Making the Test Solutions

Another possible source of error in the diphenyla-mine test is in the sulphuric acid used to make the test solution. In many cases I have found the sulphuric acid to contain sufiicient oxidizing substances to turn the entire test solution itself a deep blue. Obviously, such a solution cannot give accurate results. The test solution, when properly prepared is water white in color. 

In the variant of the procedure adapted by the European Pharmacopoeia, no amine ligand is added. Instead, calcium is added, and this gives a variant of the above complexes of a structure, which has not been elucidated. The complex is insoluble in methanol. The presence of calcium also explains the violet-blue color, since most of the complexes with amines are violet. The cobalt complexes are more likely to precipitate when making the test solution... 

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