Determination of ascorbic acid in fruit juice

The juice of many fruits and particularly those of the citrus family contain appreciable quantities of ascorbic acid (vitamin C). It is not possible to examine 

Procedure. Prepare a standard solution of ascorbic acid (ca 10 3M) by dissolving about 0.18 g (accurately weighed) in a 1 L graduated flask using oxygen-free water. 

Prepare a buffer solution (pH 4.5) by dissolving 6 g acetic (ethanoic) acid and 13,6 g sodium acetate in 1 L of distilled water. Pipette 10 mL of a commercial sample of citrus fruit juice into a 1 L graduated flask and make up to the mark with oxygen-free water, 

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Schmid et al. used the same principle to develop sensors to be incorporated into FI systems for the determination of ascorbic acid in fruit juices [38] and that of lactic acid in dairy products [39]. The membrane used in both applications consisted of decacyclene dissolved in silicone rubber that was treated similarly as the membrane in glucose sensors (Fig. 3.4.B). The oxygen optrode was coated with a sheet of carbon black as optical insulation in order to protect it from ambient light or intrinsic sample fluorescence. Ascorbic acid oxidase or lactic acid oxidase was immobilized by adsorbing it onto carbon black and cross-linking it with glutaraldehyde. The FI system automatically buffered and diluted the food samples, thereby protecting the biosensor from a low pH and interferents. 

The standard addition method can be efficiently implemented in a flow system with multi-site detection, as demonstrated by the amperometric determination of ascorbic acid in fruit juices [87], Only one sample aliquot was inserted, and the sample zone was monitored under two different conditions, before and after the standard addition by confluence. Measurements related to the first and second detection sites were normalised and the matrix effect was efficiently circumvented. About 55 samples were rim per hour, yielding precise results in agreement with a reference procedure. 

Sometimes the proposed method is an indirect procedure to monitor the excess of oxidant or the color change of an auxiliary reagent such as the method described for the assay of azathioprine, either in pure form or in pharmaceutical formulations the methods are based on the oxidation of the drug with excess N-bromosuccinimide or chloramine-T and determining the consumed reagent by the decrease in color intensity of celestine blue or gall-ocyanine, respectively. Another example is the determination of ascorbic acid in fruit juice and pharmaceuticals on the basis of its inhibition effect on the reaction between hydrochloric acid and bro-mate. The decolorization of methyl orange due to the reaction products was used to monitor the reaction at 510 nm. 

Ensafi, A. A., B. Rezaei, and M. Beglari. 2002. Highly selective flow-injection spectrophotometric determination of ascorbic acid in fruit juices and pharmaceuticals using pyrogallol red-iodate system. Anal. Lett. 35 909-920. 

A method of determining airborne iodine has also been reported.241 Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the, 31I isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242 Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243 Iodine is generated in the flow system as I3- ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloro-isocyanurate in air.244 Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and starch,245-252 as well as modified starches.245,253-255... 

Alamo, J. M., A. Maquieira, R. Puchades, and S. Sagrado. 1993. Determination of titratable acidity and ascorbic acid in fruit juices in continuous-flow systems. Fresenius J. Anal. Chem. 347 293-298. 

Jain, A., A. Chaurasia, and K. K. Verma. 1995. Determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals by flow injection spectrophotometry Matrix absorbance correction by treatment with sodium hydroxide. Talanta 42 779-787. 

Fung, Y. S. and S. Y. Mo. 1992. Application of square-wave voltammetry for the determination of ascorbic acid in soft drinks and fruit juices using a flow-injection system. Anal. Chim. Acta 261 375-380. 

Pires, C. K., A. F. Lavorante, L. M. T. Marconi, S. R. P. Meneses, and E. A. G. Zagatto. 2006. A multi-pumping flow system for chemiluminometric determination of ascorbic acid in powdered materials for preparation of fruit juices. Microchem. J. 83 70-74. 

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

In some cases, the redox reaction is one step in the whole determination process. The well-known reductive behavior of ascorbic acid on Fe(III) results in Fe(II) ions that form a chelate with o-phenanthroline. This chelate is easily adsorbed on an anion-exchange gel of dextran type. The adsorbent is packed into the spectrophotometer cell and the absorbance of the resin monitored at 567 and 800 nm. The ascorbic acid is determined without any preconcentration in fruit juices, pharmaceutical formulations, human urine, and conservative liquids. 

Otsuka et al. (1986) used TLC to isolate and partially characterize degradation products of 2,3-diketo-L-gulonic acid (intermediates in the biosynthesis of ascorbic acid) the products were characterized by different spectrometric methods. Mandrou et al. (1988) devised a TLC procedure to determine ascorbic and dehydroascorbic acids in fruit juices and wine the sample was reacted with 2,4-dinitrophenylhydrazine to form the osazones the osazones were spotted on the TLC plate and quantified by. in situ densitometry at 494 nm. 

Mandrou, B., Chariot, C., and Tsobze, A. D. (1988). Determination of ascorbic acid and dehydroascorbic acid in wines and fruit juices. Ann. Falsif. Expert. Chim. Toxicol. 81 323-332. 

Cardwell and Christophersen reported a dual channel FI system with amperometric detection for the determination of ascorbic acid and sulfur dioxide in wines and fruit juices (Cardwell and Christophersen, 2000). Here, the ascorbic acid was detected at a glassy carbon electrode polarized at 0.42 V (vs. Ag/AgCl), whereas the sulfur dioxide was detected at a Pt electrode polarized at 0.90 V (vs. Ag/AgCl) after separation of the analytes by a gas diffusion unit. The determination of ascorbic acid showed a linear range between 3 and 50 mg L with an FOD of 1.5 mg L for sulfur dioxide the linear range was between 0.25 and 15 mgF i and an FOD of 0.05 mgF" was obtained. The sample frequency achieved with the system was 30 h b The proposed method showed a good agreement with a reference method in the results obtained for white wines and juice samples, while for red wines and sweet wines an extraction procedure of the analytes by solid-phase extraction was required. 

Tang, Y. and M. Wu. 2005. A quick method for the simultaneous determination of ascorbic acid and sorbic acid in fruit juices by capillary zone electrophoresis. Talanta 65 794-798. 

Llamas, N. E., M. S. DiNezio, and B. S. Fernandez Band. 2010. Flow-injection spectrophotometric method with on-line photodegradation for determination of ascorbic acid and total sugars in fruit juices. /. Food Compos. Anal. 24 127-130. 

Greenway, G. M. and P. Ongomo. 1990. Determination of L-ascorbic acid in fruit and vegetable juices by flow injection with immobilized ascorbate oxidase. Analyst 115 1297-1299. 

Zeng, S. and H. Tan. 2000. Simultaneous determination of ascorbic acid (Vc) and L-cysteine in fruit juice by stopped-flow kinetic spectrophotometry. Shipin Kexue 21 60-62. 

Ascorbic acid level in grapes is relatively low compared with other fruits. Zubeckis (5) in Canada reported that ascorbic acid in fresh grapes varied from 1.1 to 11.7 mg/100 ml juice, except for the variety Veeport. This Canadian variety contained a high content of ascorbic acid (33.8 mg/100 ml juice). More recently, Oumac (188) determined the ascorbic acid content of seven grape juices and found a range from 2.0 to 6.0 mg/100 ml. 

For the first biosensor shown in Table 17.2, L-ascorbic acid has been determined in fruit juices and vitamin tablet, by immobilizing a slice of cucumber (Cucumis sativus) onto an 02 electrode in the pH range 6.0-6.5 (phosphate buffer solution) [11] ... 

As shown in Figure 2, the voltammetric response is significantly altered by the coupled chemical reaction and thus allows the energetics and dynamics of these homogeneous chemical reactions to be probed. This figure shows that as the scan rate is increased, the contribution from the homogeneous reaction becomes less pronounced and the voltammogram approaches the shape of that for an electrochemically reversible process. The fact that ascorbic acid is electroactive means that its concentration in fruit juices can be determined without interferences from the coloration of the sample. 

The separation by TLC of the 2,4-dinitrophenylhydrazone (DNP) of dehydroascorbic acid is extremely specific and has been employed for detection and determination of vitamin C in food and feeding stuffs, fats, fruit juices, wines and bacteria etc. In the method of Stroheckeb et al. [129, 130] the ascorbic acid in the extracts is oxidised with 2,6-dichlorophenol-indophenol (Vuilleumieb and Nobile [139] use bromine for this) the dehydroascorbic acid formed is then reacted for 3 hours at 70 C with 2,4-dinitrophenylhydrazine in the presence of a little trichloroacetic acid and thiourea [131]. After cooling 10 min in ice, the red or red-brown precipitate is collected on a sintered glass filter, washed with water and dissolved in ethyl acetate or acetone the solution is evaporated and the residue taken up in acetone. 0.1—1.0 ml (20 to 50 [xg vitamin C) of this solution is applied as a band to air dried silica gel H layers and chromatographed with chloroform-ethyl acetate (50 + 50) or, better, with chloroform-ethyl acetate-acetic acid (60 + 35 -f 5). The red DNP of the dehydroascorbic acid is thus clearly separated... 

Okafor P.N., Nwogbo E., (2005), Determination of nitrate, nitrite, N-nitrosamines cyanide and ascorbic acid contents of fruit juices maijeted in Nigeria, African Journal of Biotechnology, 4(10), 1105-1108. 

Ascorbic acid (vitamin C) appears naturally in most fresh fruits and fruit juices but is often added during the manufacture of juices or soft drinks as a preservative or antioxidant. A simple, fast, and direct SIA method for ascorbic acid determination in juices was... 

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