Perchloric Acid Method for Determination of Silicon in Ferrosilicon LOUIS WALDBAUER AND SIGURD 0. RUE', State Cniversity of Iowa, Iowa City, Iowa
is concentrated to heavy white fumes over a hot plate set a t 85" to 135" C.; the evaporation may be greatly hastened by the use of a drying lamp. The beaker, covered with a Pyrex watch glass, is then heated above 200' C. for 20 minutes, during which the acid should boil freely and reflux along the sides of the beaker. The mixture is cooled to about 100' C., diluted with 200 ml. of boiling water, and digested over a steam bath for 15 minutes. The precipitate is washed three times with hot 1 per cent hydrochloric acid, using a total of 125 ml., transferred to a 41-H Whatman filter paper (2), and washed six times with hot water. The precipitate is ignited in a platinum crucible a t 1050" C. for 30 minutes, after the paper has been charred and burned over a low flame. The crucible is cooled in a desiccator and weighed. The silica is moistened with concentrated sulfuric acid, and dissolved in 15 to 20 ml. of pure 48 per cent hydrofluoric acid. The solution is evaporated to dryness, and the crucible weighed again. The per cent of silicon in the sample is found by multiplying the weight of silica (loss by hydrofluoric acid treatment) by 203.2, which is the product of 200 and a correction factor of 1.016 (Table I).
I
N THE tentative A. S. T. M. method for the determination of silicon in ferrosilicon ( I ) , a large excess of sodium
peroxide is used t o reduce t h e violence of the reaction. T h e large quantity of sodium chloride subsequently formed in t h e solution increases t h e solubility of silica and produces low results (5). Two dehydrations with hydrochloric acid are needed to separate the silica quantitatively. This requires a great deal of time, particularly because of slow evaporations. Perchloric acid may be used as a dehydrating agent for silicon-rich ferrosilicon, if the samples are first decomposed (3, 7). A rapid empirical procedure has been evolved from these methods for t h e analysis of simple ferrosilicon samples containing approximately 25 t o 75 per cent of silicon. Precise results may be obtained in about 5 hours with a minimum amount of manipulation.
Discussion
OF CORRECTION FACTOR TABLE I. DETERMINATION
Sample0
Si Present %
Si Foundb
Error
%
%
Practically all the sources of error in t h e procedure are negative, as Hamley (4) and others (6) have pointed out in connection with similar determinations. The most significant error results from the loss of a definite amount of silicic acid which escapes dehydration, but conditions may be controlled in such a way t h a t the total loss of silica is nearly proportional to the per cent of silica in the sample. A correction factor may therefore be introduced to avoid the necessity of a second dehydration. It is evident from t h e calculations summarized in Table I that highly precise results may be obtained for ferrosilicons containing approximately 25 t o 75 per cent of silicon by applying t h e correction factor of 1.016. The factor should be checked occasionally by the analysis of a ferrosilicon of known silicon content, particularly if the procedure is changed significantly. For instance, the method may be applied to refined silicon b y reducing the size of the sample to onefourth factor weight if a correction factor of 1.008is used,
Ratio of Si Present: Found
Average correction factor 1.016 Analyzed samples 1 and 3 were obtained from the Union Carbide and Carbon Research Laboratories, Niagara Falls N. Y.; samples 2, 4. and 5 (refined silicon) are Bureau of Standards samplks 59.58. and 57, respectively. b Each value represents average of duplicate determinations. c One-fourth factor weight of sample was used. d Omitted from average. a
Procedure A fusion mixture is prepared by grinding 10 grams of anhydrous sodium perchlorate (@, 20 grams of anhydrous sodium carbonate, and 70 grams of sodium peroxide in a mortar heated to about 100" C. The mixture is dried for 1 hour a t 110' C. and kept in a desiccator. About 1 gram of the fusion mixture is melted in a 16-ml. nickel crucible, which is slowly rotated as the melt sets to produce a uniform lining about 0.47 cm. inch) in height. A 0.2336-gram sample (one-half factor weight) of ferrosilicon, ground to 200-mesh and dried for 1 hour a t 110' C., is carefully mixed with 3 grams of the fusion mixture in the lined crucible with an iron rod. Particles adhering to the rod and to the sides of the crucible are brushed down, and the mixture is covered with about 1 gram of the fusion mixture. The mixture is fused by revolving the crucible around the outer edge of a low burner flame. At the beginning of the reaction, which is detected by a characteristic sound, the crucible is set aside for about 3 minutes, after which it is heated at 700" C. for 15 minutes. The crucible when slightly cooled is inverted over the opening of a Parr bomb plate which is placed over a 200-ml. platinum dish. The melt may be removed easily by tapping the bottom of the crucible, although it is sometimes necessary to heat the crucible for a moment with a burner. The platinum dish is partly covered with a watch glass and 60 to 75 ml. of water, followed by 12.5 ml. of 12 N hydrochloric acid, are added to the melt. The crucible is rinsed with about 10 ml. of water, filled with 3 N hydrochloric acid which is left until the residue has completely dissolved, and again rinsed with water. All the washings are to be added t o the contents of the dish. The analysis is rejected if any particles of unreacted ferrosilicon are observed. The acidic solution is transferred to a 400-ml. Pyrex beaker, and 50 ml. of 70 per cent perchloric acid are added. The mixture
* Present address, Ethyl Corporation,
Summary I n a simplified method for the determination of silicon in a comparatively wide range of ferrosilicon samples, the sample is fused with a mixture of sodium peroxide, sodium carbonate, and sodium perchlorate. The melt is then decomposed with water and hydrochloric acid, and the resulting silicic acid is dehydrated with perchloric acid. A correction factor is introduced to eliminate the necessity for a second dehydration,
Literature Cited (1) Am.
SOC.Testing Materials, "Methods of Chemical Analysis of
Metals", pp. 64-5 (1939).
Clardy, F. B., Herman, R. M., and Gibbs, R. S., I N D . ENG. CHEM.,ANAL.ED.,13,88 (1941). (3) Gutman, S. M., and Rabaskin, S. E., ZavodakayaLab., 4, 292-3 (2)
(1935).
Hawley, F. G., Eng. Mininu J., 103, 541 (1917). Lenher, V., and Truog, E., J. Am. Chem. SOC.,38, 1050 (1916). Meier, F. W., and Fleischmann, O., Z . anal. Chem., 88,88 (1932). (7) Willard, H. H., and Cake, W. E., J . A m . Chem. Soc., 42, 2208-12
(4) (5) (6)
(1920). (8)
Willard, H. H., and Smith, G. F., Ibid., 44, 2816 (1922).
CONDENBATION of the dissertation submitted by Sigurd 0. Rue to the Graduate College of the State University of Iowa in partial fulfillmefit of the requirements for the Ph.D. degree.
Detroit, Mich.
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