Surprisingly, to the authors' knowledge, there are only two published kinetic spectrophotometric methods for the determination of NAC. 15,16 Also, none of the cited methods for the determination of NAC has used Fe 3+ and 2,4,6-trypyridyl-s-triazine (TPTZ) as a reagent solution. In this report, a novel, simple and sensitive kinetic spectrophotometric method with TPTZ as the chromogenic reagent for the determination of NAC is described and validated. Ease of application, sensitivity, short analysis time, low cost and reliability are its main advantages. The initial rate and fixed-time methods, after their optimization and validation, are adopted for the determination of NAC in its pharmaceutical formulations.
Experimental
ReagentsAll chemicals were of analytical-reagent grade, and solutions were prepared in Milli-Q deionized water.A stock solution of NAC, 1.0 × 10 -2 mol L -1 , was prepared by dissolving 163.2 mg of NAC (Merck, Darmstadt, Germany) in deionized water up to 100.0 mL in volume. The stock solution was stable for at least 30 days when it was stored in a dark bottle at 4 C. Working solutions of lower concentration were prepared daily by appropriate dilution of the stock solution with deionized water.A stock solution of Fe 3+ , 1.0 × 10 -2 mol L -1 , was prepared by dissolving 270.3 mg of FeCl3·6H2O (Kemika, Zagreb, Croatia) in a portion of deionized water. Concentrated hydrochloric acid (0.5 mL) was added to prevent the hydrolysis of iron, before making up to a volume of 100.0 mL.A stock solution of TPTZ (Merck, Germany), 1.0 × 10 -2 mol L -1 , was prepared by dissolving 312.3 mg in 2.0 mL of HCl (c (HCl) = 6.0 mol L -1 ), and diluted to 100.0 mL with deionized water. A stock solution of TPTZ was stored in a dark bottle at 4 C. A novel simple kinetic spectrophotometric method for the determination of N-acetyl-L-cysteine (NAC) has been developed and validated. The proposed method is based on a coupled redox-complexation reaction, the first step of which is the reduction of Fe 3+ by NAC; the second one includes the complexation of Fe 2+ , resulting from the preceding redox reaction, with 2,4,6-trypyridyl-s-triazine (TPTZ). The stable Fe(TPTZ)2 2+ complex exhibits an absorption maximum at λ = 593 nm. The initial rate and fixed-time (at 5 min) methods were utilized for constructing calibration graphs. The graphs were linear in concentration ranges from 4.0 × 10 -6 to 1.0 × 10 -4 mol L -1 for the initial rate method and 1.0 × 10 -6 to 1.0 × 10 -4 mol L -1 for the fixed-time method, with detection limits of 1.0 × 10 -6 and 1.7 × 10 -7 mol L -1 , respectively. The proposed methods were successfully applied for the determination of NAC in its commercial pharmaceutical formulations.
