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Difficulties with Direct-Current/Arc Optical Emission Spectrometric Analysis of Molybdenum Powder for Chromium, Iron, and Nickel

Volume 54, Number 11 (Nov. 2000) Page 1707-1711

Spitsberg, Anna T.; Otis, Janet S.

A new sample preparation procedure for the analysis of molybdenum powder by a direct-current/arc optical emission spectrometer for chromium, iron, and nickel is introduced. This procedure requires treatment of an oxidized molybdenum sample with nitric acid (HNO3) in order to ensure the homogeneity of the analyzed subsample. In this work, it has been shown that the Mo-Cr-Fe-Ni scales formed on the surface of the reduction tubes and boats during the hydrogen reduction process of molybdenum oxide are the main source of Cr-Fe-Ni contamination in the molybdenum powder. The formation mechanism of those scales is out of the scope of the present work. In this study, it was assumed that the chromium in Mo-Cr-Fe-Ni scales exists partly as a metal and partly as an oxide, which is why it is impossible to dissolve a contaminated sample of molybdenum powder completely in any commonly used mineral acid. The use of perchloric acid (HClO4) was not taken into consideration, since it has explosive and hazardous properties that make it difficult to use for routine sample preparation. For these reasons, the application of techniques that require a complete dissolution of the sample prior to analysis [atomic absorption (AA), inductively coupled plasma mass spectrometry (ICP-MS), or ICP atomic emission spectrometry (ICP-AES), for example] can lead to low chromium results. It has been shown that using a bigger subsample weight and an acid treatment procedure can dramatically improve the precision of chromium, iron, and nickel determination in the reduced molybdenum powder with the use of direct-current/arc optical emission spectrometry.