> "Scheme II shows an overview of the reaction mechanism to give the purple-colored complex. A 0.3 ml aliquot of sample was taken and added to a 30 ml Universal glass vial with a 24 mm polypropylene screw cap. The sample was then adjusted to pH 1 by the addition of H2SO4, to convert the GHB (I) present to the lactone, GBL (II) via intramolecular esterification. A 1.0 ml aliquot of a 1.0 M hydroxylamine HCl solution and 0.2 ml of 12 M sodium hydroxide solution was then added. The resulting mixture was then mixed gently by hand to form the hydroxamate derivative (III). Upon the addition of 0.2 ml of ferric chloride, a purple-colored complex (IV) is then formed instantly."
You'd have to carry around a little kit with all these pre-made solutions (plus some pH paper to get step 1 right). The 'app' part is just using your cell phone as a crude spectrophotometer to get a rough estimate. Also, colored drinks would be a problem. The idea here seems to be more related to 'in the field' analysis for forensic law enforcement without the need to transport samples to a lab.
Incidentally alcohol is low-quality drug with nasty side effects, even though its use is socially normalized. I group it with morphine derivatives, as alcohol triggers several opiate-response pathways, accounting for some of its addictive properties. The yearly death toll of the 'alcohol epidemic' is also comparable to the yearly death toll of the 'opiate epidemic'.