Electrochemical method for quantitative assay of peroxide compounds

Abstract

The electrochemical method is used for determining the quantity of organic peroxide compounds in food or cosmetic samples in two concentration ranges: a narrow range from 0.5 to 10 μM, applicable to non-peroxidized lipids, and a wide range from 10 to 400 μM, for moderately to highly peroxidized material. The determination is performed in a non-aqueous medium, directly, without complex pre-treatment of the sample, at low energy consumption and at room temperature. The method includes the following stages: obtaining a Co-g-C3N4 / Nation composite electrocatalyst; obtaining a catalytic peroxide electrode consisting of a glassy carbon electrode coated with the Co-g-C3N4 / Nafion composite electrocatalyst; forming an electrochemical peroxide sensor-an electrochemical cell in which the catalytic peroxide electrode is the working electrode, and the electrolyte is acetonitrile with dissolved quaternary ammonium salt; periodically adding equal volumes of the analyte standard solution to the electrolyte in the electrochemical cell; electrochemical measurements; preparing a calibration graph constructed by plotting the electrochemical response versus analyte concentration; determining the concentration of peroxides in the sample using the method of standard addition.

Description

[0001] Electrochemical method for quantitative assay of peroxide compounds

[0002] Field of Technology

[0003] The invention relates to an electrochemical method for the quantitative determination of peroxide compounds, particularly organic peroxide compounds in lipid samples, which will find application in the pharmaceutical, food, and cosmetic industries.

[0004] Prior Art

[0005] Organic peroxides are considered to be very harmful compounds to health because, upon contact with body tissues such as the gastrointestinal tract, epidermis, or mucous membranes, they tend to form free radicals responsible for the development of severe health issues, such as chronic inflammation, ulcers, or proliferating tumors. Their generation ex vivo is due to the processes of lipid peroxidation of oils, which are often components of medicines, cosmetics, or foods, as they oxidize in the air in the presence of light. Therefore, precise control of the peroxide content in commercial products is essential, containing lipids - pharmaceutical, cosmetic, or food products, is crucial.

[0006] Organic peroxides and hydroperoxides are water-insoluble compounds whose current concentration in lipid-containing samples is determined using a very limited arsenal of methods, such as: iodometric titration, chromatographic, optical, and non-enzymatic electrochemical methods.

[0007] The most commonly used method in practice is the iodometric titration method in a mixed aqueous-organic medium, according to BDS EN ISO 3960:2017. In this method, the oxidizing- reducing properties of iodide ions are used as the titrant, and starch is employed as the indicator, which changes the color of the solution from colorless to blue-violet in the presence of iodine released from the sample, and when titrating the aqueous layer with a solution of NazSzOs, the sample is decolorized upon reaching the equivalent point.

[0008] Currently, automatic titrators are commercially available https: / / blog.hannaservice.eu / peroxide-value-in-otls / and htps: / / food.metrohmusa.com / food- analy zers / peroxide-value- iodine-analy zer / for determining the peroxide value of lipid samples, referred to as PV or POV. These titrators utilize the principle of iodometric analysis for the quantitative determination of peroxides in vegetable oils and animal fats.

[0009] In the titrimetric and chromatographic methods for determining peroxide compounds in food or cosmetic samples with significant lipid content, a lengthy and labor-intensive sample preparation procedure is applied. Special preparation of the samples for analysis is required, including dissolving the fat in chloroform, extraction, and titration in a mixed medium.

[0010] There are also portable digital testers that monitor the quality of frying fats in industrial settings or restaurants , such as : htps: / / www.xylem-analytics.com.au / productsdetaiLphp7ebro- F O M-330-F ood -O i 1 - Mon i tor-335 and https: / / matiualzz.com / doc / 46961316 / testo-instruments- testo-270-cooking-oil-test-kit — testo. Their principle of operation lies in measuring the total amount of polar compounds formed during the thermal processing of frying / baking fats, expressed as a percentage. A significant advantage of these testers is that their use does not require specialized training, and they are easy to calibrate. Some models are even equipped with signal lights that indicate whether the fat is safe or hazardous, and their measurement range is quite broad, from 0 to 40%. However, these testers determine the secondary products of fat peroxidation, which are present when the rancidification of the fats has already progressed, and they do not determine peroxides. Furthermore, measurements with these testers are carried out at temperatures ranging from 50 to 200°C, which significantly exceed room temperature.

[0011] From CN115165993 (A), an electrochemical method for the quantitative determination of peroxide compounds is known, comprising the following steps: obtaining a composite electrocatalyst; forming a catalytic peroxide electrode consisting of a glassy carbon electrode coated with the composite electrocatalyst; forming an electrochemical peroxide sensor consisting of an electrochemical cell that includes the catalytic peroxide electrode as the working electrode, a reference electrode, a counter electrode, and an electrolyte; connecting the electrodes of the electrochemical cell to a direct current source and applying voltage; periodically measuring the current; and determining the concentration of peroxides. In the known electrochemical method, the composite electrocatalyst is MoSe2 / Pt — a highly sensitive material, and the electrolyte is a mixture of the analyte — a liquid containing peroxide compounds and a phosphate-buffered saline solution with a pH of 7.4. This method has relatively high sensitivity due to the composite material, but obtaining the latter requires multiple sequential operations and cannot be performed at room temperature. The electrochemical cell in the known method operates only with aqueous solutions, but not with organic solutions. Therefore, this method can only be used for the quantitative determination of hydrogen peroxide, but not for organic peroxides that do not dissolve in water.

[0012] The objective of the present invention is to develop an electrochemical method for the quantitative determination of peroxide compounds, particularly organic peroxides, that is applicable for the qualitative and quantitative assessment of food or cosmetic fats, as well as for the analysis of pharmaceutical products with a high lipid content. The aim is for the method to be fast, highly sensitive, and to have a simplified analysis procedure, allowing samples to be analyzed directly in a non-aqueous medium without complex pre-treatment, and to be performed at room temperature.

[0013] Technical Essence of the Invention

[0014] The task is solved through an electrochemical method for the quantitative determination of peroxide compounds, comprising the following steps: obtaining a composite electrocatalyst; forming a catalytic peroxide electrode consisting of a glassy carbon electrode coated with the composite electrocatalyst; forming an electrochemical peroxide sensor consisting of an electrochemical cell that includes the catalytic peroxide electrode as the working electrode, a reference electrode, a counter electrode, and an electrolyte; connecting the electrodes of the electrochemical cell to a direct current source and applying voltage; periodically measuring the current; and determining the concentration of peroxides.

[0015] According to the invention:

[0016] 1) the composite electrocatalyst is Co-g-C3N4 / Nafion, obtained by adding 1 ml of an aqueous suspension of the polymer Nation, with a concentration of 0.1% to 0.7%, to 0.5 - 5 mg of graphitic carbon nitride doped with tricobalt tetraoxide, containing 2% to 10% CO3O4, and then dispersing it ultrasonically for 30 minutes; 2) the formation of the catalytic peroxide electrode is carried out by mechanically cleaning the glassy carbon electrode, rinsing it with distilled water, and ultrasonically treating it in ultrapure water for 1-2 minutes, then, a drop of 5-8 pl of the composite electrocatalyst Co-g-CalSU / Nafion is deposited on its surface and air-dried for 2 to 24 hours at room temperature;

[0017] 3) the formation of the electrochemical peroxide sensor is carried out by positioning the catalytic peroxide electrode obtained in the previous step, serving as the working electrode, along with a reference electrode Ag|Ag+ and a counter electrode — a platinum foil — within the electrochemical cell, the electrochemical cell has a working volume of 10-50 ml and no separators, and the electrolyte consists of 10 ml of background electrolyte, which is acetonitrile in which tetra-n-butylammonium perchlorate is dissolved at a concentration of 0.1 M;

[0018] 4) a constant voltage current in the range of -500 to -100 mV is applied to the electrodes of the electrochemical peroxide sensor, preferably at -200 mV or -300 mV, and the current strength is measured;

[0019] 5) a steady-state background current with a strength of 0.2 to 0.5 pA is allowed to establish, after which 20 to 50 pl of the analyte under investigation is added to the background electrolyte in the electrochemical cell while continuously stirring;

[0020] 6) The establishment of the signal is awaited, and the new value of the current strength is recorded;

[0021] 7) the concentration of the analyte containing organic peroxide is increased by sequentially adding equal volume amounts of the analyte to the solution in the electrolytic cell;

[0022] 8) after each increase in the analyte concentration, a waiting period of 5 to 10 seconds is allowed for the signal to stabilize, after which the current strength is recorded;

[0023] 9) the concentration of peroxide in the analyte is determined using a previously prepared calibration curve constructed by the method of standard addition.

[0024] The calibration curve is prepared using a standard solution of chemically pure solid organic peroxide, either 0.003 M dibenzoyl peroxide or 0.03 M dibenzoyl peroxide or trichloroperoxybenzoic acid in chloroform. The solution is prepared just before calibration, by successively adding 5 times 20 pl, followed by 6 times 50 pl of the standard solution to the background electrolyte in the electrochemical cell. During the measurements, the solution in the electrochemical cell is continuously stirred at 450-550 revolutions per minute.

[0025] The catalytic peroxide electrode can be used for analysis up to 10 times within 1 day.

[0026] It is preferable for the catalytic peroxide electrode to be stored dried, after rinsing with ultrapure water, in air and at room temperature, covered with a plastic protective cover, for a period of one year.

[0027] If the catalytic peroxide electrode is to be stored for more than 3 days, it is advisable to reactivate it. For this purpose, it is placed in the electrochemical cell filled with background electrolyte, connected to a direct current source, along with the reference and counter electrodes, and treated by cyclic voltammetry in the potential range from -0.4 V to +0.6 V for at least three cycles, after which it is calibrated twice. The main advantage of the proposed electrochemical method is that it operates in a nonaqueous environment and provides a fast and effective method for the quantitative determination of organic peroxide compounds in food or cosmetic samples with significant lipid content. The determination is performed directly, without complex pre-treatment of the sample, as is the case with titrimetric and chromatographic methods, while consuming low amounts of electrical energy and at room temperature.

[0028] Another advantage of the method is that it allows for the electrochemical quantitative determination of organic peroxides in two concentration ranges: a narrow range from 0.5 to 10 pM, which is safe, and a wide range from 10 to 400 pM, suitable for moderately to highly peroxideladen material.

[0029] It is important to note that the proposed new composite electrocatalyst Co-g-CslSU / Nafion, used in the method, has high sensitivity and is produced quickly at room temperature, unlike the composite electrocatalyst known from CN 115165993 (A), whose production requires multiple sequential operations and preheating of the constituent materials.

[0030] The method could also find application in the determination of hydrogen peroxide in various aqueous solutions: acidic, neutral, and weakly alkaline, with a pH of up to 8.5. It can also be used to determine other water-soluble peroxides, such as tert-butyl hydroperoxide, which is a strong oxidizer and a highly flammable explosive chemical.

[0031] Explanation of the Attached Figures

[0032] Figure 1 shows a calibration graph of the correlation between measured and expected amounts of oxygen (O2) equivalents in a real oil sample enriched with peroxides in a narrow range of low peroxide concentrations, from 0.5 to 10 pM, expressed in pmol / 1.

[0033] Figure 2 displays a calibration graph of the correlation between measured and expected amounts of oxygen (O2) equivalents in a real oil sample enriched with peroxides in a narrow range of low peroxide concentrations, from 0.5 to 10 pM, expressed in meq / kg.

[0034] Figure 3 shows a calibration graph of the correlation between measured and expected amounts of O2 in a real oil sample enriched with peroxides in a wide range of high peroxide concentrations, from 10 to 400 pM, expressed in pmol / 1.

[0035] Figure 4 presents a calibration graph of the correlation between measured and expected amounts of O2 in a real oil sample enriched with peroxides in a wide range of peroxide concentrations, from 10 to 400 pM, expressed in meq / kg. Figure 5 displays a graph of the amounts of peroxides determined in a real oil sample according to the Bulgarian State Standard (BSS).

[0036] Figure 6 show a graph of the amounts of peroxides determined in a real oil sample using the electrochemical method according to the invention.

[0037] Figure 7 presents the calibration plot that is used to determine the concentration of peroxides in the analyzed samples of anti-cellulite oils.

[0038] Examples of the Invention’s Implementation

[0039] Example 1: Determination of peroxide content in a real oil sample — Rivana anticellulite oil, enriched with peroxides, using the electrochemical method for quantitative determination of peroxide compounds according to the invention. According to one embodiment of the invention, the electrochemical method for the quantitative determination of peroxide compounds includes the following steps:

[0040] Step 1: Preparation of Composite Electrocatalyst

[0041] To 2.5 mg of pre-weighed graphite carbon nitride (Co-g-C3N4), doped with cobalt tetraoxide, containing 6% CO3O4, 1 ml of an aqueous suspension of Nafion polymer with a concentration of 0.4% is added, followed by ultrasound dispersion for 30 minutes. This results in the formation of the composite electrocatalyst Co-g-C3N4 / Nafion.

[0042] Step 2: Preparation of Catalytic Peroxide Electrode

[0043] A standard glassy carbon electrode is mechanically cleaned, rinsed with distilled water, and subjected to ultrasonic treatment in ultra-pure water for 1-2 minutes. Then, a drop of 5 to 8 pl of the composite electrocatalyst Co-g-C3N4 / Nafion obtained in Step 1 is deposited on its surface and air-dried for 13 hours at room temperature.

[0044] It is important to note that the catalytic peroxide electrode, constructed from a composite material Co-g-C3N4 / Nafion layered onto a standard glassy carbon electrode, can be used for conducting analyses up to 10 times within one day and can be stored for one year or more. It is stored dried, after rinsing with ultra-pure water, in air and at room temperature, covered with a plastic protective cap. Before use after prolonged storage (more than 3 days), the catalytic peroxide electrode must be reactivated. For this purpose, it is placed in the electrochemical cell filled with background electrolyte, connected in a three-electrode configuration, along with the reference and auxiliary electrodes to the electrochemical workstation, and treated by cyclic voltammetry in the potential range from -0.4 V to +0.6 V for at least three cycles, after which it is calibrated twice, as described below.

[0045] Step 3: Formation of the electrochemical peroxide sensor

[0046] The catalytic peroxide electrode obtained in the previous Stage 2 is used as the working electrode, positioned together with the reference electrode Ag|Ag+and the auxiliary electrode — a platinum foil — within an electrochemical cell without separators, with a working volume of 30 ml. Ten milliliters of background electrolyte are added. The background electrolyte is prepared by dissolving a pre-weighed high-purity tetrabutylammonium perchlorate in acetonitrile, so that the final concentration of the quaternary ammonium salt reaches 0.1 M.

[0047] Stage 4: Electrochemical measurements

[0048] A constant voltage of -200 mV is applied to the electrodes of the electrochemical peroxide sensor, and the current is measured. The establishment of a stable background current with a strength of 0.2 to 0.5 pA is waited for. Then, a specific amount of the analyte under investigation, ranging from 20 to 50 pL, is added to the background electrolyte in the electrochemical cell while continuously stirring. The signal is allowed to stabilize, and the new value of the current strength is recorded. The concentration of the analyte containing organic peroxide is increased several times by sequentially adding equal volumes of the analyte to the solution in the electrochemical cell. After each increase in the concentration of the analyte, a waiting period of 5 to 10 seconds is observed for the signal to stabilize, after which the current strength is recorded.

[0049] Stage 5: Preparation of the Calibration Curve

[0050] The calibration curve is prepared using a standard solution of 0.003 M or 0.3 M of chemically pure solid organic peroxide (dibenzoyl peroxide or trichloro-peroxybenzoic acid) in chloroform, which is prepared immediately before calibration. To cover the widest possible range of concentrations, 5 times 20 pL, followed by 6 times 50 pL of the standard solution, are successively added to the background electrolyte in the electrochemical cell. During the measurements, the solution in the electrochemical cell is continuously stirred at 450-550 revolutions per minute. The linear range of the calibration curve is up to 0.2 mM and above, i.e., to a concentration of > 48.45 mg / L.

[0051] The calibration curve is constructed using the standard addition method and by evaluating the accuracy of the method over a range of low and high peroxide concentrations.

[0052] Example 1A) Evaluation of the accuracy of the method at low peroxide concentrations, a narrow (safe) range of 0.5 to 10 pM.

[0053] The following actions are performed:

[0054] Standard solutions of benzoyl peroxide (BPO) in chloroform (CHCI3) are prepared with concentrations of 1, 2, 3, 6, and 10 mM by diluting 0.5 M BPO in CHCI3. The standard solutions are brought to a final volume of 10 ml.

[0055] The catalytic electrode is calibrated with 0.003 M BPO / CHCI3 added to fresh oil (ACM Rivana, peroxide-free).

[0056] A stock solution with a concentration of 0.03 M BPO in CHCI3 is prepared.

[0057] 0.4 ± 0.0175 g (approximately 0.5 ml) of fresh oil is weighed and dissolved in 0.03 M BPO / CHCI3. This solution is designated as BPO / CHCWfresh oil.

[0058] The 0.03 M BPO / CHCI3 / fresh oil solution is diluted 10 times with CHCI3 to a concentration of C / = 0.003 M BPO / CHCb / fresh oil.

[0059] Calibration is performed at a working potential of -0.2 V with the following aliquots of 0.003 M BPO / CHCL / fresh oil: 5 aliquots of 20 pL each, followed by 6 aliquots of 50 pL each.

[0060] All measurements are carried out according to the description, with strict adherence to the procedure.

[0061] The equation of the calibration curve is: y = -2.62 x 10~9+ 0.00272x;

[0062] The signals of all standard solutions of BPO in CHCI3 with concentrations of 1, 2, 3, 6, and 10 mM are measured, for which 0.5 mL from each standard solution is taken, and 0.4 g of fresh oil is dissolved in each.

[0063] Table 1 presents the data for the standards used for calibrating the catalytic peroxide electrode.

[0064] Table 1

[0065] The aliquot taken for analysis from each standard solution is 0.1 ml.

[0066] Table 2 shows the theoretically calculated amounts of peroxides compared to those measured using the electrochemical method.

[0067] Table 2

[0068] Based on the data from Table 2, the graph presented in Fig. 1 has been constructed. It shows the correlation between the measured and expected amounts of oxygen equivalents (O2) in a real oil sample enriched with peroxides, at low concentrations of peroxides, within a narrow (safe) range of 0.5 to 10 pM, expressed in pmol / 1. The graph indicates that the slope of the line is 1.2547. There is a 25.5% deviation of the measured values from the expected ones. However, the response is linear and shows good reproducibility (R2> 0.99) of the analytical result.

[0069] The measured and expected levels of peroxides in pmol / 1 have been recalculated to the measured and expected peroxide values (POV), which are presented in Tables 3 and 4, respectively. Table 3

[0070] Table 4

[0071] Based on the data from Tables 3 and 4, the graph presented in Fig. 2 has been constructed. It shows the correlation between the measured and expected amounts of O2 in a real oil sample enriched with peroxides, at low concentrations of peroxides, within a narrow (safe) range of 0.5 to 10 pM, expressed in meq / kg. The graph indicates that the slope of the line is 1.253. There is a 25.3% deviation of the measured values from the expected ones. However, the response is linear and shows good reproducibility (R2> 0.99) of the analytical result.

[0072] Example IB) Assessment of the method's accuracy at high peroxide concentrations, in a wide range from 10 to 400 pM.

[0073] The following actions are performed:

[0074] 1) Standard solutions of BPO / CHCL are prepared with concentrations of 50, 100, 200, 300, and 400 mM by diluting 0.5 M BPO / CHCk The standard solutions are brought to a final volume of 2 ml.

[0075] 2) The catalytic peroxide electrode is calibrated with a 0.03 M benzoyl peroxide (BPO) solution dissolved in chloroform (CHCI3) and added to fresh oil (Rivana anti-cellulite oil) in the following steps. 3) A starting 0.03 M solution of BPO in CHCI3 is prepared.

[0076] 4) 0.4 ± 0.0175 g (approximately 0.5 ml) of fresh oil is weighed and dissolved in 0.03 M BPO / CHCI3. This solution is designated as BPO / CHCI3 / fresh oil.

[0077] 5) The 0.03 M BPO / CHCI3 / fresh oil is diluted 10 times with CHCI3 to achieve Ct= 0.003 M BPO / CHCI3 / fresh oil.

[0078] 6) Calibration is performed at a potential of -0.2 V with the following portions of 0.03 M BPO / CHCI3 / fresh oil: five times 20 pl, followed by six times 50 pl. The working conditions are identical to those described in Example 1 A.

[0079] 7) The signals of all standard solutions of BPO in CHCI3 with concentrations of 50, 100, 200, 300, and 400 mM are measured, taking 0.5 ml from each standard solution and dissolving ~ 0.4 g of fresh oil in them.

[0080] The data for the standards used to calibrate the catalytic peroxide electrode are presented in Table 5.

[0081] Table 5

[0082] The standard solutions from Stl to St5 are pre-diluted. From the already diluted solutions, an aliquot of 0.1 ml is taken for analysis. The dilution factors (DF) are presented in Table 5. The conditions for calibrating the catalytic electrode are the same as those described in Example 1A.

[0083] The calculations made to determine the concentration of peroxides in the samples, including the measured pmol, expected pmol, measured POV, and expected POV, are similar to the calculations used for evaluating the accuracy of the method in a concentration range of low concentrations. For this purpose, the equation from the calibration graph in point 1A of the same section is used, which in the range of high peroxide concentrations is: y = 2.30 x 10~8+ 0.00348x

[0084] The results of the calculations are presented in Tables 6 and 7.

[0085] Table 6

[0086] Table 7

[0087] The correlation dependencies between measured and expected amounts of O2 in a real oil sample enriched with peroxide over a wide range of peroxide concentrations, from 10 to 400 pM, expressed in pmol / 1 and meq / kg, as indicated in Table 7, are presented graphically in Figures 3 and 4. From the graphs, it is evident that the slope of the line is 1.0874, indicating a deviation of 8.7% of the measured values from the expected values. However, the response is linear and shows good reproducibility (R2> 0.99) of the analytical result.

[0088] Example 2: Comparison of the accuracy of the electrochemical method for quantitative determination of peroxide compounds according to the invention and the conventional titrimetric method for determining peroxide value (POV) (BDS EN ISO 3960:2017).

[0089] The results of measuring the peroxide value of a real sample, using the titrimetric method according to BSS (BDS EN ISO 3960:2017) and the electrochemical method for quantitative determination of peroxide compounds according to the invention, are presented in Table 8.

[0090] Table 8

[0091] The data from Table 8, obtained through the titrimetric method according to BDS and the electrochemical method implemented according to the invention, are presented graphically in Figures 5 and 6, respectively.

[0092] From the graphs, it is evident that the slope of the line obtained by the titrimetric method is 0.7795, indicating a negative deviation of 22.05%. A likely reason for this data is the extraction stage, which is part of the titrimetric method. The response is linear, and the reproducibility is (R2< 0.99).

[0093] In the electrochemical method for the quantitative determination of peroxide compounds according to the invention, the deviation is positive and significantly lower than the one of the titrimetric method - 8.7%. The response is linear, and the reproducibility of the result is good (R2> 0.99).

[0094] It is clear that the method of the invention demonstrates higher linearity and better reproducibility (R2> 0.99) of the analytical result.

[0095] Example 3: Determination of Peroxide Concentration in a Real Sample - Rancid Oil

[0096] The following actions are performed:

[0097] 1) Four samples of 1 ml each are taken from the initial solution of the rancid oil.

[0098] 2) A stock standard solution of BPO / CHCI3 with a concentration of 10 mM is prepared.

[0099] 3) Standard solutions are prepared, and the catalytic electrode is calibrated using the standard addition method. The data for the standard solutions are presented in Table 9. Table 9

[0100] 4) The signals of all standard solutions are measured, with the aliquot portion for analysis being 0.1 ml under the experimental conditions of Example 1.

[0101] 5) The dilution factor (DF) of the standard is calculated when adding a portion of it to 1 ml of the real sample and the DF of the standard after adding 0.1 ml of the sample from the standard solutions to the electrochemical cell containing 10 ml of 0.1 M solution of tetrabutylammonium perchlorate in acetonitrile (TBACICU in ACN).

[0102] 6) The data from the calculations are presented in Table 10.

[0103] Table 10

[0104] 7) The concentration C of BPO / CHCE in the electrochemical cell containing 10 ml of 0.1 M TBACIO4 in acetonitrile is calculated. Table 11

[0105] Based on the obtained concentrations C and the recorded signals S, as presented in Table 11, a calibration curve was constructed, shown in Figure 7. This curve illustrates the relationship between the signal measured with the catalytic electrode (electric current) and the peroxide concentration in the electrochemical cell when a standard peroxide solution is added.

[0106] The intercept on the calibration graph corresponds to the signal from the real sample. The concentration of peroxides in a 15 ml sample of undiluted oil, the number of moles of peroxides in this sample, and the peroxide value (POV) are calculated sequentially, using the data obtained from assessing the accuracy of the method over a wide range of POV. The resulting value is POV = 739.75 ± 64.65 meq / kg. Therefore, the sample containing a mixture of vegetable oils is very rich in peroxides and is highly peroxidized (rancid).

Claims

PATENT CLAIMS1. An electrochemical method for the quantitative determination of peroxide compounds, comprising the following steps: obtaining a composite electrocatalyst; forming a catalytic peroxide electrode consisting of a glassy carbon electrode coated with the composite electrocatalyst; forming an electrochemical peroxide sensor consisting of an electrochemical cell including the catalytic peroxide electrode as the working electrode, a reference electrode, a counter electrode, and an electrolyte; connecting the electrodes of the electrochemical cell to a direct current source and applying voltage; periodically measuring the current strength; determining the concentration of peroxides, characterized in that> the composite electrocatalyst is Co-g-C3N4 / Nafion, obtained by adding 1 ml of an aqueous suspension of the Nation polymer, with a concentration of 0.1% to 0.7%, to 0.5 - 5 mg of graphitic carbon nitride doped with cobalt(II) tetroxide, containing 2% to 10% of CO3O4, followed by dispersion using ultrasound for 30 minutes;> the formation of the catalytic peroxide electrode is achieved by mechanically cleaning the glassy carbon electrode, rinsing it with distilled water, and ultrasonically treating it in ultrapure water for 1-2 minutes, after which a drop of 5- 8 pl of the composite electrocatalyst Co-g-CsNVNafion is deposited on its surface and dried in air for 2 - 24 hours at room temperature;> the formation of the electrochemical peroxide sensor is achieved by positioning the catalytic peroxide electrode obtained in the previous step, serving as the working electrode, along with a reference electrode Ag|Ag+and a counter electrode — a platinum foil — in the electrochemical cell, as the electrochemical cell is with the volume of 10-50 ml and without separators, and the electrolyte is 10 ml of background electrolyte, consisting of acetonitrile, in which tetrabutylammonium perchlorate with a concentration of 0.1 M is dissolved;> a constant current is applied to the electrodes of the electrochemical peroxide sensor within a voltage range of -500 to -100 mV, preferably at a voltage of -200 mV or - 300 mV, and the current is measured;> a steady-state background current of approximately 0.2 - 0.5 pA is established, after which 20 to 50 pl of the analyte under investigation are added to the background electrolyte in the electrochemical cell while continuously stirring;> the signal is allowed to stabilize, and the new value of the current is recorded;> the concentration of the analyte containing organic peroxide is increased by sequentially adding equal volumes of the analyte to the solution in the electrochemical cell;> after each increase in the analyte concentration, a waiting period of 5 to 10 seconds is allowed for the signal to stabilize, after which the current is recorded;> the concentration of peroxide in the analyte is determined using a pre-prepared calibration curve constructed by the standard addition method.

2. An electrochemical method for the quantitative determination of peroxide compounds, according to claim 1 , characterized in that the calibration curve is preparedusing a standard solution of chemically pure solid organic peroxide, 0.003 M dibenzoyl peroxide or 0.3 M trichloroperoxybenzoic acid in chloroform, which is prepared immediately before calibration, where 5 aliquots of 20 pl each are successively added to the background electrolyte in the electrochemical cell, followed by 6 aliquots of 50 pl each from the standard solution, and during the measurements, the solution in the electrochemical cell is continuously stirred at 450-550 revolutions per minute.

3. An electrochemical method for the quantitative determination of peroxide compounds, according to claim 1 or 2, characterized in that the catalytic peroxide electrode can be used for conducting analysis up to 10 times within one day.

4. An electrochemical method for the quantitative determination of peroxide compounds, according to any of claims 1-3, characterized in that the catalytic peroxide electrode can be stored dry, after rinsing with ultra-pure water, in air and at room temperature, covered with a plastic protective cover, for a period of one year.

5. An electrochemical method for the quantitative determination of peroxide compounds, according to any of claims 1-4, characterized in that the catalytic peroxide electrode, when stored for more than 3 days, should be reactivated by placing it in the electrochemical cell filled with background electrolyte, connecting it to a direct current source, together with the reference and auxiliary electrodes, and treating it through cyclic voltammetry in the potential range from -0.4 V to +0.6 V for at least three cycles, after which it is calibrated twice.

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