Abstract
Tantalum formed a blue complex with Pyrocatechol Violet (PV) in acidic medium in the presence of cetylpyridinium bromide (CPB), and its absorption maximum was 605610 nm. Although similar color reaction was taken place with the same conditions in the absence of CPB, the apparent molar absorptivity at absorption maximum was one third of the former. Thus an increase of molar absorptivity was obtained by the aid of CPB, the absorption maximum being unchanged virtually.
The procedure was as follows: An aliquot of tantalum solution (tantalum : 4.545 μg, 1 : 1 sulfuric acid solution) was taken in a 10 ml beaker, and evaporated to almost dryness on a sand bath. After addition of 0.51.0 ml of concentrated sulfuric acid, the vessel was covered with a watch glass, and heated for 20 30 min. The beaker and its content was stood for 10 min, and then 3.0 ml of 0.1% PV solution and 2.0 mlof 2.0×10-2M CPB solution were added. The mixture was transferred entirely to a 25 ml volumetric flask containing preliminary 5.0 ml of 1: 1 hydrochloric acid. The content was gently mixed and diluted exactly to the mark with water. Absorbance was measured at 605 nm against to the reagents blank.
The quantities of reagents for obtaining optimum conditions were; 1: 1 hydrochloric acid 5.08.0 ml, 0.1% PV 2.0 ml. The absorbance was unchanged for at least 20 hrs. Calibration curve was straight line from 4.5 to 45 μg/25 ml of tantalum. The order of addition of reagents was essential to obtain a reproducible results.
The tantalum-PV ratio by the molar ratio method was 1: 18 which was abnormally high.
This abnormality may be in connection with (1) a dissociation of PV which is promoted by the addition of CPB and is also changed by the quantity of CPB. (2) species of PV which may exist rather aggregated than monomeric state on the miceller surface especially in this higher acidity.
The effects of further addition of PV on the absorption spectrum were also tested. When 5.0 ml of 0.1 % PV was added, two maxima, 440 and 540550 nm, appeared, while 605610 nm band was still unchanged. The 440 nm band was substantially unaffected by concentration of tantalum, but the 540550 nm band was poorly proportional with the concentration of tantalum. Although this phenomenon appeared coincidently with an appearance of visible suspended particle in the measuring solution, the phenomenon may be a so-called metachromasy induced by PV-tantalum interaction.
