Determination of vitamin C by flow injection analysis

A simple spectrophotometric method for the determination of tetracycline and doxycycline in pharmaceutical formulations using chloramine-T

A simple, rapid, accurate and inexpensive spectrophotometric method for the determination of tetracycline and doxycycline has been developed. The method is based on the reaction between these drugs and chloramine-T in alkaline medium producing red color products with absorbance maximum at the Λ = 535 and 525 nm for the tetracycline and doxycycline, respectively. The best conditions for the reactions have been found using multivariate method. Beer´s law is obeyed in a concentration ranges 1.03 x 10-5 to 3.61 x 10-4 mol L-1 and 1.75 x 10-5 to 3.48 x 10-4 mol L-1 for the tetracycline and doxycycline, respectively. The quantification limits were 5.63 x 10-6 mol L-1 and 7.12 x 10-7 mol L-1 for the tetracycline and doxycycline, respectively. The proposed method was successfully applied to the determination of these drugs in pharmaceutical formulations and the results obtained were in good agreement with those obtained by the comparative method at the 95% confidence level.

Flow-injection spectrophotometric determination of ascorbic acid in pharmaceutical products with the Prussian Blue reaction

A lot of modern analytical strategies for exploiting chemistries have been developed by using flow-injection analysis. However, even after 20 years of flow-injection evolution, there still are new quantitative procedures being established using old qualitative assays. The formation of Prussian Blue is a classical test to detect Fe(2+) using hexacyanoferrate(III) as a precipitating reagent. This reaction was evaluated for spectrophotometric determination of ascorbic acid employing Fe(3+) and hexacyanoferrate(III) as chromogenic reagents. An excess of the complexing anion avoids the formation of precipitate and forms a deep blue solution when Fe(3+) is reduced to Fe(2+) by ascorbic acid. The maximum absorbance of the colored complex occurs at 700 nm and the molar absorptivity is 3.0 x 10(4) 1 mol(-1) cm(-1). Under flow-injection conditions the Prussian Blue reaction was employed with an intermittent flow of an oxalate alkaline solution for removing the colored product adsorbed on tube and flow-cell walls. Reference solutions containing 5.0 x 10(-6)-1.0 x 10(-4) M of ascorbic acid were employed to obtain the analytical curve (r = 0.9999). For all solutions the relative standard deviation was lower than 1.0% (n=10). Results obtained for ascorbic acid determination in pharmaceutical products (Cewin, Redoxon and Cebion) are in good agreement with those obtained by using a flow-injection procedure involving the reaction between triiodide and ascorbic acid. The sampling frequency is 140 h(-1) and only 430 microl of reagents is consumed in each determination.

Determination of small amounts of L-ascorbic acid using the chemiluminescence of an iron-chlorophyllin complex

Chemiluminescence (CL) was immediately observed after an iron-chlorophyllin aqueous solution was added to an acetonitrile/water mixed solution containing hydrogen peroxide. Quenching of the iron-chlorophyllin complex CL was caused by adding L-ascorbic acid. Based on these facts, a determination method involving small amounts of L-ascorbic acid was developed. As a result, this CL system is able to determine L-ascorbic acid over a wide concentration range of 4.0 x 10(-12) to 4.0 x 10(-4) mol L(-1). Also, coexisting substances, such as sugar and vitamins, did not interfere with the determination. Moreover, the participation to the CL was not observed when using other reducing agents, such as hydroxylamine hydrochloride. As an application for practical use, L-ascorbic acid in soft-drink powder was determined. The experimental value was almost the same as the calculated one (5.30 x 10(-5) mol L(-1)).

Successive determination of thiamine and ascorbic acid in pharmaceuticals by flow injection analysis

A simple and rapid fluorimetric method for the determination of mixtures of thiamine and ascorbic acid is proposed. The procedure is based on the oxidation with mercury(II) of the B1 and C vitamins to form thiochrome (TC) and quinoxaline derivate, respectively. Both reaction products exhibit fluorescence at the same wavelengths (lambdaex = 356 and lambdaem = 440 nm). The procedure is optimised in a flow injection (FI) system and applied with excellent results in the determination of B1 and C vitamins in commercial pharmaceutical preparations. The calibration graphs were linear over the range 2-100 microg ml(-1) for thiamine and 5-100 microg ml(-1) for ascorbic acid. The throughput was 25 samples per hour.

Automatic potentiometric flow titration procedure for ascorbic acid determination in pharmaceutical formulations

A flow procedure for the determination of ascorbic acid in pharmaceutical formulations exploiting potentiometric titration is described. The method is based on the reduction of IO3- by ascorbic acid and the detection was carried out employing a flow-through ion selective electrode for iodide. The flow network controlled by a microcomputer was designed to implement multicommutation for ease of operation and robustness. The titration system allowed the determination of ascorbic acid in pharmaceutical formulations with concentrations ranging from 7.5 to 15.0 mmol l(-1). No significant differences at the 95% confidence level were observed in comparison with results obtained by a manual procedure. Merit figures of results such as a relative standard deviation of 1.0% (n=6) and a reagent consumption of 21.4 mg IO3- per determination were obtained.

Determination of ascorbic acid in pharmaceuticals and urine by reverse flow injection

Two reverse flow injection (FI) methods, using spectrophotometric detection, are proposed for the determination of ascorbic acid. Both methods are based on its reaction with the ethylenediaminetetraacetic acid-CoIII complex in a medium of 5% diethylamine. In the first method, using the peak-height FI technique, ascorbic acid is determined over the range from 2 x 10(-4) to 5 x 10(-3) mol dm-3 and in the second, using the peak-width FI method, the working range is extended (2 x 10(-3)-5 x 10(-2) mol dm-3). Both FI methods were applied to the determination of ascorbic acid in pharmaceuticals while the peak-height FI technique was also used to determine ascorbic acid in urine.

Mechanistic study and kinetic determination of vitamin C

A simple specific method for the determination of vitamin C has been investigated by elucidating the reaction mechanism of its oxidation with cerium(IV) in sulphuric acid media. The reaction kinetics and rate measurements which form the basis of this method were determined by following the decrease of the intense yellow colour of cerium(IV) at 405 nm. The fixed-time and fixed-concentration methods were investigated. The former was found to be more precise and was consequently used for the determination. Advantages of this method over the B.P. and other methods are discussed.