Since As2S3 sol is negative sol, Ba2+ is the effective ion for coagulation Amount of Ba2+ ion = 4 × 0.005 = 0.02 millimol Total volume of solution = 16 + 4 = 20 ml. If 20 ml requires 0.02 millimol of Ba2+ , 1000 ml requires 1 millimol of Ba2+ Flocculation value.of Ba2+ is 1 (Flocculation value is expressed in millimol perlitre)