[Babcock_Hall]

I have briefly surveyed some recipes for this stain.  The concentration of phosphomolybdic acid (PMA) varies at least fivefold, 2-10 grams per 100 mL.  One recipe called for absolute ethanol.  I have one recipe in my notes which indicates adding one drop of sulfuric acid.  I have not found the sulfuric acid in a book or an on-line source yet.  I did find one recipe where a plate is first sprayed with sulfuric then PMA (https://scs.illinois.edu/system/files/inline-files/ThinLayerChromatography.pdf).  Does anyone have experiences with preparing this stain that he or she would like to share, either regarding concentration or the addition of acid?

[tomek]

The recipe I use calls for 10% PMA in ethanol. So I just weigh some amount into a Erlenmeyer flask and top it up with 96% ethanol (eg. 5.6g of PMA -> 56g of final stain). While developing plates I dip them briefly, let them drain and then heat the hell out of them with a heat-gun. The fresh stain is yellow and can be used for some time (couple of weeks). Then it goes green and somewhat is less effective. I've never tried adding sulfuric acid, but maybe doing so will not require such a high temperature (prolonged heating at high temp. sort of darkens the whole plate).

[Babcock_Hall]

Thank you; I was just remarking to myself that the stain was turning green with time.  I have typically used a hot plate and fiddled with time and temperature.