One of the main sources of error in the micro determination of carbon and hydrogen is attributable to the lead dioxide used for the removal of nitrogen oxides. Some preparations yield high blanks, and in others the equilibrium between water and carbon dioxide is readily upset even though the reagent is maintained at the .optimum working temperature. The classical alternative is hot metallic copper, but its use is complicated by the fact that the surface rapidly oxidizes. Metallic silver has also been recommended even in recent timesf, although E~IcI-P as long ago as ISgz proved experimentally that it was inefficient. ELVING AND MCELROY~ examined a number of metals and metallic oxides as possible substitutes for lead dioxide. Of these only ceric oxide and manganese dioxide absorbed nitrogen dioxide efficiently.
They considered however, that both these substances would be unsuitable for micro combustion work. Ceric oxide absorbed carbon dioxide at temperatures below 300~ C ; heated manganese dioxide would have objectional properties similar to lead dioxide. The same authors finally recommended as reliable absorbents, sulphuric acid solutions of potassium dichromate and potassium permanganate. A suitable absorption vessel containing these solutions was inserted in the combustion train between the water and carbon dioxide absorption tubes, a technique previously developed by DOM-
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