A directional control valve for fluid comprising an axial chamber connected to first and second radial inlet/outlet ports and to a radial outlet port for administering said solution, a rotary crank pin housed in said chamber and through which there pass connecting ducts which are connected to axial third and fourth inlet/outlet ports intended to be connected to said second reservoir, said connecting ducts being designed to connect said first, second, third, fourth inlet/outlet ports and said administering port selectively and fluidically according to the angular position of said crank pin with respect to said body.