Commission, CD-B-S-518 (1944). Raseman, C., Keisman, J., Nucleonics 12, ( 7 ) , 20 (1954). Read, E. B., private communication, 1956. Rodden, C. J., ASAL. CHEX 25, 1598 (1953). Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” RIcGraw-Hill, New York, 1950.
(32) Rulfs, C. L., De, A. K., Lakritz, J., Elving, P. J., AKAL. CHEM. 27, 1802 (1955). Silverman, L., Moudy, L., Sucleonics 12, (9), 60 (1954). Silverman, L., Moudy, L., Hamley, D. W.,AKAL. CHEM. 25, 1369 (1953). \Tare, E., U. S.Atomic Energy Commission, MDDC-1432 (1947).
(36) Warf, J. C., J . Am. Chem. SOC.71, 3257 (1949). (37) Wright, W. B., C . S. Atomic Energy Commission, Y-838 (1952). (38) Ibid., Y-884. (39) Yoe, J. H., Will, F., Black, R. A . , AKAL.CHEST.25, 1200 (1953). RECEIVED for review January 25, 1957. Accepted April 3, 1957.
Photometric Determination of Silicon in Steel WILLIAM F. SANDERS and CHARLES H. CRAMER United Engineering and Foundry Co. , Vandergriff, Pa.
A . photometric method has been developed for determining the small percentages of silicon present during the refining period in the melting of steel. During the investigation, the method was found to b e adaptable to silicon in the regular range for plain carbon and low alloy steels. The photometric molybdenum blue method i s used with the ferrous ions of the sample as the reducing agent and with a compensating blank for reference. The method i s accurate, relatively simple, and well suited for the simultaneous determination o f silicon in a large number of samples.
S
ILICON is
usually determined photometrically by measuring the color of the yellow or blue complex formed with silicon and molybdate ions. The yellow color of the silicomolybdate complex is the basis for the work in various procedures (1, 2, 6, 7 , 10). The sensitivity of the method is increased by reduction of the yellow complex to molybdenum blue with a suitable reducing agent (4, 6, 8, 9, 11). The work presented here was undertaken to provide a rapid and accurate photometric method for the routine determination of silicon in steel by measuring the blue color of the reduced complex. The method comprises an r q a n s i o n and improvement of the original work briefly outlined by Hill ( 2 ) ) and provides a n analytical procedure which mill be of considerable value to practical analytical chemists. APPARATUS
Filter Photometer. T h e Fisher electrophotometer with t h e standard 23-ml. Fisher cells a n d t h e 650A red filter furnished with t h e instrument. Spectrophotometer. T h e Beckman Model B with t h e Beckman test t u b e holder a n d standard 23-ml. Fisher cells. T h e instrument was also used
with matched 1-em. borosilicate glass cells in t h e regular cell holder. EXPERIMENTAL
The measurement of the yelloiy color was not considered in the present work because of its low sensitivity. The far more sensitive reaction of the blue complex n-as tried and the best conditions for measurement of the blue color were established. Most of the work described was done with the Fisher electrophotometer; however, a number of absorbance values were checked on the Beckman spectrophotometer fitted with the test tube holder and the same 23-ml. cells which were used in the electrophotometer. Also, with the 1-em. cells in the spectrophotometer, the upper limit of the permissible silicon concentration could be greatly increased owing to the use of the shorter cell length and the narrow and more selective wave length of obtainable light, as compared with the filter instrument. Selection of Reducing Agent. Stanous chloride (6, Q), ferrous sulfate (8, 9), and sodium sulfite (4, 11) were tried as reducing agents h u t the ferrous ion was found to offer the best possibilities for development of the blue color. According to Usatenko and Orlova (8), there is no reduction of free molybdate with the ferrous ion, and no interference from phosphorus and arsenic, as is possible with stannous chloride; also, there is no greenish hue and no erratic color development due to slight changes in temperature as there is when sulfite is used. Furthermore, instead of requiring a separate ferrous solution, the method has been simplified by partial oxidation with persulfate. thus allowing the ferrous ions of the sample itself to become the reducing agents. This procedure was described by Hill (2, S ) , who measured the blue color at 710 mp and presumably used the same sample weight, aliquots, and reagent concentrations as for the
develonment of his yellow silicomolybdate color. Concentration of Reagents and Sample Size. A careful study of t h e variables was made in order t o arrive a t a sufficiently stable color for accurate evaluation of silicon. Sulfuric acid, 8% and 1670, was established for acid concentration and 37, ammonium persulfate for partial osidation of t h e sample. Table I shows t h a t t h e acid concentration is not critical for t h e higher percentages of silicon, but should be kept within about lOyoof the amount specified in the procedure for silicon in the low range. The persulfate is stable for a t least a week and may be varied by as much as 207, of the given concentration without affecting bhe accuracy of the results. The greatest stability of color was produced when the molybdate concentration was reduced to l.GyOrather than the 5 to 10% called for in othcr procedures. As both the molybdatc and fluoride are additions to volume. they must be added from burets or pipets.
Table 1. influence of Acid Concentration on Color Formation
Acid ConcentraSamDle tion, % HZSOI Absorbanceo -b YBS 8h, 6.5 0.028 Si, Pro- 7 . 5 0.171 n,J..,.A A 8.0 0.171L ._ 8.5 0.169 9.5 0.159 SBS 72e, 10.0 0.205 0.2!12 Si, Pro- 15.0 0.210 cedure B 16.0 0.210 17.0 0,210 20.0 0.205 25.0 0.189 Absorbance values obtained on spectrophotometer at 620 mp with 23-ml. stmdard Fisher cc,lls. Precipitate formed on addition of molybdate. bGU
