Most RP/HI reactions use the azeotrope of hydriodic acid, with a constant boiling point of 127 C and a specific gravity of 1.72.
Most of the HI I have been offered for sale has a specific gravity of 1.65, which is barely 50%. I have found that yields have been around 55% w/w, increasing to around 65%-70% when the weaker part of the acid is distilled off.
Recently, some dreams I had involved using homemade HI which had a specific gravity of 1.85, due to the way it was produced, by dissolving as much HI gas as possible under heavy cooling, and found the yields jump to around 75% w/w.
I am seeing a trend here. Has anyone tried using fuming HI with a higher specific gravity under cold reflux cooling, and have they found that their yields increased proportionally, or is there a point at which the high concentration of the HI has a negative impact on yield?
Generally speaking I use a 2.2 ml of HI for every gram of ephed, and about .25 grams of red p.