Research On Determination Of Peroxide Value Of Vegetable Oil Using Modified Electrode

This research choosed peroxide value of vegetable oil as detection index, used the biologic al and chemical modification method to make two kinds of electrodes respectively, studied their electrochemical behavior in organic phase medium, discussed the reaction mechanism and the electrod e reaction process, and determined the best technical parameters. By these parameters, a good catalytic r eaction micro environment was built to meet the practical requirement of vegetable oil testing. In the me antime. this method was applied to the micro-electrodes (screen-printed electrodes), and detection tech nology and equipment which can quantify the vegetable oil quickly and accurately were developed. It s main contents are as follows:1. The HRP enzyme electrode with a double-film structure constructed by using the Sol-Gel method can generate a significant reduction current response to lauroyl peroxide in an organic phase medium, and potassium ferrocyanide as the electron mediator can effectively transfer electrons between the HRP enzyme and the electrode surface, and enhance reduction current responses. As the added amount of lauroyl peroxide increased, the reduction current generated by the HRP enzyme electrode gradually increased, and the oxidation current gradually weakened; Through exploration of the relationship between the reduction peak current and the scanning speed, it is demonstrated that when the scanning speed is low, the HRP enzyme electrode mainly involves the mass transfer process control, that is, the reduction reaction is carried out by transfer of the HRP enzyme onto the electrode surface to generate reduction current. When the scanning speed is high, the mass transfer process control of the electrode weakens and is gradually turned into the surface process control, in which the oxidation-reduction reaction of the HRP enzyme electrode is relevant to the oxidation-reduction of the electron mediator potassium ferrocyanide on the electrode surface. The catalytic reduction current of lauroyl peroxide of different concentrations on the HRP enzyme electrode was measured by chronoamperometry, a calibration curve between the catalytic reduction current and the concentration of peroxide was established accordingly, and the apparent Michaelis constant Km of the HRP enzyme in this system is determined according to Lineweaver-Burk equation.2. The research examined that the electrode reaction process of iodine ion and chitosan on the surface of the graphite electrode belongs to the adsorptive stripping voltammetric process. For detailed process, the new material from iodine ions combine with protonized chitosan enriched on the electrode surface. Through the potential scan from negative to positive, due to lose electrons on the surface of the electrode, iodine ions is oxidized to the free iodine elemental which cannot be chitosan adsorption, and then stripping out from the electrode surface, at the same time produce the oxidation peak current. The best determination of iodine ion conditions are:0.1mol/L-citric acid sodium citrate buffer (pH5.0) as the base solution, chitosan concentration is200mu g/mL; Adsorptive stripping parameters:the enriched potential0v, the enriched time180s, the static potential0v,the static time25s; The scanning parameters of differential pulse voltammetry:beginning potential0.1V, terminate potential1.5V, potential increment0.004V, amplitude0.05V, pulse width0.1s, pulse cycle0.2s; In this condition, the iodine ion concentration producing by the oxidation peak current of graphite electrode is in range of0.07～0.49g/L, and show good linear correlation (R=0.9990). This method has a certain anti-interference ability, and show good reproducibility and stability. The used electrodes can be repeated use by some renewable activation steps.3. In the present study, mechanism of the chitosan modified screen-printed electrode catalytic reac tion was described and experimental conditions were defined for the determination of peroxide value i n vegetable oil using differential pulse stripping voltammetry. Due to the fact that protonated chitosan c an absorb only iodide ion, chitosan modified electrode was applied to determine the oxidation process o f iodide. Results showed that the peak current value decreased with increasing peroxide value in the veg etable oil and reveled a good correlation (R=0.9914) with the peroxide in the concentration range of0.01～3.2μg/mL. Moreover, the peroxide value measured in the vegetable oil using chitosan modified elect rode was in agreement with the results obtained using standard iodometric method.4. A New method in the determination of peroxide value was developed, namely by differential pulse anodic stripping voltammetry and using chitosan modified screen printing electrode scan vegetable oil samples, instead of titration to determine changes of the iodine ion concentration. In this research, the proposed method determine peroxide value of40saled vegetable oil samples, and compare with the national standard method (GB/T5538-2005). Through evaluate comprehensive deviation degree and En value to show that in the qualitative evaluation the results all satisfied. In respect of quantitative judgment have33satisfied result, the overall satisfaction rate was82.5%.Especially in the range of0.031～0.090meq/kg and0.096-0.150meq/kg the result is more accurate and satisfaction rate can reach82.35%and93.33%. This research showed that compared with the national standard method, the new method in operational, security, etc. has a certain advantage, the result accuracy was recognized within the limits of certain peroxide value range.