I am working to detect the activity of hydroxyl radical using Fenton's chemicals. The system consists of (3 mM ferrous sulfate + different conc. of H2O2). I am adding (200 uL) nanoparticles followed by the addition of NaCl (100 uM) to cause the color change of nanoparticles. I am getting a fixed color of nanoparticles (NOT gradual) which is due to NaCl reaction with ferrous sulfate. Are there any other chemicals to use other than salts and will not interfere with ferrous?