Selected Analytical Problems Involving Organic Analytes which are Major or Minor Organic Constituents of a Sample

Summary

Aspirin (acetyl-o-salicylic acid) can be selectively determined in a pharmaceutical formulation by chelation of its hydrolysis product, o-salicylic acid, with Fe³⁺. Other constituents of analgesic formulations such as paracetamol, caffeine and phenacetin will not give this reaction and hence do not interfere. The method is, however, time consuming and is ideally used when alternative instrumentation is unavailable. Other analytical methods display different selectivities. APC tablets, which contain aspirin, phenacetin and caffeine, can be assayed by solvent extraction—UV spectrophotometry by a relatively time-consuming method which involves the separation of aspirin from the other two constituents of the formulation prior to their UV spectrophotometric determination. The electrochemical oxidation of paracetamol and its degradation product (p-aminophenol) leads to a voltammetric method for their rapid and selective determination, since other analgesics do not interfere at the potentials of oxidation. NMR spectrometry involves the use of sophisticated instrumentation for the rapid and specific assay for a particular analgesic such as aspirin in a multianalgesic formulation. The optimum selectivity for the determination of multianalgesics can be obtained using HPLC, by means of which most of the analgesics can be determined rapidly in a single run. Given the availability of HPLC instrumentation in the laboratory, this would be regarded as the method of choice for the analysis of multianalgesic formulations.