C.I. pigment yellow 180, aqueous pigment dispersion for ink-jet ink, and aqueous pigment ink

The aqueous pigment dispersion for IJ inks has excellent dispersion quality as well as high color strength and light fastness and is particularly suited for use as an U aqueous pigment ink. It includes C.I. Pigment Yellow 180, a block copolymer having an acid value of 10 or more and 20 or less and an amine value of 20 or less, and water. The present invention also provides C.I. Pigment Yellow 180 having an aspect ratio of 1.0 to 2.0, a primary particle shorter diameter of 30 to 100 nm, and a crystallite size of 15.0 nm or more as calculated from the Scherrer equation based on the full width at half maximum of a peak A indicated by a maximum diffraction intensity at a diffraction angle 2θ=6.7±0.50 in a powder X-ray diffraction pattern of diffraction intensities versus 2θ measured using CuKα radiation.