Reassessment of Matrix Interference in the Pharmacopeial Limit Test for Aluminum in Citric Acid: Toward Method Revision

The pharmacopeial limit test for aluminum in citric acid intended for dialysate production commonly relies on solvent extraction with 8-hydroxyquinoline followed by fluorescence measurement. However, fluorescence intensity (F.I.) readings from citric acid extracts have frequently proven unreliable, often registering lower values than even the blank solution. This study aimed to investigate the impact of the citric acid matrix on the accuracy of the test. A comparison between standard aluminum solutions prepared in water and those in citric acid revealed significant differences in both slope and y-intercept of the calibration curves, with F.I. values markedly lower in the citric acid solutions. Further experiments demonstrated that increasing the concentration of citric acid consistently reduced the F.I. of aluminum solutions. These findings indicate that the acidic nature and metal-chelating properties of citric acid interfere with the assay. Consequently, the current pharmacopeial limit test for aluminum in citric acid requires revision, as it may otherwise underestimate aluminum content and produce misleading results.