I am following APHA Diphenyl Carbazide(spectrophotometric) method for Cr VI analysis .Different 10-100 ppm std. solutions of K2Cr2O7 are added with deionsed water upto 100 ml and then acidify with H2S04 (same quantity in all std solutions) for achiving pH ~1-2 then adding 2 ml Diphenyl Carbazide reagent, mixed it for 5 minutes getting red violet colour but this colour is disappering in next 5 -10 min. what should i do for stability of this colour? or suggest any other method for Cr VI analysis by spectrophotometer.