[Babcock_Hall]

False positives are a general problem in presumptive tests, which is why confirmatory tests are run.  The cobalt thiocyanate test is described in Suzanne Bell's textbook, "Forensic Chemistry," pp. 239-241, and one variation of this is called the Scott test.  The last step of the test is the extraction of a complex between cocaine and cobalt thiocyanate into chloroform.  I am interested in theoretical or empirical reasons why this test might provide a false positive.  Good control of pH is obviously critical.

I am just at the beginning stages of my reading on this subject, but I found this passage helpful:  "Published reports suggest that diphenhydramine hydrochloride, chloropromazine, promazine hydrochloride, scopolamine, promethazine with phencyclidine, and a combination of phenycyclidine with either promazine, dibucaine, or methaprilene as well as other medicines and designer drugs give false positives with the Scott test which is also used to identify crack cocaine. It has also been shown that too much heroin or dibucaine also give false positives. In February 2004, three boys were arrested in this manner by police in Tokyo; ultimately it became clear that their substance was not cocaine but a legal drug.⁹⁰"  From the essay, "False Positives Equal False Justice" by John Kelly.
http://www.cacj.org/documents/sf_crime_lab/studies__misc_materials/falsepositives.pdf

Reference 90 is to a newspaper report.

[Babcock_Hall]

Diphenylhydramine is an antihistamine that is sold under the trade name Benadryl, among other names.  Lidocaine is a local numbing agent.  Both of these compounds are tertiary amines, and look fairly lipophilic upon inspection.  I have seen contradictory things about whether or not lidocaine will produce a false positive in cobalt-based test.