In the medium of citric acid-NaOH buffer (pH 3.5), proteins may combine with chrome azurol S(CAS) by intermolecular forces (mainly by electrostatic force) to form macromolecular ion-association complexes, causing an elastic light-scattering signal with a maximum wavelength of about 370nm. Based upon this, a new method for the determination of serum albumin with a detection limit of 0.02μg.mL^-^1 was proposed. Its calibration curve is linear over the range of 0-1.0μg.mL^-^1. The sensitivity of this method is fifty-fold higher than that of the Coomassie brilliant blue protein assay. The scattering spectra of this system were measured using an ordinary spectrofluorophotometer. Influence of experimental conditions, such as pH, ionic strength, and concentration of CAS, on the intensity of light-scattering was studied. Effects of surfactants (cationic, anionic, and nonionic), amino acids, and metal ions on the combination of CAS with protein were also investigated. This method has been used for the determination of urinary protein with good results.

Key words: CITRIC ACID, CHROME AZUROLS, LIGHT SCATTERING, PROTEIN, SODIUM HYDROXIDE