CHEMICAL DISPENSING TAPS

We are working with a rural community who draw water from springs and wells and take it home foruse in 20 litre containers . We have in mind installing a sodium hypochlorite dispenser which can dispense only 10mls of the Hypo solution  to give 40ppm of disinfection  enough to disinfect the water in the 20 litre containers i.e 2ppm per litre. Can anyone out there advise on how we get such dispenser taps which can dispense exactly 10mls when pressed. Best Regards P.W.Karani