D. C. Ayres Sir
John Coss College London and B. E. Dawson
King's College London
I I1
Use of the Oxygen Flask in Organic Qualitative Analysis -
The extension of SchGnigerls method (1) to qualitative analysis was originally suggested by Stephen (2). The qualitative detection of nitrogen (3) and the use of the procedure in teaching organic quantitative analysis of sulfur and phosphorus (4) have also been described. Since the oxygen flask method has not been generally adopted in either teaching or r e search laboratories, we feel that our experience of the method with undergraduate and postgraduate students may be of interest. We find that the oxygen flask procedure is superior to sodium fusion (6) since it leads to complete and controlled destruction of the organic material, eliminates filtration and is less hazardous. The method has given satisfactory results for a range of halogen compounds, including volatile compounds and those of low reactivity, e.g.. chloroform and methylene chloride. Conclusive results were obtained on combustion of tertiary nitrogen compounds, e.g., triphenylamine and N,Ndiphenylacetanilide where nitrogen may escape detection by sodium fusion. For this, Middleton's method (6) is as sensitive. However, this additional procedure may he overlooked if unreactive nitrogen is uot suspected. One forn~of the apparatus is illustrated in a paper by Macdonald and Stephen (4). We use a conical flask (250 ml) with a stopper glass-blown from a B24 joint (see figure) and a basket made by heat welding a length of platinum wire to a piece of suitably formed and welded 60-gauge platinum mesh. Either a wick cut from filter paper (Whatman No. 42) or a methyl cellulose capsule' can be used to contain the sample. Procedure
The absorbent (5 ml the flask which is then flushed thoroughly with oxygen. Meanwhile,a wick is charged with the sample for analysis (ca. 3 mg), folded and inserted into the basket (cleaned previously by ignition). The taper is ignited and the stopper In U. S. A.-Coleman Instruments, Maywood, Illinois. In U. K.-A. Gallenkamp, London, E.C.2
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Journal of Chemical Educotion
inserted promptly into the neck of the flask. During the combustion the stopper must be held on against a pressure of about one atmosphere. The flask should he left to stand for 10 min and, after shaking, the elements can he determined by accepted methods. Traditional reageuts have been used since these are available in teaching laboratories. Thus, halogens call be detected directly using silver nitrate in acid solutiou since oxidized sulfur and nitrogen do not interfere. Also, sulfur can be precipitated as barium sulfate by completing the oxidation with hydrogen peroxide (1 drop 100 vol solution for 1 ml test solution). Nitrogen is detected as nitrite even when azo con~poundsare used (7): the Griess test may be used for this purpose but we prefer the nitrosation of resorcinol arid t,he formation of the green iron complex (Peach test). The practical details of this test have been described by Haslam, et al. (8),together with a number of more sophisticated reagents and procedures for less common elements. It has been noted (5) that bromine interferes wit2h the Griess test by oxidation of nitrite. We have found that intense positive tests were given for nitrogen when the Peach test was applied to a number of typical organic bromo-nitrogen compounds. For our purpose, hypochlorite oxidation followed by carbon tet,rachloride extraction proved satisfactory for the identification of bromine and/or iodine. Students may gain experience and confidence in the method by combustion of S-benz;ylthiouroniun~chloride when a 3-mg sample suffices for the detection of nitrogen, sulfur, and chlorine. Operating Precautions
There is a very slight, although real, risk of a flask bursting during combustion and for complete safety in a teaching situation, the combustion should be carried out behind a screen with the hand protected. The authors thank Dr. J. Haslam and Mr. B. T. Saunderson for helpful discussion. Literature Cited (1) SCHONIGER, W., Mikrochim. Aeta, 123 (1955). (2) STEPHEN,W. I., Ind. C h m i s l , 37, 86 (1961). (3) CAMPBELL, A. D., and MUNRO,M. H. G., Anal. Chim. A d o , 28,574 (1963). (4) MACDONALD, A. hl. G., AND STEPHEN, W. I., J. CHEM.EDIT(:., 39, 528 (1962). (5) LASSAIGNE, J. L.,Compl. Rend., 16, 387 (1843). (6) IMIDDLETON, H., Analyst, 60, 154 (1935). (7) INGRAM, G., Mik~ochim.A d a , 7 2 (1953). (8) HASUM, J., HAMILTON, J. B., AND SQUIRRELL, 1). C. M., Analyst, 86,239 (1961).
