INDUSTRIAL AND ENGINEERING CHEMISTRY
408
tested with success the possibility of using formalin for the selective precipitation of selenium. The use of sodium sulfite and formalin permits simultaneous determination of free selenium and free sulfur, the latter by the method of Bolotnikof and Gurova (S), which consists of iodometric titration of the hyposulfite formed. This method has been tested on three mixtures of commercial selenium and sulfur, with the results shown in Table 11. TABLE 11. DETERMINATION OF SELENIUM AND SULFUR IN MIXTURES Selenium Taken Gam 0.1614 0.1442 0.1700
Selenium Gram 0.1492 0.1431 0.1686
Found % 98.5 98.8 99.1
Sulfur Taken Gram 0.0648 0,0201 0.0190
Sulfur Found Grana % 0,0605 93.4 0.0190 94.5 0,0180 9 4 . 5
The method of determining free selenium b y means of extracting it with sodium sulfite has been checked b y the authors on several rubber stocks, compounds 1, 2, and 3. The rubber was milled and cut into pieces 1 mm. square with scissors. Two different curves of No. 1 were used.
Smoked sheets Zinc oxide Synthetic rubber Stearic acid Captax Selenium Sulfur
Rubber Formulas No. 2 No. 3 100 100 5
. 1. .
...
0.5 3 2
100 3 1.4 6 1
111.5
211.4
-
_ .
When using the authors’ method of degradation, approximately 30 hours are required to extract free selenium from rubber, the greater part of the selenium being extracted during the first 12 hours. For determining free selenium, about 1 gram of rubber was placed in a 500-cc. Erlenmeyer flask, 200 cc. of 10 per cent sodium sulfite solution were added, and the mixture was boiled under reflux for 30 hours. A second treatment of the rubber was made with a fresh portion of sulfite solution to insure complete extraction. After cooling, the solution was filtered and the rubber washed with hot water. The flask containing the filtrate was again connected with the condenser, 75 cc. of formalin were
VOL. 7, NO. 6
added, and the solution was boiled for about an hour. After settling, the selenium was filtered on a Schott’s filter and rinsed with water. The filtrate was transferred to another flask and the free sulfur determined. The residue on the filter was washed with hydrochloric acid and hot water until free of chlorides, then with alcohol and ether, and dried to constant weight. Table I11 gives a summary of the determination of free selenium in these rubber compounds. TABLE111. DETERMINATION OF FREESELENIUM IN RUBBER Time Rubber
of
Cure
Min
STOCKS Free Selenium Sulfur (Calculated) Found
.
%
15
..
2.71 2.71 2.71 2.71 2.69
..
2.84
40
Free Combined Selenium Selenium Found by Difference
%
%
%
....
2.38 2.36 2.17 2.15 2.29 2.30 2.27 2.15
0.33 0.35 0.54 0.56 0.40 0.39 0.57 0.69
o:is
0.16 0.15 0.18
Conclusions The total selenium in rubber compounds may be determined b y disintegrating with nitric acid and then separating the selenium b y reducing the nitric acid solution with hydrazine sulfate, or by reducing with sulfurous acid after evaporation of the nitric acid. Free selenium is determined by extracting the rubber with aqueous sodium sulfite, from which selenium is precipitated by formalin. Free sulfur is determined volumetrically b y iodometric titration after precipitating selenium from sulfite solution.
Literature Cited (1) Barkovsky, A. A., and Babalova, A. A., J. Plant Laboratory (U.S. S. R.), 4, 306 (1934). (2) Blake, J. T., IND.ENG.CHEM.,22, 737, 740, 744, 748 11930). (3) Bolotnikof, V., and Gurova, V., J. Rubber Ind. (U. S. S. R.),10, 61-2 (1933). (4) Shaw, E. H., Jr., and Reid, E. E., J . Am. Chem. Soc., 49, 2330 (1927). (5) Wagenmann and Triebel, Chem. Zentr., 11, 273 (1930). R ~ C B I V EJune D 19, 1935.
Colorimetric Determination of Manganese in the Presence of Titanium GEORGE J. HOUGH, Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
THE
manganese content of crystalline rocks, according to Hillebrand ( I ) , rarely exceeds 0.3 per cent. Long experience with soils in this laboratory shows the manganese content in the average soil of the United States to be about 0.05 to 0.06 per cent. When present in such small quantities manganese is usually determined by a colorimetric method, and the procedure generally used is oxidation to permanganic acid by means of ammonium persulfate. The method for soils commonly used in this laboratory ( 2 ) is as follows : One gram of the soil is ignited in a small platinum dish to destroy organic matter, and is then digested on the hot plate with 25 cc. of hydrofluoric acid and 5 cc. of sulfuric acid until white fumes appear. It is cooled, diluted with water, warmed gently until all salts are in solution, and then filtered through a close filter to insure a perfectly clear solution. The filtrate is transferred to a 100-cc. volumetric flask, 1 cc. of 0.5 per cent silver nitrate solution is added, and then 1 gram of ammonium persulfate. The flask is put on the steam bath for 20 to 30 minutes
until the color is fully developed. I t is then cooled, made up to volume, and compared with a suitable standard in a colorimeter. Another convenient way to prepare the sample is to fuse, after ignition, with about 10 grams of potassium pyrosulfate and 1 gram of sodium fluoride; heat to fumes for some time, cool, extract with hot water, boil, filter, and treat as above with persulfate. The standard manganese solution is best prepared from c. P. manganese sulfate and for soils and rocks shouldhave avalue of about 0.0002 gram h n 0 per cubic centimeter.
It is not good practice to take the theoretical percentage of manganese in the manganese sulfate as purchased, as the amount of water held by the salt is variable. Therefore the actual percentage of manganese should be determined gravimetrically, preferably as the phosphate. It is perhaps not generally known that if titanium oxide is present in the solution in amounts exceeding 1 per cent, the persulfate method is useless, as no color is developed unless excessive amounts of reagents are used, and even then one cannot be certain t h a t the full color has developed. This
NOVEMBER 15, 1935
ANALYTICAL EDITION
vias observed recently during the analysis of some soils that were unusually high in titanium (2 to 12 per cent). In making the determination of manganese by the persulfate method no color developed at first, although there was good reason for believing that the samples contained some manganese. When more reagents were added, to an excess of four or five times the usual amount, the color finally appeared. In order to verify the interference of titanium, which also reacts with persulfates, a sulfuric acid solution containing 54 mg. of titanium oxide and 0.2 mg. of manganese oxide was treated in the usual manner, using 1 gram of ammonium persulfate; no color developed. It should be noted here that ammonium persulfate, with or without silver nitrate, produces the characteristic brown color with titanium only in concentrated solutions strongly acid with sulfuric acid. When i t was found t h a t titanium interfered seriously with the ammonium persulfate method, an attempt was made to eliminate the titanium t o avoid its interference. Certain textbooks state t h a t titanium is volatilized by evaporation with hydrofluoric acid unless sulfuric acid or some other acid is present. This statement is not entirely accurate, as shown by the following experiments: A solution containing 28 mg. of titanium oxide was precipitated by ammonia, filtered, and washed; the precipitate was then evaporated twice to dryness with 20 cc. of hydrofluoric acid. The residue was redissolved in acid, and the titanium determined gravimetrically; 24.2 mg. of titanium oxide were found. One gram of a soil colloid sample which contained 4.51 per
409
cent of titanium oxide (determined by the color method) was evaporated twice to dryness with 25 cc. of hydrofluoric acid. The residue was then fused with potassium yrosulfate, and titanium determined gravimetrically by the sulfurous acid method; 4.63 per cent of titanium oxide was found. One hundred milligrams of finely ground rutile were evaporated three times to dryness with 25 cc. of hydrofluoric acid. The residue was then fused with potassium pyrosulfate, and the titanium determined as above; 100 mg. of titanium oxide were found. Finally, a given weight of pure potassium titanium fluoride containing 70 mg. of titanium oxide was taken, and evaporated twice to dryness with 20 cc. of hydrofluoric acid. When the residue was analyzed gravimetrically, 68.5 mg. of titanium oxide were found. These tests show that while hydrofluoric acid alone may have a tendency to volatilize titanium, the effect is so slight, even on pure, freshly precipitated material, t h a t i t would be useless as a method of eliminating titanium for the purpose of preventing its interference in the persulfate color method. It is recommended that potassium periodate be used as the oxidizing agent in the presence of much titanium. Sodium bismuthate can also be used successfully, as there is no interference with the development of the manganese color.
Literature Cited (1) Hillebrand, W. F., U.8. Geol. Survey Bull. 305 (1906). (2) Robinson, W.O., U.8. Dept. Agr. Circ. 139 (1930). RECEIVED August
2, 1935.
Determination of Selenium in Organic Matter K. T. WILLIAMS AND H. W. LAKIN Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
IN 1934 a
Robinson, Dudley, Williams, and Byers (2) published series of methods for accurate estimation of small quantities of selenium, which included a modification suggested b y Van Kleeck for vegetation. This modification has been found generally satisfactory and perhaps the best hitherto available. Several thousand determinations have been made by its use (1). The method provides for the preparation of the sample by digestion with concentrated sulfuric acid as in the Kjeldahl method for nitrogen and subsequent distillation with concentrated hydrobromic acid and bromine. There are two unsatisfactory features in this procedure: In order to estimate quantities of selenium as low as 1 to 2 parts per million i t is necessary to digest 10 grams of the air-dry vegetation, and even with the aid of mercuric oxide as a catalyzer this requires 6 to 8 hours for completion. Also, unless the digestion is carefully regulated the evolution of gases-carbon dioxide, sulfur dioxide, and water vaporis so rapid that there is danger of loss of small quantities of selenium which are not absorbed by the bromine trap. The modification described below has been found to be much more rapid in execution, lends itself to routine work more readily, and is, when properly executed, free from all danger of selenium loss. To prepare a sample of air-dry vegetation it is first ground to A weighed sample, usually 10 grams, is stirred into a mixture of 50 CC. of concentrated sulfuric acid and 100 cc. of nitric acid in a 600-CC. Pyrex beaker. The mixture is stirred with a thermometer until it becomes homogeneous, after the first few minutes with gentle heating, without allowing the temperature t o rise above 100" C. After all frothing has ceased, the temperature of the mixture is raised t o a maximum of 120" C. until all evolution of nitrogen peroxide has ceased. The end of the operation is marked also by an incipient carbonization of the mixture, although longer pass a 2-mm. mesh sieve, then mixed and quartered.
TABLE I. SELENIUM CONTENT OF VEGETATION AS DETERMINED BY KJELDAHL AND WET-COMBUITION PROCEDURE8 Lab. No. B14447
Material
Selenium Content Kjeldahl Wet oomdigestion bustion P. p . rn. P. p . m.
Astragalus grayii
14448
Mixed vegetation
14481
Corn
14482 14796A 14863 14733A 147188 16187 16190 16193 16197 16199
Corn Tium racemosum Stanleya bipinnata Tium racemosum Tium racemomm Diholcos bisulcatus Vegetation, not identi6ed Sunflower Wild aster Corn
5
450 460 45 150 370 7 3 220 4
..
160 550 18 7 3 10 4
heating at or below 120' C. does little harm. After the mixture is cooled it is transferred to an all-gIass distilling flask, 100 cc. of hydrobromic acid and 1 cc. of bromine are added, and 75 cc. of distillate are collected. Care is taken that the first portion of the distillate contains a smalI excess of bromine. The further procedure-filtration t o remove wax, ,precipitation with sulfur dioxide and hydroxylamine hydrochloride, and subsequent estimation by color or weighing-% as previously described. The only important precaution to be observed is t o remove the nitric acid, completely, by digestion. If this is not done the distillate will contain nitroxyl tribromide, which is not decolorized by sulfur dioxide and requires excessive quantities of hydroxylamine hydrochloride. If the temperature be allowed to rise until fumes of sulfur trioxide are evolved, certain loss of selenium will ensue. In Table I are given the results obtained upon identical samples by the procedure suggested and by the Kjeldahl digestion.
