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تسجيل

Interpretation of colour tests , Effects of ionic form

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  340

2026-11-01

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Interpretation of colour tests

Effects of ionic form

Salts may give different colours from those of the corresponding acid or base. In general, free acids or bases that have been isolated from the test material by an extraction process give better colours than their salts. The colour of a salt may be modified by the nature of the other ions present. For example, all hydrochloride salts give a red colour in Mandelin’s test and a blue colour with Koppanyi–Zwikker reagent (prior to adding pyrrolidine). Basic salts of weak acids may

produce different colours because of a change in pH. Where a compound has been extracted from biological material, these factors should not create any difficulty since it will be present in the form of the base. However, when applying the tests to pharmaceutical preparations, where the compounds are usually present as salts, this can cause problems. To overcome this, the material can be extracted in much the same way as for biological samples to derive the free base. Bromide and iodide salts can be converted into the nitrate before testing.