Swim knows about using anhydrous magnesium sulfate to dry store-bought IPA (which is usually in the 70-91% pure range), but is wondering how to double-check that this drying technique actually works. He wants to be sure it works, b/c water in his IPA would mean loss of product in the procedure he's currently working through.

Can anyone think of any ways to test that it works? Swim has been messing around with NaCl-saturated 91% IPA solutions and adding food colorings with the hope that the water and IPA layers would become distinct, but has not had any luck.

not easy to get it to 99% in a home "lab", nor easy to test outside lab environ

Why not? If you load up some 91% IPA with home-made drying agent (like anhyrdous mag sulfate), why shouldn't it approach 99% purity?

I believe sodim carbonate is a good test for this. It's soluble in water, but I believe not soluble in IPA, so if you put some Sodium carbonate in the IPA and it desolves, then there is still water present.

However, if you keep adding sodium carbonate, the water element should become saturated and then the IPA can be carefully decanted off the settled sodium carbonate at the bottom of the container.

There was a thread somewhere about a fellow forumer having success with this method. I can't remember the title though.