METHOD FOR MEASURING IRON WITHOUT CAUSING INTERFERENCE, AND REAGENT

PROBLEM TO BE SOLVED: To measure iron in a sample by avoiding an interference by adding to the sample a water-soluble salt composed of metallic cations of the main and the sub group III element when measuring a luminous intensity of a colored complex formed by reducing to Fe . SOLUTION: A reducing agent is added to a sample solution, thereby reducing an emitted iron in a trivalent form to a divalent form. A ferroin type complexing agent generating an iron-containing dye is used as a coloring system. A colored complex forming the dye proportionally to the quantity of iron contained in the sample is evaluated by photometry. At this time, the sample is brought in touch with a water- soluble salt of metallic cations of the main or the sub group III element of the periodic table, preferably indium or scandium salt. An interference which occurs under the presence of EDTA can hence be avoided in measuring the luminous intensity. The concentration of the salt of the main or the sub group III element is particularly preferably to be approximately 1-20 mmol/l, and the pH of the sample is maintained in the range of 0-6. As the buffering substance a buffer solution of acetic acid, citric acid or the like of a compound which does not form a complex with iron at a pK value in the range of 2.5-6.