Abstract：The interaction of neutral red and heparin was studied by UV-Vis spectrophotometry in pH 3.0 Britton-Robinson (B-R) buffer solution. Neutral red has a strong absorbance at 523 nm and the addition of heparin into neutral red solution resulted in the decrease in the absorbance value at 523 nm without the appearance of new absorbance peak. The decrease in absorbance value was linear with the concentration of heparin. The conditions for the interaction were optimized and the interferences of coexistent substances were investigated. Under the optimal conditions a linear regression equation was obtained as ΔA=0.044+0.076c(mg·L-1) (n=12, r=0.997) in the range of 0.10-15.0 mg·L-1 with the detection limit of 0.073 mg·L-1. The molar absorptivity of the method was calculated to be 2.037×106 L·mol-1·cm-1. The method was applied to determine the heparin sodium injection solution with satisfactory results. The stoichiometry of heparin with neutral red was calculated as 1∶3.
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