I recently bought some Isopropanol in the form of Iso-Heet to be used as a cleaning and reaction solvent. According to the MSDS sheet put out by the manufacturer, the product contains 99% IPA and 1% "proprietary additive". In order to separate the alcohol from the additive, a simple distillation has always been sufficient as there remains in the boiling flask a high-boiling syrupy amber liquid.
However, when I was cleaning the recently-bought Iso-Heet, it was distilling over at 45C rather than at its usual boiling point of 83C. Checking the distillation periodically I did not notice any of the "lines" normally observed when two liquids of different densities mix (I have heard them called Schlieren but I have no idea if this is an accurate term). Despite this one sticking point the liquid I distilled had a density within 0.01 of pure IPA, it was oxidized by acidic permanganate, it has the characteristic odor of IPA, and I observed no change in the solution density when I mixed a small amount of this liquid with some of my old stock that I am certain is (or rather was) pure Isopropanol.
I suspect that this impurity must have a density very similar to that of pure Isopropanol such that when the two are mixed there is little to no observable change, and that I have an azeotropic mixture of the two as when doing a second distillation I only collected one fraction at 43C. The other option is that the liquid in question simply isn't Isopropanol. Does anybody know of any further qualitative tests or separation methods I could try, maybe there is something I'm missing? I am tempted to see if I can't have a sample sent to a lab to have an IR and NMR done but that would likely be rather costly, so I view it as a last resort.