ANALYTICAL CHEMISTRY
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If the original sample is large and contains much iron, it is desirable to add a little ammonium hydroxide to neutralize the acid formed during electrolysis. Addition of ammonium hydroxide is usually indicated if the solution is not colorless after 20 minutes’ passage of the current.
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CELL
In Figure 2 the right-hand cell is in the position for electrolysis, while in the left-hand cell the electrolyzed solution has been separated from the mercury and is ready to be drawn
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Figure 3. Wiring Diagram of the circuit by the snap switch on the panel above it. The switch on the right-hand side controls the main power supply. The whole assembly rests in a shallow tray, so that any mercury spilled accidentally may be recovered.
ACKNOWLEDGMENT
The writer wishes to express his thanks to George Ensell for help in constructing the cell.
OPERATION OF CELL
LITERATURE CITED
Electrolysis is usually conducted in 0.3 N sulfuric acid solution. The direct current supply is adjusted by a rheostat so that the amperage in each cell is not less than 6 and preferably 7 with a voltage of 5 to 6. Under these conditions most samples are electrolyzed completely in 30 t o 40 minutes. After testing for the complete removal of the unwanted elements, the leveling bottle is lowered and, as soon as the mercury has drained from the cell, the electrolyzed solution is run into a 400-ml. beaker. The thistle tube and cell are rinsed with water and the rinsings are added t o the beaker. If the solution contains some fine, blackish suspension (arsenic, manganese, etc.), filter pulp is added and the mixture is filtered on a KO.30 Khatman paper. Meanwhile, about 30 ml. of 2% sulfuric acid are added to the cell, the leveling bottle is raised, and electrolysis is continued for 5 minutes. This is a precautionary measure to ensure recovery of any electrolyte entrained with the mercury in the rubber tube. This wash solution is then run off, the thistle tube and cell are rinsed with distilled water as before, and the combined washings are filtered through the original paper.
(1) Brophy, Ind. Eng. Chem., 16, 963 (1924). (2) Cain, Zbid., 3, 476 (1911). (3) Craighead, IND. EXG.CHEM.,ANAL.ED.,2, 189 (1930). (4) Gibbs, Am. Chem. J., 13, 571 (1891); Chem. News, 42, 291 (1880). (5) Hillebrand and Lundell, “Applied Inorganic Analysis,” pp. 105, 106, 390, New York, John Wiley & Sons, 1929. (6) Lundell and Hoffman, “Outlines of Methods of Chemical Analysis,” pp. 94, 95, New York, John Wiley & Sons, 1929. (7) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel,” p. 47, New York, John Wiley & Sons, 1931. (8) Melaven, IND. EKG.CHEX.,ANAL.ED.,2, 180 (1930). (9) Smith, “Electroanalysis,” p. 60, Philadelphia, Blakiston Co., 1911. (10) Steinmeta. IKD. EKG.CHEX.,A s a ~ED., . 14, 109 (1942). RECEIVED Feburary 21, 1947. Submitted for publication by permission of the Director, hlines and Geology Branch, Department of Mines and Resources, Ottawa, Canada.
Phosphoric Acid Attack Method for Determination of Silicon in Aluminum Alloys GEORGE NORWITZ’ 1613 N o r t h F r a n k l i n S t . , Philadelphia 22, P a .
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ECEKTLY Lisan and Katz ( 1 ) described a rapid phosphoric acid attack method for the determination of silicon in aluminum alloys. To improve the accuracy of this method this author suggests: (1) the use of casseroles instead of beakers in order to eliminate the considerable error that can be caused by the dissolution of the glass by the phosphoric acid, if the beakers are kept on the hot plate more than 2 minutes past the point at which the elemental silicon dissolves, and (2) the use of a lower temperature (900” C.) and shorter ignition period (10 minutes) for the ignition that follows the treatment with hydrofluoric acid. The recommendation of Lisan and Katz that this ignition period be a t 1100” C. to constant weight can lead to high results, because of the slow volatilization of phosphoric acid ( 2 ) . RECOMMENDED PROCEDURE
Weigh the sample in a 500-ml. casserole: 2 grams, 1,5 to 4.5% silicon 1 gram, 4 . 5 to 107Gsilicon 0 . 5 gram, over 10% silicon Add 80 ml. of acid mixture (made by mixing 750 ml. of phosphoric acid, 1000 ml. of nitric acid, and 260 ml. of sulfuric acid) to the 1- or 2-gram samples, or 60 ml. of acid mixture to the 0.5gram samples. Cover with a watch glass and warm on the hot plate until the sample is completely in solution. (If the sample is very finely divided, allow the initial reaction to subside before heating.) Remove the watch glass and evaporate until the solu1 Present address, 577 77th St., Brooklyn, N. Y.
tion clears. The end point is not critical, and the casserole may be kept on the hot plate indefinitely. Add a pinch of ammonium nitrate, swirl, and remove from the hot plate. Allow the solution to cool somewhat, add 60 ml. of 70% perchloric acid, and evaporate to strong fumes of perchloric acid. Cover with a watch glass and fume strongly for 8 to 10 minutes. Remove the casserole from the hot plate and allow to cool somewhat. .4dd 300 ml. of hot water and 40 ml. of hot gelatin solution (l%),and stir well. Filter through an llrcm. KO.41 Whatman filter paper containing paper pulp. Swab the casserole, and wash the precipitate 12 times with hot dilute sulfuric acid (lY0) and finally a few times with hot water. Transfer the paper and precipitate t o a clean platinum crucible and char o f f the paper carefully. Ignite the silica a t 1100” to 1200” C. to constant weight (for routine work 35 minutes are ample). Add 3 drops of dilute sulfuric acid (1 to 1) and 5 ml. of 48% hydrofluoric acid, and evaporate carefully to dryness. Ignite at about 900” C. for 10 minutes. The difference in the two weights represents silica. The factor for converting silica to silicon is 0.4672. The phosphoric acid attack method is not recommended for the analysis of aluminum alloys containing less than 1.5% silicon (1). Results obtained for the above procedure averaged 0.02% higher than the results obtained for the alkali-attack singledehydration method. LITERATURE CITED
(1) Lisan, P., and Katz, H. L., BNAL. CHEM.,19,252 (1947).
(2) Willard, H. H., and Diehl, H., “Advanced Quantitative Analysis,’’ p. 184,New York, D. Van Nostrand Co., 1946. RECEIVED April 28, 1947.
