ANALYTICAL CHEMISTRY
1054
ple vaporizing, and 15 minutes’ sweeping. Large or volatile eamples r uire a longer vaporizing time but rarely as long as 20 minutes. small, coated ma et inside the tube, and propelled ush the boat into osition for by an external magnet, is use& vaporizing the sample. The smalrmagnet is returnefto the end of the reaction tube before the s a m le is vaporized. Elaborate steps need not be t&en to ensure that all connections are laas to glass. The very low blank value of 0.08 mi. of 0.02 N so%um thiosulfate sometimes obtained is evidence that no appreciable difTusion takes place through the various rubber connections.
?
Table 111. Comparison of Unterzauoher and ter Meulen Methods 1
Sample Bensoio acid
Nitrobenzaldehyde
TER MEULEN METHOD
This method involves the catalytic hydrogenation of the pyrolyzed sample, followed by absorption and weighing of the water formed. In carrying out the determination it was found that the thoria-promoted catalyst used by Russell and Fulton (6) was the most satisfactory. A nickel-chromite catalyst was tried, but it gave incomplete hydrogenation and poor recovery of water. It appeared to Russell and Marks (6) in a study of the analysis of organic compounds containing nitrogen that in the absence of oxides of carbon in the effluent gases the ter Meulen procedure aould be simplified if an efficient water absorbant could be found which mould not absorb ammonia. To this end reagent grade eodium hydroxide pellets were shown to have met their requirements. The autuors of this paper tried several desiccants and combinations for selective absorption of water including Dehydrite, Drierite, native gypsum calcined at 250” C., plaster of Paria made into a paste and calcined a t 250’ C., mixtures of plaster of Paris with 1 and 5% of sodium hydroxide, pellets of potassium hydroxide, and sodium hydroxide flakes. The hydroxides of potassium and sodium were the only compounds found to be selective in that they did not absorb ammonia gas, but they also proved to be inefficient for the complete absorption of water. The ter Meulen method suffers other handicaps which further limit its usefulness. One great disadvantage was found to be the short life of the catalyst, which limits the number of determinations that can be made with a fresh charge to about 12,and this number decreases with successive regenerations. Sulfur as reported by Dinerstein and Klipp ( 9 ) and by Elving and Ligett ( 4 ) was found to poison and hasten the deactivation of catalyst. Another disadvantage was the large and varying blank, probably due to the slow rate of reduction of the cores of the catalyst particles. Depositing the activated nickel on the surface of an inert support reduces the high blank but also shortens the life of the catalyst packing because of the small amount of material used. Table I11 shows the oxygen content of samples as determined by the two procedures in question; the data presented make the distinct advantage of the Unterzaucher method readily apparent.
Unterzaucher 26.07 26.09 26.09 26.19 Av. 26.11 31.33 31.65 31.80 Av. 31.69
Di-o-tolylguanidine
Oxygen, % ter Meulen 26.43 26.32 25.99 26.83 26.14 37.51
0.08
0.08
10.76
Theory 26.20
31.76
0.00
in a catalytic process-namely, varying catalytic activities and an acute sensitivity of the catalysts toward poisons. The presence of inorganic oxygen compounds such as carbonates and the oxides of iron and lead will cause an error in the values for oxygen by the Unterzaucher method when oxygen is determined in organic compounds. It has been shown that sulfur-containing compounds yield hydroxen sulfide, carbon disulfide, and carbonyl sulfide when the Unterzaucher method is employed, and the latter two gases (carbon di ilfide and carbonyl sulfide) are not retained by an hcarite or potassium hydroxide scrubber. Carbon disulfide and carbonyl sulfide, if present in considerable amount, are detrimental because they have been found to liberate iodine from iodine pentoxide, thereby causing high results for oxygen. On the contrary, it is apparent that carbonyl sulfide formed during the determination has removed some of the oxygen sought, thus causing low oxygen results. For this reason the carbon disulfide and carbonyl sulfide should be retained in a liquid nitrogen trap and for a very accurate determination of oxygen the amount of carbonyl sulfide condensed should be determined and a correction applied accordingly. The Unterzaucher method has proved more reliable than the ter Meulen method, both for uniformity of operation and for accuracy. It is also superior to the Liebig method. LITERATURE CITED
(1) Aluise, V. A , , ANAL.CHEM.,21, 746 (1949). (2) Aluise. V. A., Hall, R. T., Stash, F. C., and Becker, W. W., Zbid.. 19, 347 (1947). (3) Dinerstein, R. A., and Klipp, R. W., Zbid., 21,546 (1949). (4) Elving, P.J., and Ligett, W. B., Chem.Rem, 34, 2 (1944). (5) Russell, W. W., and Fulton, J. W., IND. ENQ.CHEM.,ANAL.ED., 5, 384 (1933). (6) Russell,W. W., and Marks, M. E., Zbid.,6,381 (1934). (7) Verdery, R. B., Jr., ChemistAnaZwt, 38, No. 3, 88 (September 1949). (8) Walton, W. W., McCulloch, F. W., and Smith, W. H., J. Research Natl. Bur. Standiardn, 40,443 (1948).
SUMMARY
The indirect determination of oxygen (determination by difference) is susceptible to large errors and upcertainties due to the fact that all the errors in the elemental analysis are additive. Moreover, analysis of synthetic samples for carbon and hydrogen by several laboratorieq, all uaing the Liebig method, has shown results of great variance on the same sample and of considerable divergence from theory. In the ter Meulen method it is necessary to use an absorbant that will absorb water and exclude ammonia, unleea proviaion is made similar to that originated by Cr Meulen for a combination absorbing tube for both compounds followed by a quantitative determination of the ammonia. The hydroxides of potassium and sodium in pellet form do not absorb ammonia, but neither are they efficient reagents for completely retaining water. The ter Meulen method, while proving satisfartory for some compounds, has the disadvantages inherent
b O I B I V E D May24,1949. Presented before the Division of Petroleum Chemiatry, Symposium on Microchemistry and the Petroleum Industry, at the CHBMICAL SOCIETY, San Francisco, Cblif. 116th Meeting of the AM~BIOAN
COWCTION. In the article on “Colorimetric Microdetermination of Rhodium with ZMeroaptobenzoxazole” [Ryan, D. E., ANAL.C ~ Y . 22, , 599 (1950)l on page 599, fist column, under the heading “Development of Method” 240 micrograms of rhodium should have been specified, not 24, aa stated. D. E. RYAN 23 Montem Road
For& Hill,London, 8. E.23
England
.
