Iron as a disturbing factor in the determination of phosphate by the molybdenum blue method

The interference of ferric and ferrous iron in the determination of phosphate by the molybdenum blue method has been studied. It was found that the presence of ferric iron in the solutions could cause either an increase or a decrease in the colour intensity depending on the amount of stannous chloride applied and on the acid and molybdate concentrations in the reagent. Also the phosphorus concentration exerted its effect upon the course of the errors. If the original modification of Truog and Meyer was employed, generally, the most convenient way for the elimination of the interference of ferric iron was to dilute the solution. An increase in the amount of stannous chloride largely helped to prevent the fading effect of ferric iron, provided the phosphorus concentration was not lower than 0.25 ppm. When the effect of ferric iron upon the development of molybdenum blue at various concentrations of sulphuric acid and ammonium molybdate was studied, the observation was made that at each acidity there could be found a concentration of molybdate in which the effect of even fairly high amounts of ferric iron was almost negligible. In lower molybdate concentrations the presence of ferric iron caused an increase in the colour intensity, in higher molybdate concentrations the fading effect of ferric iron was marked. This most suitable level of the molybdate concentration depended to a certain degree on the phosphorus concentration of the solution and on the amount of stannous chloride applied. Fairly good results could be obtained, if the ratio of molybdate (expressed as mg/ml) to acid (expressed as normality) in the solution to be reduced was five times as high as the acidity of the solution to be reduced (expressed as its normality), e.g. 4 in 0.8 N acid, 3.5 in 0.7 N acid, 3 in 0.6 N acid etc. Although it seemed to be fairly possible to avoid the interference of ferric iron by a proper choice of the concentrations of acid and molybdate and of the amount of stannous chloride applied, the fading effect of ferrous iron could not be prevented, if only sulphuric acid was used in the reagents. But the substitution of sulphuric acid by hydrochloric acid totally prevented the fading effect of ferrous iron. On the contrary, a slight increase in the colour intensity was demonstrated. This was true also when only one half of the acid present was hydrochloric acid. It was found that this mixture of sulphuric acid and hydrochloric acid in the molybdate reagent offers an available way for the elimination of the disturbing effect of iron.