Then add 2 ml of Na2CO3 7% w/ v (in distilled water), homogeneously mixed for 30 seconds, then paid back the volume to 5 ml with distilled water in a pint flask in order to obtain a final concentration of 2, 3, 4, 5, and 6 mg / ml. Allowed to stand for 95 minutes. Measured on a UV-Vis spektrofometer the maximum wavelength