Abstract: The drug sample was dissolved in water and diluted to 100 mL in a volumetric flask. The solution was filtered with filtering membrane of 0.22 μm and the filtrate was used for IC determination of free sulfate ion. The Dionex AS11-HC column was used for separation with 22 mmol·L-1 KOH solution as mobile phase with a flow-rate of 1.0 mL·min-1. Conductivity detector with depression current of 55 mA was used in the determination. Linear relationship between values of peak area and mass concentration of SO2-4 ion was obtained in the range from 0.50 to 10.0 mg·L-1, and detection limit (3s/k) found was 0.003 μg·g-1. Precision was tested at 3 concentration levels and values of RSD′s (n=7) found were less than 2%. Test for recovery was made by standard addition method using 3 drug samples as matrixes. Values of average recovery found were from 102.0% to 106.0%.