Dragendorff's Reagent

[Dinah Thiers Farrington]

     I am a graduate student trying to identify some herbicidal compounds using   TLC. My system uses Dragendorff's reagent (potassium iodobismuthate). I am using the Munier modification and would like to hear from anyone with extensive experience using this reagent. In trying to make it, following a textbook recipe, I am having trouble getting bismuth subnitrate to dissolve. I would be very glad to hear anyone's suggestions on this. Thank you.