Cations are segregated in 5 groups depending upon their solubility with the help of various precipitating reagents. As per the segregation, ${\mathrm{Zn}}^{2+}, {\mathrm{Co}}^{2+}, {\mathrm{Ni}}^{2+}$ belongs to the 4th group and ${\mathrm{Fe}}^{3+}$ belongs to the 3rd group.