137. Solving a chemistry problem

In the determination of lead in urine by the atomic absorption method in an acetylene-air flame, the additive method was used. The same urine samples, 50.00 ml each, were introduced into three separating funnels, and 0.00 0.25 and 0.50 ml of a standard lead solution (50 mg / l) were added to them, respectively. The samples were acidified to pH 2.8 and extracted with 1.0 ml of a 4% solution of ammonium pyrrolidinedithiocarbamate in methyl n-amyl ketone. The organic phase was separated, sprayed into an acetylene-air flame, 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 urine (mg / l).