The Zwikker reagent is used as a simple spot-test to presumptively identify barbiturates. It is composed of a mixture of two solutions. Part A is 0.5 g of copper (II) sulfate in 100 ml of distilled water. Part B consists of 5% pyridine (v/v) in chloroform. One drop of each is added to the substance to be tested and any change in colour is observed.
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. 
- O’Neal, C. L.; Crouch, D. J.; Fatah, A. A. (2000). "Validation of twelve chemical spot tests for the detection of drugs of abuse". Forensic Science International 109 (3): 189–201. doi:10.1016/S0379-0738(99)00235-2. PMID 10725655.
- "Color Test Reagents/Kits for Preliminary Identification of Drugs of Abuse" (PDF). Law Enforcement and Corrections Standards and Testing Program. July 2000. Retrieved 2011-07-24.
- Brandenberger, Hans; Maes, Robert A. A. (1997). Analytical toxicology: for clinical, forensic, and pharmaceutical chemists. Walter de Gruyter. p. 353. ISBN 978-3-11-010731-9. Retrieved 2012-02-28.
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