A liquid dispenser employs passive driving force resulting from water level variations in the water tank of a toilet to dispense a quantitative amount of liquid into toilet bowl of each Flush is disclosed. A sanitary concentrate detergent of a selected formula is filled and sealed in a bottle and mounted to a float quantitative dispensing device in a dilute buffer chamber. Through up and down movement of a float quantitative barrel, a constant amount of concentrate liquid is squeezed and isolated into the dilute buffer chamber for dilution. The whole dispenser apparatus is hung inside. the toilet tank at a desired elevation by means of suction force of a sucking disc. A dispensing probe is provided which has an inlet/outlet submerged in the low portion of the toilet tank. When toilet is flushed, most diluted detergent in the dispenser is flushed into the toilet bowl for cleaning use, and only a small amount of the diluted detergent is retained in the toilet tank. The liquid dispenser uses the principle of liquid partial pressure difference and gravity force feeding. The precision of sampling volume is not changed along with the residual liquid in the liquid bottle.