Ascorbic acid standard curve

Free radical scavenging activity was assessed using the DPPH (1, 1-di-phenyl-2-picrylhydrazyl) assay as described by Harbilas et al. [22] with incubation time increased to 65 min. Briefly, 250 tiL of 100 timol/L DPPH dissolved in methanol was added to 40 p.L of extract (tested at 5 concentrations) in a microplate well. A standard curve of ascorbic acid (positive control) was included as a reference and all data were blanked against a treatment with only methanol. Absorbance was read with a microplate reader at 517 nm. The inhibitory concentration for 50% scavenging (IC50) of each extract was calculated and compared to the IC50 of the ascorbic acid standard curve. 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

Correlate the decrease observed in a dose-response curve with a standard antioxidant (e.g., trolox, ascorbic acid), expressing the antioxidant activity as equivalents of standard antioxidant, a well-established parameter in this respect being Trolox Equivalent Antioxidant Capacity (TEAC). ... 

To quantitate the ascorbic content of various tissues, a standard curve (3.0-50 /xg/mL ascorbic acid) was prepared in 3% metaphosphoric for each analytical run. A typical standard curve is shown in Figure 4. Correlation coeflBcients of 0.998 or better were consistently obtained for the standard curve. Pooled serum samples were used to measure day-to-day and within-run precision. The coeflBcient of variation for within-run... 

Figure 3.1Z( Thermogravimetric curves for the degradation of standard ascorbic acid samples under three different atmospheres (a) N2, (h) dry air, and (c) humid air.

Figure 3.15 DSC curves for standard ascorbic acid samples at three different heating rates.

Figure 17-10 Voltammogram of 50.0 ml of orange juice and standard additions of 0.279 M ascorbic acid in 0.029 M HNOb. Voltage was scanned at -t-33 mV/s with apparatus in Rgure 17-9. Peak position marked by arrows in the iowest and highest curves changes slightly as standard is added because the solution becomes more acidic.

The peak current is proportional to the concentration of ascorbic acid in the orange juice. We measure peak current at the arrow in Figure 17-10 relative to the baseline extrapolated from the region between —0.4 and 0 V, where little reaction occurs. Any species in juice that is oxidized near +0.8 V will interfere with the analysis. We do not yet know the proportionality constant between current and ascorbic acid concentration. To complete the measurement, we make several standard additions of known quantities of ascorbic acid, shown by the dashed curves in Figure 17-10. 

A standardized NajS solution in sulfur antioxidation buffer (SAOB, consisting of 2N NaOH, IN EDTA and 4% ascorbic acid) was used to established a standard curve. Samples were diluted with the SAOB to an electrode potential within the range of the standard curve. The HjS concentration in the samples could then be calculated from the electrode potential and the dilution factor. 

Standard Curve Ascorbic acid solutions of concentrations ranging from 0.01 to 0.09 mg/ml were prepared from a stock solution by diluting with cold, CO2 - saturated glass distilled water to the requisite concentration. 

Another way to represent the results is in equivalent antioxidant capacity (EAC) of standard antioxidants, such as ascorbic acid, gallic acid, and Trolox. Usually, the acronym referring to the standard antioxidants is put before EAC (TEAC is acronym of Trolox, for example). A calibration curve of the standard antioxidant concentration versus DPPHj (%) or AA% is necessary to obtain the EAC. The result can be expressed as equivalents of the standard antioxidant (e.g., in grams) per gram of the sample. The results of the sample can be obtained in two ways ... 

Figure 4. HPLC standard calibration curve obtained for ascorbic add dissolved in 3% metaphosphoric acid see text for conditions used for ascorbic add y = 0.3547x + 0.620, r = 0.9983.

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