Colorimetric Determination of Silicon Tetrafluoride in Hydrocarbons GEORGE N. CADE Research Department, Phillips Petroleum Company, Bartlerville, Okla.
A method that is free from interference b y small amounts of hydrofluoric acid has been developed for the determination of 0.0005 to 0.015% of tilicon tetrafluoride in hydrocarbons. A n y hydrofluoric acid i s preliminarily converted to fluoboric acid to prevent The silicon tetrafluoride is converted to silicomolybdetermined ~olorimetrica~ly. to p.5 mg. of silicon tetrafluoride i s determined with a precision Of '0.05 mg. in a Fisher Eloctrophotometer or with a precision of -0.1 mg. in Nessler tubes.
attack On glass.
up
dic acid, which
T
HE following method was developed to meet the need for determining 0.0005 to 0.015% of silicon tetrafluoride in hydrocarbons, such as those obtained in hydrofluoric acid alkylation plants in which organic fluorine is removed with silica-containing bauxite. The silicon tetrafluoride is absorbed in sodium bicarbonate solution and converted to silicate by heating a t afterneutralization of the bicarbon1000 c. (7). The ate solution and sonversion of fluoride to fluoborate ( 1 , 6), is converted to silicomolybdic acid, which is determined c010rimetrically (3,4,6, 7,8). A P P A R A T U S AND REAGENTS
Sample to vaporize and bubble through the liquid in the ab-
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~ ~ ~ ~ t ~~ t ~~ ~ heat the sample bomb to about 60" c. by immersion in hot water. When evolution of vapor has again ceased, close the valve. Transfer the contents of the absorbers to a 100-ml. d u metric flask. If any silica is deposited in the end of the inlet tube of the first absorber, scrape it out with a small piece of Saran or hard rubber and wash it into the volumetric flask. Remove the valve from the bomb, rinse the interior of the bomb with 20 ml. of the sodium bicarbonate solution, and add the solution to the volumetric flask. If liquid hydrocarbon remains in the bomb, transfer it, together with the sodium bicarbonate rinse solution, to a separatory funnel, shake vigorously, and transfer the aqueous phase to the volumetric flask. Dilute the contents of the volumetric flask to the mark and mix. Pipet a 50-ml. aliquot into a 100-ml. platinum dish, cover with a watch glass, and heat for 1 hour a t 100" C. on a steam bath. Add 2.4 ml. of l.o N sulfuric acid, cool to temperature, and add 10 ml. of the boric acid solution. Filter if turbidity is noticeable. Transfer to a 50-ml. volumetric flask calibrated to deliver, dilute to the mark designated "delivers", and mix. Transfer to a 125-m1. glass-stoppered Erlenmeyer flask, add 5 ml. of the ammonium molybdate solution, mix, and allow to stand for at lemt 10 minutes. Obtain the Scale A reading on the trophotometer, and read on the calibration curve the weight of silicon tetrafluoride present, Calculate as follows:
A photoelectric colorimeter, suitably a Fisher A.C. Electrophotometer with 425-mp blue filter (4, 6, 8), 23-ml. cylindrical
cells, and instructions for operation. Cells put together with optical cement should not be used. When a photoelectric colorimeter is not available, a set of 50-ml. Nessler tubes may be used. Sample bomb, 100-ml., preferably of silver, Monel, or stainless steel, with Hoke needle valve and adapter for 0.6-cm. (0.25-inch) copper tubing. The interior of the bomb must be as free aa possible from moisture and metal oxides. Absorbers. Two 50-ml. test tubes, each fitted with a rubber stopper through which pass a Saran outlet tube and a Saran inlet tube, which extends nearly to the bottom of the test tube. Sodium bicarbonate solution, 10 grams per liter. Sulfuric acid, 1.0 N . Boric acid, saturated solution (1, 6). Ammonium molybdate solution. Dissolve 30 grams of C.P. amrllonium molybdate tetrahydrate in 400 ml. of water and add 200 ml. of 1 to 1 hydrochloric acid (8). Potassium chromate solution, 0.63 gram per liter (8). Sodium tetraborate decahydrate buffer solution, 1% (8). PROCEDURE FOR ELECTROPHOTOMETER
CALIBRATION CURVE. Prepare five standard solutions by mixing for each a known volume (0 to 25 ml.) of the potassium chromate solution, 25 ml. of the buffer solution, and enough water to make the total volume exactly 55 ml. (8). Determine for each the Scale A reading of the electrophotometer in accordance with the directions supplied by the manufacturer. Make a calibration curve by plotting Scale A readings against milligrams of silicon tetrafluoride on the basie of each milliliter of potassium chromate solution being equivalent to 0.173 mg. of silicon tetrafluoride. TREATMENT O F SAMPLE. When the hydrocarbon is normally liquid, extract an approximately 50-gram sample, whose weight is kiiown to 0.1 gram, with two 20-ml. portions of the sodium bicarbonate solution in a separatory funnel; run the aqueous extracts into a 100-ml. volumetric flask. When a normally gaseous hydrocarbon is present, draw a sample of 30 to 45 grams in the liquid phase into the sample bomb, previously evacuated and tared; weigh to the nearest 0.1 gram. Pipet 20 ml. of the sodium bicarbonate solution into the first absorber and 20 ml. of water into the second. Dry the inlet tube of the first absorber arid connect the absorbers in series. Clamp the bomb valve upward and connect it with the inlet tube of the first absorber. Open the valve and allow the lower-boiling components of the
B - in which
C 6 0
= tetrafluoride content* % C = silicon tetrafluoride found, mg. D = weight of sample, grams PROCEDURE FOR NESSLER TUBES
STANDARDS (8). Pipet known volumes of the potassium chromate solution, ranging from 0 to 15 m1.9 into 50ml. Nessler tubes, add 25 ml. of the buffer solution, dilute to the 50-ml. marks, add 5 ml. of water, and mix. One milliliter of the potassium chromate solution so treated corresponds to 0.17 mg. of silicon tetrafluoride. TREATMENT OF SAMPLE. Proceed as described in the procedure for the Electrophotometer until the boric acid has been added. Filter if necessary, transfer the solution to a 50-ml. Nessler tube, and dilute to the mark. Add 5 ml. of the ammonium molybdate solution, mix, and allow to stand for 10 minutes. Determine the amount of silicon tet,rafluoride present by comparing with the standards. Calculate as described above. PREPARATION OF
RMINATION OF
Table I. Determination of Known Quantities of Silicon Tetrafluoride
487
NaF
Electrophotometer SiF4 found Difference
SiF4 Present
Present
Mu.
Mu.
MU.
MU.
2.28 2.28 2.28 2.28 1.65 1.65 1.65 1.65 1.65 1.65 0.66 0.66 0.66 2.48 0.74 0.50 1.24 0.25
0
2.17 2.24 2.26 2.26 1.65 1.62 1.60 1.70 1.72 1.60 0.77 0.68 0.72 2.42 0.75 0.49 1.32 0.27
-0.11 -0.04 -0.02 -0.02 0.00 -0.03 -0.05 -I-0.05 +0.07 -0.05 +O.ll +0.02 +0.06 -0.06
10
25 25 0 5 10 25 25 50 0 25 50 0
0 0 0 0
+0.01
-0.01 +0.08
+0.02
Nessler Tubes SiF4 found Difference MQ. dig.
2.4 2.4 2.5 2.3 1.7 1.7 1.7
1.7 1.7 1.7 0.68 0.68 0.68 2.2 0.76 0.60 1.4 0.34
+0.1 1 +0.2
+o.
0.0 1 1 +0.1
+o. +o.
+O.l +o. 1 +0.1
4-0.02
+0.02 4-0.02 -0.3 +0.02
+0.10 +0.2 fO.09
~
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 - 0.0004 +0.0003 +0.0006 -0.0002 -0.0001
0.0144
0.0054 0.0007 0.0021
Neaeler Tubea SiFd found Differenoe
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., (4) Knudson,
and Neurath, F., 2. angew. Chem., 11, 315-16 (1898) 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 b y 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.
