A micropumping multicommutation manifold to perform turbidity determinations in waters is described. The procedure is based on the use of a combination of hydrazine sulfate and hexamethylenetetramine, to obtain an external standard of nephelometric turbidity units (NTU), which could compare the absorbance measurements at high wavelengths for samples with a calibration line obtained from a concentrated formazine standard diluted on-line. To minimize sample and reagent consumption and waste generation, the flow system was designed with two solenoid micro-pumps, one of them for the alternative introduction of the formazine standard and samples and the other one for the water carrier. The multicommutation approach makes possible the on-line dilution of a single standard to obtain the external calibration. The linear response was ranged up to 160NTU. The coefficient of variation was estimated as 1.6 and 3.2% for 10 and 100mm flow cell, respectively, for solutions containing 40NTU (n=10). Approximately, 60 determinations can be carried out per hour with limit of detection values of 1 and 0.1NTU, consuming only 160 or 240muL formazine solution and generating 1.8 or 2.0mL waste per determination, using measurement cells of 10 and 100mm optical pathlength, respectively. The procedure was successfully applied to 11 different water samples. Recovery studies were carried out and results obtained were between 97.5+/-0.2 and 100+/-1%. The development of a homebuilt light emitting diode (LED)-based portable flow analysis instrument was checked for in situ turbidimetric measurements, providing this equipment a LOD value of 0.09NTU working with a blue LED at 464nm and a LOD value of 0.1NTU working with an IR LED.