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Indirect method for spectrophotometric determination of ascorbic acid with 2, 4, 6-tripyridyl-s-triazine by flow injection analysis

A flow-injection indirect spectrophotometric method for the determination of ascorbic acid (AA) is proposed. The method is based on the coupled redox-complexation reaction, the first step of which is the reduction of iron (III) by ascorbic acid ; the second one includes the complexation of the reduced iron by using 2, 4, 6-tripyridyl-s-triazine (TPTZ) as chromogenic reagent in buffered medium (pH = 3.6). The stable Fe(TPTZ)22+ complex exhibits an absorption maximum at λ = 593 nm. The three-line manifold with one reaction coil was used. Optimization of manifold parameters and experimental conditions were carried out by means of univariate method. A linear calibration curve is established in a concentration range of 8×10−8 to 1×10−5 mol L−1 AA with regression equation y = 3.14 × 104 x  0.0007 (R2 = 0.9997) (n = 11) and detection limit of 2.4×10−8. The analytical frequency was 180 h−1. Also, the reagent consumption is minimal, only 0.5 mL per sample. The proposed method is simple, rapid, sensitive and reproducible (RSD 0.8%, n = 100). The proposed method with TPTZ as cromogenic reagent is very selective, because only the reducing substances with standard (formal) potentials lower than 0.6 V would have the thermodynamic prediction to interfere in the proposed method. In addition, although some reducing substances have the thermodynamic prediction to interfere in the proposed method, because of the specifity of flow injection analysis, the slow reactions are kinetic discriminated, because the main reaction is allowed to proceed for only few seconds. Tested reducing substances (thiol compounds) did not give serious errors when present at same concentrations as the ascorbic acid. The proposed method can be applied for the determination of AA in pharmaceutical preparations, down to picomole quantity. Usual excipients and other present active substances in pharmaceuticals do not interfere with the proposed method.

ascorbic acid; flow injection analysis; spectrophotometric determination

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