Yard, Portsmouth, Va., for his critical review and comments on the preparation of this paper. LITERATURED CITED (1) Bastos, W. C., Chem. Zentr., 1942, I, 2685-6. (2) Federal Specification TT-T-425, Par. F-2k, May 2, 1944. (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganio Chemistry”, p. 76, New York, John Wiley & Sons, 1929.
(4) Jamieson, G. S.,and Wrenshall, R. J., J. IND.ENO.CHEM.,6.
205 (1914). (5) Koenig, E. W., IND.ENQ.CHEM., A N ~ LED., . 11, 532-5 (1939). (6) Prodinger, W., “Organic Reagents Used in Quantitative Inorganic Analysis”, p. 143, New York, Elsevier Publishing Co., 1940. (7) Scott, W. W., “Standard Methods of Chemical Analysis”, Vol. I, p. 980, New York, D. Van Nostrand Co., 1939. (8) Thornton, W. .M.,Jr., “Titanium”, p. 106, New York, Chemical Catalog Co., 1927. (9) Weiss, L., and Kaiser, H., 2. anorg. Chem., 65, 345 (1910). THEviews presented in this article are those of the writers and are not t o be construed as the official viewe of the Navy Department.
