Electrochemical determination of ascorbic acid at the surface of a graphite electrode modified with multi-walled carbon nanotubes/tetradecyltrimethylammonium bromide

Voltammetric Determination of Ascorbic Acid Content in Cabbage Using Anthraquinone Modified Carbon Paste Electrode

In this study, a low-cost, sensitive, and efficient voltammetric method based on anthraquinone modified carbon paste electrode was developed for determination of ascorbic acid in cabbage samples. After cyclic voltammetry was used to investigate the electrochemical behavior of ascorbic acid and to study dependence of oxidative peak current on scan rate and pH, square wave voltammetric method was developed for direct determination of ascorbic acid in cabbage samples. In contrast to the unmodified carbon paste electrode, a remarkable enhancement in oxidative peak current at anthraquinone modified carbon paste electrode confirmed electrocatalytic property of the modifier towards oxidation of ascorbic acid. A better correlation coefficient for the dependence of peak current on the square root of scan rate ( $R^2=0.9963$ ) than on the scan rate ( $R^2=0.9781$ ) indicated that the oxidation of ascorbic acid at anthraquinone modified carbon paste electrode is predominantly governed by diffusion-controlled process. Square wave amplitude, square wave step potential, and square wave frequency are optimized for the investigation of AA in cabbage. The optimized values are 30 mV, 7 mV, and 35 Hz, respectively. Under the optimized method and solution parameters, an excellent linear response was observed between square wave voltammetric peak current of AQMCPE and concentration of ascorbic acid in the range $5\times {10}^{-5}$ to $4\times {10}^{-3}$  M with a better correlation coefficient ( $R^2=0.9993$ ) and detection limit ( $\text{LOD}=1.84\times {10}^{-6}$  M). The ascorbic acid content of the three cabbage samples from three different cabbage growing areas was found in the range $3.686\pm 0.026$ – $8.298\pm 0.011$  mg/g of powdered cabbage. Excellent recovery results between 95.042 and 96.139% for spiked ascorbic acid in cabbage samples confirmed the potential applicability of the developed method based on AQMCPE for the determination of ascorbic acid in real samples like cabbage.

Determination of ascorbic acid content of some fruit juices and wine by voltammetry performed at pt and carbon paste electrodes

A method was developed for assessing ascorbic acid concentration in fruit juices and wine by differential pulse voltammetry. The oxidation peak for ascorbic acid occurs at about 530 mV (versus SCE) on a Pt strip working electrode and at about 470 mV on a carbon paste working electrode. The influence of the operational parameters like the pulse amplitude and the pulse period on the analytical signal was investigated. The obtained calibration graph shows a linear dependence between the peak height and ascorbic acid concentration within the range 0.31-20 mM with a Pt working electrode, and within the range 0.07-20 mM with a carbon paste working electrode. The equation of the calibration graph was y = 21.839x + 35.726, r² = 0.9940, when a Pt strip electrode was used (where y represents the value of the current intensity measured for the peak height, expressed as µA and x the analyte concentration, as mM). R.S.D. = 2.09%, n = 10, C_{ascorbic acid} = 2.5 mM. The equation of the calibration graph was y = 3.4429x + 5.7334, r² = 0.9971, when a carbon paste electrode was used (where y represents the value of intensity measured for the peak height, expressed as µA and x the analyte concentration, as mM). R.S.D. = 2.35%, n = 10, C_{ascorbic acid} = 2.5 mM. The developed method was applied to ascorbic acid assessment in fruit juices and wine. The ascorbic acid content determined ranged between 6.83 mg/100 mL juice for soft drinks (Fanta Madness) and 54.74 mg/100 mL for citrus (lemon) juices obtained by squeezing fruit. Different ascorbic acid concentrations (from standard solutions) were added to the analysed samples, the degree of recovery being comprised between 94.74 and 104.97%. The results of ascorbic acid assessment by differential pulse voltammetry were compared with those obtained by cyclic voltammetry. The results obtained by the two methods were in good agreement.