SPECTROPHOTOMETRIC DETERMINATION OF NIOBIUM IN CARBONATITES

The determination of Nb in carbonatites having a predominant content of calcite, as well as a high P-content, requires decomposition of the sample in acetic acid. The insoluble, Nb-containing residue is dissolved and the spectrophotometric determination performed by using NH/sub 4/SCN as agent. 0.4 g of the rock is heated for 1 hr with 20 ml of 25% acetic acid on a water bath with stirring. The insoluble residue is filtered off, and washed out with 0.5% acetic acid. Filter plus residue are incinerated in a quartz crucible, and the ashes are fused with 1.25--2.5 g Na/sub 2/S/sub 2/O/sub 7/ or K/sub 2/S/sub 2/O/sub 7/. The melt is dissolved in 12.5--25 ml tartaric acid (15%), the residue filtered off, and the filtrate is diluted with water to 25 or 50 ml. Five ml of a 10% solution of SnCl/sub 2/ in concentrated HCl and 15 ml of a freshly prepared mixture of 23% solution of NH/sub 4/SCN with acetone (1:2) are added to 5 ml of the resulting solution. The spectrophotometric determination follows after a period of 30--40 min by using a spectrophotometer at 405 m mu in 1 cm cells, employing the solution of a control test for purposesmore » of comparison. The color is stable for a period of 5 hr. A calibration curve is plotted by using a standard solution of Nb-tartrate + 100 gamma /ml of Nb/sub 2/O/sub 3/. The analysis takes half the time of that with tannic acid. (Referativnyy Zhurnal, Khimiya, No. 19, 1961.)« less