Recently, polyester fiber has been widely used around the world. This fiber has its own advantages and disadvantages. One of the disadvantages of this fiber is the difficulty of the dyeing process. The most important dye that is applicable on this fiber is disperse dye. Disperse dyes due to their hydrophobic nature have proper affinity to polyester fibers. Disperse dyes are usually small and have few polar groups, so they are hydrophobic and have little solubility in water. The little solubility of these dyes in water, while the dyeing process normally occur in aqueous environment, causes these dyes not to properly follow the Bouguer-Beer-Lambert’s law and makes it difficult to measure the oncentration of these dyes online. In the present research, three disperse dyes with different chemical structures and molecular weights were used. These dyes were dissolved in water with different concentration of a dispersing agent and with or without being treated with ultrasounic operation.Then, the absorbance of each dye solutions was measured by UV-Vis spectrophotometer. The absorbace curve of the solutions were compared with the ones for dyes in water and in dimethylformamide/water 50/50% solution. The rsults of the experiments showed that using the combination of ultrasound operation with the addition of the proper amount of a dispersing agent gives comparable results to the conventional method for which a 50/50% DMF/Water solution is used.
