The o-phenanthroline method was tested as to its applicability to the separate determination of iron in natural water: ionic ferrous iron, reducible iron, snd colloidally dispersed iron. In connection with this, the results of the following experiments are given in this paper. 1. Determination of the absorption curve of o-phenanthroline ferrous complex. (Figure 1.) 2. Construction of the calibration curve for a Purfrich's photometer with filter S 50, 15 cm cuvettes. (Figure 2.) 3. Examination of the effects of coexisting anions on the determination of iron by the method of three way layout. (Table 1., Table 2.) 4. Trial determinations of iron in three different states in solution. (Table 3.,
Table 4. ) 5. Test of the applicability of this method to the determination of ionic ferrous
iron and reducible iron in thermal water. (Table 5., Table 6.) From the above, it was revealed that ionic ferrous iron and reducible iron in natural water are separately determinable with sufficient accuracy by the o-phenanthroline method, and that ionic iron and colloidally dispersed iron are also separately determinable, when the dispersed iron is present in amount more than 1.3% of the total iron.