Stability-Indicating Polarographic Determination of Acyclovir Through Chelation with Nickel(II)

A simple and sensitive, stability-indicating polarographic method was developed for the determination of acyclovir (ACV) in raw materials and dosage forms. The proposed method relies on the chelation of ACV with nickel(II) in Britton Robinson buffer (pH 5) and measuring the resulting polarographic wave either in the direct current (DCt) or differential pulse (DPP) modes. The polarographic wave has been characterized as being catalytic reduction prewave. Different experimental parameters affecting the formation of the NiACV chelate and its polarographic activity were studied and optimized. The current concentration relationship was found to be linear over the range of 0.88 and 18 g/mL, with minimum detectabilities of 0.10 and 0.19 g/mL using DPP and DCt modes, respectively. The method was used to investigate the kinetics of the acid-induced degradation of the drug. The apparent first-order rate constants and half-life times were calculated.

Determination of Magnesium in Foods by Single-Sweep Polarography

In KOH, the Mg(II)bromopyrogallol red (BPR) complex produced a very sensitive polarographic wave at 1.30 V. The wave height was linear with the concentration of Mg(II) in the range of 0.05 to 2 μg/mL. The detection limit was 0.01 μg/mL. The electrochemical behavior of Mg(II)BPR was studied by electrochemical and spectrophotometric methods. Experiments proved that the polarographic wave of Mg(II)BPR was due to the reduction of BPR in the Mg(II)BPR complex. The method, which was sensitive, selective, and simple to perform, was used to determine magnesium in foods, and the results were consistent with those obtained by atomic absorption spectroscopy.

Determination of Nitrite in Foods by Single-Sweep Polarography

In an acid medium, nitrite diazotized with p-rosaniline and then coupled with 8-hydroxyquinoline in a weak alkaline medium produces azo compounds. The azo compounds produce a very sensitive polarographic wave at −0.70 V (versus the saturated calomel electrode). The height of the peak is linear with the concentration of nitrite in the range of 5 × 10−9 to 5 × 10−7 g/mL. The detection limit is 3 × 10−9 g/mL. The electrochemical characteristics of the polarographic wave are also discussed. The method was used to determine nitrite in sausage. The results agree well with those obtained by spectrophotometry.

Cathodic Reduction of Isatin-3-oxime

The molecule of isatin-3-oxime was studied polarografically over the region of pH 1 to 13. Cathodic reduction within the region of pH 1 to 10.5 gives rise to a single four-electron polarographic wave due to the reduction of the oxime group, whereas two polarographic waves are observed at pH 10.5 to 13: one is due to the reduction of the isatin-3-oxime anion, the other, more negative wave is associated with the reduction of the oxime group in the α-oxime isatinic acid anion, which is the product of hydrolytic cleavage of the isatin ring in the basic pH region. The experimental results give evidence that the substance can exist as seven ionic and molecular species in dependence on pH. The species have been confirmed polarographically, by IR and electronic spectroscopy, potentiometrically and by paper electrophoresis.

Some Aspects of the Electroreduction of Niazid to Nicotinamide on Mercury Electrodes in Acidic Media

The electroreduction of niazid on mercury electrodes has been studied in acidic media (pH < 6). Tafel slopes and reaction orders were obtained at potentials corresponding to the foot of the first polarographic wave. On the basis of both polarographic and voltammetric results it has been shown that the waves appearing at more negative potentials correspond to the reduction of nicotinamide. Protonation of niazid plays an essential role in its reduction and pK values of 1.4, 3.2 and 11.5 were obtained by UV spectroscopy. The process corresponding to the first wave is irreversible, being the second one-electron transfer the rate determining step. Above pH 4 the process is complex due to the overlapping of the waves caused by the occurrence of protonation reactions.

Preparation, chemical and electrochemical reduction of pyrido[2,3-b]quinoxalines and pyrido[3,4-b]quinoxalines

The reaction of 2,3-diaminopyridine with the dimeric 4,5-dimethylcyclohexa-3,5-dien-1,2-dione gives 7,8-dimethylpyrido[2,3-b]quinoxaline, 1, in good yields; in the same way 3,4-diaminopyridine gives the 7,8-dimethylpyrido[3,4-b]quinoxaline 2. The electrochemical reduction of 1 and 2 in hydroorganic medium gives the 5,10-dihydro compounds 6 and 7; 1 and 2 present a single 2e− polarographic wave, in contrast to phenazine which shows two monoelectronic waves. The catalytic hydrogenation of 1 and 2 gives 6 and 7 and does not involve the pyridinic ring as in the case of pyridopyrazines. AlLiH4 does not react with 2 but 1 is reduced into the 1,2-dihydro derivative 8. The behavior of 1 and 2 is thus different from that of pyridopyrazines (which give the 1,2,3,4-tetrahydro compounds) and from that of phenazine (which gives the 5,10-dihydro derivative). NaBH4 reacts with pyridopyrazines to give, according to the substituents, 1,2- or 5,6-dihydro or 1,2,3,4-tetrahydro derivatives. Methylmagnesium chloride reacts with 1 to give a mixture of 2-methyl-1,2-dihydro, 2,6,7-trimethyl, and 4,6,7-trimethylpyrido[2,3-b]quinoxaline. In the case of 2, 4,6,7-trimethylpyrido[3,4-b]quinoxaline is obtained.