The determination of nitrogen in niobium and tantalum

As the literature contains only few references it seems justified to treat niobium and tantalum together, especially since they are mostly considered together. Suitable methods are reducing fusion, Kjeldahl and alikali fusion. 

Compared with titanium, the solubility of nitrogen in both metals is low. According to Fromm and Gebhardt (1), the figures at I000°C are 0.3 at.% for niobium and 5 at.% for tantalum. The nitrogen content of industrial metals is usually very low ( 20 Mg/g) and only seldomly reaches values above 100 t g/g. 

Booth et al. (3) stated in 1957 that the vacuum fusion technique as at present operated, does not furnish very reliable nitrogen figures for those elements forming very stable nitrides and these elements undoubtedly include niobium and tantalum. 

Mallett (4) described as follows the situation in 1962 In general, vacuum fusion nitrogen values for metals other than steels amount to no more than numbers obtained incidental to oxygen determinations. The findings were never properly verified by work with standards . 

In 1968 Friedrich et al. (23), who also tested flux methods (Pt flux or bath, or Ni-Ce-Mg) and used an empty crucible for each sample, were still unable to report any improvement in this situation. Their conclusion was that since none of the techniques investigated up to the present time has given satisfactory nitrogen results, we must advise against the determination of nitrogen in niobium and tantalum by vacuum fusion at present, unless an empirical factor can be tolerated . 

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The conditions for the determination of nitrogen in niobium and tantalum (42)(45)(46) using the N(p,n) 0 reaction are similar to those described under 2.3 except for the irradiation (12 MeV protons, 0.5 lA for Nb and 3 to 4 /.A for Ta) and measuring conditions (No lead absorber needed, detector with 5 % relative detection efficiency). 

The N(p,n) 0 reaction is most suitable for the determination of nitrogen in niobium and tantalum, because ... 

As discussed in section 2.2, the N(p,a) c reaction is interfered with by B(p,n) C. An upper limit for the boron concentration was determined by instrumental deuteron activation. Upper limits for boron of 0.50 txq/q and 0.018 Aig/g for niobium and tantalum resp. were obtained corresponding to an upper limit for the interferences of boron of resp. 0.68 and 0.025 Mg/g of nitrogen. 

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