When Rose Bengal is added to fentanyl; it should show a color change from red to purple due to the new complex made by binding O- of RB and N of FTN (depicted in a black arrow). But it does not show any color change. What may be the reason for this? Is there any specific ratio Rose Bengal or Eosin Y should be mixed with fentanyl to get a color change? Do you recommend any special chemical to be added to get the expected visual change?