Analysis of Organoselenium Compounds JAMES D. MCCULLOUGH, TOD W. CAMPBELL,
AND
N. J. KRILANOVICH
Department of Chemistry, University of California, Lor Angeles 44, Calif.
A procedure developed for the determination of selenium in organic compounds involves combustion of the substance in a hydrogen-oxygen flame, followed b y collection and titration of the selenium dioxide formed. lodometric methods for the determination of the equivalent weights of certain types of organoselenium compounds reduced b y potassium iodide and of diary1 diselenides are also described. The methods are all rapid and accurate.
I
KVESTIGATION of the known methods for the analysis of organic compounds of selenium shoir-ed these procedures to 4e either tedious and time-consuming or inaccurate. In testing the available methods, best results were obtained by use of a modification of the combustion method of Niederl and Siederl (4) which is based on the methods of Alber and Harand ( 1 ) and Umezawa (5) and involves combustion of the substance in oxygen gas on a glowing platinum surface. However, it was found more expedient and fully as accurate to determine the selenium dioxide formed by an improved iodometric method based on that of van der Meulen (3). The titration as described by van der Meulen is erratic, particularly where hydrochloric acid is used or where chlorine is present in the substance being analyzed. This erratic behavior is due to oxidation of iodide by oxygen of the air which is apparently catalyzed by something present in the solution. This difficulty was overcome by modifying the method as described in the procedure below. In spite of this and other improvements, the method was still lengthy and required constant attention and skill during the combustiori. tt gives good results, however, in the hands of a skilled operator. The new analytical methods described in the present paper are uf two types: (1) the flame combustion of the organoselenium compound which leads directly to the determination of the selenium content and (2) volumetric methods which yield the equivalent weights of the compounds being analyzed. FLAME COMBUSTION METHOD
.1 method for the determination of halogens in organic compounds by flame combutjtion has been described by Winter (6).
The simplicity and accuracy of this procedure led the present workers to investigate its possibilities as a method for selenium analysis. Preliminary tests showed that selenium was oxidized completely to the tetravalent state when the organoselenium compound was burned in a flame supplied with an ample quantity of oxygen. In an ordinary air-supplied gas flame, part of the selenium came through as the element and formed a red deposit in the chimney.
.Ilthough only solid samples have been analyzed in this laborstory, there is no apparent reason why liquid and gaseous samples could not be handled if one modified the apparatus as described by Winter for halogen analysis. The sample size was adjusted to contain 20 to 80 mg. of selenium, the governing factors being the normality of the sodium thiosulfate, the size of the buret, and whether or not a method of aliquots is employed in the titration. The sample may be compressed into a pellet or weighed directly into the small platinum-foil boat, B . The weighed sample is introduced a t A and shaken down to B. Hydrogen gas is then introduced a t D a t a rate of about 200 ml. per minute and after the original air has been swept out, the gas is lit at C. The flame should be about 1 em. high. The oxygen is then slowly turned on until a flow rate of about 100 ml. per minute is reached. Once started, the flame is Very stable and does not go out during an analysis. The furnace and chimney assemblies are then brought into place and suction applied at J sufficient to give an air flow through the chimney of about 1 liter per minute, which is rapid enough to sweep all products of combustion into the absorber. Although successful analyses have been performed Tvithout the use of flowmeters, it is recommended that some kind of gage be placed in each of the three lines mentioned. The cutrent to the heater is then started and the voltage raised t o the point where a suitable rate of volatilization of the sample is obtained. Experience alone will tell the most satisfactory temperature for the vaporization of a given substance, but the operator soon becomes skilled in this regard. The otherwise colorless flame assumes a blue color, the intensity of which is proportional to the rate et which the selenium compound is being burned. This color serves as a good indication of the onset of vaporization as well as of the completion of the analysis. Most of the selenium dioxide formed in the reaction deposits in the tube at F , but a significant quantity makes its way to the glass-wool in the absorber, H. il large proportion of the compounds analyzed vaporized cleanly without decomposition or residue. However, in some cases decomposition aceompahies volatilization, so that a dark, nonvolatile, selenium-containing residue remains. When this is the case, the thermometer is removed and the temperature is gradually raised to the point where the compound slowly burns in the oxygen stream. This is the advantage of introducing the sample into the oxygen rather than into the hydrogen. In some cases the decomposition can be prevented by a vaporizing a t a higher temperature, the rate of vaporization being inF creased more than the rate of decomposition as the temperature rises. Increasing the rate of oxygen flow also helps in these cases.
00
APPARATUS AND PROCEDURE. The apparatus is shown in Figure 1. The burner, ABCD, is made of transparent quartz tubing. Although the dimensions do not seem to be critical, the inside diameters of the tubing actually used are 6 mm. for arm A B , 3 mm. for side arm D and the inner jet of burner C, and 10 mm. for the outer tube of the burner. The chimney and absorption tube, EFGH, are of Pyrex, the inside diameters being 30 mm. a t E , 6 mm. a t F , and 15mm. at H with a 10/30 f joint a t G. Other dimensions are shown roughly to scale in the figure. The absorption tube, H , is tightly packed with Pyrex glass-wool which had been soaked in chromic acid cleaning solution and thoroughly rinsed with water. I n use, the burner, thermometer, and lower part of the chimney are placed inside an electric furnace, L, contained in a 10 X 20 cm. Pyrex electrolytic beaker, K . The heater consists of 5.4 meters (18 feet) of 22-gage Chromel-A wire (1.0 ohm per foot) wound on a Transite frame and is energized through a variable transformer. The apparatus is assembled into three independently movable units consisting of the quartz burner and thermometer, the furnace, and the chimney and absorption apparatus.
L
Figure 1.
638
Flame Combustion Apparatus
ANALYTICAL EDITION
October, 1946 Table
I. Analysis of Organoselenium Compounds b y the Flame Combustion M e t h o d Selenium Formula Tbeors .inalssii
Compound Di-p-tolylselenium Dibenzyl diselenide Diphenyl diselenide Bis ( p - meihoxyphenyl) selenium Bis ( o - chlorophenyl) selenium p-Tolyl selenocyanate p Kitrophenyl selenocyanate Phenylseleninic acid .r (o - Biphenylylseleno) propionic acid
-
-
Table
n
%
Cl*HI+Se C~rHlrSe C1zHlaSez
30.23 46.42 50.60
30.29 46.53 50.51
-0 03 0.24 -0.18
Cr4H1&zSe
26.93
26.83
-0.37
C1zHsCIzSe CsH:NBe
26.14 40.27
26.14 40.3;
CiHJzOzSe CsHaOzSe
34.77 41.76
34,60 41,70
-0.49 -0.14
ClsHlr0,Se
25.87
25, i 7
-0.39
0.00 0.25
When combustion is complete, which requires i 5 to 45 minutes, che hydrogen and oxygen are stopped and the furnace is shut off and lowefed to permit the apparatus to cool. The suction is left on during the cooling process. When cool, the chimney is removed from the absorber and the selenium dioxide rinsed into a 250- or 500-ml. volumetric flask with water. By alternately applying suction to G and J,the glass-wool is rinsed with three or Four 25-ml. portions of water, each portion being drawn up and down slowly severak times, then added to the volumetric flask. This rinsing must be carefully done in order to remove all the selenious acid. The final rinse rvater should not show a color when added to an acidified solution of starch and potassium iodide. After rinsing is complete, the volumetric flask is filled to the mark and aliquots are titrated by the procedure below. TITRATION OF SELENIOCS ACID. The solution containing the selenious acid is boiled gently for a few minutes to remove dissolved oxygen, then cooled to room temperature in an ice-water mixture. Air is subsequently kept out of the flask by addition of small pieces of dry ice from time to time or by passage of carbon dioxide or nitrogen gas into the flask. hlthough this latter precaution is not absolutely necessary, more reliable results have been obtained in this way. To the solution are then added t5 ml. of 2% starch solution, 10 ml. of 1.5 .M potassium iodide, and 10 ml. of 6 sulfuric acid in the order named, mixing thoroughly after each addition. The solution is then titrated a t once with approximately 0.05 -V sodium thiosulfate. The end point is sharp and is marked by a change from a turbid brown to a transparent red color. 1.000 ml. of 0.05000 .V L-a&Oa = 0.9870 mg. of Se.
II. Reduction Products and Equivalents per M o l e for Various Types of Organoseleniurn Compounds Type of Reduction Equivalenta Caompound Product per Mo!e RzSeXz RzSe RzSeO RzSe RSeXa RzSez RSeX RzSer RSeOzH RzSer
Deviation
%
639
Table
Ill. Equivalent Weights of Some Selenium Compounds Reduced b y Potassium Iodide Equivalent Weight ExperiFormula Theory mental di-
C 5 rnpound Diphenylselenium chloride diDiphenylselenium bromide Di-p-tolylselenium dibromide Dibeumoselenophene diClnHsSeBrn bromide 4 bfeihoxydiphenylselenium dichloride C~aHlzOSeClz 4 - Bromodiphenylseleniuin dibromide ClzHpBrSeBrz 4 Methyldiphenylseleniuin dibromide CjaHnSeBr~ 4,4‘ Diethoxydiphenyl selenuxide (CsH9O)nSeO Phenylseleniurn tribroiuide CaHaSeBra Phenylseleniuni monobromide CaHaSeBr Phenylseleninic acid CsHaSeOzH
70
-0.13
152.1
151.9
196.5
196.6
210.5
210.3
-0.10 -0.05
-
195.5
195.4
167.1
167.7
-
236 0
235,l
203.5
203.6
168. 6
169.0
131 9
131.4
236 0 63.0
236.3 63.2
-
Deviation.
0.05
0.36 -0.38 0.05 0.24 -0.30 0.13 0.32
IV.
lodometric Titration of Diary1 Diselenides Equi\,aleni Weight Experi- Deviation, Conipound Formula Theor:: mental % Diphet?yi diselenide (CBHdzSez 52 01 51.97 -0.1 Table
Di-p-tolyl diselenide (C;H7)2Sez Di;o-biphenylyl) diselenide (CizHdzSel
56.70 77.35
51.53 56.98 76.84
-0.9 0.5 -0 7
A \ 7
RESULTSASD DISCLXSIOS.The accuracy of the flame combustion method is indicated by the results given in Table I for the analysis of a number of carefully purified substances. The above examples shoxv’thatthe method yields excellent rewlts for compounds of widely differing types containing seLenium together with carbon, hydrogen, oxygen, nitrogen, and chlorine. The method in its present form does not give good results when bromine, iodine, or sulfur is present, since these elements yield substances which reduce tetravalent selenium in the absorber. However, bromine and iodine also interfere with other combustion methods, although sulfur does not because of its oxidation to sulfur trioxide (rather than dioxide) on the platinum contacts. It is this interference by sulfur in the flame combustion method which makes it advisable to use hydrogen as a fuel rather than commercial gas which is apt to contain sulfur. The flame combustion method has a distinct advantage over other methods in being more rapid and requiring less skill and attention on the part of the analyst. VOLUMETRIC METHODS
X number of types of organoselenium compounds which contain halogen or oxygen directly bonded to selenium lend themselves to iodometric procedures for determination of equivalent might. These compounds react with aqueous potassium iodide to form triiodide, which may then be titrated with sodium thiosulfate. The titration of diarylselenium &halides has been described in a previous communication (2). The reduction products and the equivalents per mole for the various types of cornpounds are indicated in Table 11.
P R O C E D CFOR R ~COMPOUNDS REDUCIBLE BY POTASSIUM IoThe weighed sample (0.1 to 0.5 gram) is placed in a glass-
DIDE.
stoppered flask containing 5 ml. of carbon tetrachloride, 25 ml. of 0.3 ‘11 potassium iodide, and 2 ml. of 6 ill sulfuric acid. After shaking, the mixture is titrated with standard sodium thiosulfate. Starch solution is added just before the end point.
TITRATIOS OF ARYLDISELENIDES.Aryl diselenidej may be titrated iodometrically to the iodine nionochloride end point if the hydrochloric acid concentration is kept a t 5.0 to 5.5. F a t the end point. Because of a small amount of side reaction, possibly oxidation of the diselenide to selenonic acid, the results are sometimes low by 0.5 to l . O % , Although the reactions are probably more complex than indicated, the stoichiometry is represented by the equations:
+ 6 IC1 = 2 RSeCIJ + 3 I, 2 Iz + IO, + 6H’ + 5 C1- = 3 HZO + 5 IC1 RzSez
(1)
(2) PROCEDURE FOR ARYL DISELENIDES,A solution of iodine nionochloride is prepared by titration of 1 ml. of 1.5 M potassium iodide in 50 ml. of 12 J1 hydrochloric acid, using 5 ml. of carbon tetrachloride as an indicator. When the carbon tetrachloride is just colorless, 25 ml. of 12 .V hydrochloric acid and the weighed sample of diselenide are added. After shaking, the mixture is titrated with standard potassium iodate solution.
Tables 111 and IV shox results from volumetric determination of equivalent weights of a number of different compounds. LITERATURE CITED (1) .\lber, H. K., and H a r a n d J J . Franklin Inst., 228, 243 (1939) ( 2 , SIcCullough, J , D., J. sot., 64, 2672 (1942). (3) Sleulen, J. H., v a n d e r , Chem. Weekblad, 31, 333-5 (1934). (4) S i e d e r l , J. B., a n d Niederl, V., “ M i c r o m e t h o d s of Q u a n t i t a t i v e O r g a n i c .4nalysis”, 2nd ed., p. 208, X e w York, John Wiley Q Sons, 1942. ( j j cmel;awa, B U Z Z . hem. SOC. an, 14, 153 (1939). (6) W i n t e r , P. K., IND. ENG.CHEM., ANAL.ED.,15, 571 (1943).
s.,
