19. Solving a chemistry problem

198. The atomic absorption method was used to determine trace amounts of lead in natural water. In three separating funnels, identical samples of natural water, 50 ml each, were introduced and 0.00 was added to them; 0.25 and 0.50 ml of lead standard solution (50 mg/l), respectively. The samples were acidified to pH 2.8 and extracted with 1.0 ml of a 4% solution of ammonium pyrrolidine dithiocarbamate in methyl n-amyl ketone. The organic phase was separated, atomic vapor was obtained from it and the optical density was measured at 283.31 nm. The measured values were 0.214; 0.435 and 0.650 respectively. Determine the concentration of lead in natural water.