A new method for determining acid values in vegetable oils has been developed for a nontitration pH-metric technique with a reagent consisting of 0.20 M triethanolamine in a 1:1 solution of water and isopropanol. The emulsive properties of this reagent provide rapid (within a minute) and quantitative extraction of free fatty acids from an oil into the solvent phase. Acid values were determined by measuring conditional pH of the emulsified reagent in a pH-meter with an aqueous reference electrode. Three different applications of this technique are discussed. IAOCS 73, 295-301 (1996).
The acid value (AV) of vegetable oils is determined without titration by using a new reagent consisting of triethanolamine in a solution of water and isopropyl alcohol. When the oil sample is mixed with the reagent in the pH-metric cell, free fatty acids from the sample are extracted into the reagent ( 3 - 4 min). The initial pH, called conditional pH´, is measured, a standard acid (HC1) is added, and the final pH, pH´2, is measured. AV is calculated from the difference between pH´! and pH´2. The method is applicable for quality control of vegetable oils during their production, trade, and use
The rapidity and uncertainty of the acid value determination in oilseeds are defined by the rate and completeness of the free fatty acid extraction. The strategy of reagent development for extraction has been discussed, and a set of reagents that provide rapid and complete extraction has been obtained. The set consists of two reagents. Reagent A contains triethanolamine in a mixture of water, isopropanol, and heptane. Reagent B contains a strong acid and inorganic salt in water. Reagent A allows the carrying out of rapid (1-2 min) solid-liquid extraction of the free fatty acids and some other acids from oilseeds. Reagent B provides the separation of the free fatty acids only into the heptane phase (-5 min), which can be used directly for the free fatty acid determination. Two techniques for this determination have been applied: pH-metric and titrimetric. The advantages of the proposed set of reagents are described.
A novel pH-metric method is described for the determination of acid values (AV) in vegetable oils without titration. The method is based on a reagent containing triethanolamine, isopropanol, and water to which an oil sample is added before measuring pH. Oil samples with AV in the range 0.006-0.107 mg KOH/g oil were prepared from commercial soybean oil by treatment with a strong-base anion exchanger in OH − form and addition of oleic acid. Compared to the standard titrimetric method, significantly greater AV were obtained at less then 0.02 mg KOH/g oil. This was due to the influence of triethanolamine hydrolysis on the acid-base equilibrium in the mixture "oil-reagent." Thus, the AV 0.02 mg KOH/g oil is accepted as the limit of quantitation. Because refined oils usually have AV of 0.05 mg KOH/g oil or more, this method should be suitable for practical oil analyses. JAOCS 74, 1339-1341 (1997).KEY WORDS: Acid value method, pH-metric, quantitation, vegetable oils.A new method for pH-metric acid value (AV) determination in vegetable oils without using titration has been developed (1). The advantages in comparison to standard techniques (2) are the reduction of time and potential for automation. The new method is based on extraction of free fatty acids from an oil sample into a reagent containing triethanolamine (TEA), water, and isopropanol. The extraction is rapid (3-4 min) since TEA is a weak base and reacts easily with free fatty acids. The process involves mixing an oil sample with the reagent, measuring pH prior to (pH 1 ′) and after addition of HCl (pH 2 ′) using a commercial pHmeter with an aqueous reference electrode [version II of the method in (1)]. AV is calculated from the difference between pH 1 ′ and pH 2 ′:where 56.11 is the molecular weight of KOH; N st is the standard acid concentration (M); V st is the volume of the standard acid added (mL), which is considerably less than the volume of the reagent; ∆pH = pH 1 ′ − pH 2 ′; and m is the weight of the oil (g). The accuracy, reproducibility, and repeatability of this method satisfied AOAC requirements (3,4). The objective of this study was to determine the limit of quantitation (LOQ) for AV by the method. Since LOQ is the lowest level of analyte that can be accurately measured (5,6), AV LOQ was determined with a bias (deviation from standard titrimetric results) not more than the expanded uncertainty of the method as calculated with a coverage factor 2 of 12-14% (7,8). The LOQ evaluation established the relevance of the method for commercial food oil analysis. EXPERIMENTAL PROCEDURESChemicals. TEA, methyl isobutyl ketone, and oleic acid were obtained from Merck (Darmstadt, Germany); isopropanol and potassium hydroxide from Frutarom (Haifa, Israel); hydrochloric acid, phenolphthalein, and buffers from BDH (Poole, England); potassium nitrate from Baker (Phillipsburg, NY); and strong-base anion exchanger Amberlite IRA-400 (OH) from Supelco (Bellefonte, PA). Soybean oil was purchased from local suppliers.Apparatus. The 632 Metrohm titroprocessor (Metrohm Ltd...
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