February 28, 2020, 07:12:35 PM
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Topic: Cobalt thiocyanate complexes and the presumptive Scott test for cocaine  (Read 311 times)

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Offline Babcock_Hall

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I have been reading Suzanne Bell's textbook Forensic Chemistry, 2nd ed., pp. 239-241 with the hope of understanding this chemistry.  Let us represent the conjugate base of cocaine as NR3.  The test has three reagents added successively in a test tube: cobalt thiocyanate, Co(SCN)2(H2O)3, HCl, and chloroform.  In the presence of cocaine a blue complex is extracted into the organic layer.

Under acidic conditions {Co(NR3)2}(SCN)2 is favored and under neutral or basic conditions (HNR3)2){Co(SCN)4} is favored.  The latter form is blue and is not soluble in water (it forms crystals), but it is soluble in chloroform.  The other complex, which is water-soluble, and Co(SCN)42- are both pink or pinkish.

Does anyone know of a good discussion of this chemistry?  For one thing, it is not entirely clear where the extra thiocyanates are coming from.  Presumably other cobalt ions are providing them.  For another thing it is unclear to me why the blue complex is more organic-soluble than the pinkish complex.  I imagine that I could think of other questions...

Offline Babcock_Hall

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Re: Cobalt thiocyanate complexes and the presumptive Scott test for cocaine
« Reply #1 on: January 16, 2020, 07:58:47 PM »
The textbook had two references, and I am obtaining both of them.
Schlesinger HL "Topics in the chemistry of cocaine," Bull Narc. 1985 Jan-Mar;37(1):63-78.
Elsherbini SH, "Cocaine Base Identification and Quantification," Forensic Sci Rev. 1998 Jun;10(1):1-12.

Offline Babcock_Hall

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Re: Cobalt thiocyanate complexes and the presumptive Scott test for cocaine
« Reply #2 on: January 18, 2020, 12:35:58 PM »
I am in the process of obtaining additional references.  However, the two that I have consulted so far claim the same thing with respect to the paradoxical acid-base chemistry I discussed in my initial post, namely that the protonated form of the cocaine complex with cobalt and thiocyanate is favored under basic conditions and the deprotonated form is favored under acidic conditions.  I am puzzled about this because it seems backward.
EDT
This is a passage from a 1985 article by Schesinger. https://www.unodc.org/unodc/en/data-and-analysis/bulletin/bulletin_1985-01-01_1_page006.html
"An elucidation of the reaction mechanisms involved has been published [56] . lt assigns the formula
(alkaloid H)2 [Co(SCN)4] to the relatively water-insoluble blue complexes formed in neutral-to-basic solutions, and the formula [Co(alkaloid)2] (SCN)2 to the more water-soluble, brownish-red to pink complexes formed in acid-to-neutral solutions. Solubilities in water and chloroform are given for complexes formed with 29 alkaloids and nine metal thiocyanates."
Presumably alkaloid H is the protonated form of cocaine or other compounds.  Experimentally, there are blue crystals initially formed in the reaction, that the solution turns pink upon addition of HCl and that addition of chloroform leads to a blue organic layer.
« Last Edit: January 18, 2020, 04:06:00 PM by Babcock_Hall »

Offline Babcock_Hall

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Re: Cobalt thiocyanate complexes and the presumptive Scott test for cocaine
« Reply #3 on: January 21, 2020, 11:34:16 AM »
Oguri K et al. (1995) Japanese J. Toxic. Environ. Health 41(4):274-279 were not able to isolate a cocaine-containing complex by silica gel chromatography.  They suggested that the complex is too labile.  However, they determined the stoichiometry between cocaine and cobalt to be 2:1 in one complex.  They proposed a structure involving two bidentate cocaine ligands and two isothiocyanate (Figure 2 in their paper).  My only reservation about their proposed structure is that the isothiocyanate may coordinate through nitrogen, not sulfur.  I am still working on the acid-base aspects of this question.

Offline Babcock_Hall

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Re: Cobalt thiocyanate complexes and the presumptive Scott test for cocaine
« Reply #4 on: January 25, 2020, 02:08:39 PM »
Tsumura et al., 2005 Forensic Science International 155:158-164.  doi:10.1016/j.forsciint.2004.11.011.  I did some calculations after I made some assumptions about the final step, the addition of chloroform.  Let us start with with 1 mg of cocaine, as in Figure 1 of this paper.  I assumed that the absorbance of the solution was 0.5 and that the path length was 1 and that the volumes of the two layers were 1 mL each.  I also assumed that the molar absorptivity of the complex was 50,000.  Under these assumptions the amount of cocaine in the chloroform layer was 0.3% of the starting mass.  Obviously this is only a crude calculation, but I think it is helpful in illustrating that the chloroform layer probably contains only a small fraction of the total amount of cocaine.  Most of the cocaine is almost certainly in the hydrochloride form in the water layers, but a small amount might be the conjugate base complexed with cobalt and thiocyanate in the chloroform layer.

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