An automatic kinetic spectrophotometric method is described for the ultramicro determination of manganese and iridium, based on their catalytic effect on the periodate - phosphinate reaction. The method is also used for the determination of nitrilotriacetic acid (NTA) and 1,2-diaminocyclohexane- NNN′N′-tetraacetic acid (DCTA), on the basis of their activating and inhibiting effect, respectively, on the manganese-catalysed periodate - phosphinate reaction. The time required for the reaction to consume a fixed amount of periodate is measured automatically and related directly to the concentration of catalyst, activator or inhibitor. Manganese and iridium at the 10-8-10-7 M level and NTA and DCTA at the 10-6 M level were determined with average errors of about 2%. The method has been applied to the determination of manganese in natural waters
