V O L U M E 21, NO. 5, M A Y 1 9 4 9
625,
for the determination of nitrite, because in the warm acid medium used the nitrous acid disproportionates rapidly into nitrate ion and nitric oxide and it is difficult to prevent the escape of the latter from the solution. The reaction between nitrous acid and ammonium ion to produce elemental nitrogen is another prohibitive factor. LITERATURE CITED
(1) (2) (3) (4)
( 5 ) Kolthoff, I. M., and Lingane, J. J., “Polarography,” New York, Interscience Publishers, 1946. (6) Kolthoff, I. hf., Sandell, E. B., and Moskovitz, B., J . Am. Chem, Soc., 55, 1454 (1933). (7) Lingane, J. J., ANAL.C m x . , 20,797 (1948). ( 8 ) Lingane, J. J., and Pecsok, R. L., Ibid., 20,425 (1948). (9) Szebelledy, L., 2.anal. Chem., 73, 145 (1928) ; 74, 232 (1928). and Schall, B.-M., Ibid., 86, 127 (1931). (10) _. Szebelledy, L., and Passarge, W., Ber., 49, 1692 (1916). (11) Traube, W., (12) Vortmann, G., Ibid., 23,2798 (1890). Trans. Faraday SOC.,28,665 (1932). (13) Wellings, A. W., (14) Wiercinski, J., PTzernysZ Chem., 17, 57 (1933). I_
Arndt, Th., 2.angew. Chem., 30,169 (1917) ; 33,296 (1920). Bottger, JV., 2.Elektrochem., 16, 698 (1910). Devarda, A., Chem. Ztg., 16, 1952 (1892). Knecht, E., and Hibbert, E., “New Reduction hfethods in Volumetric Analysis,” London, Longmans, Green and Co., 1925.
RECEIVED July 15, 1948.
Microdetermination of Sulfur in Organic Compounds A Simplified Gravimetric Method G. L. STKAG.4IND AND H. W. SAFFORD The Cnicersity of Pittsburgh, Pittsburgh, P a . A modified gravimetric method has been devised by which the sulfur content in organic compounds may be determined quantitatively on a micro scale. Nitrogen and the halogens (except fluorine) do not interfere. The sample is burned in an atmosphere of oxygen using a platinum catalyst; the sulfur trioxide formed is absorbed by a silver gauze with the quantitative formation of silver sulfate. When halogens other than fluorine are present, the silver sulfate is extracted in water and the percentage of sulfur is calculated from the loss in weight of the gauze. In the absence of halogens, the gain in weight of the gauze (as sulfate) may be used to estimate the amount of sulfur. Samples of sulfur content from less than 10 up to 100% (pure sulfur) have been analyzed by this method.
T
HE conventional quantitative microanalysis of sulfur ‘in organic compounds is considered essentially to be the conversion of “organic” sulfur to the sulfate ion. K e t and dry combustion methods are employed in carrying out this oxidation and the final sulfate estimation may be volumetric or gravimetric (11, IS). Although the x-ell-known Carius determination has many staunch supporters, the time consumed in this and most other gravimetric microprocedures has prompted many investigators to propose modified titrimetric methods of sulfur analysis ( I , S, 6, 1B, 16, 17). S o existing procedure seemed to be without one or more attendant difficulties or objections. The requirements for speed, simplicity, and accuracy that are to be found in a modern microanalytical service laboratory led to the search for fundamental reactions upon which might be based a new or improved method incorporating these and other advantages. The combustion of the sulfur in an organic compound a t atmospheric pressure and in the presence of oxygen yields for the most part sulfur dioxide and sulfur trioxide, the relative amounts of which depend largely upon temperature, the amount of oxygen present, and the presence or absence of a catalyst. Uhl (16) has indicated that metallic silver heated in a stream of sulfur dioxide forms silver sulfate and silver sulfide, together with small amounts of sulfur trioxide. Dennstedt ( 5 )was perhaps the first to employ silver t o retain sulfur trioxide quantitatively in his elemental organic analyses. Kirner (8) almost 40 years later recognized the importance of Dennstedt’s pioneering work, and proposed the silver-sulfur
trioside reaction as the basis for a quantitative determination of sulfur. Huffman ( 7 ) applied the suggestions of Kirner by establishing a method for the determination of sulfur in yhich the osides of sulfur formed during a combustion reaction y i t h metallic silver, yielded silver sulfate quantitatively; the electrodeposition of silver from the silver sulfate so formed, followed by conversion of the weight of silver to sulfur by the appropriate factor, completes the analysis. Compounds containing halogen cannot be analyzed for sulfur by this method and inherent errors necessitate the application of a correction factor in calculating the amount of silver deposited. Belcher and Spooner ( 2 ) adapted their procedure for the ultimate analysis of coal to the microanalysis of organic compounds. The simultaneous determination of carbon, hydrogen, and sulfur is effected by combustion a t 800” C. with an oxygen flow of 50 nil. per minute in a tube containing no catalyst or oxidant other than oxygen itself. A flowmeter is required to measure the fast rate of oxygen flow and a transparent silica combustion tube is employed to Fyithstand the minimum temperature designated. These authors indicate that sulfur is quantitatively absorbed and retained as silver sulfate by a roll of silver gauze inserted near the exit end of the combustion tube. The silver sulfate is extracted with boiling water and the loss in weight of the silver multiplied by the appropriate factor gives the percentage of sulfur. Considerable experimentation is necessary to position the gauze properly in the combustion tube. Moreover, with samples that are ton large or high in sulfur content, an empirical factor is necessary in
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ANALYTICAL CHEMISTRY
converting the JTeight of silver sulfate into a corresponding weight of sulfur. The authors have attempted to duplicate the A CI D E F B experiments of Belcher and Spooner, but with only moderate success. With compounds of relatively simple structure, precision is difficult to H attain, and for samples containing large amounts J of sulfur, the same low results of these authors Figure 1. Combustion Apparatus were noted. Furthermore, with liquids and solids of low melting points, difficulty lvas encountered a result of t,he study, a normal combustion temperature of 650" C. in ensuring complete combustion a t the fast rates of flow is maintained a t E, t,he center and hottest zone of the furnace. used. The lack of consistent accuracy and precision possi\rang authors quote and practice the findings of Knietsch ( 9 ) ,who ldy may be attributed to the high speed of the oxygen, to the observed t,hat at 400' C. there is approximately quantitative high temperature which is well above that which favors the maxiconversion of sulfur dioxide t,o trioxide in the presence of oxygen and a platinum catalyst. However, Yost and Russell (18) indimum formation of sulfur trioxide, and to the absence of a platicate that the optimum temperature with respect both to rate of num catalyst TThich i\-ould ensure complete conversion of sulfur reaction and yield is about 665" C. Thus, a close approach t o
