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A Study
of Acids Used for the Acidified Cobalt Thiocyanate Test for Cocaine
Base Anna
L. Deakin ABSTRACT: Four acids (hydrochloric, sulfuric, nitric, and acetic) were used as acidifying reagents in the "one well" cobalt thiocyanate test for cocaine base. Concentrated sulfuric, nitric, and acetic acids were found to be equally fast as concentrated hydrochloric acid (the standard acid used in the test). In addition, dilute (down to 0.1 N) hydrochloric acid was found to be as effective as concentrated hydrochloric acid. Only concentrated hydrochloric acid gave a transient blue color upon addition to the cobalt thiocyanate reagent. A number of other controlled substances, adulterants, and diluents were also tested and confirmed to not give false positives with sulfuric, nitric, acetic, or dilute hydrochloric acids. KEYWORDS: Cocaine, Cobalt Thiocyanate, Acidified Cobalt Thiocyanate, Spot Tests, Color Tests Introduction The cobalt thiocyanate color test is widely used in forensic laboratories to determine the presence of cocaine salt, i.e., cocaine hydrochloride (1,2). However, the test requires a water soluble form of cocaine, and is ineffective for testing cocaine base. Therefore, a modified version of the test, the acidified cobalt thiocyanate test, is used to determine for the presence of cocaine base. The addition of an acid to the reagent allows the cocaine base to dissolve, and the color reaction can proceed. A sustained blue colored precipitate is a positive test. There are two general procedures for running these tests. The first is to have two separate solutions prepared (one "normal" and the second acidified) and use them in two separate spot wells of a standard porcelain spot plate. The other is to run the normal (non-acidified) test first, observe for any color change, and if none then add a small amount of acid to the spot well, and again observe for any color change. This latter technique is referred to as the "one-well" method. A literature search found that the only documented acid used for this "one well" test is concentrated hydrochloric acid (HCl). However, there is a complication when using this acid in that when it is first introduced to the cobalt thiocyanate solution, the color of the solution temporarily turns from pink to blue even if cocaine base is not present - and blue is also the characteristic color change observed for cocaine. Although this change is only temporary (as well as distinguishable to the trained eye), and there is no blue colored precipitate, it can be confusing to novices, and can potentially give ambiguous results with samples containing only trace amounts of cocaine. The latter problem can be an issue with commercial field test-kits. In this study, a series of acids commonly utilized in most forensic/analytical laboratories were used to perform the "one well" test for cocaine base. A variety of other controlled and non-controlled substances were also studied using the same acids. In addition, the concentration of HCl used for the "one well" test was also studied to determine if the test would still be effective if a diluted version was used. Experimental Chemicals Chemicals were purchased from the following vendors.
Prepared Reagents Cobalt thiocyanate reagent: 2 grams of cobalt thiocyanate were dissolved in 100 mL distilled water. Acidified cobalt thiocyanate reagent: 2 mL of concentrated HCl were added to 98 mL of above cobalt thiocyanate reagent. Procedure Several controlled and non-controlled substances were studied, as well as numerous case samples of cocaine base. For each sample, the following procedure was followed:
The effect of the concentration of HCl added to the cobalt thiocyanate solution was separately studied. Two to three drops of the cobalt thiocyanate reagent were added to several wells of a spot plate. One drop of HCl (of varying concentrations) was added to each well. Results and Discussion It was found that all four acids (hydrochloric, sulfuric, nitric, and acetic) produced the same test results for cocaine base (see Table 1, below). All four concentrated acids were equally fast. In addition, no false positives were observed with any of the other controlled substances, adulterants, and diluents tested when sulfuric, nitric, or acetic acids were substituted for concentrated HCl. Notably, only concentrated HCl gave the transient blue-colored solution when added to the "normal" (non-acidified) cobalt thiocyanate reagent that did not contain cocaine.
Dilute HCl (from 1:1 down to 0.1 N) produced the same results as concentrated HCl, but also did not give the transient blue-colored solution when added to the "normal" (non-acidified) cobalt thiocyanate reagent that did not contain cocaine (see Table 2). When cocaine base was present, it was noted that the weaker the HCl solution, the slower the color reaction, but it never took more than a few seconds for the blue precipitate to form, and the overlaying solution did not turn blue even when cocaine was present. Thus, dilute HCl is as effective as concentrated HCl for the test. The collective results suggest that substituting an alternative acid or a diluted form of HCl for concentrated HCl for the acidified cobalt thiocyanate test would be advantageous. Table 2. Effects of Hydrochloric Acid Dilution
Acknowledgements Thanks to Sandy Kassner and the members of the Chemistry Section at the Florida Department of Law Enforcement, Tampa Regional Crime Laboratory, for their help and contributions to this project. References (Not Cited in Text) Drug Enforcement Administration, Basic Training Manual for Forensic Chemists. Velapoldi RA, Wicks
MS. The use of chemical spot test kits for the presumptive identification
of narcotics and drugs of abuse. Journal of Forensic Science 1974;19(3):636-656. * * * * *
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