Air-Adsorption Error in Micro-Dumas Method for Nitrogen

Abstract

THE air-adsorption error arising from the copper oxide in the temporary filling of the combustion tube is given by Trautz (7) as 0.006 ml. The authors have long believed this value to be much too large. In order to measure such quantity accurately it is almost imperative that the generator and diffusion error should remain essentially constant throughout the entire investigation.Eight years ago a large-capacity, all-glass carbon dioxide generator (6) was constructed to meet this requirement. Air-adsorption values for the temporary filling were found to be 0.001 to 0.003 ml. As these values are of the same order of magnitude as the sum of the two unavoidable (±0.001 ml.) reading errors of the conventional microazotometer, these results were not published. It was observed, however, that the same adsorption values were obtained with 10-gram portions of either fine or coarse air-ignited copper oxide as with 10-gram portions of copper oxide which had been deaerated by heating in high vacuum at 550°C ., cooled, and preserved in pure carbon dioxide in glass ampoules.It thus becomes apparent that the observed air adsorption might arise from air adsorbed on the wall of the combustion tube, rather than from the copper oxide, since, in removing the old temporary filling and inserting the new filling, most of the carbon dioxide in this part of the tube will be replaced by air. This research was recently continued using the same carbon dioxide generator in conjunction with a specially constructed ultramicroazotometer which could be read to ±0.0001 ml. Generator and diffusion blanks and empty combustion tube and copper oxide absorption blanks were determined independently.

Dumas Nitrogen Determination on Decimilligram Scale

“…Then the potassium hydroxide is filtered through a sintered-glass filter. The amount used in every nitrometer is mixed by shaking with a few drops of isomyl alcohol before it is introduced (5).…”

Organic Microchemistry

“…He recommended the use of nickel oxide because it can be used at higher temperatures and does not liberate oxygen on heating, as does copper oxide. Pagel and Oita (139) discussed the errors in the Dumas method and particularly those due to air absorption.…”