latter stages of elution) are reproduced in Figure 9. Polarograms of the unoxidized samples are also included. The pronounced increase in what has been tentatively designated as diaryl sulfide sulfur (the second wave) as the aromaticity of the sample increases is noteworthy. There is a remarkable parallelism between the total sulfur content as determined directly and the sum of the percentages of sulfur of each type (Table XI). The only gross diecrepancies appear in the cases of sample 4, fraction 2, and sample 4, residue. These data suggest that there may be only three main classes of sulfur compounds in crude petroleum. Thiols and disulfides, therefore, are probably not major sulfur types in crude oil, but are produced through thermal degradation during refining. Application of this method to most refinery distillates probably would require the preliminary removal of hydrogen sulfide, elemental sulfur, thiols, disulfides, and polysulfides by standard procedures. It is possible that extensive alkyl substitution of dibenzothiophene may shift the half-wave potentials for the corresponding sulfones to more negative values. If this is the case, the sulfur compounds which are tentatively considered as diaryl sulfides may be dibenzothiophene derivatives or a mixture of both. Chemically, diaryl sulfides and dibenzothiophenes are very similar. If the second wave is indeed caused entirely by the dioxides of dibenzothiophenes, the weight per cent sulfur, as reported in this paper, should be divided b y 2, as i t was assumed that 4, not 2, electrons are transferred in the electrode reaction. Because no good independent analytical methods are available for comparison, the ultimate accuracy of this procedure cannot be determined. How-
ever, the surprisingly good agreement between the total determined sulfur and the sum of the percentages of sulfur by compound type attests to the potentialities of the method. Even though interpretation of the polarograms is open t o question, it is hoped that the study reported herein may serve as a basis or inspiration for a more exact procedure. ACKNOWLEDGMENT
The authors wish to thank the Xlultiple Fellowship on Petroleum sustained at LIellon Institute by the Gulf Research and Development Co., which supported this work, for permission to publish this material. Thev also wish t o thank R. T. Wendland who supplied the molecular distillates and Dorothy Taylor who analyzed the samples by the spectrophotometric iodine complex method. LITERATURE CITED
(1) Bergman, E., Engle, I.., Sandor, S., 2. physik. Chem. 10,397 (1930). (2) Bordwell. F. G.. McKellin. IT. H.. J . dm.‘Chem. Boc. 72, 198h 11950).’ 131 Tbid.. 73. 2252 ~- 119511. (4)Bordwell, F. G., RlcKellin, IT. H., Babcock, D., Ibid., 73,5566 (1951). (5) Challenger, F., Clapham, P. H., J . Chem. SOC. 1948; 1615. (6) Charles, R. G., Freiser, H., J . -4tn. Chem. SOC.72.2233 (1950). ( 7 ) Cohen, J. B., ‘Skirrow, F. IT., J . Chem. SOC. 75,889 (1899). (8) Drushel, H. V., Rliller, J. F., ASAL. CHEM. 27, 495 (19%). (9) Druehel, H. V., Miller, J. F., Huhie, IT.. Clark. R. 0.. Anal. Chini. Acta 15, 394 (1956): (10) Eccleston, B. H., llorrieon, II., Smith, H.hI., .ANAL. CHEif. 24, 1745 (1952). f l l i Elliott. A . . “Electrochemical Analvsis of Phenyl Sulfides, Benzothiophenes, and Thiophenes,” Southxide Chemical Conference, IIemphis, Tenn., December 1956. (12) Gol’dfarb, Ya. L., Kirmalova, 11. L., \ - 3
>
.
-
I
~
\ - - - - ,
Doklady
Akad. .Vaztk
91, 539 (1953).
S.S.S.R.
(13) Hastings,
S. H., ANAL. CHERI. 25, 420 (1953). (14) Hastings, S. H., Johnson, B. H., Ibid.. 27. 564 11955). (15) Hastings, ’S.H:, Johnson, B. H., Lumpkin, H. E., Ibid., 28, 1243 (1956). (16) Heilbron, I., “Dictionary of Organic Compounds,” Vol. 11, p. 65, Oxford University Press, New York, 1953.
(17) Ibid., p. 414. (18) Hinsberg, O., Ber. 43, 289 (1910). (19) Johnson, C. \I-., Overberger, C. G., Seaeers. W.J.. J . .4m. Ckem. SOC. 75. -149:i f195i). (20) Karc’hmer, ‘J. H., Walker, 31. J., ANAL.CHEM.26, 277 (1964). (21) Keswani, R., Feiser, H., J . Am. Chem. SOC. 71, 1789 (1949). (22) Kolthoff. I. I f . , Linnane. J. J., “Polarography,” Voly I, pp. 2667 . Interscience. New York. 1952. Levin, E. S., Shekov, ,4.P.,’Doklady d k a d . S a i i k S.S.S.R. 96, 999 (1954). Lumbroso, H., RIarschalk, C., J . chim. phys. 48, 123 (1951). LumDkin. H. E.. Johnson. B. H..’ A i . 4 ~ CHEM. . 26, 1719 (1954). lIairanovskii, S. G., Seiman, 11.B., Doklady A k a d . .Yaiik S.S.S. R. 87, 805 (1952). Moore, R. J., Greensfelder, B. S., J . A m . Chem. SOC.69, 2008 (1947)
(28) O’Donnell, G., AYAL. CHEU. 23,
894 (1951). (29) Rolla, XI., Sanesi, M., Leandri, G., -4nn. chim. (Rome) 42, 664 (1952). (30) Smith, P., Sprague, H. G., Elmer, 0. C.. AXAL. CHEK 25. 793
(1953):
Smyth, C. P., “Dielectric Behavior and Structure.” p. 303, 1IcGrawHill, New York, 1955. Thompson, C. J., Coleman, H. J., Rall, H. T., Smith, H. AI., Av.4~. CHEM.27, 175 (1955). Werner, E . G. G., Rec. trazi. chim. 68, 509, 520 (1949). RECEIVEDfor review June 10, 1957. Accepted January 18, 1958. Group Session on Analytical Research, 22nd hlidYear hleeting, Division of Refining, American Petroleum-_ Institute, Philadelphia, Pa., 3lay 190,.
Spectroscopic Detection of Silicon in Organic Silicon Compounds JACK RADELL, P. D. HUNT, E. C. MURRAY, and W. DICKINSON BURROWS Aeronautical Research laboratory of Wright Air Development Center, Air Research and Development Command, Wright-Patterson Air Force Base, Ohio
F A qualitative spectroscopic test for silicon in organosilicon compounds has been developed. The method employs the Spectranal for detection of emission lines a t 6347 and 6371 A. and has proved satisfactory for a wide range of organosilicon compounds.
1280
ANALYTICAL CHEMISTRY
R
efforts necessitated the development of a simple qualitative test for silicon in organosilicon compounds, a more rapid test than existing chemical methods, The spectroscopic procedure described proved satisfactory for a \vide range of organic ESEARCH
materials incorporating silicon. The method involves decomposition of the insoluble organosilicon compound in an aqueous acid solution by a spark discharge between platinum electrodes. The Todd Spectranal was employed successfully, although the nianufac-
turers indicated that the instrument detects silicon only in the elementary or alloyed state (6). Silicon derivatives exhibit four persistent emission lines a t 5056, 5041, 6347, and 6371 -4.( 7 ) . The most carefully prepared blank test solution displayed a number of emission lines coincident with the 5036- and 5041-A. lines of silicon. At the high level of excitation required to decompose the organosilicon compounds, the interfering lines become so prominent that the 5036and 5041-A. lines of silicon are unreliable for analytical purposes. H o u ever, the 6347- and 6371-A. lines are qualitatively unambiguous. The only coninion element that should interfere significantly is zinc, which ekhibits a line of moderate intensity a t 6362 A. The exclusion of inorganic zinc froni test samples should not be difficult. The method \I ill not quantitatively determine silic-on in organic materials, because the intensities of emission lines of these conipounds are a function of structure as n-ell as concentration. Further, deconiposition of the organic material is frequently acconipanied by darkening of tlie test solution, n hich results in diminution of eniission intensity. The method does permit the semiquantitative determination of certain organic material. containing silicon. Thus, the concentration of a specific silicon compoiuid in an organic sample could be dettwiiined by a single-line extinction procedure, if a concentration standard is established ( 3 ) . Material. tested fall into three classes : compound- yielding a positive test in 10% nitric acid solution, conipounds giririg a positive test in nitric acid only in the presence of carbon tetrachloride, and (.ompounds yielding a negative t w t under both conditions. The first clas- (I) includes niost of the organosilicon compounds rommonly encountered, in particular the alkyl- and alkoxysilane.: tile second (11) includes the triarylsilanol., niost of the tetraarylsilanes, and the peraryldisilanes; and the third (111) includes the tetraarylsilanes with four identical aryl groups. T h a t a positive test is given bj- the phenylbiphenylj-Manes but not by the more synimetrical tetraphenylsilane and tetra-p-biphenylylsilane is probably a consequence of the very slight qolubility of the latter compounds. The silicone fluids providi. a strong positive test, suggesting application to the detection of small amounts of silicone contaminants. I n Table I. the relative intensities of the 6347- and 6371-A. lines and the relative excitation intensities are listed for representative compounds. I n doubtful case‘, chemical methods may be substituted (1, 2 ) , but the authors believe that the generality of tlie spectroscopic method is not impaired by the
liniited number of cases for which it fails. EXPERIMENTAL
Instrumentation. T h e Todd hlodel D Spectranal (Todd Scientific Co., Springfield, Pa.) with platinum electrodes and a calibrated long wave length spectroscope (Bausch & Lomb) were used for all measurements. Materials. T h e silicon compounds were commercially available or were DreDared bv methods described Drei-ioisly (4, i).
Test Procedure.
CLASS I CON-
A test solution was prepared b y dissolving 1.0 gram of reagent grade potassium nitrate and 10 ml. of reagent grade nitric acid in 90 nil. of distilled water. From 2 to 4 diops of liquid or about 30 nig. of solid sample containing silicon were suspended in 2.0 ml. of test solution in a 3 x 18 em. test tube of borosilicate glass (excitation chamber). The platinum excitation electrode was immersed about 2 mm. in the solution and the other electrode was immersed nearl) to the bottom of the chamber. The relatire excitation intensity of thr Ppectranal was set a t six. -2 persistent band a t 6347 A. accompanied by a less intense band at 6371 A. indicatd the presence of silicon. I n some cabw. it n a s expedient to set the excitation intensity a t 10, the maviniuni l t v l , to promote decomposition of the silane prior to analysis; the excitation level n as reduced to six as soon as the 6347-d. line became prominent. CLASSI1 COMPOUKD~. K h e n niaterials gave a negative test under the procedure for class I compounds, about 30 mg. of sample were dissolved in 1.0 nil. of carbon tetrachloride in the elcitation chamber. Then 2.0 nil. of test solution, prepared as described above, were added. K i t h this procedure, the excitation electrode n as first dipped into the organic layer; then n hile applying current, the electrode n-as sloivly withdrawn until the tip I\ as about 2 mm. below the surface of the teqt solution. The other electrode vias immersed nearly to the bottom of the chamber, and the excitation intensity was maintained a t maxinium level throughout the measurement. Test solutions containing thienylsilanes rapidly turned opaque, sharply reducing the intensity of the emission bands. POUSDS.
Table I. Relative Intensities of Cornpounds Tested Relative Intensity Excitation, 10 = Band max. Moderate Moderate \Teak Strong Strong
6 8 6
Strong Strong Strong Xoderate Moderate Strong
6 6 6 6
Strong
6
JIoderate
6
6
6
6
10
Moderate Strong \-cry strong
6 6
Strong
6
6
Moderate Xoderate Moderate
10 10 10
Moderate
10
Strong
10
Strong
10
Keak
10
Keak
10
Weak
10
Keak
10
n’eak
10
Xegative
10
Negative Xegative
10
10
Kilev, R. E., J. Am. Chem. SOC.
6 8 , 2 i 2 8 (1946).
ACKNOWLEDGMENT
The authors gratefully acknowledge the preparation of some of the samples by G. H. Doriun, C. K.Harris, D. C. Priest, and Leonard Spialter of this laboratory; and Harold Rosenberg and Christ Tamborski of the Materials Laboratory, Vright Air Development Center. LITERATURE CITED
(1) Clark, R. S . , Diehl, H., Gilman, H.,
Gilman, H., Gorsich, R. D., Ingham, R. K., Ibid., 76,918 (1954). Kress, K. E., A p p l . Spectroscopy 6 , 20 (1952). Post, H. W.: “Silicones and,, Other Organic Silicon Compounds, Reinhold, X e w 1-ork, 1949. Rochon., E. G., “Introduction to the Chemistry of the Silicones,” TTiley, Xew Tork, 1951. Todd Scientific Co., Springfield, Pa., Bull. 140. Todd Scientific Co., Springfield, Pa., “Operating 1nst;;uctions for the Todd Spectranal. RECEIVEDfor review July 20, 1957. Accepted March 6, 1958. VOL. 30, NO. 7, JULY 1958
1281
