OK, here is the procedure:
Take 25 mL of milk, add 5-10 mL of conc. HCl, filtrate it (this took quite a while in my case...this goddam solution didn't want to go through the funnel...it took more than 20 minutes...
Make sure that 100% of the milk goes thoruogh the funnel!
Pour the filtrate in a 100 mL volumetric flask and fill it.
Now take 20 or 25 mL with a transfer pipette and put it in an Erlenmeyer flask, add ammonia (c=0.1 mol/L) until you reach a pH of 9-10 and titrate with EDTA (c=0.01 mol/L)