A review of a new voltammetric method for determining acids

Voltammetric determination of palmitic acid by electrode modified with reduced graphene oxide

Palm oil is one of the most produced and traded vegetable oils in the world recently. The quality of palm oil is very important to be examined and one of the quality indices is free fatty acid (FFA) content. Thus, in this study, an electrochemical technique for the determination of FFA as alternative to conventional method (titration method) has been explored. The electrochemical method was developed based on electrochemically reduced graphene oxide (rGO) deposited onto screen printed carbon electrode (SPCE) via drop-casting technique. The modified electrode was characterized by physico-chemical and electrochemical methods, respectively. The voltammetric behaviour of 2-methyl-1,4-naphthaquinone (VK₃) in the presence of palmitic acid at the modified electrode was investigated in an acetonitrile/water (3:1) mixture containing 2.5 M lithium perchlorate (LiClO₄). The electrochemical detection of palmitic acid was based on the voltammetric reduction of VK₃ to form corresponding hydroquinone which is proportional to the concentration of palmitic acid. Under optimum condition, the developed method showed a good linear relationship in the concentration ranging from 0.192 mM to 0.833 mM with the detection limit of 0.079 mM. The developed sensor illustrates high sensitivity and rapid detection towards determination of FFA content in palm oil.

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

The bis[(trifluoromethyl)sulfonyl]methyl (Tf2CH; Tf=SO2CF3) group is known to be one of the strongest carbon acid functionalities. The acidity of such carbon acids in the gas phase is stronger than that of sulfuric acid. Our recent investigations have demonstrated that this type of carbon acids work as novel acid catalysts. In this paper, recent achievements in carbon acid chemistry by our research group, including synthesis, physicochemical properties, and catalysis, are summarized.

Rapid separation of free fatty acids in vegetable oils by capillary zone electrophoresis

INTRODUCTION

Olive oil is a very important product to human health since it inhibits formation of free radicals, tumour growth, lesions and inflammatory substances. High concentrations of free fatty acids in olive oils results in lipid deterioration due to oxidative or hydrolytic rancidity.

OBJECTIVE

To optimise an alternative capillary zone electrophoresis methodology, under ultraviolet indirect detection and to determine free fatty acids in edible vegetable oils without derivatisation steps in sample preparation.

METHODS

The condition used consisted of 15 mm NaH2 PO4 -Na2 HPO4 at pH ~6.86, 4.0 mm of sodium dodecybenzenesulphonate, 8.3 mm of Brij 35®, 45% v/v of acetonitrile and 2.1% of 1-octanol, injection at 12.0 mbar of pressure for 4 s, +19 kV of applied voltage and indirect detection at 224 nm, within an analysis time of 11 min.

RESULTS

The capillary zone electrophoresis method was successfully applied to determination of free fatty acids in extra virgin olive oil, virgin olive oil and soybean oil samples. The comparison with the official volumetric titration method showed no significant difference within the 95% confidence interval.

CONCLUSION

The main advantage to the proposed method is the possibility to observe the individual amount of the free fatty acids, which would be useful for researchers interested in studying the effect of the free fatty acids profile on oxidative process in food.

Synthesis of δ-oxo-1,1-bis(triflyl)alkanes and their acidities

The reaction of 1,1-bis(triflyl)ethylene generated in situ with enolizable carbonyls yielded δ-oxo-1,1-bis(triflyl)alkane derivatives. Their acidities in both the gas and solution phases were determined.

Determination of short-chain fatty acids in rat and human feces by high-performance liquid chromatography with electrochemical detection

A simple method for determining short-chain fatty acids (SCFAs) in rat and human feces was developed using high-performance liquid chromatography with electrochemical detection (HPLC-ECD). A two-channel HPLC-ECD system was fabricated using an ion exclusion column and an electrochemical detector with a glassy carbon working electrode. Aqueous solutions of 0.1 mM HClO(4) and of ethanol containing 2-methyl-1,4-naphthoquinone served as a mobile phase and a quinone solution, respectively. Peak areas for lactic, acetic, propionic, butyric, isovaleric, and valeric acids at a detection potential of -0.9 V vs. an Ag/AgCl electrode showed a linear relationship with the acid amount in the range 0.1 to 40 nmol. Standard acids at 4 nmol were determined ten times with relative standard deviations (RSD) of less than 2.0%. The analytical results of healthy human feces were measured within 35 min. RSD (n = 5) in all SCFAs were less than 2.7%, and recoveries of SCFAs were more than 92%. The present method was characterized by reproducibility with the simple and rapid procedure without derivatization of analytes, and it has the potential for clinical and biomedical applications.

Rapid determination of free fatty acids in poultry feed lipid extracts by SB-ATR FTIR spectroscopy

A simple, rapid, and reproducible method has been developed for the quantitative determination of free fatty acid (FFA) content in lipids extracted from poultry feeds by Fourier transform infrared (FTIR) spectroscopy with the use of a single-bounce attenuated total reflectance (SB-ATR) accessory. An FTIR calibration curve was prepared by gravimetrically adding oleic acid (15-37%) to pure refined, bleached, and deodorized (RBD) canola oil and measuring the area of the COOH absorption band at 1710 cm-1. The oil from each of 12 poultry feed formulations was extracted using conventional Soxhlet extraction, and after evaporation of the solvent, the FFA content was determined by the conventional AOCS titrimetric procedure and by the SB-ATR/FTIR method. The SB-ATR/FTIR FFA predictions were related to those determined by the AOCS titrimetric method by linear regression, producing an R value of 0.999 and a SD of +/-0.28% FFA. Time-course spectra collected as lipids extracted into hexane indicated that a 15 min extraction was adequate to obtain a representative sample for FFA determination, with further extraction resulting in little, if any, change in the proportion of FFA in the lipid extract. Only a small volume of the hexane extract ( approximately 20 mL) yielded sufficient material for the SB-ATR/FTIR analysis. Thus, by shortening the extraction time and taking a small sample so as to reduce solvent removal time, the SB-ATR/FTIR procedure provides a very simple and rapid means of determining the FFA content of poultry feed lipids.

Electro-deposit polyaniline on carbon electrode for voltammetric detection of lipase

Normally, the pancreatic digestive enzymes are created and carried into the duodenum in an inactive form. During pancreatitis attacks, these enzymes are inhibited from reaching the duodenum, become activated while still in the pancreas, and begin to autodigest and destroy the pancreas. In order to accurately detect lipase within seconds, a square wave technique was explored without immobilizing enzyme and any other biomolecules over the electrode surface. The cyclic voltammetric (CV) technique was used to coat a carbon electrode with a thin layer of polyaniline. This modified electrode proved to be highly sensitive and specific to detect lipase between 0 and 225 IU/L, which is well within the normal physiological range. The consistency and reliability of this technique proved to be a very formidable detector of lipase present in abnormal quantities due to pancreatitis and other pathological disorders. The interference test has been performed to detect the lipase in the presence of amylase 50, 100, and 150 IU/L to understand the interference phenomenon with lipase detection. There is no significant effect of the amylase concentration over lipase detection resulting from using a modified carbon electrode. The response time of the modified carbon electrode is approximately 5s and the electrode has shown to be stable for 4 weeks at room temperature without special storage condition.

Determination of organic acids by high-performance liquid chromatography with electrochemical detection during wine brewing

Voltammetric determination of acids by means of the electrochemical reduction of quinone was applied to high-performance liquid chromatography (HPLC) with electrochemical detection (ED) for determining organic acids in fruit wines. A two-channel HPLC-ED system was fabricated by use of an ion-exclusion column and an electrochemical detector with a glassy carbon working electrode. Aqueous solution of 0.1 mM HClO(4) and ethanol containing 2-methyl-1,4-naphthoquinone served as a mobile phase and reagent solution, respectively. Determination of acetic, citric, lactic, malic, succinic, and tartaric acids was made by measuring the peak areas of the flow signals due to the reduction current of quinone caused by the eluted acids. The peak area was found to be linearly related to the acid amount ranging from 0.1 to 40 nmol per 20 microL injection. The present method was characterized by reproducibility with the simple and rapid procedure without derivatization of analytes. The method was shown as an effective means for following acid contents in fruit juices during fermentation with wine yeast.

A Disposable Voltammetric Cell for Determining the Titratable Acidity in Vinegar

A disposable voltammetric cell using three pencil leads as working, reference, and counter electrodes was developed for determining the titratable acidity, i.e. the acid content in vinegar. The materials of the pencil leads were graphite-reinforcement carbons (GRCs). A voltammetric determination of acid was made by measuring the reduction prepeak current of 3,5-di-t-butyl-1,2-benzoquinone (DBBQ) due to the presence of acids in unbuffered solution. The potential stability of the pseudo-reference electrode of GRC was examined. The prepeak current was found to be proportional to the acetic acid concentration from 0.05 to 2.7 mM with a correlation coefficient of 0.999. The cell-to-cell reproducibility for 1 mM acetic acid was evaluated with ten individual disposable cells. The RSD of the prepeak current and the SD of the prepeak potential were 2.56% and 0.008, respectively. The titratable acidity in five vinegar samples was determined by voltammetry using disposable cells and compared with that of the titratable acidity determined by the conventional potentiometric titration method. We then observed the results by both methods, and found a correlation coefficient of 0.972. As such, the voltammetry using disposable-cell required only one thousandth the volume of a vinegar sample for the titration method. The disposable cell was superior to the conventional electrochemical cell, in terms of facility, environment-friendly, and economy, and thus a sensor using the present cell would be useful for routine work in the quality control of vinegar.

Determination of aliphatic organic acids by high-performance liquid chromatography with pulsed electrochemical detection

A new ion exclusion HPLC procedure accomplished with a pulsed electrochemical detection for the determination of several common aliphatic acids is described. A triple-step waveform of the applied potentials, based on the formation/inhibition of PtOH species on the electrode surface, is successfully used for sensitive detection of several aliphatic acids in flowing systems avoiding pre- or postcolumn derivatization and/or cleanup procedures. Under optimal chromatographic conditions (i.e., 50 mM HClO(4)) the proposed method allowed detection limits between 0.5 and 7 microM for all investigated acids, and the dynamic linear range spanned generally over 1 or 2 orders of magnitude. Determination of citric, malic, tartaric, lactic, formic, and acetic acids in several foods and beverages was performed, in approximately 15 min, without the necessity of any sample pretreatment.