The determination of oxygen in niobium and tantalum

Niobium and tantalum are so similar that they can be dealt with together here. As metals of the 5th sub-group in the periodic system of the elements, they are distinguished from the elements of the 4th sub-group, titanium and zirconium, by markedly lower oxygen contents. The solubility limits for oxygen at 1000°C are approx. 2 at.% in niobium and about 2.5 at.% in tantalum. 

It should be noted that, above about 1600°C, oxygen is released from the Nb-0 or Ta-0 mixed crystals in the form of the volatile oxides NbO and Nb02 or TaO and Ta02, respectively (48) however, this process is of no importance in connection with the analytical determination of oxygen. 

The oxygen concentration likely to be found in the compact metals is in the range of 10 to 100 Aig/g. 

Chemical methods have only been suggested for the determination of oxygen in niobium and tantalum in very few instances (151) and remained of no practical importance. 

The first comprehensive surveys are those of Mallett (152) in 1961 and, in particular, of Friedrich and Lassner (153) in 1967. In essence, the conclusions reached in the latter paper are as follows  

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