Photometric titration of anionic polyelectrolytes using the cationic dye as a color indicator

The cationic dye, crystal violet (CV) is applied as the indicator for the determination of anionic polyelectrolytes such as potassium poly (vinyl sulfate) (PVSK) by photometric titration, using a poly (diallyldimethylammonium chloride) (Cat-floc) solution as a titrant. Before the titrant is added to the sample solution containing CV as an indicator, the absorbance of the sample solution is relatively low because the attachment of CV to the anionic polyelectrolyte results in a decrease of the absorbance of CV at the maximum absorption wavelength (590 nm). As the amount of added titrant increases, the absorbance of the sample solution increases because the combined CV is liberated by the exchange with the titrant. After the end-point, the absorbance of the sample solution decreases slightly due to dilution with the titrant. The end-point of the titration is detected as the break point of the titration curve. The effects of the concentration of CV, coexisting electrolytes in the sample solution on the degree of the change of absorbance at the end-point were examined. A linear relationship between the concentration of anionic polyelectrolyte and the end-point volume of the titrant exists in the concentration range from 6.25 × 10^-6 to 5 × 10^-5 M for PVSK.