A method for the determination of iron(I1) in water in the presence of iron(I1I) with 1,lO-phenanthroline is described, in which the interfering effect of iron(II1) ions is suppressed by masking with complexones. In the absence of a chelating agent for iron(II1) the colour intensity of samples being analysed is unstable, owing to redox processes induced by the effect of 1,lO-phenanthroline on the iron(I1)iron(II1) system, causing reduction of iron(II1) and formation of the corresponding coloured iron(I1) chelate compound in a stoicheiometrically proportional concentration. The advantages of complexones, especially nitrilotriacetic acid, over other chelating agents are discussed.The method for the photometric determination of iron, after converting it into iron(I1) ions, with 1 ,lo-phenanthroline has been thoroughly investigated and proved to be satisfact0ry.l If, however, it is required to differentiate between the two valency forms of iron by using this method, the omission from the basic procedure of the addition of reductant used in the determination of iron(I1) in the presence of iron(II1) ions is recommended.2 The results thus obtained are, however, rather unsatisfactory, especially because of the instability of the resulting colour intensity with time.3 This drawback also becomes apparent in the method in which iron(I1) is determined indirectly by calculation from the difference between the absorbances of the iron(I1) -iron(II1) -1,lO-phenanthroline system at two specific wavelengths .4The applicability of the methods for the determination of iron(I1) with 1 ,lo-phenanthroline in the presence of excess of iron(II1) ions is restricted by the following requirements: the iron(I1) concentration must be higher than 1 mg l-l, an amount of reagent equivalent to a t least 30 times the total iron content must be used and the absorbance read within 10 to 15 min; also, the sample to be analysed must be free from all foreign matter and protected against direct sunlight .3 However, even when these conditions are satisfied, the results obtained are not reliable.5 Similarly, the use of 2,9-dimethyl-l,lO-phenanthroline instead of