Electrochemical studies of ascorbic acid, dopamine, and uric acid at a dl-norvaline-deposited glassy carbon electrode

The electrochemical behavior of ascorbic acid (AA), dopamine (DA), and uric acid (UA) on a dl-norvaline-modified glassy carbon electrode (GCE) was studied by cyclic voltammetry. The bare GCE failed to distinguish the oxidation peaks of AA, DA, and UA in phosphate-buffered solution (pH 5.0), while the dl-norvaline-modified GCE could separate them efficiently. In differential pulse voltammetric (DPV) measurements, the modified electrode resolved the overlapped voltammetric responses of AA, DA, and UA into three well-defined voltammetric peaks. Under optimum conditions, the anodic peak currents of DPV for AA, DA, and UA were proportional to the concentration in the range of 20–400, 1–40, and 15–180 μmol/L, respectively, with a correlation coefficient (r) of around 0.998. The detection limits were 5, 0.3, and 10 μmol/L (S/N = 3) for AA, DA, and UA, respectively. Satisfactory results were achieved for the determination of AA in vitamin C tablets, DA in a dopamine ampoule sample, and UA in human blood serum samples.

Experimental and theoretical study of the electrochemical behavior of N-protonated dopamine at Nafion multiwalled carbon nanotubes (MWNTs) modified glassy carbon (GCE) electrode

The electrochemistry of N-protonated dopamine (DAH+) was studied by cyclic voltammetry (CV) at a glassy carbon electrode modified by Nafion multiwalled carbon nanotubes (MWNTs) in phosphate buffers (pH 5.50) and the results show that the standard electrode potential of half reaction for DAH+(O),H+/ DAH+(R) is 0.74 V. Calculations are performed using DFT B3LY/6-31G(d,p) for nine conformers of N-protonated dopamine (DAH+(R)) and three conformers of oxidized N-protonated dopamine (DAH+(O)). The energies of solvation and sum of electronic and thermal free energies of DAH+(R), DAH+(O), p-benzoquinone (p-BQ), and p-hydroquinone (p-HQ) are calculated at same level. The theoretical weighted average of the standard electrode potential (0.732 V) for DAH+(O),H+/ DAH+(R), using the transformed Gibbs free energies of the stable DAH+(R) and DAH+(O) with a p-BQ, H+/p-HQ reference electrode, is consistent with the experimental one of 0.74 V.Key words: dopamine, DFT, cyclic voltammetry, standard electrode potential.

Indirect potentiometric titration of ascorbic acid in pharmaceutical preparations using copper based mercury film electrode

A simple and rapid potentiometric method for the estimation of ascorbic acid in pharmaceutical dosage forms has been developed. The method is based on treating ascorbic acid with iodine and titration of the iodide produced equivalent to ascorbic acid with silver nitrate using Copper Based Mercury Film Electrode (CBMFE) as an indicator electrode. Interference study was carried to check possible interference of usual excipients and other vitamins. The precision and accuracy of the method was assessed by the application of lack-of-fit test and other statistical methods. The results of the proposed method and British Pharmacopoeia method were compared using F and t-statistical tests of significance.

Simultaneous Determination of Dopamine and Ascorbic Acid at a Poly(Toluidine Blue) Modified Electrode

A novel polymer modified electrode is discussed in this paper. This resulting electrode can catalyze remarkably toward the electrochemical oxidations of dopamine (DA) and ascorbic acid (AA). Moreover, it can clearly discriminate the electrochemical oxidations of DA from that of AA based on their semi-derivative voltammograms. Hence, a simultaneous determination of DA and AA based on semi-derivative voltammetry at a poly(toluidine blue) modified electrode is suggested. The detection linear range for DA is 0.4 micromol L(-1)-1.5 mmol L(-1), and AA 0.2 micromol L(-1)-2.4 mmol L(-1), respectively. The resulting modified electrode was tentatively used to determine DA and AA in brain tissue.

Electrocatalytic Determination of L-Ascorbic Acid by Modified Glassy Carbon with Ni(Me2(CH3CO)2[14]tetraenoN4) Complex

A novel modified glassy carbon electrode containing Ni(Me2(CH3CO)2[14]tetraenoN4) complex was used as an electrocatalytic sensor for the determination of L-ascorbic acid in pH = 6.6. The peak potential shifted to negative by 205 mV compared with that for a bare electrode in cyclic voltammograms. The calibration curve was linear up to 6.2 x 10(-3) M with a detection limit 3.1 x 10(-7) M and an RSD% better than 2.47%. This newly modified electrode was applied to commercial pharmaceutical tablets, injections and foods. The obtained results were identical to those obtained by the classical 2,6-dichlorophenolindophenol method.

Electrochemical behavior of dopamine and ascorbic acid at osmium(II) complex cationic monolayer modified gold electrodes

The electrooxidation of dopamine (DA) and ascorbic acid (AA) was studied using gold electrodes modified by a cationic self-assembled monolayer of [Os(bpy)2(bpy-(CH2),3SH)]2+ by cyclic voltammetry. At an [Os(bpy)2(bpy-(CH2)13SH)]2+/Au electrode, the oxidation peak of DA shifted to a much more positive potential as compared with that of a bare gold electrode, while the oxidation peak potential of AA showed a slightly negative shift due to their different electrostatic interactions with the cationic monolayer. Thus, a sufficient potential difference was achieved for distinguishing the electrochemical responses of DA and AA. However, when CH3(CH2)11SH was mixed into the cationic monolayer, the enhanced packing of the mixed monolayer blocked the access of DA or AA to the electrode, resulting in further positive shifts for both oxidation-peak potentials.