Unit for extemporaneous generation of a quantity, variable on request, of an isotonic solution based on water of a isotonic concentration, isotonic concentration means the known concentration of NaCl characteristic of cell cytoplasm or the intracellular space save compatible tolerances, the unit being characterised in that it comprises: - at least one choice interface for allowing the user to set a desired quantity of solution to be generated from a plurality of possible quantities; - purified water supply means comprising at least one water purification station adapted to receive water from outside the unit and to purify it, - means for supplying at least one solute, - at least one mixing station adapted to mix the purified water with said at least one solute in quantities such as to form a set quantity of solution; - management means comprising calculation means communicating with the said choice interface for calculating the quantity of purified water and the quantity of solute to be mixed for producing the quantity of solution set by the user, wherein the management means are in operating communication with the purified water and solute supply means for ordering them to supply to the mixing station said calculated quantities of purified water and solute wherein the management means are programmed to control the concentration of the solution to be generated by calculation of osmolarity of the solution, the management means are programmed to stop the mixing in the mixing station when both the number of molecules deriving from a stoichiometric check and the dissociation constant deriving from a pH check have reached a predefined target value. the unit being characterised in that it comprises: - at least one choice interface for allowing the user to set a desired quantity of solution to be generated from a plurality of possible quantities; - purified water supply means comprising at least one water purification station adapted to receive water from outside the unit and to purify it, - means for supplying at least one solute, - at least one mixing station adapted to mix the purified water with said at least one solute in quantities such as to form a set quantity of solution; - management means comprising calculation means communicating with the said choice interface for calculating the quantity of purified water and the quantity of solute to be mixed for producing the quantity of solution set by the user, wherein - at least one choice interface for allowing the user to set a desired quantity of solution to be generated from a plurality of possible quantities; - purified water supply means comprising at least one water purification station adapted to receive water from outside the unit and to purify it, - means for supplying at least one solute, - at least one mixing station adapted to mix the purified water with said at least one solute in quantities such as to form a set quantity of solution; - management means comprising calculation means communicating with the said choice interface for calculating the quantity of purified water and the quantity of solute to be mixed for producing the quantity of solution set by the user, wherein the management means are in operating communication with the purified water and solute supply means for ordering them to supply to the mixing station said calculated quantities of purified water and solute wherein the management means are programmed to control the concentration of the solution to be generated by calculation of osmolarity of the solution, the management means are programmed to stop the mixing in the mixing station when both the number of molecules deriving from a stoichiometric check and the dissociation constant deriving from a pH check have reached a predefined target value.