Jayalak

Hello all, I'm doing sorption studies of certain heavy metals (Pb, Cu, Zn, Ni, Cd, Cr) in soil (natural clay and kaolinite) with ph as a variable (2,4,7,10,12).On adjusting pH to reach a constant value using NaOH and HCl, my heavy metals in nitrate salts form are getting precipitated. thus unable to maintain the concern concentration (50ppm, 250ppm, 500ppm, 750ppm, 1000ppm). help me out please.. thanks in advance.