I'm currently trying to make a standard curve for potassium ion detection. For my reagent I'm using 25 mM of sodium cobalthexanitrite with potassium dihydrogen phosphate as a substrate in a 40:60 ratio and measuring at 420nm after incubating the mixture in the dark for 10 minutes. Unfortunately, it's not really working and I'm not getting any precipitation. Is there something more I need to add or a step I've missed? Is my wavelength incorrect?