Electrochemical determination of the loop diuretics piretanide and furosemide in pharmaceutical formulations and urine

The oxidation of the loop diuretics 4-phenoxy-3-(1-pyrrolidinyl)-5-sulphamoylbenzoic acid (piretanide) and 4-chloro-2-furfurylamino-5-sulphamoylbenzoic acid (furosemide) was studied in methanol-water (10:90) in the pH range 0.3–13 (piretanide) and 0.5–13 (furosemide), with 0.04 mol/l Britton-Robinson buffers as supporting electrolytes and 0.5 mol/1 KĆl as ionic medium at a glassy carbon electrode. Oxidation processes have shown to be irreversible and predominantly diffusion controlled over the whole pH range studied for both diuretics. Voltammetric methods have been developed for the determination of piretanide at pH 5.0 and furosemide at pH 4.5 using differential pulse and square wave voltammetry. The peak current showed a linear relationship with concentration up to 25 mg/l (6.9 × 10−5 mol/l for piretanide and 5.5 × 10−5 mol/1 for furosemide) with a reproducibility in terms of standard deviation (n = 10) lower than 5.6%. A detection limit of 50 ng/l was obtained (1.38 × 10−7 mol/l piretanide and 1.51 × 10−7 mol/l furosemide) for both electroanalytical techniques. Methods proposed were applied to pharmaceuticals and spiked urine samples.