Several presumptive tests are available for barbiturates and benzodiazepines. These are not as 'general' as those used for other drug classes (for example, the Marquis test for opiates and amphetamines (including ring-substituted species)), but have the disadvantage that they do not discriminate between the drugs within the specific class. These tests are described in the following sections.
This colour test involves the use of two reagents. The first of these is a solution of 0.1 g of cobalt acetate and 0.2 ml of glacial acetic acid in 100 ml of methanol, with the second comprising 5 ml of isopropylamine in 95 ml of methanol. To test materials for the presence of barbiturates, three drops of the first reagent, followed by three drops of the second, are placed on the test material contained in the well of a spotting plate (positive and negative controls should also be undertaken). A purple colour develops if barbiturates are present in the test sample. This test is convenient because there are very few other substances which give such a colour reaction. However, it does not discriminate between specific barbiturates and thus further analyses are required.
Again, two reagents are required for this test. The first of these is a 1% (wt/vol) solution of 2,4-dinitrobenzene in methanol, with the second being a 15% aqueous solution of KOH. One drop of the first reagent, followed by one drop of the second, is added to the test substrate. The development of a red-pink colour putatively indicates the presence of diazepam or some other benzodiazepine. However, it should be noted that a number of other materials will also result in a similar colour reaction.
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