ABTS ELISA PEROXIDASE SUBSTRATE is a ready to use substrate - No dilution required. Read absorbance of blue green color at 405 nm. The color formation as a function of time can be recorded at intervals or the reaction may be stopped by addition of acid after 30 mins. A 0.625 Mol L-1 Oxalic Acid stop solution is recommended, color is stable for 1hr.