The determination of carbon in niobium, tantalum, molybdenum and tungsten

4 THE DETERMINATION OF CARBON IN NIOBIUM, TANTALUM, MOLYBDENUM AND TUNGSTEN 

Mallett (31) recommends combustion in a high-frequency furnace in pure oxygen, followed by conductometric determination of carbon dioxide after absorption in barium hydroxide solution. The combustion can only be carried out when fluxing agents are used. For a 1 g sample, 1 g iron and 1 g tin are generally needed. For materials that are difficult to combust the amount of sample is reduced to 0.5 g. If the sample combusts easily, the amount of iron can be reduced to 0.5 g, the amounts of sample and tin being 1 g. 

Elwell and Wood (39) burn niobium, tantalum and tungsten in a tube furnace at 1200°C. The sample weight is 2 g, 3 g lead being used as flux. For zirconium and hafnium containing alloys, 1 g iron and 2 g bismuth are used as flux. 

A6AR0 (40) discusses the problem of the analysis of carbon in refractory metals in detail. As far as the choice of the combustion furnace is concerned, the following is stated  

Thereby a range of application of 10 to 120 jug/g carbon is taken as a basis, or even lower when reaching low enough blank values. Millings, turnings or fragmented pieces, degreased and etched in hydrochloric acid (1+1), are used. 

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