Cobalt(II) thiocyanate


Cobalt thiocyanate is an inorganic compound with the formula Co2. It is a layered coordination complex and its trihydrate Co23 is used in the cobalt thiocyanate test for detecting cocaine. The test has been responsible for widespread false positives and false convictions.

Structure and preparation

The structures of Co2 and its hydrate Co23 have been determined using X-ray crystallography. Co2 forms infinite 2D sheets in the mercury thiocyanate structure type, where as Co23 consists of isolated tetrahedral Co22 centers and one equivalent of water of crystallization.
The hydrate may be prepared by the salt metathesis reactions, such as the reaction of aqueous cobalt sulfate and barium thiocyanate to produce a barium sulfate precipitate, leaving the hydrate of Co2 in solution:
or the reaction of the hexakisacetonitrile cobalt tetrafluoroborate and potassium thiocyanate, precipitating KBF4
The anhydrate can then be prepared via addition of diethylether as an antisolvent.

Cobalt thiocyanate test

Detailed procedures for the cobalt thiocyanate test are available.
Addition of the cobalt thiocyanate reagent to cocaine hydrochloride results in the surface of the particles turning a bright blue. The solution changes back to pink upon adding some hydrochloric acid. Addition of chloroform, results in a blue organic layer for both cocaine hydrochloride and cocaine base. Diphenhydramine and lidocaine also give blue organic layers. These compounds are known false positives for cocaine. Lidocaine is commonly used to adulterate or mimic cocaine due to its local anaesthetic effect.
If the procedure is adjusted to basify the sample rather than acidifying it, the test can be used to test for ketamine hydrochloride.