Copper forms coloured complexes with the ore dyes of the pyridine series, e.g., 2-(pyridyl-2-azo)- chromotropic acid (A) and 2-(pyridyl;-3-azo)ehromotropie acid (B). Both the reagents combine with copper in a metal to reagent ratio of 1:3 with instability constants in the order of 10-13 and 10-15. The systems obey Beer's law a 580 in with the optimum ronge from 0.5 to 2.0 p.p.m . with Reagent A and at 670 mµ. with the optimum range from 1.0 to 4.0 p.p. m. with Reagent B. The reagents are extremely sensitive towards copper and the sensitivities, calculated according to Sandell, are 0.00283 \(\gamma\)/cm2 , and 0.00930 \(\gamma\)/cm2, respectively,