926
INDUSTRIAL AND ENGINEERING CHEMISTRY
The recovery of boric acid is accomplished by acidifying the boric acid sludge with sulfuric acid or sulfuric acid sludge, followed by recrystallization. Approximately 1.3 pounds of 95 per cent sulfuric acid or its equivalent are necessary for each pound of sodium h droxide removed from the oil. The acidified s ent boric acid? solution containing sodium sulfate is kept a t a&out 200’ F. for approximately 60 minutes in order to effect separation of oily matter with which it is usually contaminated. After the oil is skimmed off, the solution is cooled to about 90” F. to precipitate most of the boric acid. This tem erature was chosen to prevent simultaneous crystallization o r sodium sulfate. The solubility characteristics of boric acid in water and in sodium sulfate solutions are illustrated in Figures 4 ( 1 ) and 5, respectively. The precipitate is separated from the remaining solution by filtration. This procedure yields wet boric acid containing approximately 30 per cent by weight of the mother liquor. The recovered acid on a moisture-free basis is thus composed of 91 to 92 per cent boric acid and 8 to 9 per cent sodium sulfate. The presence of mater and sodium sulfate in the recovered acid does not interfere with the de-ashing process, as shown by experimental data given in Tables I V and V. The mother liquor is concentrated to 50 to 55 per cent of its original volume and the preci itated sodium sulfate is separated by filtration. The remaining {quid is recycled in order to increase the boric acid recovery. Total recovery of boric acid is approxi-
Vol. 14, No. 12
mately 90 per cent by weight of that originally charged to the unit.
Acknowledgment The authors wish to express their appreciation to J. B. Rather, in charge of the General Laboratories of the SoconyVacuum Oil Company, Inc., for permission to publish this manuscript.
Literature Cited (1) (2) (3) (4)
(5)
Comey, A. M., and Hahn, D. A., “Dictionary of Chemical Solubilities (Inorganic)”, 2nd ed., New York, Macmillan Co., 1921. Fischer, H. G . M., Canadian Patent 281,168 (June 19, 1928). Hendrey, W. B., U. S. Patent 2,133,094 (Oct. 11, 1938). Kalichevsky, V. A,, and Stagner, B. A., “Chemical Refining of Petroleum”, rev. ed., p. 163, New York, Reinhold Publishing Corp., 1942. Scafe, E. T., U. S. Patent 2,222,596 (Nov. 26, 1940).
PBEBENTED before the Division of Petroleum Chemistry at the 104th Meeting of the AMBRICAN CHEMICAL SOCIETY, Buffalo, N. Y.
Determination of Selenious Acid and Selenites A Rapid Volumetric Method D. F. ADAMS AND L. I. GILBERTSON State College of Washington, Pullman, Wash.
A
LTHOUGH adequate colorimetric methods have been developed for the determination of minute quantities of selenium in organic matter, the present methods for the determination of selenious acid and selenites are not entirely satisfactory. Of the available methods, the method of Norris and F a y (8), titration with standard thiosulfate, described b y Coleman and McCrosky (3) appears t o be most commonly used. Coleman and McCrosky (4)have described the quantitative oxidation of colloidal selenium t o selenite with a number of oxidizing agents. The gravimetric determination of selenium described b y Lenher and Kao (7), involving sulfur dioxide reduction of selenite to selenium, is long and is subject to the same criticism in so far as the determination of selenite in the presence of selenate is concerned. The method described here was developed to provide a rapid determination of selenite in salts or solutions of selenious acid. This determination can be used in the presence of the corresponding selenates. The method consists in the oxidation of the selenite ion to selenate ion in nitric acid solution by an excess of standard bromate solution, which is in turn backtitrated with standard arsenite solution, using Fast Red B indicator. That a bromate in nitric acid solution would oxidize selenite to selenate was shown by Blumenthal ( 1 ) in 1913. The use of bromic acid in volumetric analysis was sug ested by Kratschmer (6) in 1885. Smith (10)has demonstrate$ the stability of bromate solutions in the resence of nitric acid and bromine. Shaver and McCrosky (9) Bescribed a method by which selenium is oxidized quantitatively to selenite by bromate in hydrochloric acid solution. An electron beam sectrometer was used to determine the end point of the titration. Feit and Xubievschky (6) described a method by which an excess of bromate in acid solution was used as a volumetric oxidant, boiling off the bromine formed
in the reaction and back-titratin the excess of bromate iodometrically. The method describefis a modification and improvement on this method.
Preparation of Reagents ARSENITESOLUTION.Prepare a 0.1 N solution by dissolving 4.95 grams of arsenious oxide in 50 ml. of 1 N sodium hydroxide solution. Neutralize to litmus with 1 N sulfuric acid and dilute to 1 liter. POTASSIUM BROMATE SOLUTION.Prepare a 0.1 N solution by dissolving 3.34 grams of potassium bromate and make up to 1 liter, using 50 ml. of concentrated nitric (sp. gr. 1.42) acid and 950 ml. of distilled water. STANDARD SELENIOUSACID SOLUTION.Standard selenious acid solutions were prepared by two methods. (1) Resublimed selenium dioxide ( 8 ) was dissolved in distilled water and the selenium content determined gravimetrically. (2) Reprecipitated selenium was weighed out, oxidized to selenious acid with concentrated nitric acid, and diluted quantitatively to a known volume. PRIMARY STANDARD SELENIUM.Resublimed selenium dioxide was reduced by sulfur dioxide to selenium, which was washed with water containing sulfur dioxide, then with alcohol and ether, successively displacing each other to avoid oxidation. The selenium was dried a t 105’ C. FAST REDB INDICATOR SOLUTION.The indicator solution was prepared by dissolving 0.5 gram of Fast Red B in 100 ml. of distilled water.
Procedure DETERMINATION OF ARSEINITID-BROMATE RATIO. Measure about 35 ml. of the arsenite solution into a titration flask,and add 10 ml. of concentrated hydrochloric acid and 2 drops of Fast Red B indicator solution. Add potassium bromate solution, from a buret, until a fading of the indicator warns that the end point is being approached. Add another drop of indicator and titrate dropwise until the red color is destroyed. Calculate the ratio of arsenite to bromate,
December 15, 1942
ANALYTICAL EDITION
STANDARDIZATION OF POTASSIUM BROMATE SOLUTIO.?'.Weigh out about 0.12 to 0.15 gram of reprecipitated selenium and oxidize with 1 ml. of concentrated nitric acid. The solution may be heated to hasten the dissolving of the selenium, but boiling must be avoided to prevent the sublimation of selenium dioxide. Dilute with 25 ml. of water and add a calculated excess of potassium bromate solution. Boil until all the bromine has been expelled. Cool the flask somewhat and add a quantity of arsenious acid solution calculated to be sufficient to reduce the excess bromate. Add 10 ml. of concentrated hydrochloric acid and 2 drops of Fast Red B indicator solution. Titrate with potassium bromate until the red color is destroyed. Calculate the selenium and selenium dioxide titer of the bromate solution. BrOa-
+ 3As02-
+
Br-
+ 3AsOa-
DETERMINATION OF SELENIOUS ACID. Weigh or aliquot a sample of selenious acid containing about 0.10 to 0.30 gram of selenium dioxide, add a calculated excess of potassium bromate solution, and boil until bromine-free. Add from a buret a quantity of arsenious acid calculated to be sufficient to reduce the excess bromate. This may involve a preliminary determination to find the quantities of bromate and arsenite required for a given sample. Add 10 ml. of concentrated hydrochloric acid and 2 drops of Fast Red B indicator solution and titrate with bromate to a colorless condition. The volume of solution used should be kept to about 50 ml. and the final titration should be carried out a t or below a temperature of 80" C. 2BrOa-
+ 2H+ + 5SeOa--
-+
Br2
+ 5SeO,-- + HZ0
TABLE I. DETERMINATION OF SELENITE HNOs in 50 XI1 of Solution Ml. 0.5 0.6 0.7 0.8 0.9 1.0 1.1
1.2 1.3 1.4 1.5
SeOz
Found Gram
Eirur
0.3134 0.3188 0.3192 0.3185 0.3192 0.3189 0.3188 0.3194 0.3189 0.3189 0.3191
- 0.0056 - 0.0002
Gram f0.0002
-0.0005 +-0.0001 0.0002 - 0.0002 +-0.0001 0.0004
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A sample of selenious acid was prepared from resublimed selenium dioxide and was analyzed gravimetrically by precipitating and weighing t h e selenium. The results compared with the volumetric determination as follows : Gravimetric Gram
Volumetric Gram
0.1000 0.1000
0 1000 0.0996 0.1002
Certain cations, which form insoluble selenates, were found to interfere with the determination. Barium and lead selenites could not be analyzed by this method. Other cations which were easily oxidized and reduced, such as ferric and chromic ion, interfered. Colored ions, such as nickelous and cupric ion, made it difficult to observe the end point of the determination. Hon-ex-er, if an additional amount of indicator nere added analyses of these selenites could be carried out satisfactorily. I n the presence of 0.4 to 0.5 gram of copper selenate, aliquots containing 0.1009 gram of selenium dioxide were analyzed, using an excess of the indicator to make the end point observable: taken, 0.1009 ?am; found, 0.1011, 0.1011, and 0.1009 gram of selenium dioxide. Colorless cations which form soluble selenates did not interfere with the determination. Anions forming soluble salts with the cations present in solution were found not to interfere, with the exception of the halide ions, which mere required to be absent from the original sample because of the reaction of halide ions with bromate ion. An excess of nitrate ion did not interfere. 4 solution of selenious acid containing 0.3240 gram of selenium dioxide was analyzed, 2.0-gram portions of several salts being added to the respective aliquots with results as follows: Salt Added
SeOz Found Gram
- 0.0001
+0.0001
0.3240 0.3236 0,3242 0,3236 0.3238 0.3236 0,3242 0 . sa46 0,3240 0.3244
Experimental EFFECTOF KITRATE CONCEXTRATIOS.Blumenthal (1) found t h a t nitric acid must be present if bromate is to oxidize selenite t o selenate. Smith (10) has shown that nitric acid does not decompose boiling solutions of bromate. To determine the limiting concentration of nitric acid needed for the oxidation, 25-ml. aliquots of standard selenite solution containing 0.3190 gram of selenium dioxide per aliquot mere analyzed using varying amounts of concentrated nitric acid (Mallinckrodt, c. P . analytical reagent, specific gravity 1.42, 70 per cent). Selenite content was determined as previously described with results shown in Table I. It is evident that only a small amount of nitric acid is necessary for the reaction and t h a t large amounts of acid do not interfere. It was found convenient to add the nitric acid directly to the bromate solution before standardization as has been described. RESULTS OF ANALYSISOF SELEXITE SOLUTIOIUS.Sainples of selenious acid mere prepared by weighing reprecipitated selenium, dissolving i t in nitric acid, and diluting. Aliquots were analyzed with results as follows: SeOz Taken Gram
SeOn Found Gram
0,2507
0.2505 0.2507 0.1010 0.1010 0.1010 0.3189 0.3188 0 3192
0.1009 0.3190
Addition of large quantities of alkali and alkaline earth cations, except barium, did not interfere nor did the presence of selenate, sulfate, or tellurate ion.
Discussion This method affords a means of determining selenites and selenious acid in the presence of selenates. It is rapid and accurate, comparing favorably in accuracy with the gravimetric methods. The determination is particularly well adapted to the analysis of selenious acid solutions and has been found useful for the determination of purity of selenates which contain selenites.
Literature Cited (1) Blumenthal, P. L., Am. J . Sci., 4, 35 (1913). (2) Booth, H . S., "Inorganic Syntheses", T'ol. I, pp. 119-20, New
York, McGraw-Hill Book Co., 1939. (3) Coleman, TV. C., and McCrosky, C . K., IND.ENG.CHEM.,ANAL. ED.,8, 196 (1936). (4) Coleman, W.C., and McCrosky, C . R., J . Am. Chem. Soc., 59, 1458-60 (1937). (5) Feit and Kubievschky, Chem.-Ztg., 15,351 (1891). (6) Kratachmer, Z . anal. Chem., 24, 546 (1886). (7) Lenher, V., and Kao, C., J . Am. Chem. Soc., 47, 2454 (1925). (8) Norris and Fay, Am. Chem. J . , 18,705 (1896). (9) Shaver and McCrosky, I N D .ENG.C H E h f . , ANAL.ED., 12, 74 (1940). (IO) Smith, G . F., J . A m . Chem. Soc., 45,1115-21 (1923).
