Qualitative Method for Selenium in. Organic Compounds M. J. HORN,Protein and Nutrition Division, Bureau of Chemistry and Soils, U. S. Department o l Agriculture, Washington, D. C.
W
HEAT plants grown by the Bureau of Plant Indus-
try a t the Arlington Experiment Farm on normal soil to which had been added from 1 to 20 parts per million of selenium in the form of sodium selenate were believed to contain small quantities of selenium. Only a limited supply of these plants was available for analysis, and a quick method which would require only a gram or so of material was desired to test this material for selenium. The method of Schmidt (S), used by Stover and Hopkins (4seemed , to be the quickest procedure available. These workers digested powdered, dried leaves with 10 cc. of concentrated nitric acid until only 2 or 3 cc. remained. This operation with nitric acid was repeated. Then 5 cc. of sulfuric acid were added and the mixture was boiled, a few drops of nitric acid being added from time to time t o hasten the oxidation. When a straw-colored solution was finally obtained, the boiling was continued until dense white fumes were evolved. The solution was cooled and brought up t o definite volume and 0.01 gram of codeine was added. The presence of selenium was indicated by the formation of a green color, changing gradually to a blue-green and finally t o a pure blue color. The disadvantages of this method are (1) the possibility of losses of selenium on digestion with nitric acid, (2) the difficulty of removing all traces of the nitric acid, and (3) the time consumed in first digesting the material with nitric acid and then removing the nitric acid with sulfuric acid. It is necessary to remove all traces of nitric acid in the digest; otherwise a deep red color is obtained when codeine is added. It was desired, therefore, to eliminate the use of nitric acid if possible, and the Kjeldahl digestion method for nitrogen determination offered possibilities. Besides giving a smooth, quick method of oxidizing the organic material, digestion of the material with sulfuric acid and mercuric oxide offered advantages in the probable formation of a double salt of mercruy with selenium, which lessens the possibility of a loss of selenium by volatilization, and in the change of the inorganic salts occurring in the plants to a state of oxidation that does not give a blue color with codeine. Lyons and Shinn (I) showed that added salts, such as mercuric chloride and zinc chloride, appear to form a double salt with the selenium holding it in solution. Schmidt (3) showed that ferric iron in quantities of 1 drop of strong ferric chloride solution in 10 cc. of concentrated sulfuric acid will interfere with the test. Weighed uantities (0.01 gram) of metallic selenium were digested for I l o u r with concentrated sulfuric acid and a little mercuric oxide, after the manner of Kjeldahl nitrogen digestions. The sulfuric acid digest was cooled and made up t o 50 cc. with concentrated sulfuric acid. An aliquot of 5 cc. was taken and 4 drops of a 3 per cent aqueous solution of codeine sulfate were added. The color developed was compared in a Bock Benedict colorimeter against the color similarly developed in a standard solution containing 0.01 gram of metallic selenium dissolved at room temperature in 50 cc. of concentrated sulfuric acid. The standard solution was set in the comparator at 20 mm. The comparison of the color intensities showed that no loss of selenium had occurred during the digestion. Further comparisons made against the same standard with 5-00. aliquots of the digest diluted 50 er cent and 100 per cent with concentrated sulfuric acid gave tge same result. Two grams of ferric sulfate were digested for 1 hour with 50 co, of sulfuric acid and mercuric oxide in the manner described. A test of this digest with codeine sulfate gave no blue color, the solution remaining colorless t o light yellow. 34
Solutions were made up with concentrated sulfuric acid containing 0.0005 t o 1.0 gram per liter of selenium. These were digested with sulfurlc acid and mercuric oxide, and a 5-cc. aliquot of each was tested in a test tube with 2 drops of 3 per cent codeine sulfate solution. All gave visible blue colors, the limit of detection by this method being less than 0.5 part per million. Plants grown on soil to which selenium had been added and which were known to contain selenium and similar plants grown on normal soil were furnished by the Bureau of Plant Industry. I n every test with these plants those known to contain selenium gave a blue color on addition of codeine sulfate solution, while the plants which contained no selenium remained colorless. Tests were then made on wheat straw, corn, wheat flour, wheat leaves and stalks, and soils, only 1 gram of the dried material being used. The test requires no more time than that required for a Kjeldahl digestion for nitrogen. In order to ascertain just what salts might interfere with this test under the conditions of the experiment, Daniel Ready, of the Bureau of Chemistry and Soils, tested 17 inorganic elements: chromium, nickel, titanium, beryllium, molybdenum, thallium, tellurium, vanadium, boron, antimony, bismuth, arsenic, iodine, manganese, iron, silicon, and tungsten. Only vanadium interfered with the test, giving a dark greenish blue solution on addition of codeine sulfate. Arsenic on digestion with plants gave no color, but on digestion with soils it gave a blue color which rapidly disappeared, leaving the solution a light yellow. There is no evidence that vanadium salts are present in plant material, although they are present in some soils. Ready added 2 mg. of the salt of each element to 5 grams of the soil, and digested the mixture with 50 cc. of sulfuric acid. This method can be made fairly accurate as a quantitative method if fresh solutions of definite quantities of selenium are made up in sulfuric acid and comparison is made with the unknowns. For quantities ranging from 1 to 5 parts per million it is quite accurate, but when larger quantities are present the color is not permanent. Also, on standing the codeine is gradually acted upon by the sulfuric acid, giving a brown color which interferes with the test. A digest on standing in the open air for several hours absorbs sufficient moisture to interfere with the test. It is of interest to note that the blue color developed by adding 2 drops of codeine sulfate solution in 5 cc. of concentrated sulfuric acid containing 10 parts per million of selenium will gradually disappear if water is added drop by drop. Digests which will not give a test with codeine sulfate, because of absorption of water on standing in the air, can be heated for 15 minutes to distill off the water. The digest will then give the test in the usual way. It is important, therefore, that the codeine be added to the cooled digest as soon after the digestion as possible.
EXPERIMENTAL One gram of the dried sample is put in a Kjeldahl flask and 40 cc. of concentrated sulfuric acid are added, together with 0.2 gram of mercuric oxide. The mixture is then digested until colorless, and the digest is cooled and made up t o definite volume if desired, with concentrated sulfuric acid. To 5 cc. of the cooled digest in a test tube are added 2 dro s of B 3 er cent aqueous solution of codeine sulfate, with cooli)ng and &&king after each drop. If the digest contains selenium, a green color develops
January 15, 1934
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
and then rapidly changes to blue. When the method is applied to soils, the cooled digest should be centrifugalized, and the test made on the clear supernatant solution. Table I shows the results of tests made on samples obtained from the Bureau of Plant Industry. The plus marks denote the relative intensities of color developed by the above test; the figures denote an approximate quantitative estimation made by comparison with fresh standards. All samples taken were 1 gram each in 50 cc. of sulfuric acid. Quantitative determinations made on some of these samples by other methods show the method to be fairly acaurate.
35
TABLE11. TESTS ON MATERIALS KNOWN TO CONTAIN SELENIUM AND OTHERS KNOWN TO CONTAIN No SELENIUM Straw (R) Normal straw Corn (R) Normal corn Wheat (R) Normal' wheat Soil (R) Normal soil
Positive Negative Positive Negative Positive Negative Positive Negative
A quick accurate method has been developed for the detection of selenium in organic compounds. The results on samples known to contain selenium and on those containing no selenium are described. Vanadium interferes with the TABLEI. TESTSON WHEATLEAVES AND STALKS CONTAININGtest. VARIABLEAMOUNTSOF SELENIUM APPROXIMATE SAMPLE
101 Wheat leaves 102 Wheat leaves 103 Wheat leaves 104 Wheat leaves 105'1 Wheat leaves 108 Wheat leaves 112 Stalks
UALITATIVE %TIMATION
+++++ ++ +j-c+++
++++++ ++++++
UANTITATIVE %TIMATION P.I). m.
io
10 2 20 0 25 25
a Sample 105 was grown on soil containing no selenium, the others on soil containing different quantities of selenium.
Table I1 shows a comparison of substances containing selenium and the normal substance. (R) denotes samples containing selenium determined by Robinson's gravimetric method (2).
ACKNOWLEDGMENT
The author wishes to express his thanks to A. M. HurdKarrer for the numerous samples supplied in connection with this investigation.
LITERATURE CITED (1) Lyons, R . E.,and Shinn, F. L., J. Am. Chem. Soc., 24, 1087 (1902). (2) Robinson, W. O., J . Assoc. Oficial Agr. Chem., 16,423 (1933). (3) Schmidt, E.,Arch. Pharm., 252, 161 (1914). (4) Stover, N. M., and Hopkins, B. S.,
[email protected]., 19, 510 (1927). RECEIVED September 13, 1933. Presented before the Division of Agricultural and Food Chemistry a t the 86th Meeting of the American Chemical Society, Chicago, Ill., September 10 to 16, 1933.
Platinized Silica Gel as a Catalyst in Gas Analysis 11. Oxidation of the Methane Hydrocarbons KENNETH A. KOBEAND E. BRUCEBROOKBANK Department of Chemical Engineering, University of Washington, Seattle, Wash. N A PREVIOUS paper (3)it has been shown that the copper oxide tube on the gas analysis apparatus may be replaced by a similar tube containing a commercial platinized silica gel which is an efficient catalyst for the oxidation of hydrogen and carbon monoxide. Hydrogen may be quantitatively oxidized at 100" C.; carbon monoxide acts as a catalyst poison at this temperature, making it necessary to raise the temperature to 300" C. to oxidize the carbon monoxide quantitatively. Although methane is not oxidized a t 300" C., nothing is known of the oxidation of ethane and higher hydrocarbons under these conditions. It is common practice to determine hydrogen and carbon monoxide together by oxidation at 300" over copper oxide. In order to determine the possibility of oxidizing hydrogen and carbon monoxide together at 300" in the catalyst tube in the presence of hydrocarbons found in technical gases, the conditions under which methane, ethane, propane, and butane are catalytically oxidized over platinized silica gel were determined.
of methyl iodide on a zinc-copper couple (7). Ethane was prepared by the hydrolysis of ethyl magnesium iodide. The propane was commercial Shellane obtained from the Shell Oil Company and was approximately 97 per cent propane. Butane was prepared by the hydrolysis of butyl magnesium bromide. The hydrocarbons after being prepared were analyzed for hydrocarbon content and then diluted with oxygen before use.
OXIDATIONOF HYDROCARBONS
The hydrocarbon-oxygen mixture was passed through fuming sulfuric acid to remove any unsaturates, then over the catalyst a t 100" C. to remove any hydrogen. The procedure in using the catalyst tube for hydrocarbon oxidation was exactly the same as that used in the determination of hydrogen. The gas mixture was passed at a rate of 30 t o 50 cc. per minute through the catalyst tube heated to the desired temperature. Eight passes were made, as this is in excess of the number needed for the oxidation of hydrogen. The results are shown in Table I. APPARATUSAND GASES The results with methane show that it is possible to oxidize The apparatus was that used in the previous work ( 3 ) . hydrogen and carbon monoxide in the catalyst tube a t a The catalyst is the commercial platinized silica gel contain- temperature of 300" C. without oxidizing methane. The oxidation products from methane and ethane were ing 0.075 per cent of platinum produced by the Silica Gel Corporation for the oxidation of sulfur dioxide to trioxide. examined for unsaturated compounds, but none were found. Methane was prepared by the action of an alcohol solution Thus the high ratio of contraction to carbon dioxide is not
