Neo-Thorin was previously presented by the other authers as a color-developing reagent for the spectrophotometric determination of zirconium. 1) To determine the optimum condition for the use of this method, the effects of pH, time and temperature, and the interference of several ions were checked. The following conclusions were derived from the experimental results: Absorption curve-- The zirconium Neo-Thorin complex salt shows a maximum absorption at 580 mμ against Neo-Thorin (Figs. 1 and 2). Effect of pH-- The complex salt gives a maximum absorption at pH 1. 7 (Fig. 3). Effect of time and temperature-- Color absorption is stable for a period of 15 to
200 minutes after color development at room temperature. Heating over 40°C is harmful, because of the formation of a purple precipitation. Interference by several ions-- Cations UO(2)(2+) and Fe(3+) besides Th(4+), considerably interfer with color development. The absorption of 2000 μg U corresponds to that of
10 μg Zr (Fig. 4). However, interference by Fe(3+) becomes negligible if hydroxylamine
hydrochloride is added. 2) As a result of the above conclusions, the following procedure is recommended: Procedure recommended-- A few ml of sample solution, 1 ml of 20% hydroxylamine
hydrochloride and 1 ml of dilute acid, if necessary, are mixed and diluted to 9 ml. To
this solution, 1 ml of 0.05% Neo-Thorin is added. The pH value of the final solution is 1.7. From 15 to 200 minutes after mixing, color absorption is measured at 575 mμ. In the range of 0 to 150 μg Zr per 10 ml, the color absorption of the complex salt obeys Beer's law (Fig. 5). 3) In demonstration, this method was applied to the determination of zirconium in a uranium mineral. Table 1 shows the zirconium content of beta-uranophane from Katamo Mine, Tottori-ken.