The Zwikker reagent is used as a simple spot-test to presumptively identify barbiturates.
Part A is 0.5 g of copper (II) sulfate in 100 ml of distilled water.
[1][2] One drop of each is added to the substance to be tested and any change in colour is observed.
The test turns phenobarbital, pentobarbital and secobarbital light purple.
[1] The test's lack of specificity and tendency to produce false positives means it is not widely used for presumptive drug testing, although it does still play a role as a thin layer chromatography stain.