196
INDUSTRIAL AND ENGINEERING CHEMISTRY
if present in large amounts as the sulfate, is occluded in the barium sulfate precipitate in sufficient quantity to cause an appreciable error. Because of the high 804 content of such waters, however, small volumes of the sample are usually taken for the volumetric determination and diluted to 100 ml., thus minimizing the chance of occlusion. The result then obtained is comparable to that obtained gravimetrically on a similarly diluted sample. High results are obtained if phosphate is present in excess of 5 mg. in the sample tested. Boiler waters are almost the only waters that contain phosphate in quantities approaching this magnitude. They may contain 50 parts of phosphate to more than 1000 parts of sulfate and consequently the amount of phosphate present in the
VOL. 8, NO. 3
sample used for the sulfate test would be negligible. Of the other acidic constituents, nitrite is the only one that might have disturbing effects, and this is seldom present in natural waters in quantities sufficient to affect the results, because it is so readily oxidized to nitrate after exposure of the sample to air.
Literature Cited W-., Am. Chem. J.,11, 567 (1889). (2) Komarowsky, A., Chem.-Ztg., 31, 498 (1907). (3) Schmidt, R., 2.anal. Chem., 82, 353-61 (1930). (1) Andrews, L.
RECEIVEDApril 3, 1936. Published by permission of the Director, U. S. Geological Survey.
Determination of Selenium in Steel W. C. COLEMAN
AND C. R. MCCROSKY Syracuse University, Syracuse, N. Y.
II
C
FIGURE 1. SOLUTION APPARATUS A . 500-ml. Erlenmever Erlenmeyer flask containing 50 ml. of 1.18 hvdrochloric hydrochloric acid 0.1 r 1 N iodine in potassium iodide and 1 ml. of O B. 250-ml. Erlenmeyer flask containing one-fourth filter paper (Whatm a n No. 40, ? om.) finely macerated, and 6 ml. of 0.1 N iodine in potassium iodide diluted t o 100 ml. C. 250-ml. Erlenmeyer flask containing 200 ml. of water.
W
ITH the advent of the use of selenium in steel, the desirability of an accurate, inexpensive, and rapid method for its determination becomes increasingly important. As the result of work in this laboratory, the authors developed a method for the determination of selenuim in steel and submitted it to the Carpenter Steel Company in the spring of 1935. Marvin and Schumb (2) have recently published a volumetric method in which they use perchloric acid as a solvent. The present method makes use of hydrochloric acid as a solvent and a modification of the Norris and Pay (3) volumetric procedure for selenious acid. Briefly outlined, the method consists in the solution of the steel sample in hydrochloric acid, trapping the hydrogen selenide evolved in a potassium iodide solution of iodine, combining the selenium precipitated in this solution with the bulk of the selenium in the hydrochloric acid, and filtering through macerated filter paper. The precipitate is then dissolved by a solution of bromine in hydrochloric acid, refluxed, the excess bromine discharged with alcoholic acetanilide solution, ferric ion suppressed by the addition of sodium fluoride, and the selenious acid titrated by the Norris and Fay method.
The apparatus used for the solution of the sample is shown in Figure 1. A 5-gram sample of steel is placed in flask A , containing hydrochloric acid and a potassium iodide solution of iodine. The iodine solution prevents the loss of hydrogen selenide while connecting this flask, and iodide insures complete reduction of any selenite. The flask is heated with a microburner, at first gently, and finally with boiling until solution is complete and the volume is reduced to 25 to 30 ml. Flask B, containing a potassium iodide solution of iodine for oxidizing hydrogen selenide, is kept cooled in a water bath. Flask C, containing water, maintains a desirable back pressure. (The water bath and flask C, as well a9 the refluxing operation following, are refinements that may be omitted where percentages are reported to only two decimal places.) The contents of flask A are carefully transferred to flask B, rinsing the connecting tube into this flask, removing any selenium from the end with moist filter paper. The solution, containing the selenium precipitate and macerated filter paper, is digested on a hot plate for 15 minutes, then filtered through a Gooch containing a circle of filter paper over which is a matte made from one-quarter of a macerated filter paper (Whatman No. 40, 9 cm.). A qualitative analysis showed that this precipitate contains nickel, silicon, and iron as impurities. The precipitate is washed with warm water and then transferred to flask B and 5 ml. of bromine in hydrochloric acid (1 ml. of bromine in 100 ml. of 1.12 acid) are added. If, after shaking, the bromine color disappears and black particles still remain, more bromine solution is added, dropwise, until the bromine color is permanent. The mixture is refluxed in a 40-cm. (16-inch) condenser for 5 minutes, with a connecting tube from the top of the condenser dipping into a test tube containing 5 ml. of water to trap bromine. The tube is disconnected a t the top of the condenser, the burner removed, and through the condenser tube are poured 50 ml. of water t o which has been added 1 ml. of a saturated solution of acetanilide in alcohol. Excess bromine not removed by refluxing is discharged by the acetanilide. (Of several reducing agents tried, acetanilide was found most satisfactory.) After removing the flask, 20 ml. of 2.5 per cent sodium fluoride and starch solution are added, and the solution is cooled to Z O O , diluted to 150 ml., and titrated by the Norris and Fa method, using approximately 0.02 N solutions of sodium thiosul&te and iodine.
As a means of evaluating this work, a gravimetric method was developed which depends on separating the selenium from the steel by distillation as selenium tetrabromide, its reduction to elemental selenium, and weighing as such. The selenium tetrabromide distillation method was first used by Gooch and Pierce (1) and its reliability as an accurate method ably demonstrated by the work of Robinson, Dudley, Williams, and Byers (4). A 5-gram sample of steel is treated with 50 ml. of 1.18 hydrochloric acid in a special distilling flask fitted with a ground-glass funnel tube (Figure 2). A low flame is used until the steel is
MAY 1.5, 1936
:
ANALYTICAL EDITION
decomposed and then thk flame is increased and acid distilled until only 5 to 10 ml. remain in the flask. The distillate is caught in a flask under biomine water (20 ml. of saturated bromine diluted to 50 ml.). Air is aspirated through the apparatus and bromine added to the cooled distilling flask through the funnel tube until its vapors appear in the delivery tube. Suction is discontinued, and 10 ml. of 1.49 hydrobromic acid are added and distilled down t o the former volume. This distillation is repeated two more times using 10 ml. of 1.49 hydrobromic acid and 0.5 ml. of bromine each time. The receiving flask is disconnected, the acidity made 4 t o 5 N (adding hydrochloric acid if necessary), and the solution filtered. Bromine is discharged with sulfur dioxide, 2 ml. of 25 per cent hydroxylamine hydrochloride are added, and the flask is placed on a hot plate to diges;t at 80” overnight. The precipitate is collected on asbestos in a Gooch crucible, washed with warm water and then with ethyl alcohol, dried at 80°, and weighed with a tare. (Besides lowering the temperature of drying, alcohol aids in protecting finely divided selenium from oxidation.) To test the accuracy of the distillation method, 0.02 mg. of selenium in solution as selenious acid was added to a distillstion residue shown to be free from selenium and the distillation method employed. A pink coloration on the subsequent treatment with hydroxylamine hydrochloride showed to the authors’ satisfaction that the amount of selenium not removed in an average sample of steel by this method is less than 0.02 mg. By similar experiments Robinson, Dudley, Williams, and Byers demonstrated the accuracy of this method for removing minute amounts of selenium from other materials. Furthermore the insoluble portions of residues from the distillations of selenium steels were carefully tested for selenium with negative results. A selenium steel with the manufacturer’s analysis of 0.254 per cent of selenium was analyzed by the distillation method. A percentage of 0.241 was obtained for each of three determinations when the values were rounded off a t the third place. On the same sample the control method used by the Carpenter Steel Company was employed, except t h a t the selenium precipitate formed was redissolved with bromine in hydrochloric acid solution and reprecipitated with hydroxylamine hydrochloride. I n the authors’ opinion reprecipitation is necessary to free the selenium from impurities. For each of two determinations a percentage of 0.241 was found, which is in excellent agreement with those obtained by the distillation method. The authors’ volumetric method was now used on this sample, 50 determinations giving an average of 0.240 per cent with an average deviation less than 0.002 per cent. This method is considerably more rapid than the gravimetric methods, several analyses being easily accomplished in 1.5 hours. Another specimen of steel, which according to the manufacturer contained 0.230 per cent of selenium, was analyzed by the disdillation method. The values obtained-0.216, 0.216, 0.218, and 0.216 per cent-are lower, and in the authors’ opinion this is additional evidence that the manufacturer’s analyses give results that are consistently too high. Carefully purified potassium iodate, bromate, and dichromate were used to standardize the thiosulfate solution. It was found that the Korris and Fay method could be modified to the extent of carrying on the titration a t room temperature (20”) if the excess of sodium thiosulfate solution was not more than 5 ml. of 0.02 N solution. I n all titrations the concentration of hydrochloric acid was 0.2 N in a volume of 150 cc Two standard solutions of selenium were used for reference: (1) a solution of selenious acid standardized by precipitating selenium with hydroxylamine hydrochloride and weighing, and (2) a solution of sodium selenocyanate made by dissolving a known weight of highly purified selenium in sodium cyanide and diluting to a definite volume. The selenium precipitated from the selenious acid solution was determined under the same conditions used in the steel analysis and found
197
FIGURE 2. DISTILLATION APPARATUS A . 500-ml. distilling flask B . 250-ml. Erlenmeyer flask containing 20 ml. of saturated bromine water diluted to 50 ml. C . 250-ml. Erlenmeyer flask containing 60 ml. of water
to vary from the standardization value by less than 1 part per 1000. Again on precipitating the selenium from the standard sodium selenocyanate solution and carrying on the procedure used in the steel analysis, results were obtained agreeing as closely with the standardization value. In 150 ml. of solution containing 10 mg. of ferric chloride, 20 ml. of 2.5 per cent sodium fluoride, 0.2 N with hydrochloric acid, gave no starch iodine color when 2 grams of potassium iodide were added. This amount of iron is in excess of that always present with the selenium precipitate. Filtrates were examined for selenium by allowing them to stand for 12 hours and then filtering through filter paper. Appreciable amounts were recovered until the method of digesting for 15 minutes in the presence of macerated filter paper was adopted. Six successive filtrates were then found to be free from selenium. The authors are convinced that small amounts of selenium are commonly lost on filtration unless special precautions are taken. Digestion in the presence of macerated filter paper effectively prevents the loss of colloidal selenium.
Acknowledgment The steel samples used in this work with the analyses reported by the company’s laboratory were supplied through the courtesy of the Carpenter Steel Company, Reading, Pa.
Summary A modification of the Norris and Fay volumetric method has been developed for the determination of selenium in steel. The reagents used are inexpensive; the cost for an analysis is less than 4 cents. The method is rapid and accurate, and could be applied in any determination of selenium when it is possible to precipitate the free element.
Literature Cited (1) Gooch and Pierce, “Methods in Chemical Analysis,” New York, John Wiley & Sons, 1912. (2) Marvin and Schumb, IND. ENQ.CHEX.,Anal. Ed., 8 , 109 (1936). (3) Norris and Fay, Am. Chem. J . , 18, 705 (1896). (4) Robinson, Dudley, Williams, and Byers, IND.EZG.CHEM.,Anal. Ed., 6, 274 (1934).
RECEIVED December 2, 1936
