A robotic procedure for measuring benzalkonium chloride preservative in ophthalmic formulations was developed. The benzalkonium chloride was complexed with picric acid under basic conditions and the colored complex extracted into chloroform. After washing to remove excess picric acid, the chloroform was drawn into a spectrophotometer cell and the absorbance measured at 360 nm. Response was linear over a concentration range of 0.0050-0.0015 mg/mL with coefficients of variation <2%.