Abstract
Niobium formed a purple complex with Pyrocatechol Violet in acidic medium, (e.g. hydrochloric acid-potassium chloride buffer system, pH; 0.90) and its absorbance maximum was 575 nm. On the other hand, in the presence of cetylpyridinium bromide (CPB), niobium formed also the purple complex in the same acidity, and its absorbance maximum was 565 nm. However, its apparent molar absorptivity at absorbance maximum was 5.3×104 which was nearly 2 times that of the complex in the absence of CPB. The latter complex was stable for at least 19 hours, but the former precipitated gradually within several hours. Since the improvements were obtained by addition of CPB in the system as described above, further experiments were carried out.
The procedure was as follows; A 0.21.0 ml aliquot of a standard solution of niobium containing 40% (v/v) sulfuric acid (niobium: 5.050 μg) was taken and 0.51.0 mlof 40% (v/v) sulfuric acid was added. Then 2.0 ml of 0.1% PV solution, 2.5 ml of saturated potassium chloride solution and 3.0 ml of 5.0×10-3M CPB solution were added in this order. The solution was diluted to 1520 ml, pH was adjusted and diluted exactly to 25 ml with a volumetric flask. Absorbances were measured at 565 nm against the reagents blank.
For determining the molar ratio of niobium to PV in the complex, continuous variation method was carried out. The results showed that the molar tatio was 1: 4 regardless the presence of CPB. Calibration curves were linear from 0.2 to 2.0 μg/ml of niobium. The optimum pH range was 0.811.6.
For examining influence of ionic strength and salt species, potassium chloride, potassium nitrate, sodium sulfate were added in the system. Absorbance increased as increase of potassium chloride concentration up to 0.4 M, but remained constant beyond this value. Although potassium nitrate indicated also the same effects as potassium chloride up to 0.4 M, decrease of absorbance then precipitation of CPB occured by further addition. Sodium sulphate did not alter the absorbance up to at least 1.3M. Perchlorate precipitated by formation of ionic pair with CPB.
Auxiliary complexing agents including EDTA, oxalate, tartrate, citrate, and phosfate appreciably reduced the absorbance.
Effects of diverse metalic ions were also examined.
